SEAGRASSES, MANGROVES AND SALTMARSHES€¦ · SEAGRASSES, MANGROVES AND SALTMARSHES Author: Paul...

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7 SEAGRASSES, MANGROVES AND SALTMARSHES Author: Paul O’Neill Reviewers: Warren Lee Long, Dr Rhonda Melzer, Dr John Robertson

Transcript of SEAGRASSES, MANGROVES AND SALTMARSHES€¦ · SEAGRASSES, MANGROVES AND SALTMARSHES Author: Paul...

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7 SEAGRASSES, MANGROVES AND SALTMARSHES

Author: Paul O’NeillReviewers: Warren Lee Long, Dr Rhonda Melzer, Dr John Robertson

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180 | Seagrasses, Mangroves and Saltmarshes

THE ENVIRONMENT AND ITS CONDITIONThe extensive mangrove communities of the Shoalwater Bay Training Area (SWBTA or the Area) have long been recognised, with both Flinders (1802) and King (1827) noting their extent but without extolling their values. King (1827) for example, noted that the area including Port Clinton and Shoalwater Bay was ‘… low and overrun with mangroves.’ After European settlement, large areas of seagrass and mangroves in Australia were cleared or altered to make way for development. The general understanding of the importance of not only mangroves, but also the dynamic natural processes that occur in shallow coastal areas, is now far greater than it was in the early nineteenth century. Some areas that were originally cleared of mangroves in Australia have even been replanted. The mangrove, saltmarsh and seagrass communities of SWBTA remain in their natural state. Unlike communities elsewhere, they have never been cleared or signifi cantly altered, and are isolated from the elevated sediment and nutrient infl uences that characterise developed catchments. Scientifi c interest in these areas is likely to increase as an understanding of natural coastal processes, including the role played by marine vegetation, takes on a new relevance with the looming threat of sea level rise brought about by climate change.

The impenetrable nature of much of the mangrove forests in SWBTA was well documented by Flinders (1802). While exploring Island Head Creek he wrote ‘The shores are almost every where low and covered with mangroves, sometimes to the distance of miles from the water, and there are very few places where a boat can land without great inconvenience’. The Area’s very large tidal variation of more than seven metres has also produced extensive intertidal and subtidal sand and mud fl ats where seagrasses grow. Extensive saltmarsh areas have formed in the Shoalwater Bay area, on the dry sediments that occur immediately adjacent to mangrove forests and above the infl uence of most tides.

MARINE BIOREGIONS

The Great Barrier Reef Marine Park (GBRMP) was divided into 40 non-reef bioregions (and 30 reef bioregions) as part of the Representative Areas Program (RAP) completed by the Great Barrier Reef Marine Park Authority (GBRMPA) in 2003. Much of the marine area of SWBTA is within the High Nutrients Coastal Strip Bioregion (Figure 7.1; GBRMPA 2008). This bioregion runs along the inshore coastal areas in the north near Cooktown south to the southern boundary of the marine park just north of Bundaberg. It is characterised by terrigenous mud sediments and high levels of nutrients from adjoining lands. Seagrass is also a common feature in sheltered sites, and this provides good turtle and dugong habitat. Although this bioregion comes under a wet tropical infl uence along much of the coast, it is subject to a dry tropical infl uence in the Shoalwater Bay area.

The other large non-reef bioregional component of SWBTA is the Inshore Terrigenous Sands Bioregion. This bioregion surrounds most of the islands in the north-western parts of Shoalwater Bay and is under a strong Broad Sound tidal infl uence. The substrate is composed largely of highly mobile sands, and contains little algae or seagrass.

Ross Creek

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FIGURE 7.1

0 10 205 15

Kilometres

N

Marine Bioregions (non-reef)(Source: GBRMPA, developed for

Representative Areas Program 2001)High Nutrients Coastal Strip

Inshore Terrigenous Sands

Inner Shelf Lagoon Continental Islands

Capricorn Bunker Lagoon

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SEAGRASS COMMUNITIES

Seagrass communities of SWBTA were surveyed and mapped during September 1995 and April 1996 as part of a Queensland-wide seagrass mapping project by QDPI Fisheries (Lee Long et al 1997). The Area contains just over 13 000 hectares of seagrass meadows with eight species represented, compared to 15 species known from Queensland (Table 7.1, Coles et al 2004). Shoalwater Bay is recognised as the largest area of seagrass habitat in the southern Great Barrier Reef (GBRMPA 1997; Marsh et al 1996; Waycott et al 2005). Approximately 62% of known seagrass resources in the GBRMP south of Mackay are located in SWBTA (Lee Long et al 1997). Other large areas of seagrass do not occur until about 300 kilometres to the north around the Whitsunday Islands, and 150 kilometres to the south near Gladstone.

