APPENDIX S: TECHNICAL R B H C · by filter feeders, but mixed communities include occasional...

APPENDIX S: TECHNICAL REPORT FOR BENTHIC HABITATS AND

COMMUNITIES

CLIENT: TNG Limited STATUS: Rev A REPORT No.: R190216 ISSUE DATE: 23 October 2019

Darwin Industrial Processing Facility

Benthic Habitat and Communities

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Important Note

This report and all its components (including images, audio, video, text) is copyright. Apart from fair dealing for the purposes of private study, research, criticism or review as permitted under the Copyright Act 1968, no part may be reproduced, copied, transmitted in any form or by any means (electronic, mechanical or graphic) without the prior written permission of O2 Marine.

This report has been prepared for the sole use of the TNG Limited (herein, ‘the client’), for a specific site (herein ‘the site’, the specific purpose specified in Section 1 of this report (herein ‘the purpose’). This report is strictly limited for use by the client, to the purpose and site and may not be used for any other purposes.

Third parties, excluding regulatory agencies assessing an application in relation to the purpose, may not rely on this report. O2 Marine waives all liability to any third-party loss, damage, liability or claim arising out of or incidental to a third-party publishing, using or relying on the facts, content, opinions or subject matter contained in this report.

O2 Marine waives all responsibility for loss or damage where the accuracy and effectiveness of information provided by the Client or other third parties were inaccurate or not up to date and was relied upon, wholly or in part in reporting.

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WA Marine Pty Ltd t/as O2 Marine ACN 168 014 819 Originating Office – Southwest Suite 5, 18 Griffin Drive, Dunsborough WA 6281 T 1300 739 447 | [email protected]

Version Register

Version Status Author Reviewer Change from Previous Version

Authorised for Release (signed and dated)

Rev A Draft T Hurley C Lane

Transmission Register

Controlled copies of this document are issued to the persons/companies listed below. Any copy of this report held by persons not listed in this register is deemed uncontrolled. Updated versions of this report if issued will be released to all parties listed below via the email address listed.

Name Email Address

Mitch Ladyman [email protected]

Sharon Arena [email protected]

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Acronyms and Abbreviations

Acronyms/Abbreviation Description

APM Animal Plant Mineral

BHC Benthic Habitat and Communities

BPI benthic position index 100 square radius

Chl-α Chlorophyl-a

Development Envelope Designated area for site construction

EIS Environmental Impact Statement

EPBC Act Environment Protection and Biodiversity Conservation Act

GL Gigalitres

ha Hectares

km Kilometers

m Meter

m2 Square meter

MBG Marine Biodiversity Group

MPB Microphytobenthos

NOI Notice of Intent

NT Northern Territory

NT EPA Northern Territory Environmental Protection Authority

O2M O2 Marine

PSU Practical salinity units

ROV Remotely Operated Vehicle

SE Standard Error

SSS Acoustic Sidescan Sonar

Study Area Assessment area for this survey, environment that may be impacted by an aspect of a proposal

SW Diversity Shannon Weiner Index for Diversity

t tonne

t/ha Tonner per hectare

t/yr Tonne per year

ToR Terms of Reference

The Project Construction of a Magnetite concentrate processing facility

TNG TNG Limited

UAV Unmanned Aerial Survey

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Executive Summary

TNG Limited (TNG) is proposing to develop a new magnetite concentrate processing facility at Wickham in the Northern Territory (NT) (the Project). The processing facility and associated access roads, supporting infrastructure and services comprise a development footprint of approximately 264 ha. The magnetite concentrate will be produced at TNG’s proposed Mount Peake Project approximately 1,400 km south of Darwin and transported by rail from the mine site to the Processing facility. An ocean outfall is required as part of the Processing facility construction to dispose of waste by-products. The two options being considered for the oufall are:

Option 1: discharges into Elizabeth River adjacent to the Development Envelope; or Option 2: extends out the mouth of Elizabeth River into soft substrate within East Arm of

Darwin Harbour.

This report has been prepared to characterise the Benthic Habitat and Communities (BHC) that may be impacted by the proposal to support the Environmental Impact Statement and Project approvals.

To characterise BHC in the Project area, a desktop review of existing information relevant to BHC was undertaken combined with field survey assessments to validate the desktop information. The works separated studies undertaken to characterise intertidal and subtidal BHC. Intertidal BHC field surveys involved onsite assessment of canopy cover, species diversity, seedlings and samplings and epifauna at three locations along the intertidal zone consistent with the sites and methods described in Cardno (2013a) to enable comparison with existing data and included one additional site adjacent to the Development Envelope. Aerial imagery was also collected along a transect from each site. The subtidal BHC field survey involved side scan sonar of the seabed to identify seabed features of interest using the reflective image and compare to existing maps.

Intertidal BHC

The mangroves in Darwin Harbour are amongst the most floristically diverse in Australia with 36 species and covering an area of approximately 20,400 ha, which constitutes ~5% of mangrove habitat in the Northern Territory (Brocklehurst & Edmeades 1996). The floristic diversity and extent of mangroves in Darwin Harbour are a significant feature contributing to Commonwealth listing of this area in the Directory of Important Wetlands in Australia (DEE 2019) and Northern Territory Government listing as a Site of Conservation Significance (NRETAS 2019).

Eight of the 10 existing mangrove associations have been identified and mapped within the Study Area, comprising a total of 4,178 ha (~91%) of intertidal BHC and ~20% of mangrove habitat within Darwin Harbour. The Tidal flat (Map Unit 4) is the most widespread community, forming a monospecific Ceriops

australis closed forest, low closed forest and open-forest, comprising 1,710 ha or 37% of the intertidal BHC in the Study Area. Rhizophora stylosa closed-forest is also common, with the Tidal Creek (Map Unit 2) encompassing 1,564 ha (~34%) and Shoreline forest (Map Unit 1) only 40 ha or <1% of intertidal BHC. Two mixed species mangrove assemblages occur on the Hinterland Zone: a low closed forest (Map Unit 6) and a low woodland (Map Unit 7) comprising 367 ha or 8% and 97 ha or 2%, respectively. The Seaward margin (Map Unit 8) open forest/woodland is dominated by Sonneratia alba.

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Studies conducted in Darwin Harbour have estimated the productivity of mangroves through the collection and measurement of leaf litter fallen from particular area over time (Metcalfe 1999) and estimates of the standing above-ground biomass (DLRM 2013). The leaf litter rates are typically highest near the Seaward margin in Sonneratia trees and decline onto the Tidal flat, although increase to moderate rates again on the Hinterland margin. Leaf litter rates range from 3.9 t/ha/yr (Mid-tidal flat) to 12.6 t/ha/yr (Seaward). Conversely, the above-ground biomass rates range from 5 t/ha (Seaward) to 346 t/ha (Shoreline forest), where the biomass is regulated by a combination of tree size (basal area and height) and density. Productivity estimates indicate the Study Area represents approximately 21-23% of mangrove productivity within Darwin Harbour.

The diverse assemblages of mangroves form communities that provide a profound array of ecosystem services to estuarine and coastal habitats. Some of the key services mangroves provide include:

stabilisation of coastal foreshores and habitats; both from coastal erosion and by moderating discharge loads from the terrestrial environment

incorporation of carbon and primary biomass to the estuary provision of a substrate for epiphytic and periphytic flora and fauna provision of three-dimensional habitats and ecosystem services to a multitude of fauna the abundance of associated fauna are important areas for commercial fisheries, popular areas

of community activities such as recreational fishing and birdwatching have cultural significance of historical use.

Small areas of salt flat (Map Unit 11) and beach (Map Unit 12) are found within the intertidal zone of the Study Area, although are not considered to provide areas of any significance to the marine ecosystem of Darwin Harbour. Salt flats are found in Ceriops forests (Map Unit 4) or adjacent to the hinterland and are generally devoid of vegetation with hypersaline soils, although in some instances may contain samphire species, comprise a total area of 383 ha or 8% of the intertidal BHC within the Study Area. The Beach mapping unit is located at Bladin Point and Whickham Point adjacent to existing LNG plants, comprising 26 ha or <1% of intertidal BHC within the Study Area.

Subtidal BHC

The Study Area and fieldwork validation for this evaluation was originally restricted to the subtidal BHC of the Elizabeth River although numerical modelling results of the produced water discharge indicated marine water quality target objectives would not be achieved at the outfall Option 1 location and a second Option 2 was proposed. The discharge pipeline for Option 2 continues out the mouth of the Elizabeth River extending into the centre of East Arm estuary, so the outer boundary of the Study Area was expanded to include East Arm. Therefore, the information is presented in two sections; Elizabeth River and East Arm.

The seabed BHC of Elizabeth River is predominantly mapped as Bare sand and mud (either flats/bars) (404 ha or 44%) and Bare coarse sediment (377 ha or 40%). Bare sand and mud (either flats/bars) occur on the intertidal zone to the lowest astronomical tide (LAT) typically characterised by flat, bare and mobile muds or sand with occasional infauna (e.g. polychaetes, gastropods and bivalves) or mobile fauna (fishes, gastropods, crabs, prawns and shrimp). Bare coarse sediment includes the occasional rocky outcrops with medium to high rugosity habitat in the aphotic zone and variable low cover filter feeder habitat.

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A series of broad scale depressions in front of and north of Short Islet with complex bathymetry showing numerous rocky outcrops towards the mouth of the Elizabeth River represents filter feeder and mixed communities of varying density. The complex BHC towards the mouth of Elizabeth River is identified as an area of importance within the mapped unit. Occasional rocky outcrops were also observed throughout the Elizabeth River and at the mouth of Slack Creek. Filter feeder habitat comprises only 0.3% of subtidal BHC within Elizabeth Creek wheras mixed epibenthic communities contribute approximately 10% of mapped BHC within Elizabeth River. The communities are typically dominated by filter feeders, but mixed communities include occasional photosynthetic epifauna such as hard corals, macroalgae, sponges and soft corals.

A total of 16,377 ha (~75%) of subtidal BHC occurs within “high confidence” mapped areas of Darwin Harbour, including East Arm, consisting of soft bottom benthos communities living in unconsolidated sediments. These habitats generally show low bioturbation (~10 burrows/m2) and a low abundance of animals and plants. The hard substrate comprises ~17% of the mapped area and consists of outcropping bedrock (reef) that varies in rugosity. The deeper exposed reef substrates are dominated by filter feeding communities comprising ~6% of subtidal BHC. Small communities of hard coral dominant communities (1%) are found in the shallow subtidal and intertidal reefs mixed with the filter feeder habitat.

The BHC mapped as mixed community and <10% macrobiota occurs adjacent to Bladin Point, East Arm Wharf and Wickham Point, comprising ~2% and ~8% of mapped area in Darwin Harbour, respectively. These areas generally describe low to moderate faunal cover of soft corals, zoanthids, sponges, bryzoans, hydroids and sea squirts. Wrecks in East Arm also exhibit heavy growth cover of mixed community with a low abundance of solitary hard corals. No seagrass or other epifauna were recorded growing in the sediments around East Arm. The complex benthic habitats between Wickham Point and East Arm Wharf on the outer border of the Study Area identify the most significant area of subtidal BHC within East Arm.

The taxonomic groups and marine communities described in this section are well represented throughout marine environments of the Anson-Beagle Bioregion (DSEWPaC 2012).

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Table of Contents

1. Introduction 10

Project Overview 10 Environmental Approvals 12 Environmental Factors and Objectives 13 Objectives 13 Policies and Guidance 14

2. Methods 15

Desktop Review 15 Targeted Field Surveys 16

3. Results 26

Intertidal BHC 26 Subtidal BHC 37

4. Discussion 43

Intertidal BHC 43 Subtidal BHC 54 Conservation, ecological or social values of the BHC 61

5. Conclusions 65

Intertidal BHC 65 Subtidal BHC 65

6. References 67

Figures

Figure 1 TNG Darwin Processing Facility 11 Figure 2 Mangrove monitoring transect site locations and example of the quadrats recorded within different

assemblages at site C2 18 Figure 3 The equipment used for the survey: a) Starfish Side Scan Sonar and b) OpenROV Trident 19 Figure 4 Side Scan Sonar transects of the survey area 21 Figure 5 A) Backscatter image of seabed B) Classified acoustic image C) Validation via underwater video of classified

target. 22 Figure 6 Existing mangrove assemblages mapped within the study area showing the locations of survey sites 28 Figure 7 Mean percent canopy cover (±SE) in the four main mangrove associations from landward to seaward

recorded at the four validation monitoring locations 31 Figure 8 Number of trees per hectare in the four main associations from landward to seaward recorded at the four

validation monitoring locations 31 Figure 9 Canopy height (m) in the four main associations from landward to seaward recorded at the four validation

monitoring locations 32 Figure 10 Mean number of seedlings/saplings per m2 (±SE) in the four main associations from landward to seaward

recorded at the four validation monitoring locations 32 Figure 11 Mean number of individuals per m2 (±SE) in the four main associations from landward to seaward recorded at

the four validation monitoring locations 35

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Figure 12 Mean species richness per m2 (±SE) in the four main associations from landward to seaward recorded at the four validation monitoring locations 36

Figure 13 Existing Seamap subtidal BHC map showing locations and classifications of the targeted ROV video locations 38

Figure 14 Revised subtidal BHC map following site validation 42 Figure 15 Schematic profile diagram of the typical pattern of mangrove zonation in Darwin Harbour, showing the

predictable sequence of mangrove assemblages from landward to seaward (L to R). The percentage of the total mangrove area, tidal elevation (AHD or Australian Height Datum) and inundation frequency is also indicated. 44

Figure 16 BHC mapping of Darwin Harbour prepared for the Ichthys Project (Geo Oceans 2011) 58 Figure 17 Conceptual diagram showing the primary productivity processes in Darwin Harbour described by Burford et al.

(2008) 60

Tables

Table 1 Environmental factors and objectives for the theme ‘Sea’ to be addressed by the EIS for the Project 13 Table 2 Site locations for the intertidal BCH field survey 17 Table 3 Coordinates for targeted ROV locations 23 Table 4 Subtidal BHC classes classified within the habitat map of the survey area from Smit et al. (2012). 24 Table 5 Intertidal BHC mapping unit classifications of the Study Area indicating total area in hectares 27 Table 6 Forest structure and composition information recorded at locations/quadrats, including no. of trees, canopy

height, composition and comparison to the mapped mangrove association 30 Table 7 Comparison of the 2019 results for with previous reported during Inpex Ichthys Project surveys in 2012 and

2014 (Cardno 2013a, 2015a) 33 Table 8 The total number of burrows, individuals and species, the Shannon Weiner Index and total numbers of

taxonomic groups recorded 35 Table 9 BHC classifications of the study area indicating total area in hectares 37 Table 10 ROV Video depths, mapped class in Smit et al (2012), analysis observations and a representative image40 Table 11 Estimates of productivity based on leaf litter (Metcalfe, 2011) and above-ground standing biomass (DLRM

2013) 45 Table 12 The community classifications from Geo Oceans (2011), total area and relative composition of subtidal BHC

within “high confidence” areas of Darwin Harbour 57 Table 13 Directory of Important Wetlands in Australia – Information Sheet for Port Darwin NT029 62

Appendices

Appendix A Mangrove Site Images A

Appendix B Mangrove Fauna Results D

Appendix C Information Page: Darwin Harbour Site of Conservation Significance E

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1. Introduction

Project Overview

TNG Limited (TNG) (the Proponent), proposes to construct and operate the Darwin Processing Facility (the Processing facility) at Wickham in the Norther Territory (NT) (the Project). The Processing facility is proposed to process magnetite concentrate (concentrate hereafter) to produce:

vanadium pentoxide (V2O5) flake– for use in steel, non-ferrous alloys, chemicals, catalysts and energy storage (vanadium redox batteries);

titanium dioxide – for use in paint, plastics, paper and inks; and ferric oxide – for use in steel making.

The three products will be exported through the Port of Darwin’s East Arm Wharf to international customers.

The Processing facility is proposed to be located on Lot 1817, Hundred of Ayers, Middle Arm Industrial Precinct, Wickham, approximately 16 km south east of Darwin, in the Northern Territory (Figure 1). The site is located adjacent to the Elizabeth River and encompasses 507 hectares (ha), of which 40 ha is mangrove forest and intertidal zone, and 467 ha is terrestrial land. The Project and associated access roads, supporting infrastructure and service comprise a development footprint of approximately 264 ha.

The magnetite concentrate will be produced at TNG’s proposed Mount Peake Project approximately 1,400 km south of Darwin, which is the subject of a separate environmental assessment and approvals process. The Mount Peake Project will involve the mining of a polymetallic ore body (enriched with vanadium, titanium and iron) and beneficiation of the ore to produce magnetite concentrate. The concentrate will be transported by rail from the mine site to the processing facility.

An ocean outfall is required as part of the Processing facility construction to dispose of waste by-products. The ocean outfall is proposed to discharge approximately 12 GL/year of treated process water into the Elizabeth River or East Arm. The two options being considered for the oufall are:

Option 1: discharges into Elizabeth River adjacent to the Development Envelope; or Option 2: extends out the mouth of Elizabeth River into soft substrate within East Arm of

Darwin Harbour.

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Figure 1 TNG Darwin Processing Facility

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Environmental Approvals

TNG submitted a notice of Intent (NOI) to the Northern Territory Environment Protection Authority (NT EPA) and Department of Lands Planning and Environment (DLPE) on 27 October 2015 for consideration under the Environment Assessment Act 1982 (EA Act).

On 15 January 2016, the NT EPA determined the Project requires assessment at the level of an Environmental Impact Statement (EIS). The NT EPA decision (NT EPA 2016a) was based on the following risks and potential environmental impacts:

risks to biodiversity values1 of adjacent areas, including nationally significant mangrove communities2, marine ecosystems, local fisheries and listed threatened species;

environmental risks associated with vegetation clearing, erosion and sediment control, uncontrolled discharges, dust, spills, disturbance of acid sulfate soils, contamination of soils, surface water and / or ground water;

potential for Project noise, lighting and emissions to impact on existing residents, potential for future residential development of the area, and other sensitive receptor;

potential for introduction and / or spread of weeds; risks of exposure of workers to high biting insect numbers; environmental risks associated with waste streams and waste management practices; risks associated with transport, handling and / or storage of reagents, products and / or

hazardous material; public health and safety risks associated with siting a potential major hazard facility

adjacent to Darwin Harbour; and potential for impacts on service infrastructure and service supply capacities, due to Project

demands.

The Project was referred under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) and on 4 January 2016 it was determined to constitute a controlled action requiring assessment under the EPBC Act, under the bilateral agreement between the Australian and Northern Territory (NT) Governments. The controlling provisions included the likely significant impact on:

listed threatened species and communities (sections 18 & 18A); and listed migratory species (sections 20 & 20A).

1 Smit, N., Penny , S.S. and Griffiths, AD., 2012. Assessment of marine biodiversity and habitat mapping in the Weddell region, Darwin Harbour. Report to the Department of Lands, Planning and Environment. Department of Land Resource Management, Palmerston

2 Northern Territory Government. Sites of Conservation Significance Darwin Harbour.

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Environmental Factors and Objectives

The NT EPA Environmental Factors and Objectives (NT EPA, 2018) was developed to improve certainty and increase transparency within the Environmental Impact Assessment (EIA) process. The 13 environmental factors are categorised under five themes of: Land, Water, Sea, Air and People and Communities.

