MSc Env Sci Dissertation - Simon Grennan, 09324780

Trinity College Dublin

MSc Environmental Sciences

Research Project

2014

‘An evaluation of the pollution status of the

inner and outer Malahide estuary’

Simon Grennan

09324780

Supervisor: Professor Jim Wilson

Word Count: 14,967

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Declaration

I declare that this dissertation has not been submitted as an exercise for a degree at

this or any other university and it is entirely my own work. I agree to deposit this dissertation

in the University’s open access institutional repository or allow the Library to do so on my

behalf, subject to Irish Copyright Legislation and Trinity College Library conditions of use and

acknowledgement.

______________________

Simon Grennan

18/09/2014

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Abstract

Pollution levels in many of the world’s estuaries have increased in the recent past as

a direct result of human activities. Increased estuarine pollution levels are having numerous

adverse impacts, with both biodiversity and human populations suffering the consequences.

This project focused on Malahide estuary, an estuary on Ireland’s east coast, just north of

Dublin city. This estuary is a protected site, with it currently designated as a Special

Protection Area, a Special Area of Conservation and a Ramsar site. The purpose of this

project was to determine the pollution status of Malahide estuary, and to compare this status

with its corresponding status in 1999 and 2009 to ascertain whether or not it had changed,

and if it had, by what extent it had done so. Two indices of estuarine quality, namely the

Biological Quality Index (BQI) and the Pollution Load Index (PLI) were used to determine the

pollution status of the estuary. The methods behind these indices incorporated both site

surveying and sediment sampling and analysis.

The findings from this study indicate that the pollution status of Malahide estuary has

improved in recent years, with the results from both the BQI and the PLI surveys signifying a

decrease in pollution levels in the estuary since the previous studies that were carried out

there in 1999 and 2009. Positive outcomes, such as increased levels of biodiversity, are

likely to have arisen as a result of the improved pollution status of the estuary. This study

established the inner estuary as a considerably more polluted area than the outer estuary.

The inner estuary obtained a BQI score of 2.86 and a PLI score of 1.29, whereas the outer

estuary obtained a BQI score of 10 and a PLI score of 5.28. Probable causes of the higher

levels of pollution in the inner estuary include the effluent entering into this part of the

estuary from the Swords wastewater treatment plant and the runoff entering into this part of

the estuary from the agricultural land to the north of the inner estuary. Future studies will

need to analyse these two sources to determine the extent to which they are contributing to

pollution levels in Malahide estuary. A further general study on the pollution status of

Malahide estuary using the indices employed in this study should also be conducted in the

near future so as to determine whether or not the promising improvements that were

observed in this study have continued.

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Acknowledgements

Firstly, I would like to thank my supervisor for this project, Professor Jim Wilson. His

help in setting me in the right direction and giving me advice throughout the project was

invaluable. In particular, the assistance that he provided me with in the field in Malahide

estuary during the early stages of my fieldwork was of a significant benefit to me.

I would also like to thank Mark Kavanagh from the lab in the environmental sciences

department of college. The directions he gave me throughout my lab work were always

simple to follow and very helpful. I am extremely grateful to Mark for all the help that he

offered me when any problems arose in the lab during both the preparation and the analysis

of my samples.

Finally, I would like to thank my friends and family for assisting me with my fieldwork

in Malahide estuary and for helping me to proof read the final draft of my dissertation.

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Table of Contents

DECLARATION ..................................................................................................................... i

ABSTRACT .......................................................................................................................... ii

ACKNOWLEDGEMENTS .................................................................................................... iii

TABLE OF CONTENTS ...................................................................................................... iv

LIST OF FIGURES .............................................................................................................. vi

LIST OF TABLES .............................................................................................................. viii

1. INTRODUCTION ............................................................................................................. 1

1.1 ESTUARIES .................................................................................................................. 2

1.2 ESTUARIES AND POLLUTION.......................................................................................... 4

1.2.1 HUMAN IMPACTS .............................................................................................. 4

1.2.2 NUTRIENT POLLUTION – NITROGEN AND PHOSPHORUS ........................................ 5

1.2.3 LEGISLATION ................................................................................................... 7

1.2.4 PREVIOUS STUDIES ON IRISH ESTUARIES ........................................................... 8

1.3 INDICES OF ESTUARINE QUALITY ................................................................................... 9

1.4 SURVEY SITE ............................................................................................................. 11

1.5 CURRENT KNOWLEDGE .............................................................................................. 16

1.6 AIMS ......................................................................................................................... 17

2. METHODOLOGY .......................................................................................................... 18

2.1 FIELDWORK ............................................................................................................... 19

2.1.1 GENERAL TASKS AND DIVISION OF ZONES ........................................................ 19

2.1.2 BIOLOGICAL QUALITY INDEX ............................................................................ 21

2.1.3 POLLUTION LOAD INDEX .................................................................................. 23

2.2 POLLUTION LOAD INDEX – LAB WORK .......................................................................... 24

2.3 DATA ANALYSIS METHODS .......................................................................................... 27

3. RESULTS ..................................................................................................................... 29

3.1 BIOLOGICAL QUALITY INDEX ....................................................................................... 30

3.1.1 RESULTS FROM THIS STUDY ............................................................................ 30

3.1.2 COMPARISON WITH RESULTS FROM 1999 AND 2009 STUDIES ............................ 33

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3.2 POLLUTANTS ............................................................................................................. 35

3.2.1 LOSS ON IGNITION .......................................................................................... 37

3.2.2 NITROGEN ..................................................................................................... 39

3.2.3 PHOSPHORUS ................................................................................................ 41

3.2.4 CADMIUM ....................................................................................................... 43

3.2.5 CHROMIUM .................................................................................................... 45

3.2.6 COPPER ........................................................................................................ 47

3.2.7 IRON ............................................................................................................. 49

3.2.8 LEAD ............................................................................................................. 51

3.2.9 ZINC .............................................................................................................. 53

3.3 POLLUTION LOAD INDEX ............................................................................................. 55

3.3.1 RESULTS FROM THIS STUDY ............................................................................ 55

3.3.2 COMPARISON WITH RESULTS FROM 1999 AND 2009 STUDIES ............................ 59

4. DISCUSSION ................................................................................................................ 63

4.1 INITIAL HYPOTHESES .................................................................................................. 64

4.2 POLLUTION STATUS OF THE OUTER ESTUARY – INDICATED BY THE BQI ........................... 65

4.3 POLLUTION STATUS OF THE INNER ESTUARY – INDICATED BY THE BQI ............................ 66

4.4 MOST SIGNIFICANT POLLUTANTS IN THE ESTUARY ........................................................ 67

4.5 POLLUTION STATUS OF THE OUTER ESTUARY – INDICATED BY THE PLI ........................... 69

4.6 POLLUTION STATUS OF THE INNER ESTUARY – INDICATED BY THE PLI ............................ 71

4.7 IMPLICATIONS OF THIS STUDY ..................................................................................... 73

4.8 LIMITATIONS AND WEAKNESSES ASSOCIATED WITH THIS STUDY .................................... 74

5. CONCLUSION .............................................................................................................. 75

REFERENCE LIST ............................................................................................................. 78

APPECNDICES .................................................................................................................. 86

APPENDIX A ......................................................................................................................... A-1

APPENDIX B ......................................................................................................................... B-1

APPENDIX C ......................................................................................................................... C-1

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List of Figures

Figure 1.1 – Malahide estuary and its surrounds ................................................................. 11

Figure 1.2 – Aerial photograph of Malahide estuary, looking east. ...................................... 12

Figure 1.3 – Designated SPAs and SACs in Malahide estuary ............................................ 13

Figure 1.4 – Land use types surrounding Malahide estuary ................................................ 15

Figure 2.1 – Location of zones in the outer estuary and location of sampling points in these

zones .................................................................................................................................. 20

Figure 2.2 – Sampling locations in the inner and outer estuary ........................................... 23

Figure 3.1 – Comparison of the result obtained from this BQI survey on the outer Malahide

estuary with the results obtained from similar surveys completed by O’Brien (1999) and

Walsh (2009) ...................................................................................................................... 33

Figure 3.2 – Comparison of the result obtained from this BQI survey on the inner Malahide

estuary with the result obtained from a similar survey completed by O’Brien (1999) ........... 34

Figure 3.3 – LOI values (expressed as % organic matter in the sediment) of the samples

from the fifteen designated sampling sites in Malahide estuary ........................................... 37

Figure 3.4 – Interpolation map illustrating the organic matter content (%) in the sediment in

Malahide estuary ................................................................................................................. 38

Figure 3.5 – Nitrogen concentration values (ug g-1 N) in the samples from the fifteen

designated sampling sites in Malahide estuary. .................................................................. 39