SWBTA seagrass meadows are regionally important as nursery areas and habitat for prawns and fi sh, and are nationally important feeding grounds for dugongs Dugong dugong and green turtles Chelonia mydas (Lee Long et al 1997). Beam trawl samples from the seagrass meadows in SWBTA produced large numbers of juvenile fi sh, prawns and other invertebrates, indicating a healthy basis for local marine food webs

(Lee Long et al 1997). Seagrasses are mostly restricted to the intertidal fl ats where they occur on soft muddy substrates, but small areas of subtidal meadows occur to the south-west of Townshend Island and in Canoe Passage (Lee Long et al 1997).

Twelve seagrass community types were originally identifi ed in SWBTA (Lee Long et al 1997). For this report these community types were amalgamated into six communities characterised by their most dominant species (Figure 7.2). Seagrass communities dominated by Zostera capricorni are the most extensive and generally contained the highest above-ground biomass (Lee Long et al 1997). Generally, Zostera dominated communities are most common in eastern parts of the Area, with Halodule uninervis and Halophila ovalis dominated communities more common in the west. A small community of Syringodium isoetifolium dominated seagrass was located off the south-west corner of Townshend Island, and small communities dominated by Cymodocea serrulate were found to the east of Akens Island and at the southern end of Townshend Island.

TABLE 7.1Seagrass species recorded in SWBTA. (Lee Long et al 1997)

Family Species

Family ZOSTERACEAE Zostera capricorni

Family CYMODOCEACEAE Cymodocea serrulate

Halodule uninervis

Halodule pinifolia

Syringodium isoetifolium

Family HYDROCHARITACEAE Halophila decipiens

Halophila ovalis

Halophila spinulosa

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FIGURE 7.2

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Kilometres

N

Seagrass communities of SWBTA(Source: Lee Long et al 1997)

Cymodocea dominant

Halodule dominant

Halodule/Halophila

Halophila dominant

Syringodium dominant

Zostera dominant

Mangrove forests / saline flats

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Distribution and abundance of seagrass communities in SWBTA is infl uenced by many factors, including water clarity, sediment and tidal range. Strong tidal currents and abundant fi ne sediments result in high water turbidity, limiting subtidal seagrass to small areas. The high turbidity restricts light penetration, limiting the depth to which seagrasses can grow in SWBTA to about 8.2 metres, compared to a maximum recorded depth of 53 metres in other offshore areas in Queensland (Lee Long et al 1997). Shoalwater Bay also has a large tidal range of approximately seven metres. This restricts the upper distribution limits because seagrasses growing in this area would be desiccated during extended periods of low tide. However, in some areas the effect is moderated by water ponding on the fl ats as the tide drops, allowing seagrass to grow in places up to about 0.6 metres above Mean Sea Level (Lee Long et al 1997).

A four-year study showed that seagrass biomass is not high in SWBTA compared to some other areas in Queensland, but it is consistent throughout the year, providing a reliable food source for both green turtles and dugong (CQU 2007). The extent of seagrass communities did not change markedly between 1995 and 2007, but some species distribution changes were detected. An example is the Halodule–Halophila meadow at Ross Creek changing to one dominated by Zostera capricorni (CQU 2007). The seagrass immediately to the south of Sabina Point showed a sudden decline in biomass between February and June 2004, but these communities have since recovered (CQU 2007). This temporary decline was attributed to natural causes, brought about by a wave-induced change in sediment composition after a prolonged period of strong south-easterly winds (CQU 2007). Sabina Point is fully exposed to such winds and in adjacent, less exposed locations, there were no detectable changes in seagrass biomass. Seagrass cover in Shoalwater Bay fl uctuates naturally, and subtle changes to intertidal topography play a major role in determining the species mix on seagrass meadows over time (CQU 2007). The study concluded that seagrass communities in the Area remain in excellent condition (CQU 2007).