The Terms of Reference (ToR) for the Project (NT EPA, 2016b) was released by the NT EPA prior to the publication of the NT EPA Environmental Factors and Objectives. Therefore, the ToR does not identify specific Environmental Factors that the EIS must address. Discussions between the NT EPA and the Proponent subsequent to publication of NT EPA (2018) have identified the EIS must address the three factors and corresponding objectives identified by the NT EPA within the theme Sea presented in Table 1.

Table 1 Environmental factors and objectives for the theme ‘Sea’ to be addressed by the EIS for the Project

Theme Factor Objective

Sea Marine Flora and Fauna Protect marine flora and fauna so that biological diversity and ecological integrity are maintained


Protect benthic communities and habitats so that biological diversity and ecological integrity are maintained

Marine Environmental Quality

Maintain the quality and productivity of water, sediment and biota so that environmental values are protected.

Objectives

TNG commissioned O2 Marine (O2M) to undertake desktop review and marine environmental investigations of Sea relevant themes to support the EIS and Project approvals. Impact and assessment rely on knowledge of the existing environment of the Study Area. For instance, in order to determine which habitats and biota are being affected by disturbance, their distribution and diversity has to be understood. Furthermore, the types of impacts and their severity and consequence for the whole ecosystem can only be evaluated on the background of a comprehensive understanding of the ecological context.

This document is intended to characterise the factor Benthic Habitat and Communities (BHC) that may be impacted by the proposal. This document provides an account of the BHC of the Study Area using desktop investigations and site-specific surveys.

The specific objectives of this report are to:

Identify existing and historical mapping of the Study Area, adjacent potentially impacted areas and reference areas to temporally and spatially characterise the known distribution and diversity of BHC within the Study Area;

Ground-truth and validate the predicted distributions and diversity of BHC within the study area; Describe the current understanding of the ecological role and value of the BHC in the Proposal

area; and Review of any conservation, ecological or social values of the BHC that should be considered.

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Policies and Guidance

The following policies and guidance have been consulted in the evaluation of the environmental factor marine fauna, which may potentially be impacted by the proposed project:

NT EPA Environmental Factors and Objectives (NT EPA 2018); Guideline for Reporting on Environmental Monitoring (NT EPA 2016c); and Coastal and Marine Management Strategy (DENR 2019).

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2. Methods

Desktop Review

A number of studies have been undertaken to characterise the extent and better understand the environmental value and significance of intertidal and subtidal BHC within Darwin Harbour. The results of many of these studies are publicly available and have been used to characterise, map and describe the environmental values and significance of the intertidal BHC in the Study Area.

O2M completed a comprehensive desktop review of the intertidal and subtidal BHC in the Study Area as a preliminary component of this report, using information derived from surveys undertaken for previous coastal development projects in Darwin, relevant scientific journal literature, and other studies commissioned by government authorities.

2.1.1. Study Area

The assessment of BHC in the Study Area is primarily focussed on the intertidal and subtidal zones from East Arm and the Elizabeth River. The Study Area is intended to represent the environment that may be impacted by an aspect of a proposal, which extends approximately 10 km either side of the Development Envelope up the river and to between East Arm Wharf and Wickham Point. The intertidal and subtidal Study Area for this assessment is presented in Figure 1.

The Study Area for this evaluation was originally smaller with the field investigation focussed on the potential for construction or operational impacts from the Development Envelope and outfall Option 1. Outfall option 2 was subsequently proposed extending further towards East Arm and post-hoc consideration of a broader Study Area was included using review of existing information collected within Darwin Harbour.

The field assessment of intertidal BHC includes sites outside of the Study Area of Middle Arm to place the relative ecological values of intertidal habitats within the Study Area in local and regional context. The intertidal BHC incorporates a strip of coastline from mean low water neap tides (~3 m lowest astronomical tide (LAT)), where the shoreline edge of the mangrove community starts, to the intertidal habitats of highest astronomical tidal level (HAT) or the landward edge of the mangroves. The remaining area of the intertidal zone to the lowest astronomical tidal level (LAT) has been classified within subtidal maps. The intertidal zone typically extends approximately 500 m inland, although may extend up to ~2 km inland at the southern portion of the Development Envelope.

In this report, East Arm describes estuarine waters between East Arm Wharf and Wickham Point and includes the mouth of the Elizabeth River and associated creeks. Elizabeth River is considered to be defined by the southern end of the Study Area to Bladin Point.

2.1.2. BHC Mapping

Seamap Australia (Seamap) was used to provide an existing map of marine benthic habitats of the study area. Seamap provides a nationally consistent spatial layer of benthic marine habitats which have been provided to the Australian Ocean Data Network portal.

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Key data layers relevant to characterising the intertidal and subtidal marine BHC in the Study Area which support the Seamap include:

DENR (2012). Mangroves of Darwin Harbour; Smit et al. (2012). Assessment of marine biodiversity and habitat mapping in the Weddell

region, Darwin Harbour; Heap et al. (2006). Geomorphic Features 2006; Dyall (2005). Australian Coastal Waterways geomorphic habitat mapping (national aggregated

product); and Griffin et al. (2012). A Nationally Consistent Geomorphic Classification of the Australian

Coastal Zone.

The maps were updated by clipping any areas that have since been developed from the mapping provided and habitat associations were validated using the methods described below.

Targeted Field Surveys

2.2.1. Intertidal BHC

O2M and Animal Plant Mineral (APM) undertook a field survey of intertidal BHC between 21 and 24 March 2019 to collect information on mangrove tree and associated invertebrate fauna health to validate the desktop review findings and enable an investigation into the functional ecological value and regional significance of intertidal BHC adjacent to the Study Area.

The key indicators applied during the field survey attempt to enable comparison with existing methods and data collected which included:

Forest structure and composition; Canopy cover – in 20 m x 20 m plots and remote sensing; Seedling and sapling density and recruitment; and Mangrove fauna – total species richness and total abundance of benthic fauna.

The four sites selected for the field survey are listed in Table 2 and shown in Figure 2. Three sites were used during the Inpex Ichthys Project mangrove community health monitoring program (Cardno 2013a), and an additional site was included to locate a site either side of the Development Envelope within the Elizabeth River. The zones selected for survey are intended to represent the typical pattern of zonation in assemblages from the hinterland to the seaward edge (Metcalfe 2007). An example of the quadrat plots along one of the sites within each assemblage is presented in Figure 2.

At each site, proposed transects were traversed on foot, proceeding from the landward access point to the seaward margin using a GPS and compass. Indicators were measured within 20 m x 20 m monitoring plots marked using flagging tape from each zone. The survey included targeted Unmanned Aerial Survey (UAV) imagery capture of intertidal BHC at each location.

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Table 2 Site locations for the intertidal BCH field survey

Site Name Site Code Zone Eastings Northings

East Channel Island C2

6 710000 8608178 4 704428 8610747 2 704289 8610738 8 704265 8610663

Middle Arm C3

6 710000 8608178 4 709931 8608103 2 709800 8607964 8 709754 8607865

Elizabeth River - Creek I5

6 712882 8611866 4 712796 8611953 2 712671 8611976 8 712590 8612009

Elizabeth River – River1 I7

6 713354 8612883 4 713438 8613041 2 713601 8613121 8 713745 8613185

1 Only site not surveyed previously as part of the Inpex Ichthys Project mangrove community health monitoring program (Cardno 2013a).

Forest Structure and Composition

The number of trees, species composition and canopy height were recorded by using estimates from a 10 m x 10 m subplot of the 20 m x 20 m plot. Species of mangrove present were classified using the Key to the mangroves of Australia (Clifford & Jessup, 2006) combined with habitat notes described in Brocklehurst and Edmeades (1996).

Canopy Cover

Forest canopy cover was measured using a Stickler’s modified (17-point) spherical forestry densiometer (Stickler 1959) to provide estimates of foliage cover (combined leaf and branch cover). Three replicate canopy cover measurements were taken from two 10 m x 10 m subplots in each 20 m x 20 m plot. Each measurement involved readings at a randomly selected point (located using random number tables), whilst facing north, east, south and west, with a total 48 readings from 12 replicates obtained per plot.

Field readings were converted to percentage canopy cover by multiplying by 100/17. The overall canopy cover for sites in the different assemblages was calculated as the mean of densiometer readings recorded from each zone.

Seedling and Sapling Density

The density of existing populations of seedlings and saplings within each 20 m x 20 m monitoring plot was measured by direct counts within 1 m x 1 m quadrats. Four replicate quadrats were placed at randomly selected points in each plot marked using fluorescent cotton string. The abundance of seedlings and saplings within each random quadrat was recorded and averaged across the site. The number of seedlings and saplings were then added together to provide an indication of recruitment.

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Figure 2 Mangrove monitoring transect site locations and example of the quadrats recorded within different assemblages at site C2

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Fauna

Assessment of mangrove fauna was undertaken using four 1 m x 1 m quadrats randomly deployed on the surface of the mud within the 20 m x 20 m plot to sample the epifauna present. Most invertebrates in the mangrove forests live in burrows which are used for refuge and are easily startled. The following techniques were employed to avoid disturbing the fauna and ensuring repeated representative counts were achieved:

The mangrove fauna was assessed first prior to disturbing the site to undertake the flora assessment;

Each quadrat was surveyed for a minimum period of 10 minutes; and Commencement of the survey time for each quadrat was delayed by a minimum of 5 minutes

after establishing the quadrat.

2.2.2. Subtidal BHC

O2M undertook a field survey of subtidal BHC using two separate techniques:

Acoustic Sidescan Sonar (SSS) to identify consistent habitat features; and Remotely Operated Vehicle (ROV) to ground-truth areas of interest based on identified SSS

features.

The SSS survey was undertaken on 24 March 2019 and the ROV survey images were collected on 26 September 2019 to validate existing mapped benthic classifications.

The equipment used for each technique is presented in Figure 3.

a)

b)

Figure 3 The equipment used for the survey: a) Starfish Side Scan Sonar and b) OpenROV Trident

Side Scan Sonar

The side scan sonar system used in this survey was a portable dual channel 450 kHz system (Tritech Starfish 452F) that was operated through a topside control unit and acquisition software Scanline V2.1. While immersed in the water column, the side scan sonar records acoustic waves (i.e. backscatter data)

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that are reflected by the seabed. High accuracy (<1 m) geographic positioning of the side scan sonar data was acquired with a GPS antenna Starfish.

The SSS was towed behind the vessel along parallel transects within the designated area at slow speed (~4 knots) and ~50 m spacing. The data was collected at 450 kHz with a swath width four to five times the water depth, which was the optimal setting for the acquisition of high-quality data in these shallow regions. In deeper waters (>6 m) a broader swath width of 50 m (i.e. ~25 m on either side of the towed unit) was acquired. An image of the SSS transects undertaken are presented in Figure 4.

The SSS data was loaded into Sonarwiz.7.2 software to identify acoustic features of interest to validate existing mapping. The analytical method of acoustic image classification uses square windows in the acoustic image and the range of statistics about the pixels in the window to measure ‘texture’ for each pixel location, which is a function of the refracting substrate (i.e. seabed). Fourteen (14) target locations were subsequently selected for validation using the ROV.

‘Acoustic supervised classification’ (MESH (2009)) was then performed using the ROV to collect imagery of the BHC at 14 target locations to assign a habitat classification to the backscatter data classes (Figure 5).

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Figure 4 Side Scan Sonar transects of the survey area

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Figure 5 A) Backscatter image of seabed B) Classified acoustic image C) Validation via underwater video of classified target.

Remotely Operated Vehicle (ROV) Video

A Trident ROV with real-time video streaming and recording to a topside JXD s192k android tablet controller was deployed from the survey boat at each target location.

Captured camera footage (video and stills) were viewed and classified into BHC classes and manually recorded at each of the 14 target ROV locations with the following details: GPS coordinates, depth, and time. The coordinates for the targeted ROV locations are presented in Table 3. The target area was surveyed until BHC classification could be validated with confidence (e.g. bare sand was quick to survey whilst a more intensive survey was required for variable coral reef cover). The footage was stored on android tablet and backed up on computer.

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Table 3 Coordinates for targeted ROV locations

Target ID Latitude Longitude

T1 -12.53905875 130.9757134 T2 -12.53352411 130.9533388 T3 -12.53262878 130.9420797 T4 -12.53191569 130.9388387 T5 -12.53403469 130.9430388 T6 -12.53396144 130.9448512 T7 -12.53511714 130.9585219 T8 -12.53651831 130.9560005 T9 -12.53608678 130.9398566

T10 -12.54570864 130.9550562 T11 -12.54768728 130.946488 T12 -12.54630992 130.946776 T13 -12.5453832 130.9471018 T14 -12.53123389 130.9411552

To facilitate comparison, the habitat classification is based on the 10 community classes used in Smit et al. (2012). The subtidal BHC classes used in Smit et al. (2012) and descriptions applied to this survey to map the proposed Study Area are presented in Table 4.

The underwater video imagery captured during the survey was reviewed in the office to validate BHC field classification. The validated video classification at each of the 14 locations was then used to assist the ‘supervised’ classification of the SSS data and the spatial area of the identified features.

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Table 4 Subtidal BHC classes classified within the habitat map of the survey area from Smit et al. (2012).

Code Classification Depth Substrate Geomorphology Description

1 High Density Filter Feeders

Subtidal aphotic

Hard: high-medium rugosity; flat to steep sloping

Reef High abundance of heterotrophic non-photosynthetic BHC; Community composition dominated by sponges (encrusting & erect), azooxanthellate gorgonians & soft corals, ascidians, hydroids. Lacks macro algae, corals. Infauna None

2 Medium Density Filter Feeders

Subtidal aphotic

Patchy hard with coarse mixed sediment; medium to low rugosity

Reef, reef-flat, tidal channel

Moderate diversity and cover of non-photosynthetic BHC Community composition dominated by sponges (encrusting & erect), azooxanthellate gorgonians & soft corals, ascidians, hydroids. No macroalgae or corals. Infauna Polychaete worms Mobile fauna Shrimps

3 High Density Mixed Epibenthic community

Subtidal photic

Hard: high-medium rugosity; flat to steep sloping

Reef Diverse and high cover photosynthetic/non-photosynthetic BHC; Dominated by varying degrees cover & abundance of corals, macro algae, sponges (encrusting & erect), soft corals, ascidians, hydroids. Infauna None

4 Medium Density Mixed Epibenthic community

Subtidal photic

Patchy hard with coarse mixed sediment; medium to low rugosity

Reef, reef-flat, tidal channel

Moderate diversity and abundance of photosynthetic/non-photosynthetic BHC Dominated by varying degrees cover & abundance of corals, macro algae, sponges (encrusting & erect), soft corals, ascidians, hydroids. Infauna Polychaetes, gastropods & bivalves Mobile fauna Abundant fishes, gastropods & penaeoid crustaceans

5 Low-Medium Density Mixed community

Intertidal Patchy hard with mobile substrate; generally flat or sloping

Reef-flat Low to moderate diversity & cover of photosynthetic/non-photosynthetic BHC Dominated by varying degrees cover & abundance of corals, macro algae, sponges (encrusting & erect), soft corals, ascidians, hydroids. Infauna Polychaetes, gastropods & bivalves Mobile fauna Crustaceans (crabs, amphipods, shrimps and prawns), gastropods & bivalves

6 Bare Rocky Reef Intertidal Hard, can be patchy with mobile substrate; rugosity varies

Reef, intertidal platform

Bare

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Code Classification Depth Substrate Geomorphology Description

7 Bare coarse sediments Subtidal aphotic

Coarse sediment with rocky outcrops. Medium to high rugosity

Tidal channel, reef slope

Bare in mobile substrate, although filter feeder habitat where rocky outcrops occur. Diversity is variable & cover typically low where BHC present

8 Bare Sandy / Mixed sediments

Subtidal aphotic/ photic

Sand/mixed sediment; Low rugosity

Sandy flat Generally bare. Diversity is variable & cover typically low where BHC present. Infauna Polychaetes Mobile fauna Fishes, crabs & prawns

9 Bare Coarse sediments - rippled

Subtidal photic

Sand; Medium/variable rugosity

Rippled sand flats Bare Infauna Limited Polychaetes & bivalves Mobile fauna Crustaceans (prawns, shrimps & Mysids),

10 Bare sand / mud Intertidal Muds/sands; low rugosity Intertidal flats Bare Infauna Polychaetes, gastropods & bivalves Mobile fauna Fishes, gastropods, crabs, prawns & shrimp

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3. Results

Intertidal BHC

3.1.1. Habitat Mapping

The classification of intertidal BHC mapping units within the Study Area applies the same 12 classes consistent with that reported in Brocklehurst and Edmeades (1996) and has since been widely applied in subsequent research and monitoring studies of Darwin Harbour (Comley, 2002, Metcalfe 2007, Cardno 2013a, 2015a). Mangrove communities fringe the embayment of Darwin Harbour and therefore 10 of the 12 intertidal BHC mapping units are recognised mangrove associations. The distribution of the mapping units found in the Study Area are presented in Figure 6.

Eight of the 10 mangrove associations have been identified and mapped within the Study Area and the composition of the assemblages are summarised in Table 5. This classification and numeration of mangrove assemblages is not listed consecutively from seaward to landward margins although is consistent with historical mapping classifications (Brocklehurst and Edmeades 1996). The Study Area comprises a total area of intertidal zone of 4587 ha.

The dominant mangrove associations are Ceriops australis (=C. tagal) low closed forest (1710 ha) and Rhizophora stylosa/Camptostemon schultzii closed forest (1564 ha). The Mid-tidal flats mapping unit (C. australis) is widespread within the Study Area (37%) forming low closed forest occurring in mid to high tidal flat regions. The Tidal creek mapping unit (R. stylosa/C. schultzii) forms much of the seaward edge and low tidal flat mangroves in the protected environments of the Elizabeth River and tidal creeks, comprising 34% of the Study Area. Whereas in more exposed environments of East Arm and the mouth of the Elizabeth River the Seaward edge is typically characterised by Sonneratia alba (6%). Mixed species closed forests typically occur as a narrow band on the hinterland margin of the mangrove zone (8%). The Shoreline forest, Transition, High tidal flat and Low woodland mangrove associations are interspersed between other dominant assemblages forming only 7% combined of the intertidal BHC within the Study Area. The R. stylosa low woodland mapping unit was not recorded within the Study Area. Habitat previously mapped as Low tidal mudflat in 1996 represented areas re-vegetating following destruction by Cyclone Tracy (1974). Review of aerial imagery re-classified these small areas of BHC in the Study Area as Map unit 2: Tidal creek.

Small discrete areas of samphire/mudflats are found in the Elizabeth River adjacent to the Development Envelope on the mid to high intertidal zone, with larger salt flats located on the north side of the river mouth and Wickham Point. The Beach mapping unit is located at Bladin Point and Wickham Point, with the majority of this habitat located behind Seaward mangroves (Map unit 8).