Figure 3.6 – Interpolation map illustrating nitrogen concentration values (ug g-1 N) in the

sediment in Malahide estuary .............................................................................................. 40

Figure 3.7 – Phosphorus concentration values (ug g-1 P) in the samples from the fifteen


Figure 3.8 – Interpolation map illustrating phosphorus concentration values (ug g-1 P) in the

sediment in Malahide estuary. ............................................................................................. 42

Figure 3.9 – Cadmium concentration values (ug g-1 Cd) in the samples from the fifteen


Figure 3.10 – Interpolation map illustrating cadmium concentration values (ug g-1 Cd) in the


Figure 3.11 – Chromium concentration values (ug g-1 Cr) in the samples from the fifteen


Figure 3.12 – Interpolation map illustrating chromium concentration values (ug g-1 Cr) in the


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Figure 3.13 – Copper concentration values (ug g-1 Cu) in the samples from the fifteen


Figure 3.14 – Interpolation map illustrating copper concentration values (ug g-1 Cu) in the


Figure 3.15 – Iron concentration values (ug g-1 Fe) in the samples from the fifteen


Figure 3.16 – Interpolation map illustrating iron concentration values (ug g-1 Fe) in the


Figure 3.17 – Lead concentration values (ug g-1 Pb) in the samples from the fifteen


Figure 3.18 – Interpolation map illustrating lead concentration values (ug g-1 Pb) in the


Figure 3.19 – Zinc concentration values (ug g-1 Zn) in the samples from the fifteen


Figure 3.20 – Interpolation map illustrating zinc concentration values (ug g-1 Zn) in the


Figure 3.21 – PLI scores of the fifteen designated sampling sites in Malahide estuary........ 57

Figure 3.22 – Interpolation map illustrating PLI scores in Malahide estuary. ....................... 58

Figure 3.23 – Comparison of the result obtained from this PLI survey on the outer Malahide

estuary with the results obtained from similar surveys completed by O’Brien (1999) and

Walsh (2009) ...................................................................................................................... 61

Figure 3.24 – Comparison of the result obtained from this PLI survey on the inner Malahide

estuary with the result obtained from a similar survey completed by O’Brien (1999) ........... 62

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List of Tables

Table 1.1 – Results of BQI and PLI surveys conducted on Malahide estuary by O’Brien

(1999) and Walsh (2009) .................................................................................................... 16

Table 2.1 – Method used to assign ratings to species according to their abundance in each

zone .................................................................................................................................... 21

Table 3.1 – Sediment type, classification type and outer estuary proportion of each zone in

the outer Malahide estuary .................................................................................................. 30

Table 3.2 – Sediment type, classification type and inner estuary proportion of each zone in

the inner Malahide estuary .................................................................................................. 31

Table 3.3 – Sediment type, classification type and estuary proportion of each zone in the

Malahide estuary ................................................................................................................. 32

Table 3.4 – Mean pollutant values ± standard deviations at each site in the outer estuary .. 35

Table 3.5 – Mean pollutant values ± standard deviations at each site in the inner estuary .. 36

Table 3.6 – Mean PL ± the standard deviation of each parameter studied at each site and

mean PLI ± the standard deviation of each site in the outer Malahide estuary .................... 55

Table 3.7 – Mean PL ± the standard deviation of each parameter studied at each site and

mean PLI ± the standard deviation of each site in the inner Malahide estuary..................... 56

Table 3.8 – PLI scores calculated for the outer estuary, the inner estuary and the entire

estuary ................................................................................................................................ 56

Table 3.9 – Sign test analysing the relationship between geometric mean PL values of the

different pollutants studied in this study and in the 2009 study (Walsh, 2009) ..................... 59

Table 3.10 – Sign test analysing the relationship between geometric mean PL values of the

different pollutants studied in this study and in the 1999 study (O’Brien, 1999) ................... 60

Table A1 – Completed BQI data sheet from an outer estuary site ...................................... A-2

Table A2 – Completed BQI data sheet from an inner estuary site ...................................... A-3

Table A3 – Key used to help fill out the BQI data sheets .................................................... A-4

Table B1 – Pollutant values at each site in the outer estuary – original and replicate analyses

.......................................................................................................................................... B-2

Table B2 – Pollutant values at each site in the inner estuary – original and replicate analyses

.......................................................................................................................................... B-3

Table B3 – Threshold and baseline values for each pollutant used to calculate the PLI ..... B-3

Table B4 – PL values of each parameter studied at each site and PLI values of each site in

the outer Malahide estuary – original and replicate analyses ............................................. B-4

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Table B5 – PL values of each parameter studied at each site and PLI values of each site in

the inner Malahide estuary – original and replicate analyses .............................................. B-5

Table C1 – ANOVA results for mean loss on ignition values .............................................. C-2

Table C2 – ANOVA results for mean nitrogen concentration values ................................... C-2

Table C3 – ANOVA results for mean phosphorus concentration values ............................. C-2

Table C4 – ANOVA results for mean cadmium concentration values ................................. C-2

Table C5 – ANOVA results for mean chromium concentration values ................................ C-3

Table C6 – ANOVA results for mean copper concentration values ..................................... C-3

Table C7 – ANOVA results for mean iron concentration values ......................................... C-3

Table C8 – ANOVA results for mean lead concentration values ......................................... C-3

Table C9 – ANOVA results for mean zinc concentration values ......................................... C-4

Table C10 – ANOVA results for mean PLI site scores ........................................................ C-4

1

Chapter 1

Introduction

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1.1 Estuaries

A number of different definitions have been put forward in an attempt to provide an

adequate and thorough description of what an estuary is. Cameron and Pritchard (1963)

presented what is arguably the most commonly cited and accepted definition of the term

estuary. They suggested that an estuary is “a semi-enclosed coastal body of water which

has a free connection with the open sea and within which sea water is measurably diluted

with fresh water derived from land drainage” (Cameron and Pritchard, 1963: 306). A more

recent definition has been put forward by the European Commission which defines estuaries

and other transitional waters as “bodies of surface water in the vicinity of river mouths which

are partly saline in character as a result of their proximity to coastal waters but which are

substantially influenced by freshwater flows” (Vincent et al., 2002: 20).

Estuaries are highly complex, dynamic and biotically rich systems (Day et al., 2013)

that are shaped by a number of different physiochemical and biological processes (Marshall

Adams, 2005). The complexity of estuarine systems means that they can be classified

according to a variety of different factors. Estuaries are most commonly classified on the

basis of geomorphology, water balance, vertical structure of salinity or the hydrodynamics

present within the estuarine system (Valle-Levinson, 2010). Estuaries are, for the most part,

physically dominated systems. The abiotic characteristics of estuaries, including rising and

falling tides, complex water movements, high levels of turbidity, and altering levels of salt

concentration, all have significant impacts on the shapes of estuarine systems, often

resulting in the formation of distinguishable landforms such as beaches, barrier islands, mud

flats and deltas (Day et al., 2013).

A defining characteristic of estuaries is that they are areas which are extremely

productive, with large amounts of biomass and high levels of primary production (Hobbie,

2000). A number of reasons have been put forward to explain the high level of productivity

that is found in estuaries. Some of the most common reasons suggested include the

different primary production units present in estuaries, the movement of water in and out of

estuaries as a result of tidal action, the large supply of nutrients in estuaries, and the speed

at which nutrients are regenerated as a result of the presence of microorganisms and filter

feeders in estuaries (Day et al., 2013). While the above reasons are commonly identified by

3

many, they are not accepted by all. Alternative, less common theories suggest that factors

such as organic detritus and fisheries play a considerable role in causing the high level of

productivity that is found in estuaries (Day et al., 2013).

Despite having high levels of productivity, biodiversity levels in estuaries are

comparatively quite low (Costanza et al., 1997a). This is due to estuaries being

environmentally naturally stressed locations, primarily as a result of the high physio-chemical

variability which they are exposed to (Elliot and Quintino, 2007). Highly variable salinity,

current speed and direction, temperature, nutrient levels and pH places stress on estuarine

systems (Elliot and Quintino, 2007). Stress causes these systems to become dominated by

a relatively small amount of highly adapted, generalist species which are able to cope with

such environments (Costanza et al., 1997a). These generalist species are one of the primary

reasons as to why estuaries are important and why they should be protected. Estuaries yield

a significant amount of fish and shellfish harvests, while they also provide important habitats

for both waders and waterfowl (Hobbie, 2000).