Port Clinton seagrass

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Mudfl ats along western Shoalwater Bay

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MANGROVE AND SALTMARSH COMMUNITIES

SWBTA contains about 24 279 hectares of mangrove and saltmarsh communities, just over 8% of the total land area (Table 7.2). Regionally, this is about 16% of the total mangrove and saltmarsh communities present between Mackay and the northern end of Curtis Island, demonstrating the vast extent of these communities in the Area (Bruinsma 2000). Mangroves and saltmarshes within SWBTA are recognised as having particular regional signifi cance because they and the surrounding terrestrial vegetation remain in a natural and unaltered condition (Bruinsma 2000).

Mangrove and saltmarsh communities are important habitats for a variety of organisms, including many commercially and recreationally important fi sh species. Mangroves also play an important role in accumulating and stabilising coastal sediments, restricting erosion and reducing turbidity (GBRMPA 2008).

Highly detailed mapping of the mangrove and saltmarsh communities of SWBTA was completed in 1997 and is presented in Figures 7.3 to 7.8 (Byron & Hall 1998). This mapping was a response to the 1994 Commission of Inquiry recommendation that ‘Agencies cooperate on gathering baseline data on the marine and estuarine environment of the Area, with the objective of ensuring future reliable assessment of the impacts of fi shing and Defence use on the National Estate and World Heritage Areas…’. Thirteen species of mangrove were recorded during the fi eld component of this study, with at least a further eight species likely to be present (Table 7.3). Four of these species, Bruguiera exaristata, Rhizophora lamarckii, R. apiculata and Sonneratia alba are potentially at their southern distributional limit in Port Clinton (Bruinsma 2000). Rhizophora stylosa, Avicennia marina, Aegiceras corniculatum and Ceriops tagal are the most common mangrove species and are important structural components of most mangrove communities (Byron & Hall 1998).

TABLE 7.2Mangrove and saltmarsh communities of SWBTA and their extent. (source: Byron & Hall 1998)

Mangrove Community Hectares

Avicennia marina low open forest 2–10 m 39.5

Rhizophora spp and A. marina low closed forest 2–10 m 479.6

Rhizophora spp closed forest >10 m 159.2

Rhizophora spp low closed forest 2–10 m 15 187.6

Rhizophora spp closed thicket 0–2 m 58.2

Mixed mangrove low closed forest 2–10 m 3 635.7

Mixed mangrove closed thicket 0–2 m 6.2

Aegiceras corniculatum closed thicket 0–2 m 300.8

Ceriops tagal low open forest 2–10 m 1 441.2

Ceriops tagal open thicket 0–2 m 178.2

Sub-total — mangroves 21 486.2

Sporobolus virginicus meadows 48.4

Saltpan/saltmarsh community 2 744.1

Total 24 278.7

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0 1 2

Kilometres

N

Akens Island

Sabina Point

Avicennia marina low open forest 2-10m

Rhizophora stylosa & A. marina low closed forest 2-10m

Rhizophora spp closed forest >10m

Rhizophora spp low closed forest 2-10m

Rhizophora spp closed thicket 0-2m

Mixed mangrove low closed forest 2-10m

Aegiceras corniculatum closed thicket

Ceriops tagal low open forest 2-10m

Ceriops tagal open thicket 0-2m

Sporobolus virginicus meadows

Saltpan/Saltmarsh alliance

Rocky foreshore

Terrestrial vegetation

Intertidal flats

Ocean

SWBTA boundary

KEY TO INTERTIDAL FLORA ALLIANCES

INTERTIDAL VEGETATION OF WESTERN SHOALWATER BAY (NORTHERN SECTION) (After Byron & Hall 1998)

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FIGURE 7.3

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0 1 2

Kilometres

N

ShoalwaterCreek

Note: See Figure 7.3 for key to vegetation types.

FIGURE 7.4

INTERTIDAL VEGETATION OF WESTERN SHOALWATER BAY (CENTRAL SECTION) (After Byron & Hall 1998)

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0 1 2

Kilometres

N

Note: See Figure 7.3 for key to vegetation types.

Head Creek

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FIGURE 7.5

INTERTIDAL VEGETATION OF WESTERN SHOALWATER BAY (SOUTHERN SECTION) (After Byron & Hall 1998)

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Note: See Figure 6.3 for key to vegetation types.

Townshend Island

0 1 2

Kilometres

N

FIGURE 7.6

INTERTIDAL VEGETATION OF NORTHERN SHOALWATER BAY AND TOWNSHEND ISLAND (After Byron & Hall 1998)

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0 1 2

Kilometres

N

IslandH

eadC

reek

Note: See Figure 6.3 for key to vegetation types.