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Table 5 Intertidal BHC mapping unit classifications of the Study Area indicating total area in hectares

Habitat Code

Mapping Units Common Name

Study Area (ha)

Composition (%)

1 Rhizophora stylosa closed forest Shoreline forest 40 <1% 2 Rhizophora stylosa/Camptostemon schultzii

closed forest Tidal creek 1,564 34%

3 Rhizophora/Bruguiera/Ceriops closed‐forests Transition 20 <1% 4 Ceriops australis low closed forest Mid tidal flat 1,710 37% 5 Ceriops australisl/Avicennia marina low

closed forest High tidal flat 130 3%

6 Mixed species low closed forest Hinterland margin

367 8%

7 Mixed species low woodland Low woodland 97 2% 8 Sonneratia alba woodland Seaward 251 6% 9 Rhizophora stylosa low woodland Islands, rocky

shore - -

10 Low open woodland Low tidal mudflat

- -

11 Samphire/salt flat Salt flat 383 8% 12 Beach Beach 26 <1%

Total 4,587

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Figure 6 Existing mangrove assemblages mapped within the study area showing the locations of survey sites

29 TNG Processing Facility Darwin Benthic Habitat and Communities

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Forest Structure, Composition, Canopy Cover and Recruitment

The mangrove species composition recorded within quadrats generally represents the community association that has been mapped within the designated area. Sites at East Channel Island and Middle Arm tend to offer a more defined zonation of habitats occurring in these areas. The Tidal creek assemblage of the Elizabeth River locations (I5 & I7) adjacent to the Development Envelope, as well as the Seaward zone at I7, is mapped as the Tidal creek unit. This mapping unit is comprised of a greater variety of species compared to the Shoreline forest mapping unit at C2/C3, and the locations of the quadrats at I7 are positioned in areas of transition between other mapping units: R. stylosa, C.

australis and A. marina habitats. For example, at Elizabeth River (I7) where the Tidal creek is mapped along the seaward edge, A. marina forms a thin fringe along the seaward margin of the R. stylosa community, and scattered S. alba were also noted outside the plot boundary. Therefore, the Elizabeth River locations are generally comprised of mixed assemblages compared to C2/C3. However, C.

australis was dominant at the Hinterland margin and Mid-tidal flat at all locations.

Canopy cover typically ranged between 60% and 85% (Figure 7). Lower canopy cover was generally recorded within Zones 4 or 5. The canopy cover at Elizabeth River - Creek (I5) in Zone 4 was notably lower at 23%, which can be viewed in aerial imagery. Canopy cover recorded at this site declined by ~22.1% between 2012 and 2014 during monitoring for the Inpex Ichthys Project, with the last post-dredge measurement recorded at 48.9% (Cardno 2015a). These results indicate the canopy cover at this site has continued to decline following 2014. Tree density was highest in Zone 4 or 5 (10,800 to 30,000 trees/ha) and lowest in Zone 1 (700 to 1,500 trees/ha) (Figure 8). The canopy height ranged from 2 m at I5 Zone 4 to 15 m at C3 Zone 1 (Figure 9). The highest canopy was recorded within Zone 1 and Zone 4 or 5 typically recorded the lowest canopy height. The data indicates the density of the trees is inversely correlated to the maturity of the trees, where Zone 4 or 5 on the tidal flat are typically comprised of juvenile C. australis at high density and Zone 1 or 2 on the slope is comprised of mature R. stylosa at low density. However, with the exception of Zone 4 at I5 where the density and height of juvenile C. australis was lower, this relationship between tree density and maturity maintains a comparably consistent canopy cover across the intertidal area.

The regeneration density was highest in the Seaward assemblage at locations C2, C3 and I5, recording densities of 18, 43 and 8 seedling/sampling per m2, respectively (Figure 10). Seedlings/saplings of R.

stylosa or A. marina were not recorded on the seaward margin at Elizabeth River – River (I7), although 7 and 8 seedling/samplings per m2 were recorded in R. stylosa/C. schultzii mapped Zone 2 quadrats at I5 and I7, respectively. Six C. australis seedling/samplings per m2 were recorded in the mixed mangrove hinterland margin at location I5, although Zone 6 did not record any seedling/saplings at the other locations and numbers remined low (<5 per m2) in the other Zones.


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Table 6 Forest structure and composition information recorded at locations/quadrats, including no. of trees, canopy height, composition and comparison to the mapped mangrove association

Site Name

Site Code

Mapped Zone

No. of Trees

(per ha)

Canopy Height

(m)

Canopy Cover (± SD)

(%)

Regeneration Density (±

SD) (per m2)

Composition (%)

Dominant Others

East Channel Island

C2

4 10,800 8 69.4 (8.5) 0 C. australis (90%) B. exaristata (10%)

4 30,000 4 66.7 (11.6) 1 (0.3) C. australis (90%)

1 700 10 77.9 (8.0) 1 (0.5) R. stylosa (90%)

8 9,700 6 63.7 (13.2) 18 (1.8) S. alba (60%) A. corniculatum (20%)

Middle Arm C3

6 9,000 8 76.2 (12.7) 0 (0.3) C. australis (90%) L. racemosa (10%)

5 21,000 4 68.9 (13.5) 4 (1.2) C. australis (90%) B. exaristata (10%)

1 1,500 15 71.3 (9.4) 2 (0.9) R. stylosa (70%)

8 8,700 4 65.4 (22.5) 43 (9.2) S. alba (50%) A. corniculatum (20%)

Elizabeth River - Creek

I5



2 4,000 4 83.3 (15.6) 7 (3.3) R. stylosa (60%) B. parviflora (10%), C. schultzii (5%)

8 7,700 4 77.5 (18.4) 8 (4.4) S. alba (70%) R. stylosa (10%), A. corniculatum (10%)

Elizabeth River – River

I7


5 15,000 3 59.8 (13.7) 3 (1.2) C. australis (80%) A. marina (5%)

2 5,000 10 78.2 (8.2) 8 (3.8) R. stylosa (40%) B. parviflora (40%), C. australis (30%)

2 6,000 5 70.1 (16.4) 0 R. stylosa (40%) A. marina (40%)


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Figure 7 Mean percent canopy cover (±SE) in the four main mangrove associations from landward to seaward recorded at the four validation monitoring locations

Figure 8 Number of trees per hectare in the four main associations from landward to seaward recorded at the four validation monitoring locations


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Figure 9 Canopy height (m) in the four main associations from landward to seaward recorded at the four validation monitoring locations

Figure 10 Mean number of seedlings/saplings per m2 (±SE) in the four main associations from landward to seaward recorded at the four validation monitoring locations

The canopy percent cover values and seedling/sapling densities recorded at monitoring locations visited in 2019 were compared with previous surveys undertaken in 2012 (Table 7). The canopy cover in 2012 was generally notably higher than the cover recorded during 2019 by an average of ~21% at control locations C2 and C3, and except for Zone 4, an average of 7% at I5. The discrepancy from these results is likely to be more attributable to a lower level replication used in the present study, small differences in the quadrat locations and sampler bias rather than tangible changes in these parameters between surveys. This conclusion is enhanced by the fact that only small changes in these measures were observed during quarterly monitoring with the last survey completed in April 2014 (Cardno 2015a). The exception is perhaps Elizabeth River - Creek (I5) in Zone 4 where the canopy cover appears to


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have continued to decline which was a trend identified between 2012 and 2014 (Cardno 2015a). The discrepancy in the datasets suggests that the existing survey data cannot be used as a continuation of the previous monitoring program, although the information is still valuable to inform the EIS for evaluation of the value of mangroves within the Study Area.

The difference in regeneration density was variable between survey years. Spatial and temporal patterns in seedling and saplings were also observed during the Inpex Ichthys monitoring program. The higher density of seedlings/saplings near the Seaward assemblage was also observed during previous monitoring, with seedlings observed to peak during the wet season, indicating the high numbers recorded in Seaward communities may be reflective of seasonal patterns. However, previous surveys identified the density in the Tidal Flat community was typically higher than other zones, although this trend was not observed during the 2019 survey, possibly due to the variability of the results.

Table 7 Comparison of the 2019 results for with previous reported during Inpex Ichthys Project surveys in 2012 and 2014 (Cardno 2013a, 2015a)

Site Name Site Code Zone Canopy Cover (%) Regeneration (seedlings/saplings/m2)

2019 2012 2014 2019 2012

East Channel Island C2

6 69.4 96.4 92.3 0 0.3 4 66.7 95.8 94.4 1 1.8 2 77.9 97.4 96.4 1 0 8 63.7 86.7 84.2 18 15.1

Middle Arm C3

6 76.2 94.4 93.8 0 3.3 4 68.9 90.4 92.4 4 6 2 71.3 89.2 84.2 2 0.4 8 65.4 76.5 64.4 43 13.8

Elizabeth River - Creek I5

6 81.1 88.8 90.8 6 4.1 4 23.0 66.4 51.9 1 6.2 2 83.3 93.8 90.4 7 2.2 8 77.5 80.6 73.8 8 4.7

Mangrove Fauna

A total of 610 organisms from 23 species were recorded during the field survey at four locations. A summary of the results is presented in Table 8 and a more comprehensive list of species recorded is provided in Appendix B.

Approximately 72% (439 individuals) of total fauna numbers across all locations are crustaceans, followed by mudskippers comprising 16% (98), molluscs at 12% (74) and three worms. Of the crustaceans, sesarmid (304) and fiddler crabs (113) composed 95%. Molluscs recorded were predominantly (~75%) comprised of Telescopium and Terebralia sp. common in mangroves, and Ellobiidae taxa such as Pedipes and Melampus. Mudskippers are commonly counted due to being less timid and often may be recorded more than once during the survey due to movements.


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The total number of burrows is lower at site I7 (189) compared to between 399 and 544 at the other locations. However, only a very weak linear correlation (r2 = ~0.35) was found between the number of burrows and Sesarmid or fiddler crab numbers. The number of individuals and species recorded between locations are comparable, with individuals ranging between 131 at Elizabeth Creek to 190 at East Channel Island, and 8 species at East Channel Island to 13 at Middle Arm. The Shannon Weiner Diversity Index (SW Index) was relatively low (1.1 to 1.9) at sites due to the dominance of sesarmid and fiddler crabs at the sites. However, across all sites the SW Index increases likely due to higher species richness present in mangrove communities.

The mean number of individuals and species richness are presented in Figure 11 and Figure 12 respectively, to evaluate the changes in fauna across the intertidal zone. The highest mangrove fauna numbers are typically found in zones towards the seaward edge, with the tidal creek or seaward zones recording the highest fauna average density. The species numbers are generally comparable between zones likely due to the dominance of sesarmid and fiddler crabs, although a few quadrats on the tidal flat recorded higher diversity associated with molluscs and hermit crabs recorded within this zone.

Significantly higher mangrove marine fauna species richness and abundance were recorded from sites surveyed in 2012, due predominantly to a more comprehensive fauna sampling monitoring program implemented. The three dominant taxonomic groups were recorded in this study although the lower relative abundance of worms and molluscs are likely due to the more cryptic nature of their habitation requiring more intrusive sampling techniques. The 2012 investigations found both species richness and abundance increased in the seaward assemblage and was lowest in the hinterland margin. Species richness increased progressively from landward to seaward, although results can be quite variable between locations, particularly within the tidal flat and seaward associations. Mean species richness ranged from 3.8 (± 0.7 SE) on the hinterland margin to 28.2 (± 2.2 SE) on the seaward edge at Elizabeth Creek (I5). In contrast with species richness, mean invertebrate abundance was higher in the Tidal Flat assemblage than in the Tidal Creek. Species abundance recorded in 2012 ranged from 5.5 individuals (± 1.2 SE) on the hinterland margin at Elizabeth Creek (I5) to 93.5 (± 12.6 SE) on the seaward edge at East Channel Island (C2). Ordinations based on the presence/absence of all invertebrate species found clear distinctions among different assemblages, which was noted as being characteristic of patterns recorded from previous mangrove surveys in Darwin Harbour (Metcalfe 2007).


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Table 8 The total number of burrows, individuals and species, the Shannon Weiner Index and total numbers of taxonomic groups recorded

Group Family Common Name Location Total

C2 C3 I5 I7

Number of Burrows 399 402 544 189 1534 Number of Individuals 190 144 131 145 610 Number of Species 8 13 11 10 23 SW Diversity Index -1.4 -1.9 -1.1 -1.4 -2.8 Crustaceans Sesarmidae Sesarmids 82 53 97 72 304

Ocypodidae Fiddler crab 38 23 14 38 113 Macropthalmidae Sentinel crabs 0 0 3 0 3 Camptandriidae Red silt crab 0 2 5 0 7 Grapsidae Grapsid crabs 0 0 3 1 4 Menippidae Stone crab 0 0 1 1 2 Anomura Hermit crab 0 3 1 1 5 Other 0 0 1 0 1

Molluscs Potamididae Mangrove whelk 10 27 4 15 56 Neritidae Nerite snails 0 2 0 1 3 Littorinidae Periwinkles 1 0 0 0 1 Melongidae Spiral melongena 2 0 0 0 2 Ellobiidae Hollow-shelled snails 7 4 0 1 12

Fish Periopthalmus Mudskippers 47 30 6 15 98 Worms - 2 0 1 0 3

Figure 11 Mean number of individuals per m2 (±SE) in the four main associations from landward to seaward recorded at the four validation monitoring locations


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Figure 12 Mean species richness per m2 (±SE) in the four main associations from landward to seaward recorded at the four validation monitoring locations


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Subtidal BHC

The subtidal BHC is presented using the data layers sourced from Seamap. The existing Seamap BHC map is presented in Figure 13.

The composition within the study area of broad intertidal BHC classes are presented in Table 9. The Elizabeth River subtidal seabed BHC within the Project area is predominantly mapped as Bare coarse sediment (377 ha or 40%), with the intertidal zone to the lowest astronomical tide (LAT) mapped as Bare sand and mud (either flats/bars) (404 ha or 44%). Towards the mouth of the Elizabeth River adjacent to Short Islet on the bend there is an area of complexity mapped as patches of:

medium density mixed community (corals, algae, sponges, & soft corals); high density mixed communities (corals, algae, sponges, & soft corals); medium density filter feeders (Sponges & soft corals) Bare coarse sediment with rippled sand; and Bare rocky reef.

Small discrete areas of medium density mixed community (corals, algae, sponges, & soft corals) and Bare coarse sediment with rippled sand occur on the northern side and further up of the Elizabeth River.

Table 9 BHC classifications of the study area indicating total area in hectares

Habitat Code BHC Classifications Study Area (ha) Composition (%)

1 High Density Filter Feeders 0.4 <0.1% 2 Medium Density Filter Feeders 2.8 0.3% 3 High Density Mixed Epibenthic community 6.0 0.6% 4 Medium Density Mixed Epibenthic community 68.5 7.3% 5 Low-Medium Density Mixed community 18.1 2.0% 6 Bare Rocky Reef 8.8 0.9% 7 Bare coarse sediments 377.3 40.4% 8 Bare Sandy / Mixed sediments 10.5 1.1% 9 Bare Coarse sediments - rippled 28.2 2.2% 10 Bare sand / mud 404.2 44.3%


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Figure 13 Existing Seamap subtidal BHC map showing locations and classifications of the targeted ROV video locations


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ROV Video

A total of 14 locations of interest were identified following review of the side scan acoustic imagery. The results for classification and observations of the video analysis are shown in Figure 13 and described in Table 10.

The video analysis results suggest the acoustic imagery identified physical and geomorphological features such as:

Reef areas of varying rugosity supporting diverse and abundant fauna & flora (T1, T3, T4, T14);

Intertidal areas with terrigenous mangrove debris, possibly picking up mangrove branches on the substrate (T2, T7, T9, T11, T12, T13);

Areas of patchy sandy substrates with small rocky outcrops supporting low cover filter feeder BHC (T5, T6);

An intertidal bare rocky reef with high relief and rugosity (T8); and An intertidal reef/rubble and coarse sand substrate supporting low to medium BHC cover in

the upper creek (T10).

The habitat classifications assessed during review of the 14 target locations is typically consistent with the mapped areas. However, our assessment did identify some recommended local scale changes to classifications:

Target locations 5 and 6 BHC mapped as Bare sandy/mixed sediments, although substrate characteristics of small rocky outcrops and low cover filter feeder BHC displays features more aligned with habitat code 7: Bare coarse sediment. This code definition is intended to only represent areas of the aphotic depth zone. However, reclassification of this habitat is considered to better represent the BHC occurring between more complex reef systems rather than mapping as Bare substrate;

Target location 8 BHC mapped as intertidal bare sand/mud. An area of high relief and rugosity bare rocky reef was found at this location indicating this area should be designated as habitat code 6: Bare rocky reef; and

Target location 10 BHC mapped as intertidal bare sand/mud in the upper creek, although substrate characteristics of predominantly hard substrate/rubble supporting low to medium BHC displays features more aligned with habitat code 5: Low-medium density mixed community.

The ‘supervised’ classification of the SSS data was then used to determine the spatial extent of these habitats and the subtidal BHC map of the Study Area was refined. The revised version of the BCH map in the study area is presented in Figure 14. Overall, these modifications to the existing map are only likely to result in minor adjustment to the relative composition of BHC within the Study Area.

Observations from the ROV footage indicated that the condition of the BHC within the Study Area is good. A microphytobenthos (MPB) biofilm on the surface substrate was identified as a common feature of habitat class 10: Bare sand/mud in the intertidal zone. Therefore, this habitat is not strictly bare substrate devoid of a biotic community as the class description would indicate from Smit et al (2012).


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Table 10 ROV Video depths, mapped class in Smit et al (2012), analysis observations and a representative image

Site Depth (m)

Mapped Observations Image

T1 8m 3 High cover Chlorophyta macroalgae (Halimeda/Caulerpa) & soft corals

(Ellisellidae), although low diversity. Patchy hard substrate/ low relief. Coarse

sand. Some bioturbation in sand

T2 7m 10 MPB biofilm with fine sediments over sand

on substrate surface, otherwise bare. Some terrigenous mangrove debris, some

bioturbation

T3 9m 4 Low to moderate cover sponges

(encrusting/erect), soft corals& hydroids. Patchy hard substrate with coarse mixed

sands & low relief. Silty/filamentous surface with scattered terrigenous

mangrove debris, bioturbation & gobies in fish holes

T4 9m 3 Moderate to high cover & diversity of

sponges (encrusting/erect), soft corals, ascidians, hydroids. No scleractinian

corals observed. Sand overlaying a hard substrate, sloping relief.

T5 9m 7 Low density filter feeders, Vigularia soft

coral/sponges but small colonies/low cover. Fine sediment overlying hard

substrate, with patchy coarse sediments & heavy bioturbation/rubble. Low to medium rugosity. Recommend be re-classified as

8.

T6 9m 7 Similar to T5 but less hard substrate.

Some biofilm on surface, some bioturbation in coarse sediments & gobies

in fish holes.


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Site Depth (m)

Mapped Observations Image

T7 8m 10 MPB biofilm with fine sediments on substrate surface on sand and low relief.

Some terrigenous mangrove debris, limited bioturbation.

T8 4m 10 Small area of Bare Rocky Reef with high

rugosity. Recommend be re-classified as 6. MPB Chlorophyta filamentous algae

common on mobile coarse sand. Surface fine sediment with rubble & terrigenous

debris, bioturbation common.

T9 3m 10 MPB biofilm with fine sediments on

substrate surface and low relief. Some terrigenous mangrove debris, some

bioturbation.

T10 7m 10 Patchy rock/rubble interspersed with

coarse sediments/bioturbation. Low to moderate cover and diversity of soft

corals, sponges and hydroids. Recommend be classified as 5, low to

medium density mixed community Occasional filamentous fine sediment

layer on sand & rubble.