Aside from the vital role that estuaries play in supporting marine life, they also

provide a number of important goods and services to humans worldwide. Estuaries supply

food for humans, while they also provide transport routes and recreational opportunities

(Hobbie, 2000). Furthermore, they have become important sites for commerce, with many of

the world’s largest cities currently located next to estuaries (Day et al., 2013). A study

completed by Costanza et al. (1997b) demonstrated the significance of the contribution that

estuaries make towards global human welfare. It was estimated in this study that the total

value of the services provided to humans per hectare of estuary is US $22,832 per year

(Costanza et al., 1997b).

4

1.2 Estuaries and pollution

1.2.1 Human impacts

The increasing level of interaction that humans are having with estuaries worldwide is

at times having significant impacts on these estuaries. Day et al. (2013) observe that there

are almost no estuaries worldwide which have not been impacted on by humans. Human

impacts on estuaries have increased in both regularity and size over the past two centuries,

largely as a result of population growth, industrial growth and new technology developments

(Day et al., 2013). The world’s estuaries have been identified as “the ultimate repository for a

vast array of substances discharged deliberately or accidentally via human activities”

(Kennish, 1997: 1). By physically altering estuarine environments (through the reclamation of

land and by inducing hydrological changes), enriching estuarine waters (often resulting in

eutrophication), introducing toxic materials (including heavy metals and organic compounds

such as pesticides and domestic and industrial wastes) and altering estuarine community

structure (by harvesting certain species or introducing invasive species), humans are having

significant impacts on estuarine environments worldwide (Hobbie, 2000; Hartnett et al.,

2011; Day et al., 2013).

Pollution is currently one of the largest problems facing estuaries. It can arise from

numerous sources; direct discharges from coastal communities, discharges into rivers

entering into estuaries, oil and toxic spills, dumping from ships, atmospheric deposition and

agricultural runoff (Kennish, 1992). One of the main reasons that pollution is a major problem

in estuaries is that the very nature of estuarine environments makes them more susceptible

to pollution. Estuaries are areas which have a limited assimilative capacity for pollutants due

to their slow rate of exchange relative to their volume (Kennish, 1997). As a result, materials

such as toxic organic compounds can build up and remain in estuaries for significant periods

of time, potentially having drastic effects on marine life (Kennish, 1997).

5

1.2.2 Nutrient pollution – nitrogen and phosphorus

Over the past half century, human activities have both directly and indirectly resulted

in increased nutrient fluxes to estuaries worldwide (Billen et al., 2011; Howarth et al., 2011).

Excessive loading of two nutrients in particular; nitrogen and phosphorus, has been

identified as the primary driver of eutrophication in estuaries (Howarth et al., 2011; Lewis Jr.

et al., 2011; Hartzell and Jordan, 2012; Hayn et al., 2014). The harmful effects of

eutrophication have been well documented with it often causing increases in the frequency

and duration of phytoplanktonic blooms and in the growth of opportunistic macroalgae, with

this in turn leading to an oxygen deficiency in the water column and the subsequent

displacement or death of marine life (McGarrigle, 2010). Alongside the direct impacts of

eutrophication on marine life, a number of indirect impacts can also arise from this process.

Examples of some of the indirect impacts of eutrophication are given in a study completed

by Desprez et al. (1992) on the Bay of Somme estuary in northern France. This study

outlined how changes to the benthic community in this estuary which arose as a result of

eutrophication had knock on effects on the diets of the key predators of the bivalve in this

area; the oystercatcher (Haematopus ostralegus) and the common gull (Larus canus).

While both nitrogen and phosphorus are proven drivers of eutrophication in estuarine

systems, a debate has developed over the past number of decades over which of these two

nutrients is the larger contributor to this process (Howarth and Marino, 2006). This debate

began in the early 1970s (Schindler, 1974) with it being suggested at this time that

phosphorus was the primary driver. This suggestion has altered over the past three

decades, with the general consensus among scientists now being that nitrogen is the

primary driver, especially within the temperate zone (Howarth and Marino, 2006, Hayn et al.,

2014). However, some (Schindler et al., 2008) still argue against this theory. While it now

appears likely that nitrogen is in fact the primary driver of eutrophication in estuarine

systems, Howarth and Marino (2006) propose that optimal eutrophication management

should continue to control phosphorus levels as well as nitrogen levels, as phosphorus can

still limit primary production and it also has the potential to interact with nitrogen and silica

levels to have negative effects on the ecological structure of the estuarine system.

6

A number of different sources are responsible for the increased amount of nitrogen

and phosphorus pollution that has been seen in estuaries over the past half century.

Increased nitrogen levels have arisen both from the more widespread and regular use of

fertilisers and from the rise in fossil fuel emissions (Howarth and Marino, 2006; Conley et al.,

2009). Increased phosphorus levels have arisen from municipal and industrial wastewater,

while, similarly to nitrogen, they have also arisen from the increased use of fertilisers (Conley

et al., 2009). The location of agricultural land next to estuaries can have adverse impacts on

the pollution status of estuaries if the runoff from this land is not managed correctly. Due to

the application of fertilisers, agricultural runoff contains high levels of both nitrogen and

phosphorus, and can cause eutrophication in estuarine environments (Carpenter et al.,

1998). Agricultural land is often located next to estuaries as the land surrounding estuaries is

often very fertile (Day et al., 2013). The correct management of this land is therefore vital to

reduce the chances of eutrophication occurring in the adjacent estuary. However, adequate

management often fails to take place, with this resulting in the initiation or exacerbation of

many of the pollution problems that can be seen in estuaries worldwide today.

Point source phosphorus pollution from municipal and industrial wastewater

treatment plants (WWTPs) remains a problem in a large number of estuaries worldwide

(Conley et al., 2009). However, Howarth et al. (2011) observe that phosphorus inputs to

estuaries in industrialised countries have reduced significantly over the past ten years due to

the improvements which have taken place in WWTPs. Despite reduced phosphorous inputs

to estuaries over the past ten years, further efforts are needed to continue this reduction

trend and also to reduce nitrogen inputs to estuaries. Conley et al. (2009) observe that the

technologies to reduce nitrogen and phosphorus levels in estuaries often differ – for

example, the wastewater treatment technologies used to reduce nitrogen levels vary

considerably with those used to reduce phosphorus levels. As a result, it is proposed that the

way forward in reducing eutrophication levels in estuaries worldwide is to take a balanced

and carefully planned approach so as to control the levels of both nutrients as effectively as

possible (Conley et al., 2009).

7

1.2.3 Legislation

European legislation has been drafted in an attempt to protect estuarine

environments and limit the amount of pollution which they are susceptible to. A large number

of estuaries throughout Europe are considered to be of conservational importance, and as a

result, have been designated as Special Protection Areas (SPAs) (under the EU Birds

Directive) or Special Areas of Conservation (SACs) (under the EU Habitats Directive), with

some being designated as both SPAs and SACs (EC, 2011). SPAs are areas designated for

the protection of habitats of migratory birds and specific threatened birds (EC, 2009), while

SACs are designated for the protection of specific habitats due to the conservational

importance of the flora and fauna found in those habitats (EC, 1992). In Ireland, there are

currently 12 estuaries designated as SPAs and 21 estuaries designated as SACs (NPWS,

2014). Alongside the above pieces of legislation, the EU Water Framework Directive (EC,

2000) also seeks to protect estuarine environments. This Directive aims for estuaries, as

well as other surface waters and groundwaters, to achieve good ecological status and good

chemical status by 2015 (EC, 2011). It seeks to ensure that deterioration of estuaries

“should be prevented and their aquatic ecosystem status protected and enhanced” (EC,

2011: 11).

The EU Urban Wastewater Treatment Directive (EC, 1991a) and the EU Nitrates

Directive (EC, 1991b) are two further pieces of legislation which seek to protect estuarine

environments. These Directives are particularly helpful in limiting nutrient inputs to estuaries.

The Urban Wastewater Treatment Directive aims to ensure that sewage is treated to specific

standards according to the population equivalent (PE) of the surrounding urban area (EC,

1991a). It seeks to alter estuarine wastewater discharges, with this often resulting in a

reduction in nutrient inputs to estuarine environments (McLusky and Elliott, 2004). The

Nitrates Directive aims to protect both surface water and groundwater quality by ensuring

that nitrates from agricultural sources do not enter into these waters and by advocating the

use of good farming methods (EC, 2010). The Directive aims to combat eutrophication in the

estuaries where it is taking place and seeks to prevent the onset of this process in the

estuaries where it may potentially occur (McLusky and Elliott, 2004). When discussing the

pollution of estuaries, it is vital that the legal framework related to estuaries is considered, as

it is the pieces of legislation in this framework which help to ensure that pollution levels are

minimised in estuarine environments.