FIGURE 7.7

INTERTIDAL VEGETATION OF ISLAND HEAD CREEK) (After Byron & Hall 1998)

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0 1 2

Kilometres

N

Note: See Figure 6.3 for key to vegetation types.

Port Clinton

FIGURE 7.8

INTERTIDAL VEGETATION OF PORT CLINTON (After Byron & Hall 1998)

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TABLE 7.3Mangrove species of SWBTA. (source: Byron & Hall 1998 modifi ed after Melzer & Plumb 2007)

Scientifi c Name Common Name

Acanthus ilicifolius 1 holly mangrove

Acrostichum speciosum 1 mangrove fern

Aegiceras corniculatum river mangrove

Aegialitis annulata club mangrove

Avicennia marina ssp australasica grey mangrove

Avicennia marina ssp eucalyptophylla 2 -

Bruguiera exaristata 2 rib-fruited orange mangrove

Bruguiera gymnorhiza large-fruited orange mangrove

Bruguiera parvifl ora 2 orange mangrove

Ceriops tagal yellow mangrove

Cynometra iripa wrinkle pod mangrove

Excoecaria agallocha milky mangrove

Lumnitzera racemosa black mangrove

Osbornia octodonta myrtle mangrove

Rhizophora apiculata tall stilted mangrove

Rhizophora lamarckii 1 -

Rhizophora stylosa red mangrove

Scyphiphora hydrophylacea 2 yamstick mangrove

Sonneratia alba mangrove apple

Xylocarpus granatum cannonball mangrove

Xylocarpus moluccencis 2 cedar mangrove

1 Not observed in SWBTA in 1995 but refer to Lovelock (1993)2 Not observed in SWBTA in 1995 but refer to Crawford (1993)

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Six regional ecosystems (REs) covering the mangrove and saltmarsh communities of SWBTA were also mapped by the Queensland Herbarium (EPA 2007). The Regional Ecosystem mapping was carried out at a smaller scale than the earlier mapping referred to above. Regional Ecosystems containing mangroves and saltmarshes occur in both the Brigalow Belt and Central Queensland Coast bioregions. None of these REs are listed as endangered but two REs in the Brigalow Belt bioregion — 8.1.2 samphire open forbland on saltpans and 8.1.3 saltwater couch Sporobolus virginicus grassland — are listed as ‘of concern’ (Table 7.4).

Six saltmarsh species were recorded in SWBTA during the early marine vegetation mapping in 1997, with a further fi ve species listed as possibly occurring

(Table 7.5). Saltmarsh comprises a little over 11% of the total mangrove and saltmarsh communities in SWBTA. These saltmarsh environments, which are often unvegetated saltpans, are unique to the drier areas of the Queensland coast where conditions of low freshwater input favour their development (Bruinsma 2000). The total area of saltpans in central Queensland, which include large areas in the Broadsound and the Fitzroy River Delta, are the third largest in total area after similar habitats in Princess Charlotte Bay and the Gulf of Carpentaria (Bruinsma 2000).

Mangrove communities have never been grossly modifi ed anywhere within SWBTA and remain in excellent condition. Only four small patches of mangrove, each the width of a roadway, have been removed from Triangular Island (southern island), Ross Creek, Head Creek and at Seahound Hard (the southern end of Port Clinton) for boat ramp construction.

Apart from localised damage caused by feral pigs, the saltmarsh areas of SWBTA also remain intact. Vehicle tracks at two locations occur along the edge of saltmarsh landward of the mangrove zone. These tracks were created during cattle grazing operations prior to Defence acquisition, and remain visible from the air.

TABLE 7.4Description and biodiversity status of regional ecosystems occurring in SWBTA that contain mangrove or saltmarsh species.

Regional ecosystem RE description Biodiversity status as determined by the Queensland Herbarium

8.1.1 Mangrove closed-forest to open-shrubland of marine clay plains and estuaries.

Not of concern

8.1.2 Samphire open forbland to isolated clumps of forbs on saltpans and plains adjacent to mangroves.

Of concern

8.1.3 Sporobolus virginicus grassland on marine sediments.

Of concern

11.1.1 Sporobolus virginicus grassland on marine clay plains.

Not of concern

11.1.2 Samphire forbland on marine clay plains. Not of concern

11.1.4 Mangrove forest/woodland on marine clay plains. Not of concern

Mangroves in Shoalwater Bay

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TABLE 7.5Saltmarsh species of SWBTA. (source: Byron & Hall 1998)