T11 4m 10 Terrigenous mangrove detritus common. Patchy MPB biofilm/fine sediment layer on

sand & low relief.

T12 4m 10 T13 4m 10

T14 9m 4 Patchy hard substrate & coarse sand. Moderate cover & diversity of sponges

and hydrozoans on reef patches. Moderate to high rugosity. Minor

bioturbation & gobies in fish holes.


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Figure 14 Revised subtidal BHC map following site validation


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4. Discussion

Darwin Harbour is an extensive estuary with over 286 km2 of open water (depending seaward boundary) and 38 km2 of intertidal flats with three major arms fringed by some of the largest and floristically diverse mangrove communities (Wightman 2006). The harbour is a poorly flushed, semi-diurnal and macro-tidal system with a maximum spring tidal range of 7.8 m (Wang et al. 2011). Tropical monsoons provide seasonal freshwater inflows and increased sediment loads. Darwin Harbour is situated within a region recognised for its relative pristine condition (Halpern et al. 2008).

The existing maps of intertidal and subtidal BHC derived from Seamap have been generated and validation ground-truth field investigations undertaken to improve our knowledge and understanding of the factor BHC that may be impacted by the development of a new magnetite concentrate processing facility at Wickham. The Study Area of relevance was determined to comprise intertidal and subtidal habitat extending approximately 10 km either side of the Development Footprint. Further desktop review has been undertaken to provide an assessment of the diversity, distribution, ecological role and environmental value of intertidal and subtidal BHC occurring within the Study Area, which is discussed further in the following Sections.

Intertidal BHC

4.1.1. Distribution of Intertidal BHC

Mangrove community is the largest BHC type in Darwin Harbour and occupies over 60% of the available estuarine environment. In 1994, as part of the National Forest Inventory, the Northern Territory Government undertook a detailed survey of Darwin Harbour mangroves (Brocklehurst and Edmeades 1996). This detailed study mapped approximately 20,400 ha of mangroves in Darwin Harbour, which was distinguished into 10 mangrove associations or communities used in this assessment. This classification of mangrove assemblages has since been widely applied in mangrove research and monitoring studies of Darwin Harbour (Comley, 2002, Metcalfe 2007, Cardno 2013a, 2015a).

The structural features of mangrove communities in Darwin Harbour, are similar to those in many other parts of the world, showing pronounced shoreline zonation with species arranged in nearly monospecific bands, often parallel to the coast (Saenger, 2002). Each mangrove association tends to develop at a particular topographic elevation (or regime of tidal inundation), resembling strips or zones across the intertidal area. The pattern of zonation generally evident in Darwin Harbour is shown in Figure 15. The mangrove species composition recorded within quadrats during this study generally represents the community association that has been mapped within the designated area. This is not surprising given the sites have been used historically for monitoring and the level of effort previously implemented to map the mangrove assemblages within Darwin Harbour.


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Figure 15 Schematic profile diagram of the typical pattern of mangrove zonation in Darwin Harbour, showing the predictable sequence of mangrove assemblages from landward to seaward (L to R). The percentage of the total mangrove area, tidal elevation (AHD or Australian Height Datum) and inundation frequency is also indicated.

Mangrove species respond to eco physiological cues along salinity, tidal and other environmental gradients, and disturbance regimes. The hydrology of mangroves is complex. Tidal inundation, rainfall, soil water, groundwater seepage and evaporation all influence soil salinity and have a profound effect on mangrove growth and distribution. (Semeniuk, 1983, 1985). Landward mangrove habitats are characteristically more influenced by terrestrial processes (e.g. freshwater seepage, terrigenous sediment deposition, desiccation) and seaward habitats are largely shaped by marine processes (e.g. diurnal tidal movement).

Each assemblage is discussed in further detail below with consideration of information available to determine the productivity of the community, which is recognised as an important step in mangrove management. Productivity estimates are commonly measured through the calculation of leaf litter and above-ground standing biomass. Leaf litter fall provides an indication of the quantity of detritus that enters the marine food chain at the lower trophic levels. Standing biomass is a measure of the total amount of organic matter accumulated per unit area at a specified time. The estimated productivity for each community and the productivity within the Study Area are presented in Table 11. It is noted that subsequent monitoring during Cardno (2015a) determined the leaf litter biomass recorded within Metcalfe (2011) for Mid-tidal flat may be at the lower end of the typical range, although estimates were


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collected for other assemblages in this study simultaneously enabling a comparison of the productivity between assemblages.

Table 11 Estimates of productivity based on leaf litter (Metcalfe, 2011) and above-ground standing biomass (DLRM 2013)

Habitat Code

Mangrove Association

Rate Study Area Darwin Harbour

Rate (t/ha/yr)

Biomass (t/ha)

Leaf Litter (t/yr)

Productivity

(t/association)

Leaf Litter

Biomass

1 Shoreline forest 10.1 345.8 400.3 13,695.2 6,742 230,992 2 Tidal creek 8.5 137.3 13176.1 212,358.5 50,816 818,974 3 Transition 5.2 113.6 102.0 2,235.8 3,804 83,413 4 Mid-tidal flat 3.9 124.4 6739.1 212,722.1 31,366 989,952 5 High tidal flat 5.0 40.9 655.0 5,333.2 4,481 36,502 6 Hinterland margin 8.6 81.8 3152.9 29,992.5 13,114 124,808 7 Low woodland 6.6 25.1 640.5 2,442.6 1,955 8 Seaward 12.3 5.0 3149.2 1,252.7 12,167 4,830 9 Rocky shore - 14.4 - - - 40 10 Low tidal mudflat - - - - - -

Total 28,015 480,033 124,395 2,289,511

Shoreline Forest (Map Unit 1)

Map unit 1 is dominated by Rhizophora stylosa forming monospecific stands of closed forest and low closed forests. These forests show strong zonation with tidal level and are restricted to the seaward margins of the main tidal mudflats (4-6 m LAT). Soil salinity is maintained by marine water recharge and ranges from approximately 33-40 PSU at mean sea-level and 41 ±4PSU in the tidal mudflats (Semeniuk 1985). Within the Study Area, this assemblage occurs outside the mouth of the Elizabeth River backing stands of Sonneratia alba, and in discrete pockets to the West of the Development Envelope and offshore from East Arm, two of which are small islands. Thus, the Shoreline forest comprises only 40 ha or <1% of the intertidal BHC within the Study Area.

Brocklehurst and Edmeades (1996) describe characteristic traits of the Shoreline forest as a median canopy height of 10 m (2.5-15.6 m) and upper cover of 95% (30-100%). This unit was assessed at two control sites in Middle Arm outside the Study Area. The canopy height recorded was the tallest assemblage assessed at 10-15 m. These taller forests are known to occur where slopes are gently inclined, although lower canopy heights may be recorded on moderate slopes (Brocklehurst and Edmeades 1996). The upper canopy cover recorded was lower than the median of historical monitoring of the same sites at 71-78%, although was well within the range of cover that may be recorded within the assemblage. The tree density of 700-1,500 trees/ha was typically the lowest recorded within any unit, where trees are separated by stilt roots forming a large stand basal area. Seedling and sapling numbers were typically low, consistent with generally sparse regeneration recorded within this unit.


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Map unit 1 is a well-developed forest with large sparsely distributed mature trees and relatively high canopy cover, resulting in high productivity rates. However, low area coverage of this unit within the Study Area results in leaf litter of 400 t/yr (1.4%) and biomass of 13,695 t (2.9%), which is <1% of both productivity measures of mangroves within Darwin Harbour.

Tidal Creek (Map Unit 2)

The Tidal Creek is the second most common map unit comprising 1,564 ha (~34%) of the intertidal BHC within the Study Area. This is largely due to much of the intertidal BHC within the Study Area representing more protected creek and tributary habitat within Darwin Harbour. Similar to Map unit 1, the Tidal creek map unit is also found between mean sea level and mean high water neaps (4-6 m LAT). However, R. stylosa may be co-dominant or locally dominant with Camptostemon schultzii and Bruguiera parviflora. Aegiceras corniculatum, Aegialitis annulate, Avicennia marina, Xylocarpus

mekongensis and Bruguiera gymnorhiza may commonly occur within this zone. This assemblage was assessed within three quadrats adjacent to the Development Envelope, two in the Rhizophora Zone and one on the seaward fringe. Five species were recorded within assessed quadrats. R. stylosa was recorded in all quadrats, B. parviflora, A. corniculatum and C. australis occurred in the Rhizophora Zone, whilst A. marina was found on the seaward fringe. S. alba was also noted in comments occurring outside the quadrats on the seaward fringe. This unit was observed to transition between other adjacent assemblages.

The median canopy cover of 65% (2-100%) and height 8.8 m (5-12 m) is lower than the R. stylosa dominant Shoreline forest assemblage in Map unit 1 (Brocklehurst and Edmeades 1996). The canopy cover recorded within quadrats ranged between 70-83%, which is higher than the median for this assemblage, but the quadrat from site I5 (83%) was lower than cover recorded throughout the Inpex Ichthys monitoring program (90-94%) (Cardno 2013a, 2015a). The canopy height recorded (4-10 m) was generally lower in comparison to the forest in Map unit 1. The tree density ranged from 4,000-6,000 trees/ha, notably higher than the Shoreline forest although typically low in comparison to other assemblages. Seedling/sapling numbers were notably higher in the Rhizophora Zone compared to no regeneration recorded within Seaward fringe.

Map unit 2 identifies a generally closed-canopy forest of diverse mature trees, relatively high canopy cover and reasonable rates of regeneration. However, the productivity rates are less than the Shoreline forest. The high coverage of this unit within the Study Area results in leaf litter and standing above-ground biomass estimates of 212,358.5 t/yr (47%) and 50,816 t (44%), respectively. Therefore, this mapping unit in the Study Area represents between 9-11% of the productivity within Darwin Harbour.

Transition (Map Unit 3)

Map unit 3 was not recorded within any of the quadrats assessed during this survey and is not mapped to occur within mangroves immediately adjacent to the Development Envelope. The Transition assemblage is typically found within the Study Area forming a narrow zone between the Rhizophora Zone (Map units 1 & 2) and the Tidal flat (Map units 4 & 5) in the broader mangrove assemblage near the mouth of East Arm towards Wickham Point. This community is restricted to approximately mean high water neaps (6 ± 0.5 m LAT) and only comprises ~20 ha, or <1% of the intertidal BHC within the Study Area. The soil salinity on the high tidal flats is approximately 89±24PSU (Semeniuk 1985).


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Brocklehurst and Edmeades (1996) describe Map unit 3 with R. stylosa, B. exaristata and A. marina as the dominant species with C. australis and A. annulata included in the composition of lower canopy layers. The upper canopy cover is typically high 87.5% (5-100%) and canopy height is moderate 5.3 m (3.8-8 m), with height generally decreasing landward.

The Transition assemblage is comprised of relatively small scattered trees on the upper tidal flats resulting in a low productivity rate. Low area coverage and productivity result in low leaf litter and standing above-ground biomass estimates of 102 t/yr (<1%) and biomass of 2,236 t (1%) within the Study Area, which is <1% of both productivity measures of mangroves within Darwin Harbour.

Tidal Flat (Map Unit 4)

C. australis low closed-forest/open forest is the most widespread community within the Study Area, comprising 1,710 ha or 37% of the intertidal BHC. C. australis is the dominant species, the canopy height is 4.6 m (2 to 6 m) depending on salinity regimes, and upper canopy cover is high 100% (85-100%) (Brocklehurst and Edmeades 1996). Soil water salinity generally high and up to 196PSU at salt flats (Semeniuk 1985). C. australis was recorded with B. exaristata in quadrats closer to the hinterland during the present survey. The hinterland margin Zone at site C2 is mapped as Tidal flat although the community within the quadrat is more representative of the Mixed species low closed forest/open-forest (hinterland) and therefore is not discussed here. A thin strip of Map unit 6 typically occurs around the boundary of the terrestrial islands in the Study Area, although is not mapped in this quadrat but is unlikely to significantly change the estimates in the Study Area. In contrast, the quadrat in the Tidal flat zone at site C3 is mapped as Map unit 5, although no A. marina were recorded and the presence of B.

exaristata as sub-dominant suggests this community may be more representative of this assemblage.

Canopy height from quadrats in this study ranged from 2 to 4 m, although the location I5 in the Study Area was smaller (2 m) than recorded within Middle Arm (4 m). Canopy cover and tree density was particularly low at I5 (23% & 12,000 trees/ha, respectively) when compared to Middle Arm locations with ~68% cover and densities from 21,000 to 30,000 trees/ha. Canopy cover at site I5 was recorded to decline by ~22.1% between 2012 and 2014 (Cardno 2015a). Cardno (2015a) suggested the proximity to salt flats may influence sediment pore-water salinity at this site and affect seasonal and interannual growth patterns. It was noted that the assemblage here is narrow, open and with patchy cover of trees less than 2 m height, where remote sensing results indicate a reduced Normalised Difference Vegetation Index for the vegetation associated with salt flats in this assemblage. Therefore, the results suggest a continuing trend of declining cover at this site. Regeneration values ranged from 1-4 m2 and consistently recorded in each quadrat.

The small C. australis trees have low productivity rates compared to other zones, although the high density of trees and large composition of this unit within the Study Area results in leaf litter and standing above-ground biomass estimates of 6,739 t/yr (24%) and 212,722 t (44%), respectively. However, this is only approximately 5.4% and 9.3% of leaf litter and biomass productivity measures of mangroves within Darwin Harbour. The data from the present survey indicates the community immediately adjacent to the Study Area at I5 is particularly under-developed in terms of typical productivity measures.

High Tidal Flat (Map Unit 5)

The High tidal flat occurs on the same shoreline zonation unit as the previous community, although comprises significantly less extent of the Study Area, comprising 130 ha or 2.8% of the intertidal BHC.


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The primary difference between this assemblage and Map unit 4 is the greater presence of A. marina. The reason for the predominance of Avicennia is suggested to be related to a change in the soil water salinity and substrate type. However, Brocklehurst and Edmeades (1996) acknowledge the boundary for separating these communities may be subjective. As described above, the tidal flat zone at C3 better represents Map unit 4, therefore I7 is the only quadrat where this assemblage has been assessed in this study.

A. marina and C. australis are dominant, co-dominant or locally dominant. The upper canopy cover is 55% (10-90%) and canopy height is 3.7 m (2.6-6 m) (Brocklehurst and Edmeades 1996). The canopy cover recorded within the quadrat was 60% and the canopy height is low at 3 m. In this quadrat A.

marina formed only a small percent (i.e. 5%) of relative cover. The tree density was less than Map unit 4 at 15,000 trees/ha, although three seedling/saplings is comparable.

Small C. australis and slightly larger A. marina trees with a lower density results in productivity rates for this assemblage slightly higher than productivity rates for Map unit 4, although low in comparison to other assemblages. However, this assemblage comprises only a small extent within the Study Area resulting in leaf litter and standing above-ground biomass estimates of 655 t/yr (2%) and 5,333 t (1%), respectively, which is <1% of both productivity measures of mangroves within Darwin Harbour.

Hinterland Margin (Map Unit 6)

The hinterland margin supra-tidal unit is a mixed species mangrove assemblage occurring as a narrow band on the landward side of the mangrove zone varying from 10-50 m wide. The unit is typically dominated by C. australis, with L. racemosa, E. ovalis and A. marina, with B. exaristata common (Brocklehurst and Edmeades 1996). Three of these species were recorded within the hinterland margin mixed species association during the present field survey. This unit is mapped throughout the Study Area comprising a total area of 367 ha, or approximately 8% of the intertidal BHC. The zone is generally productive with leaf-litter and above-ground standing biomass estimates for the Study Area assemblage of 3153 t/yr (11%) and 29,9993 t (6%), respectively. This represents ~2.5% of leaf litter and 1.3% of above-ground biomass productivity within Darwin Harbour.

The median canopy height is 5.7 m with C. australis, generally taller in this zone than in its main tidal flat habitat (Map unit 4). The canopy cover median is listed as 90% and depending on freshwater seepage, soil salinity may vary from 14PSU up to 48PSU (Semeniuk 1985). The present study recorded canopy heights ranging from 5 to 8 m and cover from 69 to 81%. This mapping unit was typically recorded with a lower recruitment and mangrove fauna than compared to zones on the lower intertidal flats and seaward zones which is generally consistent with previous surveys (Cardno 2013a, 2015a). The information indicates that the community within the Study Area represents a well-developed and healthy mangrove association.

Low Woodland (Map Unit 7)

Map unit 7 was not recorded within any of the quadrats assessed during this survey and is not mapped to occur within mangroves immediately adjacent to the Development Envelope. Brocklehurst and Edmeades (1996) describes the Low woodland community occurring on the upper tidal reaches and edges of saline flats. Within the Study Area there are three small discrete stands at Wickham Point, East Point and in the upper reaches of Elizabeth River, comprising a total of approximately 97 ha or 2% of intertidal BHC within the Study Area.


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The assemblage is typically comprised of mixed small trees of Lumnitzera racemosa, Excoecaria ovalis, C. australis and A. marina with shrubs and samphire grasslands. It is considered to be a similar community to the Hinterland mixed assemblage (Map unit 6) although trees are generally smaller and more widely dispersed occurring on salt flats or rocky gravelly substrate (Brocklehurst and Edmeades 1996). The upper median cover is 67.5% (1-80%) and canopy height is 1.8 m (0-2.7 m) (Brocklehurst and Edmeades 1996). Low to moderate tree density and height result in relatively low productivity rates. Combined with only a small coverage of intertidal BHC within the Study Area the leaf-litter and above-ground standing biomass estimates for the Study Area are low at 641 t/yr (2%) and 2,443 t (0.5%), respectively. This represents ~0.5% of leaf litter and 0.1% of above-ground biomass productivity within Darwin Harbour.

Seaward margin (Map Unit 8)

The seaward margin is generally open forest/woodland dominated by Sonneratia alba, whilst R. stylosa and A. marina may be common and A. corniculatum thickets may also occur. The woodland may be interspersed between large areas of bare mudflat devoid of vegetation. Three quadrats assessed the Seaward margin assemblage and recorded S. alba with A. corniculatum thickets and R. stylosa was recorded in one quadrat at I5. This assemblage was generally found within the Study Area as the seaward community towards the mouth of the Elizabeth River and into East Arm, with a small pocket on the north side further up the Elizabeth River than the Development Envelope, comprising a total of 251 ha or 6% of intertidal BHC. On the seaward margin soil salinities vary from 33PSU to 40PSU (Semeniuk 1985).

The seaward margin assemblage forms an upper canopy cover of 60% (40-70%) and a median canopy height of 8.9 m (8-10 m) (Brocklehurst and Edmeades 1996). The present study recorded canopy cover ranging between 64-78%, although canopy height was lower between 4-6 m, likely due to the abundance of smaller A. corniculatum thickets within quadrats. Similarly, although S. alba trees were typically widely dispersed, the lower canopy A. corniculatum thickets resulted in moderate tree densities between 7,700-9,700 trees/ha. The highest regeneration seedling/sapling numbers were recorded within these quadrats of any assemblage.

The large diameter Sonneratia trees have been recorded with the highest leaf litter rates of productivity, although are generally hollow and widely dispersed between A. corniculatum thickets so above-ground standing biomass rates for the seaward margin is low. This results in leaf litter and biomass estimates of 3,149 t/yr (11%) and 1,253 t (0.3%), which is 2.5% and <0.1% of productivity measures of mangroves within Darwin Harbour, respectively.