8

1.2.4 Previous studies on Irish estuaries

Estuaries in Ireland have been analysed in the past so as to determine the extent to

which they are polluted. The most recent report (McGarrigle et al., 2010) produced by the

Environmental Protection Agency (EPA) presented the results of the surveying of estuarine

and coastal environments for water quality which took place between 2007 and 2009. During

this period, it was found that 5.3% of the total area that was assessed was classified as

eutrophic or potentially eutrophic, while approximately 64% of the total area was determined

to be of high or good ecological status (McGarrigle et al., 2010). Using indices which will be

used in this study; the Biological Quality Index (BQI) and the Pollution Load Index (PLI),

Wilson (2003) carried out a study which compared the pollution status of a number of

estuaries in Ireland, France and the United States. This study found that the Liffey estuary

scored lower (and is hence more contaminated) than the Seine estuary in France, which

passes through the capital, Paris, and also serves the large petrochemical port of Rouen and

the industry that is associated with this port (Wilson, 2003). Other Irish estuaries which were

given low scores in this study included the Avoca, Dodder and Tolka estuaries (Wilson,

2003). Malahide estuary, the estuary of interest for this project, was not surveyed in the

study. However, it has been surveyed twice in the last fifteen years by students; once in

1999 (O’Brien, 1999), and once in 2009 (Walsh, 2009). The results of these two studies are

analysed alongside the results of this project in the following chapters.

9

1.3 Indices of estuarine quality

There is currently little consensus among scientists as to what the most effective

indicator of the pollution status of an estuary is (Bortone, 2005). This is primarily due to the

complexity and large amount of change that can occur in estuaries over time, but also due to

the limited amount of time which has been spent assessing estuaries worldwide (Bortone,

2005). Natural stressors in estuarine environments, particularly salinity, can cause similar

responses to pollution stress, meaning that gauging the pollution status of estuaries is often

problematic (Wilson, 1994; Hartnett et al., 2011).

Two simple, comparative indices have been developed to quantify estuarine quality;

the BQI and the PLI. These indices were developed so that their methods would be simple,

relatively quick and inexpensive to carry out, while they were also devised to be used during

any season and throughout all countries in Western Europe (Jeffrey et al., 1985a). The BQI

surveys macrobiota present in an estuary to describe the biological health of that estuary,

while the PLI describes pollutant levels present in the sediments of an estuary (Jeffrey et al.,

1985a; Wilson and Elkaim, 1991). The BQI assigns a numbered score to an estuary, ranging

from 0.1 (completely abiotic) to 10 (completely unpolluted) (Jeffrey et al., 1985a). It is based

on proportions of the estuary which are classified as abiotic, opportunistic or stable, with

these categories assigned according to specific biosedimentary criteria (Jeffrey et al., 1985a;

Wilson, 1994). Biosedimentary criteria used include community type, the range and kind of

species that are found, as well as the age distribution of these species (Wilson and Elkaim,

1991). The PLI scores individual sediment contaminants on a log scale from 10 (the

baseline; unpolluted) to 1 (the threshold; harmful biological effects to be expected) (Jeffrey et

al., 1985a; Wilson, 2003). The scores obtained for individual sediment contaminants are

then combined to obtain PLI scores for specific sites, which can in turn be combined to give

a PLI score for the whole estuary (Wilson, 2003).

Other indices which attempt to measure pollution in estuaries do exist. However,

many of these are not entirely suitable. For example, the Shannon-Weiner index becomes

difficult to interpret when used in estuaries due to the low numbers, yet high densities of

some organisms (Wilson and Elkaim, 1991). This index can demonstrate a stressed

estuarine environment even when this environment is unpolluted (Wilson and Elkaim, 1991).

10

O’Boyle et al. (2013) propose the use of an index of trophic state which is based on the

variability of pH and dissolved oxygen saturation in estuaries over a three year time period.

The time constraints associated with this study meant that the use of this index was not

suitable. A Biotic Index which focuses on sedentary benthic organisms to determine the

pollution status of estuaries is suggested for use by Borja et al. (2000). While this index is

not limited by the restrictions faced by the Shannon-Weiner index or the index put forward by

O’Boyle et al. (2013), both the BQI and the PLI were used in this study due to the simplicity

and effectiveness of their methods. Furthermore, these indices were used in the similar

studies conducted on the pollution status of Malahide estuary by O’Brien (1999) and Walsh

(2009). As a result, it has been possible to compare the results obtained in this study with

those obtained from the above two studies to determine the changes in the pollution status

of the estuary which have taken place in the recent past.

11

1.4 Survey site

Malahide estuary, the estuary of the River Broadmeadow, is located on Ireland’s east

coast, approximately fifteen kilometres north of Dublin city (see figures 1.1 and 1.2) (IFI,

2010). The towns of Malahide, Swords and Donabate are positioned to the south, west and

north of the estuary respectively (NPWS, 2013a). A railway viaduct was built across the

estuary in the 1840s, creating inner and outer sections, and causing the estuary to become

lagoonal in character, with limited tidal exchange (NPWS, 2004). This viaduct collapsed in

August 2009, but has since been repaired (RAIU, 2010). The outer part of the estuary (to the

east) empties almost completely at low tide, whereas water levels in the inner estuary

change very little due to the presence of the viaduct (NPWS, 2013a). A large sand spit is

located at the mouth of the estuary, while saltmarshes are present in both the inner estuary

and the outer estuary (NPWS, 2013a).

Figure 1.1 – Malahide estuary and its surrounds

12

Figure 1.2 – Aerial photograph of Malahide estuary, looking west. Source: IFI (2010).

13

The estuary is designated as both a SPA (due to the important wintering waterfowl it

supports) (NPWS, 2013a) and a SAC (due to the range of coastal habitats it supports,

including saltmarshes and sand dunes) (see figure 1.3) (NPWS, 2013b). Furthermore, it is

also designated as a Ramsar site, as it contains wetlands of international importance

(NPWS, 2013a). During the winter months, Malahide estuary supports 1% or more of the

world’s population of Light-bellied Brent Geese (NPWS, 2013a).

Figure 1.3 – Designated SPAs and SACs in Malahide estuary

14

A number of pressures are exerted on Malahide estuary. Effluent which flows into the

estuary from two WWTPs; namely the Swords WWTP and the Malahide WWTP, exerts

pressure on the estuary in the form of increased nutrient loads. This effluent also increases

the quantity of other contaminants such as pharmaceutical compounds entering into the

estuary, although these increases are at much lower levels and are unlikely to pose any

threats to marine life in the estuary (Lacey et al., 2012). The Swords WWTP has a current

capacity to treat a PE of 60,000 (FingalCoCo, 2014a). However, the population of Swords is

increasing; it has increased by 8.6% between 2006 and 2011 (CSO, 2012). In order to cater

for the increasing population and to minimise the amount of contaminants entering into

Malahide estuary, works began to expand the Swords WWTP in April 2013 (FingalCoCo,

2014a). These works are due to be completed by April 2015, and when finished, will bring

the total capacity that this WWTP will be able to treat up to a PE of 90,000 (FingalCoCo,

2014a). The Malahide WWTP has a current capacity to treat a PE of 20,000 (FingalCoCo,

2014a). The population of Malahide has also increased in recent years; it has increased by

6.1% between 2006 and 2011 (CSO, 2012). Despite this increase in population, no plans are

currently in place to expand the Malahide WWTP.

Pressure is also exerted on Malahide estuary from the marina that is located to the

south of the outer estuary. The construction of this marina may have altered the flow of

current into and out of the estuary, which in turn may have altered sedimentation rates and

consequently impacted the survival, reproduction and distribution of marine life in the estuary

(Paterson and Black, 1999). Furthermore, the dredging activities associated with the

construction and expansion of this marina may have had negative impacts on the biota of

the estuary in the past, as contaminants may have entered into the estuary from the

dumping of the dredge spoil (Boelens et al., 1999). Contaminants may also be currently

entering into the estuary from the activities taking place in the marina. Fish waste, anti-

fouling paints, and spills, leakages and sewage from boats are all likely to be contributing to

increased contaminant inputs to the estuary (Connolly et al., 2001). The presence of the

marina in Malahide estuary has also encouraged a number of alien species to establish

themselves in this area in recent years (Minchin and Sides, 2006; Minchin, 2007; Ryland et

al., 2011). These invasive species are likely to have had negative impacts on the native

marine species in the estuary.

15

The agricultural land in the surrounds of Malahide estuary is also exerting a large

amount of pressure on the estuary. The land to the north of the estuary is predominantly

rural, with both agriculture and pasture for horses and cattle dominating (see figure 1.4) (Roe

and Lovatt, 2009). Runoff from this land into the estuary can be detrimental to its quality (see

section 1.2.2).