Scientifi c Name Common Name

Sarcocornia quinquefl ora ssp Quinquefl ora 1 samphire

Enchylaena tomentosa ruby saltbush

Einadia hastata 2 berry saltbush

Suaeda australis samphire, seablite

Tecticornia australasica 1

Centaurium spicatum spike centaury

Limonium solanderi native sea lavender

Epaltes australis 1 epaltes

Sporobolus virginicus marine couch

Sesuvium portulacastrum sea purslane

Juncus kraussii 2 sea rush

1 Not observed in SWBTA in 1995, but refer to Queensland Department of Environment (no date)2 Not observed in SWBTA in 1995, but refer to Melzer et al (1993)

Samphire fl ats at high tide, western Shoalwater Bay

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ECOLOGICAL TRANSITION ZONES

Landward edges of saltmarsh communities within SWBTA are mostly well defi ned with an abrupt change to terrestrial ecosystems. However, there are some areas where this delineation is not so clear. Such areas occur where freshwater normally seeps or fl ows directly from coastal sand masses into mangrove or saltmarsh areas. These transition zones are characterised by variations in freshwater or saltwater dominance, and appear particularly vulnerable to change during extended droughts (EPA 2005).

Recent condition surveys of the coastline and wetlands in SWBTA found about 70 hectares of severely stressed tall melaleuca swamp immediately landward of mangroves, with groves of dead trees also found at a number of sites (Jaensch 2008). The sites all occur along the eastern coastal fringe, at the base of coastal sand dunes within these transitional zones between saltwater and terrestrial ecosystems. A likely cause is the collective effect of reduced freshwater infl ow due to many years of drought and consequent saltwater intrusion (Jaensch 2008). Some of these sites have already been colonised by mangroves and other salt tolerant plants, with others now supporting regenerating melaleuca trees (Jaensch 2008).

Seagrass, mangrove, saltmarsh and the ecological transition zones between areas of saltwater and freshwater ecosystems in SWBTA are all important components of the internationally recognised Shoalwater and Corio Bays Ramsar listed wetlands. More detailed discussions of these wetlands are found in Chapter 6 ‘Forests, Woodlands and Freshwater Wetlands’ and in Chapter 9 ‘Marine Wildlife’.

LYNGBYA MAJUSCULA

Lyngbya majuscula (known as ‘mermaid hair’) is a naturally occurring toxic cyanobacterium that lives in tropical and subtropical estuarine and coastal waters around the world. When conditions cause it to bloom it can smother seagrass and benthic algae, sometimes over large areas. It has been known to cause loss of seagrass and associated crab and fi sh species in Moreton Bay on the south-east coast of Queensland (Dennison & Abal 1999; Pittman & Pittman 2005), as well as a reduction in the abundance of nematodes, copepods and polychaetes (Garcia & Johnstone 2006).

L. majuscula bloomed along an 18 kilometre stretch of Shoalwater Bay coastline in 2002. It was observed overgrowing seagrass beds in important feeding areas for green turtle and dugong (Arthur et al 2006). It was suggested that higher than average rainfall prior to the bloom and high temperatures in the previous summer may have been contributing factors. L. majuscula was found in the diet of green turtles in Shoalwater Bay (Arthur et al 2006). No further blooms have been observed in the Area, and Lyngbya is not considered to be a pressure on the condition of marine environments in SWBTA.

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MANAGED INTERACTIONSDEFENCE TRAINING

Amphibious landings are conducted occasionally at both Sabina Point and Freshwater Beach. Of the two sites only Sabina Point intertidal fl ats support seagrass communities, which are predominantly a mixture of Halophila ovalis, Zostera capricorni, Halodule uninervis and Cymodocea serrulate species (CQU 2007; Lee Long et al 1997). The vessels used during amphibious landing exercises in SWBTA are either conventional landing craft, large hovercraft known as LCACs (Landing Craft Air Cushion) operated by the United States Navy or smaller amphibious vehicles such as Amtraks (amphibious tracked vehicles) and LAVs (light armoured vehicles).

Conventional landing craft cannot be used in Shoalwater Bay during the low parts of the tide cycle because of the large tidal variation and the gently shelving intertidal fl ats. Even if conventional landing craft could reach the shoreline during low tide periods, the intertidal substrate in most places will not support the weight of offl oaded vehicles. Consequently, conventional landing craft only use Sabina Point at high tide when they can fl oat across the intertidal fl ats, reducing potential damage to the seagrass meadows. Propeller wash may temporarily disturb beach sediments over areas of two or three metres. These effects are repaired naturally by the movement of sediments during tidal cycles within a few days.