Rocky Shore (Map Unit 9)

This assemblage was not mapped within the Study Area.

Low Open Woodland (Map Unit 10)

This mapping unit identifies areas mapped as mangroves in a state of recovery following destruction by Cyclone Tracy (1974). Brocklehurst and Edmeades (1996) describe the community as generally devoid of vegetation, however mudflats on the lower tidal flat regions may contain low open-woodland with scattered juvenile S. alba, R. stylosa and A. marina in various stages of post-cyclone regeneration.


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Review of aerial imagery re-classified these small areas of BHC in the Study Area as Map unit 2: Tidal Creek. Therefore, this assemblage was not mapped within the Study Area.

Salt Flat (Map Unit 11)

Salt flats are typically found in Ceriops forests (Map unit 4) or adjacent to the hinterland and are generally devoid of vegetation with hypersaline soils, although in some instances may contain samphire species (Brocklehurst and Edmeades 1996). Small discrete areas of samphire/mudflats are found in the Elizabeth River adjacent to the Development Envelope on the mid to high intertidal zone, with larger salt flats located on the north side of the river mouth and Wickham Point, comprising a total area of 383 ha or 8% of the intertidal BHC within the Study Area.

Beach/Sand (Map Unit 12)

The beach is generally devoid of vegetation. Within the Study Area, small areas of beach occur at Bladin Point on the Southern bank at the mouth of the Elizabeth River, and at Wickham Point, generally backing the Seaward margin. The total area of beach mapped is 26 ha or <1% of intertidal BHC within the Study Area.

4.1.2. Functional Diversity and Ecological Integrity of Intertidal BHC

Three basic BHCs have been mapped within the Study Area: Mangroves, Salt Flats and Beach.

Mangroves are a diverse group of phylogenetically disparate trees and shrubs which have been separated into discrete assemblages within the Study Area. These plants have evolved to exploit the land–sea interface under conditions generally low in oxygen and highly saline that would severely stress other plants. They exist in tropical and subtropical coastal ecosystems worldwide. Their diverse assemblages form communities that provide a profound array of ecosystem services to estuarine and coastal habitats. Some of the key services mangroves provide include:

stabilisation of coastal foreshores and habitats; both from coastal erosion and by moderating discharge loads from the terrestrial environment;

incorporation of carbon and primary biomass to the estuary; provision of a substrate for epiphytic and periphytic flora and fauna; and provision of three-dimensional habitats and ecosystem services to a multitude of fauna, many

of economic significance.

Salt flats within the Study Area are characterised as a harsh environment with meagre salt-tolerant samphire vegetation offering little opportunity to form a diverse biota salt flat community. Although not contributing to vegetation biomass, salt flats have been suggested to be significant areas in terms of primary productivity for the marine ecosystem. When inundated, warm waters cause algal blooms and phytoplankton which helps feed the whole marine system (Brocklehurst and Edmeades 1996). However, these flats are generally considered important where there are extensive areas of low-lying salt flats with a cyanobacterial crust that contributes to primary production. The small areas of salt flat devoid of benthic algal mats which comprise ~8% of BHC in the Study Area are not considered to provide a significant contribution of phytoplankton or nutrients to the marine ecosystem of Darwin Harbour.


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Australian culture values beaches highly for their aesthetic qualities and recreational amenity. In addition, some beaches from Darwin Harbour form significant areas for turtle nesting and migratory birds. The beaches mapped in the Study Area are not used for recreational purpose and are not identified as significant habitat for marine turtles or migratory bird species. Given the small areas of beach BHC within the Study Area (26 ha or <1%), the following discussion is focussed on the values of the mangrove habitat.

Habitat Extent and Diversity

The mangrove communities of Darwin Harbour are a significant natural resource both locally and globally. The mangroves in the harbour are amongst the most diverse in Australia; from approximately 50 species regarded as ‘mangroves’ worldwide, 36 of these occur naturally in Darwin Harbour. Brocklehurst & Edmeades (1996) estimated mangroves cover an area of approximately 20,400 ha in Darwin Harbour, which constitutes ~5% of the Northern Territory’s mangrove habitat, of which 4,178 ha (20%) is located within the Study Area. Based on floristic diversity and extent, the mangroves of Darwin Harbour are placed as one of the three most significant mangrove resources in the Northern Territory (Wightman, 1989). Arnhem Bay and the South Alligator River containing 39 and 38 mangrove species respectively, are the other two. In Australia, higher diversity of mangroves is only exceeded in north‐eastern Queensland (Duke, 2006). From 4587 ha of intertidal BHC within the Study Area, ~91% of the area is covered in mangroves.

The mangrove communities support a diverse range of plants including palms, trees, shrubs and even ferns. Different taxa exhibit diverse morphological and physiological mechanisms to deal with environmental stresses. Some species, such as Rhizophora, have aerial prop roots that branch down from the trunk and provide structural support in the mud soils. Others, such as Avicennia, have pneumatophores: aerial roots for gas exchange that grow vertically above the mud–soil surface from a shallow extensive network of anchoring roots. These mechanisms lead to a pronounced shoreline zonation with assemblages arranged in monospecific bands along the coastal area relative to the tidal height.

Coastal Stabilisation

Mangroves play an important role in coastal protection by acting as a natural buffer to water erosion from both the land and the sea. By creating a physical barrier, slowing water movement from the land and trapping sediments, mangroves reduce erosion along estuaries and creeks. Mangroves can also dampen storm surge and in tropical Australia, which experiences periodic cyclones, their value in coastal protection may be very important.

If managed incorrectly, removal of mangroves can result in shoreline erosion and mobilisation of marine sediments. This can cause sedimentation resulting in the shallowing of estuaries and waterways and a reduction in water quality. It can also lead to exposure of potential acid sulfate soils. When exposed to oxygen in the air these soils may react to produce sulfuric acid. This can cause substantial damage to the natural environment and physical and economic damage to the built environment.

Carbon Cycle and Productivity

Mangroves play a critical role in nutrient and carbon recycling and export of organic material to food webs in coastal waters (Jennerjahn and Ittekkot, 2002). The productivity of mangroves is interlinked to


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the productivity of estuaries and coastal ecosystems through the daily contribution of carbon and nutrients from the shedding of leaves, branches and roots (Blaber, 2007). When the leaves and branches of a mangrove fall to the ground, they provide a wide variety of aquatic animals such as molluscs, crabs and worms with a primary source of food. These primary level consumers in turn support an array of secondary consumers, including small fish and juvenile predators such as barramundi which, when mature, become third level consumers.

Productivity is a concept used to describe the ecological value or function of a vegetation community. Like other plants, mangroves convert energy from the sun into organic matter through the process of photosynthesis. In general, high levels of organic matter, or high productivity, means that a larger number and more diverse array of animals can be supported within a particular ecosystem. Mangrove plants have been so successful in their development that they are among the most productive natural systems found throughout the world.

Studies conducted in Darwin Harbour have estimated the productivity of mangroves through the collection and measurement of leaf litter fallen from a particular area over time (Metcalfe 2012) and estimates of the standing above-ground biomass (the amount of living matter present) (Comley 2013). These productivity estimates within the Study Area are discussed for each map unit described in Section 4.1.1. The leaf litter rates are typically highest near the seaward margin and decline onto the tidal flat, although increase to moderate rates again on the hinterland margin. Leaf litter rates range from 3.9 t/ha/yr (Mid-tidal flat) to 12.6 t/ha/yr (Seaward). Conversely, the above-ground biomass rates range from 5 t/ha (Seaward) to 346 t/ha (Shoreline forest), where the biomass is regulated by a combination of tree size (basal area and height) and density.

Based on the distribution of mangrove assemblages within the Study Area, 47% of leaf litter derives from the Tidal creek, 24% from the Mid-tidal flat and 11% from each of the Hinterland and Seaward margins. In terms of biomass, the Tidal Creek and Mid-tidal flat communities dominate the productivity estimates with 45% and 44% of total biomass from the Study Area, respectively, whilst the Hinterland margin comprises only 6% and the seaward margin <1%. These findings are very similar to the leaf litter and above-ground biomass proportional estimates on a broader spatial scale for Darwin Harbour. However, less dominant habitats such as the Shoreline forest, Transition and High tidal flat assemblages contribute slightly higher relative productivity to the region.

Ecological Diversity/Abundance

The complex matrix of mangrove community types enhances the faunal community structure living within the ecosystem, including enhancing the species diversity and abundance of epibenthic fauna within both the mangrove forest and in the nearby subtidal BHC environments (Skilleter et al., 2005, Nagelkerken et al., 2008). Mangroves create a three-dimensional habitat both above and below the substrate in which they grow. Their prop roots, pneumatophores and buttress trunks create complex structures that form a refuge for a multitude of flora and fauna (Blaber, 2007; Nagelkerken et al., 2008). Below ground, their roots stabilise muddy soils and allow crabs, worms and burrowing crustaceans to construct a labyrinth of holes, tunnels and cavities that support part-subterranean fauna critical to the decomposition and nutrient release supporting ecosystem productivity (Nagelkerken et al., 2008).

Research has shown that the abundance and diversity of marine fauna found throughout mangrove areas is quite high. A comprehensive study of the biological diversity of fauna associated with mangrove


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assemblages in Darwin Harbour was undertaken by Metcalfe (2007). A total of 13 mammals, 11 bat species and 70 bird species were recorded during the study. The four main assemblages were found to support different faunal communities with species diversity of mammals and bats higher in the hinterland margin, although abundance was higher in the less diverse seaward communities. However, the seaward communities support the highest species diversity and abundance of mangrove birds. In general, most vertebrates within mangroves are opportunistic visitors from adjacent terrestrial environments, with only two mammal species considered to be resident, the Grassland mosaic-tailed rat and Common brushtail possum, and 14 birds are entirely restricted to mangrove habitat. A wide variety of bird species also inhabit mangroves. Dames and Moore (1988) have previously recorded a total of 128 birds found in Darwin Harbour.

The study also recorded a total of 254 species of invertebrates and fish from 81 families, indicating mangrove habitats of Darwin Harbour as one of the most diverse and productive estuarine systems of the IndoWest Pacific region (Metcalfe 2007). Crustaceans were most abundant comprising a relative abundance of 65.1%, molluscs 23.6% and worms 5.5%. Species richness and abundance was highest at the seaward margin and least at the landward mangrove fringe. Diversity in the tidal flat was equivalent with that of the tidal creek and exceeded it in terms of invertebrate abundance. The study concluded that the spatial and temporal variability evident in the primary productivity of the forests is reflected by patterns in secondary (faunal) productivity.

Mangrove faunal species were also monitored over seven surveys between 2012 and 2014 during the Inpex Ichthys Dredging program (Cardno 2015a). This study recorded four hundred and eleven different species, 80 new records for Darwin Harbour mangroves comprising 23 crustaceans, 21 worms, 19 molluscs, 11 ants, 4 fish and 2 other invertebrates, whilst one species of worm (Dendronereis sp.) had never been previously found in Australia. Species richness of invertebrate fauna was dominated by crustaceans (28%), molluscs (26%) and worm species (26%), collectively comprising 80% of all species. In terms of the total number of records (frequency), the invertebrate fauna was also dominated by crustaceans (53%), molluscs (25%) and worms (14%), collectively comprising 92% of all records. Species richness and abundance was generally greatest in the Seaward assemblage and decreased up the tidal profile towards the Hinterland Margin. Clear distinctions in the fauna associated among different assemblages was a noted characteristic in Darwin Harbour.

The current study undertook a brief assessment of benthic fauna living on the mud surface within 1 m x 1 m quadrats. Quadrats were sampled by passive observation for 10 minutes. A total of 610 organisms from 23 species were recorded during the field survey at four locations. Approximately 72% (439 individuals) of total fauna numbers across all locations were crustaceans, followed by mudskippers comprising 16% (98), molluscs at 12% (74) and three worms. Lower relative abundance of worms and molluscs in this study are likely due to the more cryptic nature of their habitation requiring more intrusive sampling techniques. Of the crustaceans, sesarmid (304) and fiddler crabs (113) composed 95%. Molluscs recorded were predominantly (~75%) comprised of Telescopium telescopium and Terebralia sp. common in mangroves, and Ellobiidae taxa such as Pedipes and Melampus. Mudskippers are commonly counted due to being less timid and often may be recorded more than once during the survey due to movements. Similar to previous studies, the highest mangrove fauna numbers were found in zones towards the seaward edge and lowest abundance at the Hinterland margin. The species numbers were generally comparable between zones likely due to the dominance of sesarmid and fiddler crabs


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and limited sampling techniques for broader diversity, although a few quadrats on the tidal flat recorded higher diversity associated with molluscs and hermit crabs recorded within this zone.

A variety of fish species utilise the mangroves during high tides when the tidal flats are inundated. The dense mangrove root structures provide perfect habitat for spawning and are utilised as nursery areas for juvenile fish and crustaceans. The abundance of small fish in turn provides a food source for larger predators, which may include the saltwater crocodile. Regardless of whether the mangroves are used periodically or as permanent home, they are considered to be vitally important as habitat areas for a wide array of organisms.

Commercial Fisheries

Commercially important fish and crustacean species such as banana prawns, mud crabs, barramundi and bream are strongly linked to the area of mangroves as they provide breeding, feeding and nursery areas for marine species and habitat connectivity within coastal ecosystems (e.g. spatial pattern of mangrove patches and length of creek systems) (Loneragan et al., 2005; Skilleter et al., 2005, Meynecke et al., 2008). As discussed above, export of organic matter from mangroves is important in sustaining the coastal food webs that support commercial fisheries (Jennerjahn and Ittekkot). As well, the physical structure of mangroves provides habitat and refuge for critical early life-history stages of fish and crustaceans taken by commercial and recreational fishers and Indigenous communities, usually at the sub-adult or adult stages of their life cycle (Jennerjahn and Ittekkot, 2002). Research conducted in eastern Australia has estimated that 67% of the entire commercial fish catch is composed of species dependent upon mangrove estuarine areas (Hamilton and Snedaker, 1984).

Community Values

The mangroves are used recreationally by the community for activities such as bird watching and recreational fishing. As discussed above the mangrove ecosystem attracts and supports an abundant, specific and varied wildlife. Bird watchers find enjoyment in identifying and watching the birds that inhabit mangrove areas. Recreational fishers target mangrove areas as they recognise that mangroves provide breeding and nursery habitats for prized fish such as the barramundi. Mud crabs and prawns are also often caught, along with many other marine species.

Cultural Values

Indigenous Australians living in coastal areas value mangrove communities and have utilised them for thousands of years. The mangrove communities are used as a resource for food, medicine and timber. Mud crabs, long bums, periwinkles, mangrove worms, mangrove jack, barramundi and many other marine species are collected as bush tucker.

Subtidal BHC

4.2.1. Distribution of Subtidal BHC

Previous studies of subtidal BHC within Darwin Harbour describe a complex assemblage of marine habitats consisting of hard corals, filter feeder, macroalgae, seagrass, a mixed reef community and soft sediment communities, with large differences in the extent, diversity and significance of the biological communities inhabiting them. Much of the benthic substrate of the lower intertidal zone of Darwin


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Harbour is described to be comprised of extensive mud flats interspersed with some areas of basement rock that may have a thin covering layers of sand or mud (URS 2010, Smit et al. 2002, Geo Oceans 2011).

The Study Area and fieldwork validation for this evaluation was originally restricted to the subtidal BHC of the Elizabeth River approximately 5 km either side of the Development Envelope, with the investigation focussed on the potential for construction or operational impacts including Outfall Case 1. However, numerical modelling results of the produced water discharge indicated marine water quality target objectives would not be achieved in the Elizabeth River location and an alternative location (Outfall Case 2) was proposed. The discharge pipeline for Outfall Case 2 continues out the mouth of the Elizabeth River extending into the centre of East Arm estuary, and the outer boundary of the Study Area was subsequently expanded to the mouth of East Arm between East Arm Wharf and Wickham Point (Error! Reference source not found.). Therefore, the information is presented in two sections; Elizabeth River and East Arm.

Elizabeth River

Smit et al. (2012) identified 12 subtidal BHC categories when mapping the Middle Arm/Elizabeth River region. These classifications were based predominantly on six variables identified as the most important factors that contribute towards explaining the spatial distribution of BHC: latitude, longitude, backscatter, mean depth, planar curvature, and benthic position index (BPI 100 square radius). This was followed by an assessment of the relative contribution of the physical parameters to predict biotic variables and key parameters and taxa using aggregated boosted tree and boosted regression trees. The final community maps were created by using previous benthic habitat data collected by BTM WBM (2010) and the Marine Biodiversity Group (MBG) overlaid onto the modelled species maps. The 12 categories and their general characteristics are presented in Table 4.

The mapped distribution of BHC within Elizabeth River prepared by Smit et al. (2012) was reviewed for this investigation and field validation works conducted using the combined methodology of Acoustic Sidescan Sonar (SSS) to identify consistent habitat features and Remotely Operated Vehicle (ROV) to ground-truth areas of interest based on identified SSS features. This work was conducted on the basis of proposed outfall Option 1 discharging produced liquid wastes into Elizabeth River.

Elizabeth River substrate systems include intertwining channels, ridges, reef edges, large extents of flats and many sand banks. The seabed BHC is predominantly mapped as Bare sand and mud (either flats/bars) (404 ha) on the intertidal zone to the lowest astronomical tide (LAT) and Bare coarse sediment (377 ha). These bare substrate habitats comprise approximately 44% and 40% of BHC within Elizabeth River, respectively.

At the mouth of the Elizabeth River is a series of broad scale depressions intertwined with large sandbanks and intertidal mud/sand flats. The broad depression in front of and north of Short Islet is complex with the bathymetry showing numerous rocky outcrops (ie narrow crests and depressions). These seem to be a continuation of the intertidal reefs surrounding Short Islet. Broad depressions with open bottom are observed north of Pikey and Slack Creek and on the north side of Elizabeth River adjacent to the Development Footprint. Pikey and Slack Creek are a miniature version of Elizabeth River and characterised by broad depressions; sand banks at the mouths; and with steep sloped creek banks. The upper reaches of Elizabeth River have broad flats and large broad isolated depressions.


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Epibenthic communities are generally associated with hard substrate environments at the mouth and in depressions of the Elizabeth River. In total, 93 ha or 10% of the BHC within Elizabeth River is mapped as mixed epibenthic community, 7.3% of this is medium density, 2% Low-medium density and 0.6% high density. Filter feeder BHC comprises only 3.2 ha or 0.4% of subtidal BHC in the Elizabeth River. This is slightly less than the 13% of the available hard substrate mapped in Smit et al (2012) for the broader Elizabeth River / Middle Arm region.

Elizabeth River subtidal BHC is typically less diverse than outer-harbour environments. Only two epibenthic community classes occur within Elizabeth River: filterfeeders and mixed epibenthic communities - defined as ‘reef substrates with a mix of coral, macroalgae and filter-feeders community assemblages >10% cover’. In comparison, Geo Oceans (2011) present four community classes: hard coral, macroalgae, filter-feeder and mixed communities within East Arm on the boundary of the Study Area. This is most likely reflecting differences in environmental / water quality parameters for both areas. The outer-harbour sites are predominantly located in sandy environments, with potentially clearer waters and are thus more suitable for phototrophic species like corals and algae. In Elizabeth River however, mangroves are the dominant feature, resulting in more turbid waters and thus favouring filter feeders and only those species of subtidal BHC that can tolerate higher sediment loads and reduced light availability.