Figure 1.4 – Land use types surrounding Malahide estuary

16

1.5 Current knowledge

Malahide estuary was surveyed by the EPA for water quality between 2007 and 2009

(McGarrigle et al., 2010). The results of this surveying indicated that during this time period

the inner estuary was an area that was eutrophic, while the outer estuary was an area that

was potentially eutrophic (McGarrigle et al., 2010). The most recent surveys of Malahide

estuary using the BQI and the PLI were conducted by O’Brien (1999) and Walsh (2009).

O’Brien (1999) surveyed both the inner and outer estuary in her study, whereas Walsh

(2009) surveyed just the outer estuary (see table 1.1). No known survey has been conducted

at this site since the work carried out by Walsh (2009). The results from this study help to

determine the change in the pollution status of the estuary that has occurred since this time.

Table 1.1 – Results of BQI and PLI surveys conducted on Malahide estuary by O’Brien (1999)

and Walsh (2009)

Survey

BQI

(outer estuary)

BQI

(inner estuary)

PLI

(outer estuary)

PLI

(inner estuary)

O’Brien (1999)

10

1.01

3.54

1.07

Walsh (2009)

6.39

-

3.99

-

17

1.6 Aims

The aim of this project was to determine the current pollution status of Malahide

estuary. The project also aimed to determine if this pollution status had changed, and if it

had, by how much it had done so since the last surveys of this kind were conducted by

O’Brien (1999) and Walsh (2009). The hypothesis was put forward that the pollution status of

the estuary would have changed significantly since the last survey conducted by Walsh

(2009). It was proposed that recent population increases in surrounding towns and the

associated increased sewage input to the estuary (which has only recently been addressed

by the granting of plans to extend the Swords WWTP) would have worsened the pollution

status of the estuary since 2009.

18

Chapter 2

Methodology

19

2.1 Fieldwork

2.1.1 General tasks and division of zones

Fieldwork for this project took place over a period of four days. The first day was

spent completing a survey of the site at Malahide estuary. An inspection of the site was

conducted to determine the range of access points into the estuary and to identify the

possible zones that the estuary could be divided into, as well as the most appropriate

sampling locations in these zones. Sketches were made of the estuary and the planned

locations of the zones and sampling points in them. Following the initial site survey, the

remaining three days of fieldwork were spent confirming the locations of the chosen zones,

classifying these zones according to the BQI, and sampling the sediment in the estuary.

During the fieldwork, the locations of the boundaries of each zone classified, as well as the

sampling points in these zones, were noted using a hand-held Global Positioning System

(GPS). These GPS coordinates were then used to create Geographic Information System

(GIS) maps of the estuary on which the zones and sampling locations were labelled (see

figures 2.1 and 2.2). GIS maps were also produced during this project to illustrate the

general surrounds of the estuary, the landcover type surrounding the estuary, and the SPA

and SAC designations in the estuary. GIS data was obtained from the EPA and the National

Parks and Wildlife Service to create these maps. Landsat satellite imagery was also

obtained from the United States Geological Survey to be incorporated into the GIS maps.

The process of selecting zones in Malahide estuary resulted in seven zones being

identified in the outer estuary (see figure 2.1) and eight zones being identified in the inner

estuary.

20

Figure 2.1 – Location of zones in the outer estuary and location of sampling points in these zones

The zones were chosen according to two criteria; biosedimentary boundaries and the

amount of macroalgal growth in or on the sediment (Wilson, 1987). As an example, the

varying characteristics of zones 3 and 4 in the outer estuary resulted in them being identified

as two separate zones. Zone 3 had a muddy gravel substrate. Zone 4, on the other hand,

had a sandy gravel substrate. Zone 3 also contained a large amount of the seagrass

Zostera, while it had a 50% coverage of green algae. Zone 4 had a higher coverage of green

algae at 90%, with it being dominated by the green alga Ulva lactuca. The remaining zones

were chosen in a similar manner, with natural barriers also helping to determine their

location. For example, the water channel which runs between zone 5 and zone 7 acted as a

boundary for each of these zones. The division of an estuary into zones is known as

‘stratified random sampling’ and is done so as to avoid the over-sampling of a large

homogenous area, thus obtaining more representative and therefore effective results

(Jeffrey et al., 1985b). The zones identified were chosen so as to take into account as many

different sources of pollution, sediment types and faunal communities in the estuary as

possible (Jeffrey et al., 1985b).

21

2.1.2 Biological Quality Index

Following the assignment of zones, each zone was classified according to the BQI.

Living, non-motile species were recorded to determine the percentage of substrate they

covered, with the highest being 100%. This was done by walking across the entirety of each

zone and recording the presence or absence of these species and their percentage cover in

the zone. The remaining animal species were recorded and assigned a rating from 5

(maximum) to 1 (minimum) depending on the abundance of these species in the zone (see

table 2.1). This rating was then used as an aid in formulating the BQI.

Table 2.1 – Method used to assign ratings to species according to their abundance in each zone. Adapted from: Jeffrey et al. (1985b)

Rating

Description

Cover

Large species

Small species

5

Abundant to

exclusion or

dominance of all

others

>50% 100/m2 10,000/m2

4

Common –

many small or

few large

patches

10-50% 10/m2 1,000/m2

3

Frequent –

scattered

patches

2-10% 1/m2 100/m2

2

Occasional –

very scattered

small clumps

2% 1/10m2 10/m2

1

Rare – 1 or 2

specimens only

found

1/100m2 1/m2

22

The macrofauna and some of the larger oligochaetes were collected using a large

sieve with a 1 mm mesh so that they could be more easily identified. A description of the

substrate type found at each zone was also made.

The percentage cover of living, non-motile species, the rating assigned to the other

animal species, the substrate type in the zone and any remaining noticeable features of the

zone were recorded onto a data sheet (see appendix A) which was used to help decide the

category that each zone should be assigned to. Each zone was classified as one of the

following categories.

A = Abiotic (contains no macrobiotic life)

B = Opportunistic (dominated by small, short-lived, opportunistic species)

C = Stable (considerable species diversity or evidence of stable, long-term conditions

(e.g. presence of several year classes))

(Jeffrey et al., 1985a)

The proportional areas of the above categories were determined for each zone so

that A+B+C=1 (Jeffrey et al., 1985a). The BQI of the estuary was then calculated using the

following formula (Jeffrey et al., 1985a).

C = proportional area of estuary that is stable (ie. proportional area of all zones in the

estuary that are stable)

A = proportional area of estuary that is abiotic (ie. proportional area of all zones in the

estuary that are abiotic)

23

2.1.3 Pollution Load Index

Surface sediment samples were obtained from the intertidal zone at low tide in each

designated zone (see figure 2.2). It was ensured that the sampled areas contained fine

grained sediment only. Samples were collected using a spade, with only surface samples

taken to ensure that the darker, abiotic layer beneath the substrate was not collected. The

samples were placed in separate, labelled plastic bags and transported back to the lab on

the same day so that they could be correctly stored for preparation and analysis.

Figure 2.2 – Sampling locations in the inner and outer estuary

24

2.2 Pollution Load Index – lab work

A sub-sample was taken from each sediment sample and dried at 40°C for three

days, before being dried for a further day at 105°C. To prevent absorption of moisture, the

samples were then placed in a desiccator overnight. Following this, the dried samples were

broken down using a pestle and mortar and passed through a 2 mm sieve to remove any

sediment particles greater than 2 mm. The remaining sediment was then placed in a

desiccator once more to prepare it for analysis. The PLI requires six mandatory pollutants to

be analysed; organic matter (by loss on ignition (LOI)), total nitrogen, total phosphorus,

cadmium, chromium and zinc (Jeffrey et al., 1985a). A further three pollutants were analysed

for this project; copper, iron and lead. Replication analysis was carried out on each pollutant

so as to increase the significance of the results and the confidence level with which

conclusions could be drawn (Aho, 2013).

LOI determines the percentage of organic matter present in a sample (Jeffrey et al.,

1985b). The LOI experiment was conducted by weighing 1 g of the prepared samples and

adding these to a muffle furnace set to a temperature of 505°C for three hours. The samples

were then removed from the furnace, left to cool and placed in a desiccator, before being

reweighed. The loss in weight was then calculated as a percentage of the original sample

mass.