Dugong feeding trails, Shoalwater Bay

Senescing Mangrove leaves

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The operational advantages of LCACs include their speed and ability to traverse substrates such as rocky ground, sandbanks and mudfl ats that would present obstacles to conventional landing craft. Their operation is also environmentally benign. They ride on a cushion of air and only minimal contact is made with the substrate by the rubber skirts on the outside of the vessels. When they reach the beach, the power to the hover turbines is reduced and the LCAC lowers onto the sand. Front and rear ramps are then opened, allowing loading or unloading. Studies show that seagrass over which the LCACs passed was not visibly disturbed in any way, and by the following day there was no visible evidence at all of their passing (URS 2005a). Seagrass meadows continue to thrive in the immediate vicinity of the amphibious landing site at Sabina Point.

Underwater demolitions are conducted in the intertidal and subtidal zone around the south-western and north-western parts of the Triangular Island (south) (see Figure 9.3). Details of these activities are further discussed in Chapter 9. Dunstan and Lewis (1980) determined that while underwater explosions have the capacity to kill areas of seagrass, the underwater demolition training at Triangular Island occurs in regions where seagrass is naturally scarce. URS (2005b) confi rmed that while seagrass meadows occur adjacent to the island, the primary areas where demolitions take place are intertidal sand/mud fl ats without seagrass. The study concluded that there was no evidence of any deterioration of seagrass where it occurs around the island (URS 2005b).

The localised disturbance to marine substrates caused by underwater demolitions training at Triangular Island can be likened to the impacts of trawling or harbour and channel dredging, except on a much smaller scale. Trawling and dredging is allowed in 34% (116 530 square kilometres) of the GBRMP. In comparison, only about 2.2 hectares of marine substrate has been disturbed at Triangular Island as a result of underwater demolitions training.

FERAL PIGS

Feral pigs frequent some mangrove areas in SWBTA, presumably drawn by food such as worms and crabs that live in the mud. Their diggings in mangroves and their tracks across saltmarshes are obvious but localised, and these impacts have been mostly observed in areas west of Sabina Point and in Port Clinton. There is no evidence that pig impacts are causing mortality of either mangroves or seagrasses.

Avicennia marina at Shoalwater Creek

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SABINA POINT PHOTOGRAPH SERIES

1. One of the two amphibious egress points at Sabina Point in June 2007.

3. The egress point immediately after amphibious landing activity, which took place at night on the high tide.

2. The egress point now prepared for the arrival of amphibious vessels with arrows and webbing.

5. Three months later, the same egress point, with dugong feeding trails evident in the foreground.

4. The egress point just after amphibious landing activity, and after a high tide.

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PRESSURESSeagrass, mangrove and saltmarsh communities that occur along coastal areas behind the Great Barrier Reef are subjected to impacts from both human activities and natural processes. The principal pressures facing these ecosystems along the Great Barrier Reef coast include coastal development, reduced water quality from land-based run-off, oilspills and other impacts from shipping, aquaculture, severe weather events and climate change, and human use (GBRMPA 2008).

Large tidal variations, dense mangrove forests and extensive seagrass covered fl ats make access to much of the coast within SWBTA very diffi cult. Historically such factors severely restricted human access to these environments in the Area. This together with the fact that all mangroves and seagrasses are protected by law, mean that pressures on these environments in SWBTA are low.

CLIMATE CHANGE

Climate change is shaping up to be one of the great challenges facing the earth this century. Sea-level rises, increased temperatures and rainfall patterns, and more severe weather events are some of the predicted major infl uences on coastal areas (AGO 2005). Seagrasses, mangroves and saltmarshes, are amongst the most at-risk ecosystems from these impacts (McLeod & Salm 2006). However, these communities have evolved to survive in harsh environments and have an ability to cope with large environmental variations. This ability may allow them to cope with the effects of climate change.

Mangrove ecosystems on low islands and in areas deprived of sediments will be especially vulnerable. However those in sediment-rich areas with large tidal variations or in situations allowing movement inland may better survive predicted sea-level rises (McCleod & Salm 2006). Some mangrove species may also be able to colonise new areas more quickly than others, thus leading to changes in species composition (Semeniuk 1994).