East Arm

The subtidal BHC for all of Darwin Harbour with focussed effort on East Arm was undertaken by Geo Oceans (2011). This mapping effort collated all existing data sources and undertook a range of targeted surveys to produce a total known-point dataset of available habitat data in Darwin Harbour. The known point data were used to improve the resolution and accuracy of the existing subtidal BHC maps to quantify the spatial distribution of the biological communities and substrates found in Darwin Harbour. East Arm is mapped as a “high confidence” indicating areas that were surveyed with adequate data, whereas much of Darwin Harbour outside East Arm was mapped as “inferred habitat”. Geo Oceans (2011) used the Inpex habitat classification scheme to combine the data and reclassify data from different sources which was adapted from a national habitat classification scheme (Mount et al. 2007) and developed in consultation with the Department of Natural Resources, Environment, the Arts and Sport (NRETAS). The classifications, total area and relative composition for each subtidal BHC mapped within the Elizabeth River is presented in Table 9. The BHC of East Arm are defined using eight broad biological community classes shown in Table 10 and the distribution of these classes are presented in Figure 16.

A total of 16,377 ha (~75%) of subtidal BHC within “high confidence” mapped areas of Darwin Harbour, including East Arm, consists of soft bottom benthos communities living in unconsolidated sediments. The surficial sediments and thicker sediment deposits comprise of fluvial muds (i.e. clays and silts) sands and gravels with finer fractions becoming more dominant in the intertidal zones and in areas of more sheltered waters. Comprehensive surveys of soft-bottom communities near Bladin Point and Wickham Point generally found low bioturbation (~10 burrows/m2) and a low abundance of animals and plants, typically consisting of sea whips and algal turfs (URS 2010). Benthic infauna sampled near Bladin Point identified ~18-20 individuals from up to 16 taxa within a 0.15 m2 grab. The taxa were primarily composed of Amphipods (~30%) and Polychaetes (~27%) (URS 2010).


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Table 12 The community classifications from Geo Oceans (2011), total area and relative composition of subtidal BHC within “high confidence” areas of Darwin Harbour

Habitat Code

BHC Classification

Description (area = 10 m2)

“High confidence” Mapped Area (ha)

Relative Composition

(%)

1 Filter feeders >10% cover, >25% hard substrate

1,258 5.7%

2 Hard coral >10% cover, > 25% hard substrate, <5 m depth

219 1.0%

3 Macroalgae >10% cover, >25% hard substrate, <5 m depth

134 <1%

4 Mangroves Removed as discussed with intertidal BHC 5 Mixed community Coral, macroalgae, filter feeder

>10% cover, >25% hard substrate, <5 m depth

433 2.0%

6 Seagrass >1% cover, <25% hard substrate, <5 m depth

1,734 7.9%

7 Soft-sediment benthos

<25% hard substrate 16,377 74.8%

8 <10% macrobiota <10% cover, >25% hard substrate

1,749 8.0%

Total 21,904 100%

The hard substrate comprises ~17% of the mapped area and consists of outcropping bedrock (reef) that varies in rugosity. The deeper exposed reef substrates are dominated by filter feeding communities composed of gorgonian fans and sea whips (family Gorgonia), soft corals (family Alcyonica) and sponges which thrive on the food provided by the strong currents and highly turbid waters (URS 2010). These filter feeder communities are predominantly located west of East Arm Wharf and in Elizabeth River from the Study Area, comprising ~6% of subtidal BHC within “high confidence” mapped areas of Darwin Harbour.

Small communities of hard corals (1%) are found in the shallow subtidal and intertidal reefs mixed with the filter feeder communities. Only two reefs mapped as hard coral occurs on the outer border of the Study Area; at the eastern edge of a rock platform extending from Wickham Point and at South Shell Island. These platforms support a 10-20% cover of massive Faviids, laminar Turbinaria peltata and Goniopora colonies with lower composition of Mycedium spp. and small branching Acropora spp. Sponges, soft corals (Dendroephythya spp.) hydroids, gorgonians (sea fans and sea whips) and crinoids were common and dominant in deeper areas (>-1.5 m LAT). Up to 123 species of hard corals have been recorded in Darwin Harbour (Wolstenholme et al. 1997), although subsequent studies have reported 44 hard coral species from five sites: Channel Island, South Shell Island, Weed Reef and two mixed platforms north of Bladin Point. Only nine species were recorded from Bladin Point mixed assemblages, 22 species from South Shell Island and 29 species and up to 50% cover at Channel Island (URS 2010). Monitoring of coral communities at these sites during the Inpex Ichthys Project recorded cover ranging from 13% to 23% with Dendrophylliidae, Faviidae, Pectiniidae and Poritidae dominant (Cardno 2015b).


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Figure 16 BHC mapping of Darwin Harbour prepared for the Ichthys Project (Geo Oceans 2011)


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The BHC mapped as mixed community and <10% macrobiota occurs adjacent to Bladin Point, East Arm Wharf and Wickham Point, comprising ~2% and ~8% of mapped area in Darwin Harbour, respectively. URS (2010) describes detailed surveys of these communities within East Arm. The mixed and <10% macrobiota communities adjacent to Bladin Point describe low to moderate faunal cover of soft corals (mainly Sarcophyton spp. and Dendronephthya spp.) where hard substrate is present, together with zoanthids, sponges (laminar, digitate and barrel), bryzoans, hydroids and sea squirts. At deeper sites where hard pavement is less exposed the faunal community typically comprised sea fans, seawhips, sea pens and large sponges, whilst on shallower areas (0 m LAT) mixed community platforms were dominated by green algae. The steel barge, Kelat wreck and five Catalinas (World War II flying boats) were also surveyed and observed to exhibit heavy growth cover of mixed community with a low abundance of solitary hard corals (URS 2010).

All seagrass BHC is located at the entrance and outside Darwin Harbour between Fannie Bay and Casuarina Beach comprising ~8% of mapped area. No seagrass or other epifauna were recorded growing in the sediments around East Arm.

4.2.2. Functional Diversity and Ecological Integrity of Subtidal BHC

Hard Substrates

Filter-feeder and mixed communities on subtidal hard substrate habitats are considered to be biodiversity hot spots within the Study Area and are productive and dynamic ecosystems. Besides providing structure for a wide range of benthos to attach, reef communities also form complex trophic structures with multiple pathways and interrelationships between primary producers (e.g. micro and macro algae), herbivores (fishes, crustaceans), detritivores (e.g. fishes, crabs), carnivores (e.g. fishes, worms, molluscs) and decomposers (bacteria). Further, the complex reef structures and large epibenthic fauna provides ample refuge for mobile benthos (e.g. fishes, crustaceans, molluscs).

Two prominent topographic features are identified within the Study Area; at the mouth of Elizabeth River, and on the outer border between Wickham Point and East Arm Wharf. The hard BHC near the Elizabeth River mouth contains protected small patches of hard substrates amongst larger areas of mobile sediments. These reefs are likely to be predominantly influenced by fresh-water influx and high turbidity during the Wet season. These subtidal hard substrate reefs are likely to be less diverse and attract different assemblages than similar habitat in East Arm, which is evidenced by the mapped presence of additional hard coral and macroalgae habitats outside of Elizabeth River. The benthos on these substrates may also be quite variable between seasons due to the freshwater flow during the Wet. The scattered reefs and artificial structures inspected along the edge of the main channel in East Arm identify a gradient of increasing abundance and diversity in benthic communities towards clearer waters at Wickham Point. Some of the deeper filter feeder reef from the outer Study Area, as well as some mixed community habitat surrounding Bladin Point, has already been disturbed for the Inpex Ichthys Project within the Study Area.



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Soft-bottom benthos

In contrast to reef communities, soft-bottom mobile substrates comprise low diversity and generally low overall abundance, communities are highly variable and patchy in their distribution. The importance of these communities in Darwin Harbour is still poorly understood. In general, mobile substrate ecosystems form a complex trophic structure with a multitude of nutrient pathways and interrelationships. For example, many of the detritus-eating fish, invertebrates and bacteria play an important role in recycling organic matter and detritus and making nutrients available to primary producers (e.g. algae). Conversely these species are also a food resource for the next trophic level and many predatory fish and invertebrates are attracted to soft-substrate habitats to forage.

Microphytobenthos (MPB) was observed on mudflats seaward of the mangrove zone in Elizabeth River during this investigation. Smit et al. (2012) previously characterised the intertidal muds and sands as bare, although supporting infauna and mobile invertebrates. However, a study undertaken by Burford et al. (2008) identified that primary production of the marine environment in Darwin Harbour is compartmentalised into three areas, with mangroves contributing ~65%, water column phytoplankton ~16% and MPB ~9% of carbon production. A conceptual diagram showing the primary productivity processes in Darwin Harbour described by Burford et al. (2008) is shown in Figure 17. The MPB are dominated by diatoms (Clarke 1997). Burford et al. (2008) used chlorophyll-a (Chl-α) and phaephyton measurements and incubation experiments on intertidal cores to estimate MPB biomass in the harbour at 1,120 t (40.4 ±9.4 mg m-2) and gross primary productivity at 226,210 t y-1. Monitoring of MPB during the Ichthys Project by Cardno (2013b) showed a wide variation among sites, with total Chl-α ranging from 1.7 to 31.8 mg Kg dry weight, which converted to between 25 and 477 mg m-2 over 18 sites with Darwin Harbour. Higher Chl-α levels were typically located further upstream (i.e. Elizabeth River) in comparison with sites from the main harbour. This may be related to localised runoff and associated nutrient sources.

Figure 17 Conceptual diagram showing the primary productivity processes in Darwin Harbour described by Burford et al. (2008)


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Benthic infauna was monitored at the same intertidal mudflat locations productivity was measured during the Inpex Ichthys Project (Cardno 2015c). The infauna is an important source of food for wading birds and fish that live in the harbour and also help in nutrient cycling through reworking of the sediments. The composition of infaunal assemblages varies both annually and seasonally, in relation to the intensity of the wet season and recruitment success (Metcalfe and Glasby 2008). Infaunal assemblages also vary largely on the type of sediment they are associated with (Brown and McLachlan 2006). The intertidal mudflats of the Darwin Harbour contain a high abundance of infauna from a diverse range of taxa including (in order of abundance): polychaete worms (from a variety of families), crustaceans (including isopods, amphipods, cumaceans, crabs and burrowing shrimp), molluscs (gastropods, bivalves and opisthobranchs), echinoderms (ophiuroids and holothuroids) and other worm taxa such as nematodes, nemerteans, oligochaetes, phoronids, platyhelminthes and sipunculids. Other groups recorded included anemones, hydrozoans, sponges and pycnogonids (sea spiders). The highest abundance of individuals was collected in the upstream Elizabeth River site, corresponding to elevated primary productivity in these areas.

Subtidal soft substrates in Darwin Harbour were monitored for infauna during the Inpex Ichthys Project (Cardno 2015d). As well as being a source of food for organisms higher up the food chain, such as fish and large invertebrates, they also drive nutrient cycling through reworking of the sediments (bioturbation), altering physical and chemical processes, excreting nutrient-rich wastes and feeding on phytoplankton (Pennifold and Davis 2001). Similar to intertidal infaunal communities, the soft sediments contain a high abundance of infauna from a diverse range of taxa including (in order of abundance): crustaceans (amphipods, isopods, tanaids, cumaceans, penaeids, copepods, ostracods, shrimp, hermit crabs, ghost shrimp and brachyuran crabs), polychaete worms (from a variety of families and feeding guilds), molluscs (gastropods, bivalves and opisthobranchs), echinoderms (ophiuroids, echinoids, holothurians and seastars) and other worm taxa such as nematodes, nemerteans, oligochaetes, phoronids, platyhelminthes and sipunculids.

Soft substrates are also connected to other habitats, such as mangroves, subtidal reefs, salt marshes and the open ocean. This connection assists the movement of organisms, in particular when different habitats are used within different stages of their life cycles (e.g. prawns, mud crabs). Besides the ecological importance of mobile substrates, they perform many valuable functions to humans. For example, mobile substrates have the ability of removing contaminants from the water. Plants and bacteria break down many pollutants into less harmful forms. Uptake by sediments and burial in the mobile substrates minimise the toxic effects of pollutants. However, there is a limit to this carrying capacity and excessive input of nutrient and pollutants can overburden the cleansing capabilities of mobile substrates.

Conservation, ecological or social values of the BHC

Darwin Harbour is listed in the Directory of Important Wetlands in Australia (DEE 2019), being noted as one of the richest mangrove systems in Australia. This is primarily on the basis of its floristic diversity and extent (Wightman, 1989). The Directory of Important Wetlands Information Sheet is presented in Table 12. Darwin Harbour is also listed by the Northern Territory Government as a site of conservation significance (NRETAS 2019). An information page is provided in Attachment C that shows values, management issues and current conservation programs. The tidal mudflat mangroves and associated highly specialised fauna are described as the primary ecological value of the area.


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Table 13 Directory of Important Wetlands in Australia – Information Sheet for Port Darwin NT029

Port Darwin – NT029

Level of importance:

National - Directory

Location: 12 degrees 25' - 12 degrees 45' S, 130 degrees 42' - 131 degrees 03' E; immediately south-west, south and south-east of Darwin. The site includes the entire embayment (where less than 6 m deep at low tide) of Port Darwin, to the usual high water mark; ocean limits are defined here as East Point in the north-east and West Point (Mandorah) in the north-west. Major sub-embayments are (clockwise from north-east) Fannie Bay, Frances Bay, East Arm, Middle Arm, West Arm and Woods Inlet.

Biogeographic region:

Darwin Coastal.

Shire: Admin Region: Darwin. Area: 48 800 ha (deepwater area not deducted); includes mangroves at least 16 000 ha. Elevation: Sea-level. Other listed wetlands in same aggregation:

None.

Wetland type A1: Marine waters - permanent shallow waters less than six metres deep at low tide; includes sea bays, straits. A6: Estuarine waters; permanent waters of estuaries and estuarine systems of deltas A2: Subtidal aquatic beds; includes kelp beds, seagrasses, tropical marine meadows. A9: Intertidal forested wetlands; includes mangrove swamps, nipa swamps, tidal freshwater swamp forests. A3: Coral reefs. A7: Intertidal mud, sand or salt flats.

Criteria for inclusion

1. It is a good example of a wetland type occurring within a biogeographic region in Australia. 2. It is a wetland which plays an important ecological or hydrological role in the natural

functioning of a major wetland system/complex. 3. It is a wetland which is important as the habitat for animal taxa at a vulnerable stage in their

life cycles, or provides a refuge when adverse conditions such as drought prevail. 4. The wetland supports 1% or more of the national populations of any native plant or animal

taxa. 5. The wetland supports native plant or animal taxa or communities which are considered

endangered or vulnerable at the national level. 6. The wetland is of outstanding historical or cultural significance.

Site Description

Physical features: Landform: Megascale irregular embayment, with two macroscale islands (Channel Island, unnamed island) in Middle Reach, also several microscale islands and numerous fringing reefs and subtidal coral platforms (e.g. East Point, Channel Island). Geological setting: Situated in the Pine Creek Geosyncline (inner parts) and an unnamed basin (outer parts), in marine deposits surrounded by alluvial/aeolian/residual deposits; hinterland rises gently to 20-40 m elevation. The embayment is a "ria coast", i.e. a drowned bedrock coast. Climate: Median and mean annual rainfall at Darwin (Post Office) are 1534 mm and 1535 mm respectively, mostly falling in December- March; annual evaporation is c. 2600 mm.


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Hydrological features: Hydrological Role: The mangroves that fringe the whole embayment protect the coast from storm surges (cyclones). Water supply: Whole site is tidal. Major surface inflow is to East Arm from Elizabeth River, originating 20 km south-south-east, and to Middle Arm from Berry Creek, Darwin River and Blackmore River, each originating up to 27 km south (this catchment total area c. 55 000 ha); minor inflow is from numerous creeks up to a few km long. Inundation: Tides are twice-daily (semi-diurnal inequality). Water depth: Tidal range is 8 m. Water salinity: Mean salinity is 33 ppt (22-36). Water pH: Mean pH is 8.4 (7.9-8.7). Water Colour: The harbour waters are highly turbid (visibility 0.7 m) (Wrigley et al. 1990). Ecological features: Ecological role: A major nursery area for estuarine and offshore fish and crustaceans in the Beagle Gulf area. Plant structural formations: Mangal low closed-forest (mangroves) in periform arrangement. Surrounding areas support open-forest and woodland, over grassland. Significance: A good example of a shallow branching embayment of the Top End Region, supporting one of the largest discrete areas of mangrove swamp in the NT. Notable flora: Composition: 36 species, 23 of them trees and tall shrubs, have been recorded in the mangrove communities. The 23 include the NT. endemic Avicennia integra, also three species of Bruguiera, two varieties of Ceriops tagal, and only one Rhizophora (stylosa) and Samphires recorded are Halosarcia mangrove-associated species include the climber Dalbergia candenatensis and parasite Amyema thalassium, the fern Acrostichum speciosum and the grass Sporobolus virginicus. Special Communities: The mangrove communities of this site are the most extensive and species-rich of any NT embayment (Wightman 1989). Notable fauna: Composition: At least 48 species occur, 25 listed under treaties (JAMBA, CAMBA, BONN); includes four cormorants, nine herons and allies, three rails, 23 shorebirds and six gulls and terns. Notable mangrove/mudflat inhabitants include Great-billed Heron Ardea sumatrana, Chestnut Rail Eulabeornis castaneoventris and Beach Thick-knee Esacus magnirostris. Rare Species: A Red-necked Phalarope Phalaropus lobatus was at East Point in February 1987. A Sabine's Gull Larus sabini at the Darwin wharfs in 1982 was the first record for Australia; Black-tailed Gull L. crassirostris has also been recorded at Darwin. Breeding: Pied Cormorant Phalacrocorax varius has been found breeding (12 pairs) in Haycock Reach of Middle Arm. Migration Stop-over: At least 15 migrant shorebird species occur, most of them probably on a regular basis. Roosting: Minor, discrete, shorebird roost sites probably exist at the site. Numbers: Few systematic surveys, but numbers of shorebirds at the site apparently are not high in the context of the Top End Region (PWCNT computer databases; Fox & Associates 1987; Marchant & Higgins 1990; Thompson & Goodfellow unpub.). Other Fauna: Composition: Dolphins and turtles are commonly seen; Woods Inlet is frequented by the uncommon dolphin Orcaella brevirostris. Dugong Dugong dugon occur throughout the site (medium densities). At least 72 fish species occur. Abundant species of benthic fauna include the Fiddler Crab Uca capricornis, the crabs Chiromantes darwiniensis and Macropthalmus spp., the shrimps Alpheus euphrosyne and Athanas japonicus, the Mudskipper Periopthalmus argentilineatus and the Dwarf Goby Pandaka lidwelli. Numbers of taxa within various groups include at least 11 polychaetes, 16 Mollusca, and 32 Decapoda. The reefs are unusually rich in sponges (220 species at East Point), soft coral and invertebrates and c. 30 species of hard corals occur at East Point. Breeding: The mangroves and shallows are an important nursery area for fish, crabs, prawns and other marine fauna. Numbers: Natural densities of Saltwater Crocodile Crocodylus porosus are relatively low (e.g. 1 per km) and where possible all are removed from the site (c. 100 removed in 1992) (PWCNT files; Wood & Bonnin 1987; Bucher 1989; Burke 1992; Ottley, B. pers. comm.). Social and Cultural values: Cultural: The annual cycle of hunting and gathering of the Larrakia Aboriginal people includes the site, where a wide variety of meat and vegetable foods are procured. Many wrecks of warplanes and ships (e.g. USS Peary) destroyed in the Japanese attack on Darwin in World War 2 lie in the bay. A leprosarium was formerly located on Channel Island. Education: A mangrove study centre (Education Department), with boardwalks and facilities for school visits, exists on Channel Island. Research: The mangrove plant communities have been studied in detail by PWCNT, NT University, V. Semeniuk and others; the marine ecosystem has been studied by L. Ferns (PWCNT), R. Noske (NT University) M. Burke and others. Economic: The site is used for commercial fishing pearl culturing, aquaculture, port facilities and industry, and tourism. Recreation: Hovercraft and boat tours of Port Darwin are available and there are regular ferry services from Darwin to recreation sites near Mandorah. Boat ramps are scattered around the site. Other activities include scuba diving at shipwreck sites and on reefs, fishing (high intensity; mainly barramundi, reef fish and crabs), yachting and (especially at Fannie Bay and Talc Head) beach use and swimming in season. Aesthetic: An impressive marine natural area adjacent to a major population centre, limiting the urban spread, providing attractive views and variation in the landscape, and (especially at Fannie Bay) recreation opportunities. The juxtaposition of monsoon vine thickets, low red/white cliffs, green mangroves and pale blue bay waters (e.g. parts of Fannie Bay) is particularly appealing.