Acid digestion of the sediments was carried out to analyse for total nitrogen, total

phosphorus, cadmium, chromium, zinc, copper, iron and lead. 1 g of each prepared

(previously broken up, sieved and dried) sample was weighed out and added to separate

labelled acid digestion tubes. 10 ml of concentrated HNO3 was then added to each tube

before they were sealed and placed in a microwave digester at a temperature of 240°C for

four hours. They were then left to cool overnight. Once cooled, the tubes were opened and

40 ml of deionised water was added to them. Following this, they were left to cool once

more. As soon as the samples were cool, they were filtered through Whatman filter paper

into labelled plastic containers and made up to 50 ml with deionised water. The samples

were then poured into labelled test tubes and analysed for total phosphorus, cadmium,

chromium, zinc, copper, iron and lead using inductively coupled plasma optical emission

spectrometry (ICP-OES). Quality control solutions 3 and 7 and two blanks of deionised water

25

were also analysed. The results of the analysis carried out on the quality control solutions

had to be within specific pre-established control limits for the analysis of the samples to be

deemed acceptable. The two blanks of deionised water were analysed to ensure that no

unsuspected contaminants were present in the samples and to ensure that adequate

cleaning procedures took place in the lab. The analysis of the blanks also helped to ensure

that the instruments used to analyse the samples were calibrated correctly during their

analysis. The concentration of total phosphorus and metals were obtained as mg l-1 in

digest. This was then converted to µg g-1 to obtain their concentration levels in the sediment

samples.

Total nitrogen present in the sediment samples was determined using an elemental

analyser. 50 mg of the prepared samples were weighed into pre-weighed zinc foil cups.

Standards (5 mg of sulphanilic acid weighed into eight pre-weighed zinc foil cups) and

blanks (zinc foil cups) were also used. The cups were then closed and placed into the

elemental analyser to test for total nitrogen.

Following the analysis conducted on the sediment samples, original and replicate

pollutant load (PL) values for each parameter (LOI, total phosphorus, total nitrogen, and

metals) were calculated using the following formula (Jeffrey et al., 1985b).

(

)

PL = pollutant load for parameter

CP = concentration of pollutant

BL = baseline

T = threshold

These values were then averaged to obtain the mean PL values for each parameter.

The baseline and threshold figures for each parameter are freely accessible (Jeffrey et al.,

1985b). The baseline figure corresponds to the ‘natural unpolluted estuary,’ while the

threshold figure has been identified as “the pollutant concentration beyond which deleterious

environmental change is observable” (Jeffrey et al., 1985b: 90).

26

Using the mean PL figures obtained, the mean PLI score was calculated for each

site, and subsequently the overall estuary, using the following formulas (Jeffrey et al.,

1985b).

√

√

27

2.3 Data analysis methods

The BQI scores calculated for the outer estuary and the inner estuary were

compared with the scores obtained for these areas from the studies completed in 1999 and

2009 to determine the changes that have taken place since these studies were carried out.

R, a programme for statistical computing and graphics, was used to produce two graphs;

one to compare the BQI scores calculated for the outer estuary in 1999, in 2009 and in this

study, and the other to compare the BQI scores calculated for the inner estuary in 1999 and

in this study.

In order to analyse the PLI results obtained, a number of different methods were

employed. The means of the results from the original and replicate analyses were used for

all of the below data analysis methods. The standard deviations of these means were firstly

calculated so as to indicate the amount that they varied from the average (Altman and Bland,

2005). A one-way Analysis of Variance (ANOVA) was also carried out for each of the

pollutants so as to compare their mean values between each of the different sites. The mean

PLI values calculated for each site in the estuary were also compared using a one-way

ANOVA.

R was used to produce graphs which compared the results for each parameter

measured both between the different sites in the estuary and also with the corresponding

baseline and threshold figures for that parameter. A graph was also produced to compare

the PLI scores calculated for each site in the estuary with each other and with the baseline

and threshold PLI scores. Error bars were plotted on these graphs to indicate standard

errors from the mean values. The standard error is an indicator of the precision of the

sample mean (Altman and Bland, 2005). GIS was used to produce interpolation maps of the

estuary which displayed the concentration levels of each parameter in the estuarine

sediments as well as for the individual site PLI scores. Interpolation is a GIS technique which

estimates the data values for locations which have not been sampled based on the known

data values of the surrounding sampled points (ESRI, 2014). The Inverse Distance Weighted

method of interpolation was used to create the maps produced.

28

A sign test (Siegel, 1956) was carried out to determine if any trend existed between

the individual PL scores presented by Walsh (2009) and those calculated in this study. A

similar test was carried out between the PL scores presented by O’Brien (1999) and those

calculated in this study. Finally, the PLI scores calculated for the outer estuary and the inner

estuary were compared with the PLI scores established for these areas from the studies

completed in 1999 and 2009 to determine the changes that have taken place since these

studies were carried out. Two graphs were produced using R; one to compare the PLI

scores calculated for the outer estuary in 1999, in 2009 and in this study, and the other to

compare the PLI scores calculated for the inner estuary in 1999 and in this study.

29

Chapter 3

Results

30

3.1 Biological Quality Index

3.1.1 Results from this study

The outer estuary in Malahide was divided into seven different zones. These zones

were assigned classifications, with them all being classified as stable (see table 3.1). Each of

the zones was classified as stable as they contained either high species diversity or entire

populations of long-lived species (Jeffrey et al., 1985b). For example, zone 1 contained

many of the bivalves Mya arenaria and Scrobicularia plana in its sediment, with these two

species being indicative of stable zones. The sediment type in each zone in the outer

estuary varied slightly (see table 3.1). However, most of the sediments were composed

largely of sand. The BQI score for the outer estuary was calculated as 10; a score which

indicates that the outer estuary is ‘completely unpolluted’ (Jeffrey et al., 1985a).

Table 3.1 – Sediment type, classification type and outer estuary proportion of each zone in the outer Malahide estuary

Zone

Sediment type

Proportion of outer

estuary

Classification type

1 Sandy mud/sandy mud

over stone 0.1185 C (stable)

2 Sandy mud 0.0638 C (stable)

3 Sand, mud and gravel 0.0603 C (stable)



6 Sandy mud over stone 0.0043 C (stable)


BQI outer estuary = 10(1-0)

= 10

31

The inner estuary in Malahide was divided into eight different zones. These zones

were all classified as being either stable or opportunistic (see table 3.2). None of the zones

were classified as abiotic, as macrofauna were collected in each zone. Similar to the case

for the outer estuary, the zones classified as stable were specified as such as they contained

either high species diversity or entire populations of long-lived species (Jeffrey et al., 1985b).

On the other hand, the zones classified as opportunistic were specified as such as they were

dominated by short-lived, fast spreading species which are characteristic of enriched or

mildly polluted systems (Jeffrey et al., 1985b). For example, the green alga Ulva lactuca was

found in some of the inner estuary zones classified as opportunistic. A number of

oligochaetes were also found in the sediment in these zones, with these animals also being

strongly indicative of opportunistic zones. The BQI score for the inner estuary was calculated

as 2.86.

Table 3.2 – Sediment type, classification type and inner estuary proportion of each zone in

the inner Malahide estuary

Zone

Sediment type

Proportion of inner

estuary

Classification type

8 Gravel, stones and

mud 0.0576 B (opportunistic)


mud 0.0635 C (stable)



11 Mud 0.0360 B (opportunistic)






BQI inner estuary = 10(0.4561-0)

= 2.86

32

It is noticeable from analysing the zone classification types of all of the zones in

Malahide estuary that there is a clear distinction between the inner and outer estuary. All of

the zones in the outer estuary are classified as stable, whereas more of the zones in the

inner estuary are classified as opportunistic, rather than stable (see table 3.3). This

distinction manifests itself in two very different BQI scores for the inner and outer estuary, as

noted above. The BQI score calculated for the entire estuary was 9.31.

Table 3.3 – Sediment type, classification type and estuary proportion of each zone in the

Malahide estuary

Zone

Sediment type

Proportion of entire

estuary

Classification type

1 Sandy mud/sandy mud

over stone 0.1117 C (stable)





6 Sandy mud over stone 0.0041 C (stable)





mud 0.0036 C (stable)









BQI entire estuary = 10(0.9689-0)

= 9.31

33

3.1.2 Comparison with results from 1999 and 2009 studies

Comparisons between the results of this BQI study and those obtained from similar

studies completed in 1999 and 2009 are presented in figures 3.1 and 3.2. The BQI score of

the outer estuary has fluctuated over the course of the three studies; going from 10 in 1999

to 6.39 in 2009, and returning to 10 in this study. The present BQI score of 10 for the outer

estuary indicates that it is completely unpolluted, as signified by the green line on figure 3.1.

Figure 3.1 – Comparison of the result obtained from this BQI survey on the outer Malahide estuary with the results obtained from similar surveys completed by O’Brien (1999) and Walsh

(2009). Two lines representing the BQI scores for a completely abiotic estuary (red) and a completely unpolluted estuary (green) are plotted on the figure.