A vulnerability index for mangroves to sea-level rise based on local conditions has been developed (McCleod & Salm 2006). According to this index mangroves in SWBTA fall into the Least Vulnerable category. Factors placing them in this category include:

• their location in macro-tidal, sediment-rich environments;

• availability of space to move landward (backed by low-lying areas, saltpans, undeveloped areas); and

• occurrence in a remote area with limited anthropogenic stresses.

Similar scenarios probably exist for the seagrass and saltmarsh communities of SWBTA, although saltmarsh and saltpan areas would probably take considerable time to redevelop.

Overall the marine vegetation communities in SWBTA are comparatively well-placed to respond to the effects of climate change over time.

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POLLUTION

Globally the threats to both seagrass and mangrove communities from oil spills are ever-present. Since 1987 over 600 shipping incidents were recorded in Great Barrier Reef waters, and even though 33 of these were considered signifi cant, none resulted in major oil spills (GBRMPA 2008). Nevertheless, 282 oil spills from vessels were confi rmed in the Great Barrier Reef World Heritage Area between 1987 and 2002 (GBRMPA 2008). None of these were from Defence vessels. The Capricorn Channel immediately to the east of SWBTA provides important shipping access to Queensland coastal waters at the southern end of the Great Barrier Reef, and although not considered an area of highest risk (QT & GBRMPA 2000), shipping incidents are always a possibility. There are over 10 000 shipping movements annually along the Queensland coast, with a high percentage transiting the inner route between the Great Barrier Reef and the mainland coast (QT & GBRMPA 2000). Bulk carriers comprise the greatest proportion of shipping, with oil tankers comprising less than 10% (QT & GBRMPA 2000). Because they carry as much as 5 000 tonnes of bunker fuel and are so numerous, many analysts believe that bulk carriers pose the greatest risk of a serious oil spill in Great Barrier Reef waters (QT & GBRMPA 2000). There is no history of oil spills in SWBTA.

The Inner Shipping Route is adjacent to, but outside the Area to the east, and all international shipping is required to use the services of marine pilots when passing inside the Great Barrier Reef. Diffi culties arising from the remoteness, large tidal variation, density of the mangrove forests, complexity of the coastal landforms and soft intertidal substrates would make cleanup of even a small oil spill in the mangrove areas of SWBTA very diffi cult. The identifi cation of these more environmentally sensitive areas is an important part of oil spill response planning.

Links between seagrass loss and decreased water quality associated with metropolitan, industrial and agricultural development in Australia have been established (Cambridge & McComb 1984; EPA 1998; Walker & McComb 1992). However, little is known about the direct or indirect effects of water pollutants such as herbicides or heavy metals like lead, zinc, copper or cadmium on seagrass meadows (EPA 1998; Prange & Dennison 2000). Although detailed water quality assessment of marine waters within SWBTA has not been undertaken, the waters emanating from the Area have been tested (see discussion in Chapter 5 ‘Water’). SWBTA is remote from any metropolitan, industrial or agricultural pollution sources and effects from pollutants on seagrasses in the Area from these sources are considered to be non-existent or negligible.

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RESPONSESCLIMATE CHANGE

There is little that land managers can do to control the threat to mangrove communities from sea-level rise associated with climate change (McLeod & Salm 2006). However, McLeod and Salm (2006) identifi ed the following strategies managers can apply to increase mangrove viability by enhancing their resilience:

• Apply risk-spreading strategies to address the uncertainties of climate change.

• Identify and protect critical areas that are naturally positioned to survive climate change.

• Manage human stresses on mangroves.• Establish greenbelts and buffer zones to

allow for mangrove migration in response to sea-level rise, and to reduce impacts from adjacent land-use practices.

• Restore degraded areas that have demonstrated resistance or resilience to climate change.

• Understand and preserve connectivity between mangroves and sources of freshwater and sediment, and between mangroves and their associated habitats like coral reefs and seagrasses.

• Establish baseline data and monitor the response of mangroves to climate change.

• Implement adaptive strategies to compensate for changes in species ranges and environmental conditions.

Because seagrass, mangrove and saltmarsh communities remain in a natural condition in SWBTA, the landward terrestrial vegetation is not modifi ed, and Defence activities are highly restricted and regulated in these areas, the strategies above are already applied to a great extent.

POLLUTION

Because shipping is an international industry, it can only be regulated through the implementation of international agreements. The International Convention for the Prevention of Pollution from Ships 1973 and 1978 Protocol ( MARPOL) is the most comprehensive international convention for reducing pollution from shipping (QT & GBRMPA 2000). MARPOL details standards for the discharge of oil, bulk noxious liquids, harmful substances in a packaged form, sewage and garbage from vessels.