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Land tenure: East Point Reef Fish Reserve (150 ha) is in the north-east of the site; Channel Island is freehold held by the NT Power and Water Authority; there are some leasehold areas (e.g. pearling leases in East Arm); an aquatic life reserve is at Doctor's Gully; reserves at Blackmore River and Berry Springs include tidal areas; the remainder is mainly marine waters (vacant Crown land). Current land use: Recreation, aquaculture (pearls) and industrial (gas-driven power station on Channel Island). Urban (residential and industrial, including port facilities), recreation, cropping (fruit), other rural smallholdings and high human population. Disturbance or threat: Past/present: Vegetation clearance/degradation and reclamation (mainly in north-east, in vicinity of Darwin), drainage (of tidal flats, for mosquito control) and cyclones (some areas of mangrove were destroyed and some reefs were buried in sediment by Cyclone Tracy, 1974). Potential: Water pollution (from ships, industry, run-off), industrial areas (new developments) and excessive human disturbance (over-harvesting of marine fauna). Investigations in 1990 (Wrigley et al. 1990) revealed a good ambient water quality. Conservation measures taken: Darwin Harbour is proposed for a Conservation Management Plan. Frances Bay is closed to commercial fishing. The Harbour is closed to commercial harvest of mud crab Scylla serrata, to protect recreational interests. Part of the site (Channel Island Reefs, 270 ha; Darwin Foreshores, 300 ha) is included on the Register of the National Estate. Management authority and jurisdiction: Aquatic life reserves are managed by NT Department of Primary Industries and Fisheries; other reserves are managed by PWCNT and others.


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5. Conclusions

Intertidal BHC

Three basic BHCs have been mapped within the Study Area: Mangroves, Salt Flat and Beach.

Mangroves in the Study Area comprise a total of 4,178 ha (~91%) of intertidal BHC and ~20% of mangrove habitat within Darwin Harbour. Eight of the 10 existing mangrove associations have been identified and mapped:

Shoreline forest (Map unit 1): Rhizophora stylosa closed-forest (40 ha or <1%) Tidal Creek (Map unit 2): Rhizophora stylosa/Camptostemon schultzii closed forest (1,564 ha

or 34%) Transition (Map unit 3): Rhizophora/Bruguiera/Ceriops closed‐forests (20 ha or <1%) Tidal flat (Map unit 4): Ceriops australis low closed forest (1,710 ha or 37%) High tidal flat (Map unit 5): Ceriops australisl/Avicennia marina low closed forest (130 ha or

3%) Hinterland margin (Map unit 6): Mixed species low closed forest (367 ha or 8%) Low woodland (Map unit 7): Mixed low woodland (97 ha or 2%) Seaward margin (Map unit 8) Sonneratia alba woodland (251 ha or 6%).

Studies conducted in Darwin Harbour have estimated the productivity of mangroves through the collection and measurement of leaf litter fallen from particular area over time (Metcalfe 1999) and estimates of the standing above-ground biomass (DLRM 2013). The leaf litter rates are typically highest near the Seaward margin in Sonneratia trees and decline onto the Tidal flat, although increase to moderate rates again on the Hinterland margin. Leaf litter rates range from 3.9 t/ha/yr (Mid-tidal flat) to 12.6 t/ha/yr (Seaward). Conversely, the above-ground biomass rates range from 5 t/ha (Seaward) to 346 t/ha (Shoreline forest), where the biomass is regulated by a combination of tree size (basal area and height) and density. Productivity estimates indicate the Study Area represents approximately 21-23% of mangrove productivity within Darwin Harbour.

Small areas of salt flat (Map unit 11) and beach (Map unit 12) are found within the intertidal zone of the Study Area, although are not considered to provide areas of any significance to the marine ecosystem of Darwin Harbour. Salt flats are found in Ceriops forests (Map unit 4) or adjacent to the hinterland and are generally devoid of vegetation with hypersaline soils, although in some instances may contain samphire species, comprise a total area of 383 ha or 8% of the intertidal BHC within the Study Area. The Beach mapping unit is located at Bladin Point and Whickham Point adjacent to existing LNG plants, comprising 26 ha or <1% of intertidal BHC within the Study Area.

Subtidal BHC

The seabed BHC of Elizabeth River is predominantly mapped as Bare sand and mud (either flats/bars) (404 ha or 44%) on the intertidal zone to the lowest astronomical tide (LAT) and Bare coarse sediment (377 ha or 40%) on the riverbed. Numerous rocky outcrops are found towards the mouth of the


19WAU-0007 R1900216

Elizabeth River with filter feeder (0.03%) and mixed communities (10%) of varying density. This complex BHC towards the mouth of Elizabeth River is identified as an area of importance within the mapped unit. Occasional rocky outcrops were also observed throughout the Elizabeth River and at the mouth of Slack Creek.

Similarly, ~75% of “high confidence” mapped BHC within Darwin Harbour, including East Arm, consists of soft bottom benthos communities. These habitats generally exhibit low bioturbation and a low abundance of epifauna. The hard substrate comprises ~17% of the mapped area and consists of outcropping bedrock (reef) that varies in rugosity. The deeper exposed reef substrates are dominated by filter feeding communities comprising ~6% of subtidal BHC. Small communities of hard coral dominant communities (1%) are found in the shallow subtidal and intertidal reefs mixed with the filter feeder habitat and mixed communities of corals/filter feeder/macroalgae comprise ~2% of the mapped habitat. Scatted rubble/sand habitat, which typically represents a sessile benthic community cover less than hard substrate (<10%), encompasses ~8% of the Study Area. The complex benthic habitats between Wickham Point and East Arm Wharf on the outer border of the Study Area identify the most significant area of subtidal BHC within East Arm.



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6. References

AECOM (2011). Ichthys Gas Field Development Project: the benthic environment of the Ichthys Project – invertebrate fauna, habitats and impacts.

Alongi et al. (2016). Review of research and monitoring of Darwin Harbour’s mangrove environment for the development of a long-term monitoring program.

Attiwill & Clough (1976). Darwin East New Town Mangroves: Final Report.

Blaber SJM (2007) Mangroves and fishes: Issues of diversity, dependence, and dogma. Bulletin of Marine Science 80(3), 457-472.

BMT WBM 2010. East Arm, Elizabeth River, Blackmore River and Middle Arm Marine Habitat Survey. Report for iX Survey R.B18215.001.01.doc.

Brocklehurst, P. and Edmeades, B. (1996). The mangrove communities of Darwin Harbour. Technical Memorandum No. R96/9. Resource Capability Assessment Branch, Department of Lands, Planning and Environment, Darwin, Northern Territory.

Brown, A. C. and McLachlan, A. (2006). The Ecology of Sandy Shores. Academic Press, Burlington, Massachusetts.

Burford, M.A., Alongi, D.M, McKinnon, A.D, and Trott, L.A. (2008). Primary production and nutrients in a tropical macrotidal estuary, Darwin Harbour, Australia. Estuarine, Coastal and Shelf Science 79: 440-448.

Cardno (2013a). Mangrove Community Health Monitoring Program Baseline Phase Report: Ichthys Nearshore Environmental Monitoring Program. Document prepared for Inpex: L384-AW-REP-10001

Cardno (2013b). Primary Productivity Monitoring Baseline Report: Ichthys Nearshore Environmental Monitoring Program. Report to Inpex: L384-AW-REP-10010.

Cardno (2015a). Intertidal and Mangrove Community Health Post-dredging Report: Ichthys Nearshore Environmental Monitoring Program. Report to Inpex: L384-AW-REP-10046.

Cardno (2015b). Coral Monitoring Post-dredging Report: Ichthys Nearshore Environmental Monitoring Program. Report to Inpex: L384-AW-REP-10103

Cardno (2015c). Intertidal Benthos Monitoring End of Dredging Report: Ichthys Nearshore Environmental Monitoring Program. Report to Inpex: L384-AW-REP-10232

Cardno (2015d). Subtidal Benthos Monitoring Post Dredging Report: Ichthys Nearshore Environmental Monitoring Program. Report to Inpex: L384-AW-REP-10207

Clark, R.B. (1997). Marine Pollution Oxford University Press, Oxford. p60

Clifford and Jessup (2006). Key to the mangroves of Australia. Queensland Herbarium and Museum.


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Comley and McGuiness (2005). Above- and below-ground biomass, and allometry, or four common northern Australian mangroves.

CSIRO (2015). Seagrass Dataset - CAMRIS. v1. CSIRO.

Dames and Moore, (1988). Mangrove Zone Management Plan Darwin Harbour N.T. Stage 3 Prepared for Conservation

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DEE (2019). The Directory of Important Wetlands in Australia (the Directory). Department of Environment and Energy, Australian Government. Accessed 20 Aug 2019: https://www.environment.gov.au/water/wetlands/australian-wetlands-database/directory-important-wetlands

DENR (2000). Mangrove mapping of Darwin Harbour. Department of Environment and Natural Resources, Northern Territory Government of Australia Data accessed at http://metadata.imas.utas.edu.au/geonetwork/srv/en/metadata.show?uuid=8ebd1996-c77d-4012-b409-002f4422a915 on (31 August 2019).

DENR (2019). Coastal and Marine Management Strategy: Northern Territory 2019-2029. Our coasts and seas. Department of Environment and Natural Resources, Northern Territory Government of Australia.

DLRM (2013). The Mangrove Communities of Darwin Harbour: Biomass Update. Department of Land Resource Management, Northern Territory Government.

DSEWPaC (2012). Marine bioregional plan for the North Marine Region. Commonwealth of Australia.

Duke NC (2006) Australiaʹs Mangroves. University of Queensland and Norman C. Duke, Brisbane, Queensland, Australia

Dyall (2005). Australian Coastal Waterways geomorphic habitat mapping (national aggregated product).

Dyall A, Heap AD, Tobin G, Bryce S, Ryan DA, Galinec V, Creasey J, Gallagher J, Radke L, Smith C, Smith R, Murray E, Harris PT, Heggie DT (2005). Australian Coastal Waterways geomorphic habitat mapping (national aggregated product). Geoscience Australia. Data accessed at http://metadata.imas.utas.edu.au/geonetwork/srv/en/metadata.show?uuid=9b403526-e386-47ef-8a78-be1ae179419d on (31 August 2019).

Geo Oceans (2011). Ichthys Gas Field Development Project – Benthic Habitat Mapping of the Darwin Region. Appendix S6: Supplementary Environmental Impact Statement. Prepared for Inpex: C036-AH-REP-0121

Griffin et al. (2012). A Nationally Consistent Geomorphic Classification of the Australian Coastal Zone.

Griffin C, Hazelwood M, Nicholas T, Xu J (2012). A Nationally Consistent Geomorphic Classification of the Australian Coastal Zone. Data accessed at

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http://metadata.imas.utas.edu.au/geonetwork/srv/en/metadata.show?uuid=a05f7892-fabe-7506-e044-00144fdd4fa6 on (31 August 2019).

Halpern, B. S., Walbridge, S., Selkoe, K.A., Kappel, C.V., Micheli, F., D’Agrosa, C., Bruno, J.F., Casey, K.S., Ebert, C., Fox, H.E., Fujita, R., Heinemann, D., Lenihan, H.S., Madin, E.M.P., Perry, M.T., Selig, E.R.S., Steneck, M,R. and Watson, R. 2008. A Global Map of Human Impact on Marine Ecosystems. Science 319:948-952.

Hamilton and Snedaker. 1984. Handbook for Mangrove Area Management, Commission of Ecology. International Union for Conservation of Nature and Natural Resources, Gland, Switzerland.

Heap A, Harris P, Sbaffi L, Passlow V, Fellows M, Daniell J, Buchanan, C (2006). Geomorphic Features 2006. Data accessed at http://metadata.imas.utas.edu.au/geonetwork/srv/en/metadata.show?uuid=a05f7892-eeab-7506-e044-00144fdd4fa6 on (31 August 2019).

Jennerjahn TC and Ittekkot V (2002) Relevance of mangroves for the production and deposition of organic matter along tropical continental margins. Naturwissenschaften 89(1), 23-30.

Lucieer et al. (2017). Seamap Australia - a national seafloor habitat classification scheme.

McHugh (2004). DLNG Mangrove Monitoring Program: Monitoring of mangrove community structure /composition, soil conditions and mangrove productivity in Darwin Harbour.

MESH (2009) Supervised classification using image processing tools. Developed as part of the Mapping European Seabed Habitats. Downloadable guide sections available online: https://webarchive.nationalarchives.gov.uk/20101014084538/http://www.searchmesh.net/Default.aspx?page=1870

Metcalfe, K. (1999). Mangrove Litter Production: Darwin Harbour Northern Territory: A study of litter fall as a measure of primary productivity in the mangrove communities of Darwin Harbour. Faculty of Biological and Environmental Science, Northern Territory University

Metcalfe, K. (2007). The Biological Diversity, Recovery from Disturbance and Rehabilitation of Mangroves, Darwin Harbour, NT. Charles Darwin University, PhD thesis: Darwin. (Available on-line at http://hdl.handle.net/10070/232797)

Metcalfe K, and Glasby, C. (2008). Diversity of Polychaeta (Annelida) and other worm taxa in mangrove habitats of Darwin Harbour, northern Australia. Journal of Sea Research 59: 70-82.

Meynecke JO, Lee SY and Duke NC (2008) Linking spatial metrics and fish catch reveals the importance of coastal wetland connectivity to inshore fisheries in Queensland, Australia. Biological Conservation 141(4), 981-996. Doi: 10.1016/j.biocon.2008.01.018

Mount, R.E., Bricher, P.J. and Newton, J. (2007). National Intertidal/Subtidal Benthic Habitat Classification Scheme: version 1. School of Geography and Environmental Studies., University of Tasmania, Hobart.

Nagelkerken I, Blaber SJM, Bouillon S, Green P, Haywood M, Kirton LG, Meynecke JO, Pawlik J, Penrose HM, Sasekumar A and Somerfield PJ (2008) The habitat function of mangroves for

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NRETAS (2019). Sites of Conservation Significance: Darwin Harbour. Department of Natural Resources, Environment, The Arts and Sport. Northern Territory Government.

NT EPA (2016a). Statement of Reasons: TNG Limited – Darwin Refinery. Northern Territory Environmental Protection Authority.

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NT EPA (2016c). Guideline for Reporting on Environmental Monitoring. Northern Territory Environmental Protection Authority.

NT EPA (2018). Environmental Factors and Objectives. Northern Territory Environmental Protection Authority.

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Semeniuk, V. (1985). Mangrove Environment of Port Darwin, Northern Territory:The Physical Framework and Habitats. Journal of the Royal Society of Western Australia, Vol.67, No.3 and 4, pp.81-97.

Siwabessy et al. (2015). Mapping and Classification of Darwin Harbour Seabed.

Skilleter GA, Olds A, Loneragan NR and Zharikov Y (2005) The value of patches of intertidal seagrass to prawns depends on their proximity to mangroves. Marine Biology 147(2), 353-365.

Smit, N (2011). Darwin Harbour marine habitats. Department of Environment and Natural Resources, Northern Territory Government. Data accessed at http://metadata.imas.utas.edu.au/geonetwork/srv/en/metadata.show?uuid=2e754ed7-caab-4640-a133-5ead9e077edb on (31 August 2019).

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Wang, X.H., Sidhu, H., and Williams, D. 2011. Modelling of three-dimensional tidal dynamics in Darwin Harbour, Australia. The ANZIAM Journal 52:C103-C123.

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Wightmann, G. (2006). Mangroves of the Northern Territory, Australia. Identification and Traditional Use. Northern Territory Botanical Bulletin No. 3. Department of Natural Resources, Environment and the Arts and Greening Australia, 168pp.

Wolstenholme, J., Dinesen, Z.D. and Alderslade, P. (1997). Hard corals of the Darwin Region, Northern Territory, Australia. In: Hanley, J.R., Caswell, G., Megirian, D. and Larson, H.K. (eds). Proceedings of the Sixth International Marine Biological Workshop. The marine flora and fauna of Darwin Harbour, Northern Territory, Australia. Museum and Art Galleries of the Northern Territory and the Australian Marine Sciences Association: Darwin, Australia, 1997: 381-398

A TNG Processing Facility Darwin Benthic Habitat and Communities

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Appendix A Mangrove Site Images

C3-Z6

C3-Z4

C3-Z2

C3-Z8

B TNG Processing Facility Darwin Benthic Habitat and Communities

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C2-Z6

C2-Z4

C2-Z4

C2-Z8

C TNG Processing Facility Darwin Benthic Habitat and Communities

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I5-Z4

I5-Z2

I7-Z6

I7-Z4

I7-Z2

I7-Z8

D TNG Processing Facility Darwin Benthic Habitat and Communities

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Appendix B Mangrove Fauna Results

Site East Channel Island

Middle Arm Elizabeth River - Creek

Elizabeth River – River

Zone 6 4 2 8 6 4 2 8 6 4 2 8 6 4 2 8

Mean Burrows/m2 9.0

2.5 67.0

28.3

4.3

9.3 57.0

30.0

0.0

16.5

32.8

86.8

0.0

10.0

22.0

15.3

Species Richness 4 6 4 4 6 8 5 4 5 3 4 6 1 7 5 3 Mean Organisms/m2 8.