34

The BQI score for the inner estuary has increased since the last study carried out on

this area in 1999. In 1999, the inner estuary obtained a BQI score of 1.01, while the score

obtained for the inner estuary in this study was 2.86. This indicates that the biological

stability of the inner estuary has improved since 1999.

Figure 3.2 – Comparison of the result obtained from this BQI survey on the inner Malahide estuary with the result obtained from a similar survey completed by O’Brien (1999). Two lines

representing the BQI scores for a completely abiotic estuary (red) and a completely unpolluted estuary (green) are plotted on the figure.

35

3.2 Pollutants

The pollutant values calculated from the original and replicate analyses (see

appendix B) were used to determine the mean pollutant values for each parameter studied

at each site in Malahide estuary. The standard deviations associated with these mean

pollutant values were also calculated and are presented in tables 3.4 and 3.5.

Table 3.4 – Mean pollutant values ± standard deviations at each site in the outer estuary

Site

LOI

(%)

N

(ug g-1

)

P

(ug g-1

)

Cd

(ug g-1

)

Cr

(ug g-1

)

Cu

(ug g-1

)

Fe

(ug g-1

)

Pb

(ug g-1

)

Zn

(ug g-1

)

1 3.64

±0.21

945.63

±56.11

457.56

±12.20

0.18

±0.0

12.28

±0.18

8.27

±0.29

8507.82

±215.91

8.80

±0.05

44.85

±0.71

2 8.55

±0.28

2395.82

±24.13

609.89

±6.76

0.27

±0.0

19.15

±0.37

16.68

±0.34

13854.56

±105.37

15.59

±0.15

76.44

±1.24

3 3.47

±0.02

1155.11

±118.41

405.13

±3.74

0.09

±0.0

12.03

±0.29

7.70

±0.98

7749.83

±278.11

8.43

±0.12

41.42

±2.51

4 6.21

±0.39

1505.24

±30.32

465.26

±31.81

0.16

±0.03

13.87

±0.39

10.42

±0.06

9904.76

±336.77

11.46

±0.44

53.35

±3.22

5 2.27

±0.20

538.08

±25.44

253.73

±18.83

0.07

±0.03

8.11

±0.29

5.74

±0.03

5355.56

±192.12

5.07

±0.20

29.49

±0.46

6 4.25

±0.07

996.32

±25.70

375.87

±3.49

0.11

±0.03

11.09

±0.46

7.80

±0.22

7526.98

±45.66

7.96

±0.11

41.89

±0.89

7 1.90

±0.06

520.09

±23.41

259.96

±9.62

0.0

±0.0

7.34

±0.37

3.93

±0.02

4867.02

±150.02

3.75

±0.12

21.77

±2.78

36

Table 3.5 – Mean pollutant values ± standard deviations at each site in the inner estuary

Site

LOI

(%)

N

(ug g-1

)

P

(ug g-1

)

Cd

(ug g-1

)

Cr

(ug g-1

)

Cu

(ug g-1

)

Fe

(ug g-1

)

Pb

(ug g-1

)

Zn

(ug g-1

)

8 3.54

±0.29

932.45

±40.24

613.84

±87.98

0.59

±0.14

13.07

±0.99

16.32

±0.39

14528.87

±3657.77

12.35

±2.61

61.27

±18.16

9 8.96

±0.30

2878.85

±117.37

997.95

±7.54

0.42

±0.0

14.21

±0.29

21.15

±0.32

11582.70

±174.44

20.04

±3.13

80.88

±0.68

10 4.15

±0.19

1240.40

±102.33

543.53

±18.50

0.14

±0.0

10.55

±0.42

9.82

±0.16

8686.69

±47.39

10.36

±0.14

49.45

±1.16

11 15.04

±0.65

3385.04

±37.05

1422.74

±69.00

0.89

±0.03

13.98

±0.94

27.82

±0.51

11941.20

±841.33

30.19

±0.31

137.98

±11.09

12 6.08

±0.02

2097.89

±70.55

1136.55

±236.61

0.51

±0.06

12.72

±0.89

20.20

±1.05

11908.53

±665.02

22.51

±6.62

82.85

±3.13

13 5.49

±0.24

2278.37

±3.31

662.09

±26.05

0.55

±0.06

7.84

±0.39

13.81

±0.29

7277.37

±14.29

12.49

±1.08

58.99

±5.60

14 9.21

±0.05

3655.64

±31.96

1018.44

±17.70

0.58

±0.03

10.52

±0.14

20.41

±0.12

10039.87

±229.35

12.78

±0.39

76.68

±2.37

15 30.43

±0.16

9460.31

±52.27

2871.03

±111.16

1.38

±0.14

14.70

±0.19

57.09

±1.10

13583.15

±372.14

22.20

±0.19

230.37

±6.74

As can be seen from tables 3.4 and 3.5, there was a large amount of variability

between many of the sites in the inner and outer estuary. In order to analyse the extent of

this variability, a one way ANOVA was carried out for each of the pollutants so as to

compare their mean values at each of the different sites. The complete results of these

statistical tests are presented in appendix C.

37

3.2.1 Loss on ignition

Figure 3.3 illustrates that LOI values, representing the percentage of organic matter

content in the sediment (Jeffrey et al., 1985b), were, for the most part, lower in the samples

obtained from the sites in the outer estuary than they were for the samples obtained from the

sites in the inner estuary. Five sampled sites had LOI values which exceeded the threshold

figure, with four of these sites located in the inner estuary. Site 11, and in particular site 15,

significantly exceeded the threshold figure, with this indicating that these sites are currently

subjected to organic pollution. On the other hand, the samples from sites 5 and 7 in the outer

estuary had LOI values which came quite close to the baseline figure for LOI. The one-way

ANOVA that was carried out indicated that there was a statistically significant difference

between the mean LOI values calculated for each site (ANOVA: F (1, 13) = 5.548,

p = 0.035).

Figure 3.3 – LOI values (expressed as % organic matter in the sediment) of the samples from the fifteen designated sampling sites in Malahide estuary. The error bars indicate standard error and are set at ± 1 standard error from the mean. Two lines representing the threshold

(red) and baseline (green) figures for LOI are also plotted.

38

Figure 3.4, which interpolates LOI values for the entire estuary based on the

recorded values obtained from the samples from each site, illustrates that LOI values vary

considerably across the estuary. As a whole, the outer estuary appears to display lower LOI

values than the inner estuary. The LOI values appear highest at the most western point of

the estuary and also at the south-eastern point of the estuary. The high organic content of

the sediment in these areas may be explained by the human activities taking place next to

them. The most western point of the estuary is the area into which the effluent from the

Swords WWTP flows, while agricultural land is also located to the north and north-east of it.

The south-eastern point of the estuary is located next to both the town of Malahide and the

Malahide marina, while it is also the area into which the effluent from the Malahide WWTP

flows.

Figure 3.4 – Interpolation map illustrating the organic matter content (%) in the sediment in Malahide estuary

39

3.2.2 Nitrogen

Figure 3.5 illustrates that nitrogen concentration values were, for the most part, lower

in the samples obtained from the sites in the outer estuary than they were for the samples

obtained from the sites in the inner estuary. Four sampled sites had values which exceeded

the threshold figure, with all of these sites located in the inner estuary. Site 15 significantly

exceeded the threshold figure, with the sample from this site having a concentration of

approximately 9,460 ug g-1 N. On the other hand, the samples from sites 5 and 7 in the outer

estuary had nitrogen concentration values which came quite close to the baseline figure for

nitrogen. The one-way ANOVA that was carried out indicated that there was a statistically

significant difference between the mean nitrogen concentration values calculated for each

site (ANOVA: F (1, 13) = 8.152, p = 0.013).

Figure 3.5 – Nitrogen concentration values (ug g-1

N) in the samples from the fifteen designated sampling sites in Malahide estuary. The error bars indicate standard error and are set at ± 1

standard error from the mean. Two lines representing the threshold (red) and baseline (green) figures for nitrogen are also plotted.

40

Figure 3.6, which interpolates nitrogen concentration values for the entire estuary

based on the recorded values obtained from the samples from each site, illustrates that

nitrogen concentration values vary considerably across the estuary. As a whole, the outer

estuary appears to display lower values than the inner estuary. The values appear highest at

the most western point of the estuary. This is most likely due to the large amount of

agricultural land located to the north and north-east of this part of the estuary. It may also be

as a result of the effluent which flows into this area from the Swords WWTP.