There are a number of Commonwealth and Queensland statutes that implement the provisions contained in MARPOL and other international agreements relating to shipping and the prevention of pollution at sea. The Protection of the Sea (Prevention of Pollution by Ships) Act 1983 is the primary legislation regulating pollution from ships in Australia. It has effect in the Great Barrier Reef Marine Park, except in State coastal waters (three nautical miles from the baseline) where the Transport Operations (Marine Pollution) Act 1995 takes effect. It is administered by Queensland Transport (QT), although most activities within the Great Barrier Reef Marine Park including shipping are regulated under the Great Barrier Reef Marine Park Act 1975 (GBRMP Act) by the GBRMPA. Strategies to reduce the risk of oil spills and their impacts on the Great Barrier Reef Marine Park are delivered jointly between the GBRMPA, the Australian Maritime Safety Authority (AMSA) and QT. Both Commonwealth and Queensland authorities maintain a state of preparedness in response to the risk of oil spill from shipping activities along the Queensland coast.

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As part of this response preparedness, Defence maintains containers of oil spill response equipment at The Glen (Range Control facility), Freshwater Beach and Sabina Point. Defence practises incident response and control procedures in conjunction with Maritime Safety Queensland, GBRMPA and Gladstone Harbourmaster for a range of possible scenarios including oil spills.

New regulations for the discharge of vessel sewage in the Great Barrier Reef Marine Park were introduced by the GBRMPA in 2005. The regulations are aligned with those of Maritime Safety Queensland (MSQ) and the AMSA, and stipulate that all vessels with toilets must also have a macerator fi tted to reduce waste into fi ne slurry. Vessels carrying 16 or more people need to store their sewage, but can discharge it more than one nautical mile seawards from the nearest reef, island, mainland or an aquaculture facility. Vessels that carry 15 people or less may pump untreated sewage into the Marine Park if outside of a boat harbour or marina and more than one nautical mile from an aquaculture facility. Commercial vessels are also required by MSQ to have a shipboard sewage management plan onboard and to keep sewage disposal records when discharging into a sewage disposal facility.

Defence vessels, no matter where they are located, are required to comply with the Protection of the Sea (Prevention of Pollution from Ships) Act 1983 and the Navigation Act 1912, regarding the disposal of shipborne waste. In line with these requirements, Defence policy permits the disposal of treated sewage at sea only if the vessel is more than three nautical miles from the nearest land, and if travelling at four knots or more. The distance increases to twelve nautical miles for disposal of untreated sewage. Greywater can either be treated in a sewage treatment plant, if suitable, or discharged when more than one kilometre from the coast or a reef.

The Great Barrier Reef is classifi ed as a Particularly Sensitive Sea Area and is afforded greater protection. MARPOL prohibits the discharge of untreated sewage within the Great Barrier Reef Marine Park. This requirement is binding on Defence and on naval ships of foreign nations, and is refl ected in naval operational policy. In this respect, MARPOL is more restrictive than the GBRMP Act, which allows discharge of untreated sewage into the marine park with some conditions. In line with the GBRMP Act, Defence vessels are permitted to discharge treated sewage and greywater (excluding food waste) if greater than 1 000 metres seawards from the seaward edge of the nearest reef, but only when underway. During exercises when a number of naval vessels may be operating within a particular area, further restrictions on the disposal of treated sewage and greywater waste are regularly applied in consultation with the GBRMPA.

Cameron Mulville

Rhizophora fl owers

Mudcrabs on seagrass, Shoalwater Bay

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RECOMMENDATIONSSeagrass, mangrove and saltmarsh communities in SWBTA have been mapped in detail and been the subject of ongoing studies for more than ten years. These communities have never been disturbed and studies have consistently shown that they remain in excellent condition.

1. Encourage and support continued monitoring of the extent and condition of seagrass, mangrove and saltmarsh communities in the Area as part of a three to fi ve year condition assessment program, preferably as part of State-wide or Reef-wide assessments. This monitoring will allow early detection of any emerging pressures on these communities, and more importantly, provide a reference for comparing marine vegetation condition and extent with other, more developed areas of the Queensland coast.

2. Continue to manage restrictions on Defence presence in intertidal areas to existing, tightly controlled activities. This will ensure these communities continue to thrive and are in the best possible condition to enable a strong response to likely future impacts from climate change.

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EPA—see Environmental Protection Agency

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