5 18.3

10.5

10.3

3.5

12.8

11.5

8.3 2.5

6.3 9.3 14.8

0.3

15.0

9.5 11.5

Mean Species/m2 2.5

4.3 3.0 3.0 2.8

3.8 3.0 3.3 1.8

1.8 2.5 3.8 0.3

3.8 2.5 2.5

SW Index 0.6

1.0 0.3 0.6 0.6

0.9 0.4 0.5 0.6

0.4 0.4 0.4 0.1

0.8 0.4 0.5

Crustaceans

Sesarmidae 6.3

5.5 7.3 1.5 0.3

3.8 7.3 2.0 0.5

5.5 7.3 11.0

0.0

8.5 8.0 1.5

Ocypodidae 0.5

1.8 1.3 6.0 0.0

0.3 2.0 3.5 0.0

0.3 1.0 2.3 0.0

0.3 0.5 8.8

Macropthalmidae

0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.8 0.0

0.0 0.0 0.0

Camptandridae

0.0

0.0 0.0 0.0 0.0

0.0 0.5 0.0 0.0

0.3 1.0 0.0 0.0

0.0 0.0 0.0

Grapsidae 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.3

0.0 0.0 0.5 0.0

0.0 0.3 0.0

Menippidae 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.3 0.0

0.3 0.0 0.0

Anomura 0.0

0.0 0.0 0.0 0.0

0.8 0.0 0.0 0.3

0.0 0.0 0.0 0.0

0.3 0.0 0.0

Other 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.3

0.0 0.0 0.0 0.0

0.0 0.0 0.0

Molluscs Potamididae 0.3

2.3 0.0 0.0 1.8

4.8 0.3 0.0 0.0

0.3 0.8 0.0 0.0

3.8 0.0 0.0

Neritidae 0.0

0.0 0.0 0.0 0.3

0.3 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.3 0.0

Littorinidae 0.0

0.3 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0

Melongidae 0.0

0.0 0.3 0.3 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0

Ellobiidae 0.0

1.8 0.0 0.0 0.0

0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.3

0.0 0.0 0.0

Fish Periopthalmus

1.5

6.0 1.8 2.5 0.3

3.0 1.5 2.8 1.3

0.0 0.3 0.0 0.0

2.0 0.5 1.3

Other 0.0

0.5 0.0 0.0 1.0

0.0 0.0 0.0 0.0

0.3 0.0 0.0 0.0

0.0 0.0 0.0

E TNG Processing Facility Darwin Benthic Habitat and Communities

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Appendix C Information Page: Darwin Harbour Site of Conservation Significance

Darwin Harbour Location and Description Darwin Harbour is a large indented embayment with three main arms – East, Middle and West. Two major rivers, the Elizabeth and Darwin Rivers drain into the Harbour and the city of Darwin is located on the north-eastern shore. The shoreline is dominated by mangroves, which largely remain in undisturbed condition and the Site contains more than 5% of the Northern Territory’s entire mangrove area. Darwin Harbour has one of the richest coastal environments anywhere in the Asia Pacific region, and occurs within one of the world’s least impacted marine regions. The coastal and mangrove environments are backed by savanna woodlands and patches of monsoon rainforest. Tenure and Land Use The Darwin Harbour Site, especially its west and south-west portions, is predominantly vacant Crown land. The remainder is mostly freehold land associated with the cities of Darwin and Palmerston. The land uses within the Site are many and varied - the freehold portions support a mix of commercial, residential and industrial land uses. The Harbour has port facilities and supports tourism, recreation and aquaculture. Approximately 3% of the Site is managed as conservation reserves. Significance Rating International Significance Ecological Values Darwin Harbour supports a range of estuarine, freshwater and terrestrial environments including extensive areas of tidal mudflats and one of the largest and most diverse areas of mangroves in the Northern Territory. The mangroves of Darwin Harbour support a highly specialised fauna and 14 bird species that are entirely restricted to mangrove environments (e.g. Chestnut Rail, White-breasted Whistler and Mangrove Golden Whistler). The Harbour itself supports a diverse range of marine species including dugongs, dolphins, marine turtles and a large variety of fish. A total of 15 threatened species are reported from within the Site. Management Issues Future urban and industrial developments around Darwin Harbour represent a major management issue for this Site. The north-eastern part of Darwin Harbour catchment is already highly developed and native vegetation and tidal flats have been cleared and drained. Further major industrial developments around Middle Arm are currently

being considered. Water pollution from run-off and industry and sea-level rise will also potentially affect the Harbour environment. Condition In comparison with marine areas near other major cities, Darwin Harbour is in good condition. However, a range of human activities do impact on the marine and coastal environments and these are likely to intensify in future years. Current Conservation Initiatives A regional plan of management has been developed for Darwin Harbour and its catchment area, and priority actions are being implemented. Ecosystem monitoring and research groups have been established to direct strategic research and co-ordinate monitoring activities within the Harbour.

D AR W I N H A R B O U R - S I T E O F C O N S E R V AT I O N S I G N I F I C AN C E

D e p a r t m e n t o f N a t u r a l R e s o u r c e s , E n v i r o n m e n t , T h e A r t s a n d S p o r t 9 2



SOCS Number 6 (NT Parks and Conservation Masterplan Map Number 12)

Latitude/Longitude 12º 34´ South, 130º 52´ East (at centre)

Bioregion Darwin Coastal (98%), Pine Creek (2%)

LOC

ATI

ON

Description The site includes the tidal flats (222 km²) within the Harbour from East Point around to West Point (including the major sub-embayments of East Arm, Middle Arm, West Arm, Woods Inlet, Frances Bay and Fannie Bay) and a buffering terrestrial area (527 km²). Sub-tidal waters of the Harbour are not included in this assessment. The rivers that flow into the Harbour (including the Darwin, Blackmore and Elizabeth Rivers and Berry Creek) have small catchments and lack the large floodplains and freshwater wetlands that characterise many other coastal areas around the Top End. Much of the area behind the extensive tidal flats in this site is high ground forested with woodland rather than wetlands (Darwin Harbour Advisory Committee 2003). The extensive tidal flats associated with nearby Shoal Bay and the sand sheets of the Howard River are also recognised as sites of high conservation significance in the NT.

Significance Rating International Significance

THR

EATE

NED

SPE

CIE

S

Threatened plants and animals (Listings at National/NT level CR - Critically Endangered, EN - Endangered, VU - Vulnerable, NT - Near Threatened, LC - Least Concern, DD - Data Deficient)

15 threatened species are reported from this site. Plants Cycas armstrongii (-/VU) Utricularia singeriana (-/VU)

Vertebrates Australian Bustard Ardeotis australis (-/VU) Christmas Frigatebird Fregata andrewsi (VU/-) Gouldian Finch Erythrura gouldiae (EN/EN) Masked Owl Tyto novaehollandiae kimberli (VU/VU) Partridge Pigeon Geophaps smithii (VU/VU) Red Goshawk Erythrotriorchis radiatus (VU/VU) Northern Quoll Dasyurus hallucatus (EN/CR) Merten's Water Monitor Varanus mertensi (-/VU) Yellow-spotted Monitor Varanus panoptes (-/VU) Flatback Turtle Natator depressus (VU/DD) Green Turtle Chelonia mydas (VU/LC) Hawksbill Turtle Eretmochelys imbricata (VU/-)

Invertebrates Atlas Moth Attacus wardi (-/EN)

There are only historic records of Utricularia singeriana and the Atlas Moth from this site, and suitable habitat may no longer be present.

Significance Rating Not Significant

END

EMIC

SP

ECIE

S

Notes Endemic to the bioregion: One vertebrate (Ramphotyphlops nema) and two plant species (Spermacoce phalloides, Typhonium praetermissum) recorded in this site are NT endemics and are only known from the Darwin Coastal bioregion. Endemic to the NT: 77 plant and 12 vertebrate species recorded in the site are only known from the NT. Other: 12 plant and one vertebrate species (Lewin’s Rail) are only known from the site or the Darwin Coastal bioregion within the NT, but are also found in other states. There is a collection of records of vagrant bird species from Darwin Harbour/Shoal Bay that have not been recorded elsewhere in the NT.

Significance Rating Not Significant

Marine turtles Flatback, Hawksbill, and Green Turtles frequent the waters of Darwin Harbour but the lack of sandy beaches within the Harbour inhibits nesting activity.

Seabirds Significant aggregations of seabirds are not known from this site (Chatto 2001).

Waterbirds This site lacks a large area of freshwater wetland and supports relatively low numbers of waterbirds (Chatto 2006).

Shorebirds Although large areas of mudflats occur around Darwin Harbour during periods of low tide, high numbers of shorebirds have not been recorded. The highest count is 3000 individuals in 1994 (Chatto 2003).

WIL

DLI

FE

AG

GR

EGA

TIO

NS

Other aggregations None known

Significance Rating National Significance

Ramsar criteria met Not assessed

DIWA criteria met Darwin Harbour is listed as a wetland of national significance in the Directory of Important Wetlands in Australia (DIWA: NT029 Port Darwin). The site meets criteria 1, 2, 3, 4, 5, 6 and includes wetland types: A1, A2, A3, A6, A7, and A9.

WET

LAN

DS

Notes Darwin Harbour is a good example of a shallow branching embayment of the Top End Region, supporting one of the largest discrete areas of mangrove swamp in the NT (DIWA). Within the Darwin Harbour catchment there are series of ponding systems (Dambos) that may play an important role in filtering organic material before it is delivered to the harbour. Many of these are not included within the current boundary of the site (R. Wasson, Charles Darwin University, pers. comm.).



Rivers The Darwin, Blackmore and Elizabeth Rivers and Berry Creek flow into Darwin Harbour. All are relatively small Top End rivers.

Significance Rating National Significance

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Notes Rainforest: About 1150 ha of mostly dry rainforest (or 0.4% of the NT rainforest estate) occur as small patches around the margin of the tidal flats in this site. One patch is >100 ha but most patches are small (<10 ha) (Russell-Smith 1991). Large areas of rainforest or vine-thicket habitat occur within the Harbour on peninsulas or ‘hinterland islands’ such as Blaydin Point, Wickham Point, Flagstaff Hill and Kings Table. Fire-sensitive vine-thicket communities have become particularly well developed in these habitats due to the protection offered by the surrounding mangroves, which unlike savannah woodlands, do not burn. Other: Mangroves fringe the whole embayment of Darwin Harbour and comprise one of the largest (~20 400 ha) and most floristically diverse (~41 species) areas of mangroves in the NT (Duke 2006). Mangrove communities within the Harbour have been identified and mapped by Brocklehurst and Edmeades (1996).

OTH

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The mangroves of Darwin Harbour support a highly specialised fauna including over 306 invertebrate species and 112 species of mammals, bats and birds (Metcalfe 2007). Mangroves in north-western Australia support distinctive fauna and more mangrove-endemic bird species than any other region in the world (Noske 1996). Some of the highly specialised bird species that occur in Darwin Harbour include the Mangrove Gerygone, Mangrove Robin, Mangrove Golden Whistler and Chestnut Rail. A group of colubrid snakes including the White-bellied Mangrove Snake Fordonia leucobalia, are also part of the distinctive mangrove fauna found in Darwin Harbour, and compliments the wider, but poorly-known, community of sea-snakes in the area (Whiting 2003). In terms of faunal diversity, Darwin Harbour is one of the richest mangrove systems in the Indo-west Pacific region. Each of the eight different floristic assemblages defined in Darwin Harbour mangroves (Brocklehurst and Edmeades 1996) supports a distinctive faunal community. In particular, the most seaward assemblage, with Sonneratia alba dominant, is an exceptionally productive mangrove community with the highest primary productivity (Metcalfe 1999) and faunal diversity and abundance of any assemblage in the harbour (Metcalfe 2007). Eight sites around Darwin Harbour are listed on the Register of the National Estate for their natural values including: Berry Springs Nature Park, Darwin Foreshores, Pachystoma pubescens Sites 1 and 2, Channel Island Reefs, Imaluk Creek Area, Southport Area, and the Darwin Harbour Wetlands (Australian Heritage Council). 80 species recorded from this site are listed under international conventions or bilateral agreements protecting migratory animals. Dugongs are common in the Harbour (Whiting 2004). The marine areas within this site are likely to encompass other significant biodiversity values and these are currently being explored and collated in a project by the Marine Biodiversity Group of NRETAS (K. Edyvane, NRETAS, pers. comm.).

MA

NA

GEM

ENT

ISSU

ES

Fire: The current fire regime in the Darwin region differs from that in other sparsely populated savanna areas of the NT and is ad hoc and closely linked to tenure (Price and Baker 2007). The frequency of late dry season fires is lower in the Darwin region than other areas (Price and Baker 2007), but exotic grasses are increasing fuel loads and the intensity of fires (Kean and Price 2003). In the period 1993-2004, 43% of the site was burnt in fewer than three years, and 24% was burnt in more than six years. Feral animals: Feral cat, dog, rat, pig and Cane Toad are present in the site. Marine pest incursions remain a concern given the proximity of Darwin Harbour to Asia (Smit 2003). Weeds: Four Weeds of National Significance (Lantana camara, Mimosa pigra, Salvinia molesta, Parkinsonia aculeata), 25 declared Category A and B weeds and 12 other undeclared but problematic environmental weeds (high priority weeds: Smith 2001) are recorded from this site. The aquatic weed Cabomba caroliniana is reported from Darwin River (Smith 2002). Other: The north-eastern part of Darwin Harbour catchment is highly developed and native vegetation and tidal flats have been cleared and drained. With the current rapid growth of the city of Darwin, further pressure is likely to come from future recreational, residential and industrial developments within the Harbour (Wightman 2006). Major industrial developments around Middle Arm are currently being considered. Nutrient enrichment from sewerage discharge and storm water run-off may affect mangrove communities in the Harbour (Dames and Moore 1984 in Wightman 2006). Despite having a macrotidal range of 7.8 m, the waters of Darwin Harbour are not particularly well flushed and recent research and modelling indicates that pollution may circulate within the upper reaches of the Harbour for considerable periods (Williams, 2006). Pollution and increased turbidity (e.g. from dredging) associated with future developments within the Harbour, may therefore affect water quality and the biodiversity values. The potential rise in sea level predicted in response to global climate change may affect mangrove communities in Darwin Harbour, especially in areas where coastal developments exclude the landward retreat of coastal ecosystems. Although mangroves are generally well adapted to the dynamic conditions at the land-sea interface, recovery from severe disturbance (e.g. storms, cyclones, clear-felling) may be very slow (Metcalfe, 2007). Indeed, severely damaged mangroves may take several decades to recover and such delayed recovery times increase their vulnerability to disturbance (McGuinness 1992).



NRM groups Belyuen Land Management Group (Belyuen), Larrakia Rangers (Darwin) (Northern Land Council 2006).

Protected areas Blackmore River Conservation Reserve (4 km²/ 0.6% of site), Channel Island Conservation Reserve (1 km²/ 0.1% of site), Charles Darwin National Park (10 km²/ 1% of site), Territory Wildlife Park/Berry Springs Nature Park (11 km²/ 1% of site).

Current management plans

Site-specific plans: Charles Darwin National Park Plan of Management (PWCNT undated); Darwin Harbour Regional Plan of Management (Darwin Harbour Advisory Committee 2003). National recovery plans for threatened species: marine turtles (Environment Australia 2003); Northern Quoll (Hill and Ward in prep.); Partridge Pigeon and Masked Owl (Woinarski 2004a), Gouldian Finch (O’Malley 2006); Red Goshawk (Baker-Gabb in prep.). Other management plans: Australian Weeds Strategy (NRMMC 2007); Threat Abatement Plan for Predation by Feral Cats (Environment Australia, 1999); Threat Abatement Plan for Predation, habitat degradation, competition and disease transmission by feral pigs (DEH 2005); FIREPLAN: Fire management for the savanna community (Russell-Smith et al. in prep.).

Monitoring programs and research projects

Fire in the tropical savannas is mapped continuously under the North Australia Fire Information Project http://www.firenorth.org.au/nafi/app/init.jsp Numerous programs and sites exist for monitoring water quality and ecological condition within the Darwin Harbour catchment area and a summary of them is reported by the Darwin Harbour Advisory Committee (2005). Fauna and vegetation are monitored at permanent sites in NT parks within the Darwin region including the Territory Wildlife Park and Charles Darwin National Park (Calnan et al. 2008). Populations of the rare ground orchid Nervilia peltata (D. Liddle, NRETAS unpubl.) and Cycas armstrongii (Liddle 2004) are monitored under different fire regimes at permanent plots in Charles Darwin National Park. There is an ongoing program of monitoring and removal of Saltwater Crocodiles from within Darwin Harbour and Shoal Bay (Nichols and Letnic in press). Dolphins are surveyed monthly along transects within Darwin Harbour and Shoal Bay (C. Palmer, NRETAS unpubl.). Research on the biodiversity of mangrove habitats in Darwin Harbour involving surveys of vertebrate and invertebrate fauna in disturbed and undisturbed mangroves was conducted from 1999-2002 (Metcalfe 2007). The methodology developed has since been applied for mangrove monitoring purposes. A two year study of primary productivity within the eight different mangrove assemblages was conducted at eight sites in the three arms of Darwin Harbour from 1997-1999 (Metcalfe 1999). Monitoring of mangrove productivity was continued for a 3rd year by DIPE. Recommended methodology for monitoring of flora and soils in mangrove habitats of Darwin Harbour was developed by DIPE (Moritz-Zimmeman et al.2002), developed further in a research framework (Comley 2002) and later applied at the Darwin LNG plant (McHugh 2004). Commercial mangrove monitoring programs for aquaculture developments in Darwin Harbour including Wild River and Tiger International subsequently adopted this monitoring methodology for impact assessment purposes. Research on the distribution and role of Dambo wetland systems in the Darwin Harbour catchment (R. Wasson, Charles Darwin University, pers. comm.). Research on the use of mangrove habitats by fish in Darwin Harbour was conducted from 1998 to 2001 including development of a trophic model for the harbour (Martin 2004). As part of the Environmental Management Plan for the Darwin LNG Plant, Conoco Phillips established in 2002 a mangrove monitoring program at Wickham Point with matched control sites in Darwin Harbour. The monitoring program has provided over 6 years of valuable baseline data on mangrove flora and invertebrate fauna (URS 2003; Metcalfe 2005; 2006).

MA

NA

GEM

ENT

INFO

RM

ATI

ON

Management recommendations

Continue to implement the Darwin Harbour Regional Plan of Management (NRETA 2005). Develop a fire management strategy for the Darwin region that identifies clear objectives, roles and responsibilities (Price and Baker 2007). Prevent the spread of exotic grasses, especially mission grasses and gamba grass, into new areas in the Darwin region and reduce populations in areas with high conservation value or where fires threaten properties (Kean and Price 2003). Assess the data for Darwin Harbour against Ramsar criteria and consider listing as a wetland of international significance (S. Blanch, Environment Centre NT, pers. comm.). Consider expanding the boundary of the site to the catchment boundary to incorporate Dambo wetland systems (R. Wasson, Charles Darwin University, pers. comm.).

Papers and reports Darwin Harbour Advisory Committee (2003). Darwin Harbour Regional Plan of Management. Department of Infrastructure, Planning and Environment, Darwin. Darwin Harbour Advisory Committee (2005). A Review of Environmental Monitoring of the Darwin Harbour Region and Recommendations for Integrated Monitoring. Darwin Harbour Advisory Committee, Darwin. DIWA (A Directory of Important Wetlands in Australia). Australian Wetlands Database. Department of Environment, Water, Heritage & the Arts, Canberra ACT (accessed November 2007). Metcalfe, K. (2007). The biological diversity, recovery from disturbance and rehabilitation of mangroves in Darwin Harbour. PhD thesis. Charles Darwin University, Darwin.

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Contributors Kristin Metcalfe, Consultant Environmental Scientist, Darwin

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