Figure 3.6 – Interpolation map illustrating nitrogen concentration values (ug g-1

N) in the sediment in Malahide estuary

41

3.2.3 Phosphorus

Figure 3.7 illustrates that phosphorus concentration values were, for the most part,

lower in the samples obtained from the sites in the outer estuary than they were for the

samples obtained from the sites in the inner estuary. All of the sampled sites in the outer

estuary but one (site 2) displayed values below the threshold figure. On the other hand, all of

the sampled sites in the inner estuary exceeded the threshold figure, with sites 9, 11, 12, 14

and 15 significantly exceeding this figure. The one-way ANOVA that was carried out

indicated that there was a statistically significant difference between the mean phosphorus

concentration values calculated for each site (ANOVA: F (1, 13) = 10.6, p = 0.006).

Figure 3.7 – Phosphorus concentration values (ug g-1

P) in the samples from the fifteen designated sampling sites in Malahide estuary. The error bars indicate standard error and are

set at ± 1 standard error from the mean. Two lines representing the threshold (red) and baseline (green) figures for phosphorus are also plotted.

42

Figure 3.8, which interpolates phosphorus concentration values for the entire estuary


phosphorus concentration values vary considerably across the estuary. As a whole, the

outer estuary appears to display lower values than the inner estuary. The values appear

highest at the most western point of the estuary. This is most likely as a result of the effluent

which flows into this area from the Swords WWTP. Increased phosphorus levels are known

to arise from the effluent of WWTPs (see section 1.2.2). The large amount of agricultural

land located to the north and north-east of this part of the estuary is also likely to be a cause

of the high phosphorus concentration values in this area. The high values at the south-

eastern point of the estuary are most likely due to the effluent which flows into this part of the

estuary from the nearby Malahide WWTP.

Figure 3.8 – Interpolation map illustrating phosphorus concentration values (ug g-1

P) in the sediment in Malahide estuary

43

3.2.4 Cadmium

Figure 3.9 illustrates that cadmium concentration values were, for the most part,

lower in the samples obtained from the sites in the outer estuary than they were for the

samples obtained from the sites in the inner estuary. All of the sampled sites in the outer

estuary and four of the sampled sites in the inner estuary displayed values below the

baseline figure. The four remaining sites in the inner estuary (sites 11, 13, 14 and 15)

displayed values above the baseline figure. One of these sites (site 15) exceeded the

threshold figure, but only by a very small amount. The one-way ANOVA that was carried out

indicated that there was a statistically significant difference between the mean cadmium


Figure 3.9 – Cadmium concentration values (ug g-1

Cd) in the samples from the fifteen designated sampling sites in Malahide estuary. The error bars indicate standard error and are

set at ± 1 standard error from the mean. Two lines representing the threshold (red) and baseline (green) figures for cadmium are also plotted.

44

Figure 3.10, which interpolates cadmium concentration values for the entire estuary


cadmium concentration values vary considerably across the estuary. As a whole, the outer


the most western point of the estuary, while they are also higher than most other areas at

the south-eastern point of the estuary. The high cadmium concentration values in these

areas may be explained by the human activities taking place next to them. The most western

point of the estuary is the area into which the effluent from the Swords WWTP flows. The

south-eastern point of the estuary is located next to both the town of Malahide and the

Malahide marina, while it is also the area into which the effluent from the Malahide WWTP

flows.

Figure 3.10 – Interpolation map illustrating cadmium concentration values (ug g-1

Cd) in the sediment in Malahide estuary

45

3.2.5 Chromium

Figure 3.11 illustrates that there was no noticeable trend in the results obtained for

chromium concentration values in the sampled sediment from Malahide estuary. The values

do not appear to be considerably higher or lower in the inner estuary than they do in the

outer estuary. All fifteen of the sampled sites displayed values below the threshold figure,

with all of these values being closer to the baseline figure than the threshold figure. Site 2

had the highest value, with a concentration of approximately 19.1 ug g-1 Cr, while site 7 had

the lowest value, with a concentration of approximately 7.3 ug g-1 Cr. The one-way ANOVA

that was carried out indicated that there were no statistically significant differences between

the mean chromium concentration values calculated for each site (ANOVA: F (1, 13) =

0.501, p = 0.491).

Figure 3.11 – Chromium concentration values (ug g-1

Cr) in the samples from the fifteen designated sampling sites in Malahide estuary. The error bars indicate standard error and are

set at ± 1 standard error from the mean. Two lines representing the threshold (red) and baseline (green) figures for chromium are also plotted.

46

Figure 3.12, which interpolates chromium concentration values for the entire estuary


chromium concentration values vary considerably across the estuary. The values appear

highest at the most northern point of the outer estuary. However, the values vary

considerably, with the lowest values also appearing in the outer estuary, nearby the area

with the highest recorded values. Similar to the outer estuary, chromium concentration

values also vary considerably in the inner estuary, with high values appearing in areas close

by to areas with low values.

Figure 3.12 – Interpolation map illustrating chromium concentration values (ug g-1

Cr) in the sediment in Malahide estuary

47

3.2.6 Copper

Figure 3.13 illustrates that copper concentration values were, for the most part, lower

in the samples obtained from the sites in the outer estuary than they were for the samples

obtained from the sites in the inner estuary. All of the sampled sites in the estuary but one

(site 15) displayed values below the threshold figure, with most of these values being closer

to the baseline figure than the threshold figure. One of the sites in the outer estuary (site 7)

displayed a value below the baseline figure. The one-way ANOVA that was carried out

indicated that there was a statistically significant difference between the mean copper


Figure 3.13 – Copper concentration values (ug g-1

Cu) in the samples from the fifteen designated sampling sites in Malahide estuary. The error bars indicate standard error and are

set at ± 1 standard error from the mean. Two lines representing the threshold (red) and baseline (green) figures for copper are also plotted.

48

Figure 3.14, which interpolates copper concentration values for the entire estuary


copper concentration values vary considerably across the estuary. As a whole, the outer


the most western point of the estuary, while they are also higher than most other areas at

the south-eastern point of the estuary. The high copper concentration values in these areas

may be explained by the human activities taking place next to them. The most western point

of the estuary is the area into which the effluent from the Swords WWTP flows. The south-

eastern point of the estuary is located next to both the town of Malahide and the Malahide

marina, while it is also the area into which the effluent from the Malahide WWTP flows.

Figure 3.14 – Interpolation map illustrating copper concentration values (ug g-1

Cu) in the sediment in Malahide estuary

49

3.2.7 Iron

Figure 3.15 illustrates that there was no noticeable trend in the results obtained for

iron concentration values in the sampled sediment from Malahide estuary. The values do not

appear to be considerably higher or lower in the inner estuary than they do in the outer

estuary. All fifteen of the sampled sites displayed values below the threshold figure. Sites 2

and 15 had the highest values, with concentrations of approximately 13,854 and 13,583 ug

g-1 Fe respectively, while sites 5 and 7 had the lowest values, with concentrations of

approximately 5,355 and 4,867 ug g-1 Fe respectively. The one-way ANOVA that was carried

out indicated that there were no statistically significant differences between the mean iron


Figure 3.15 – Iron concentration values (ug g-1

Fe) in the samples from the fifteen designated sampling sites in Malahide estuary. The error bars indicate standard error and are set at ± 1

standard error from the mean. Two lines representing the threshold (red) and baseline (green) figures for iron are also plotted.

50

Figure 3.16, which interpolates iron concentration values for the entire estuary based

on the recorded values obtained from the samples from each site, illustrates that iron

concentration values vary considerably across the estuary. The values appear lowest in the

outer estuary, particularly at the most eastern point of the outer estuary. However, the values

vary considerably, with some areas to the north of the outer estuary displaying high values.

When compared with the outer estuary, iron concentration values appear higher in most

areas of the inner estuary.

Figure 3.16 – Interpolation map illustrating iron concentration values (ug g-1

Fe) in the sediment in Malahide estuary

51

3.2.8 Lead

Figure 3.17 illustrates that lead concentration values were, for the most part, lower in

the samples obtained from the sites in the outer estuary than they were for the samples

obtained from the sites in the inner estuary. All of the sampled sites in the outer estuary but

two (sites 2 and 4) displayed values below the baseline figure. On the other hand, all of the

sites in the inner estuary displayed values above the baseline figure. However, these values

were still quite small; they were all much closer to the baseline figure than they were to the

threshold figure. Site 11 in the inner estuary had the highest value, with a concentration of

approximately 30.2 ug g-1 Pb, while site 7 in the inner estuary had the lowest value, with a

concentration of approximately 3.7 ug g-1 Pb. The one-way ANOVA that was carried out

indicated that there was a statistically significant difference between the mean lead


Figure 3.17 – Lead concentration values (ug g-1

Pb) in the samples from the fifteen designated sampling sites in Malahide estuary. The error bars indicate standard error and are set at ± 1

standard error from the mean. Two lines representing the threshold (red) and baseline (green) figures for lead are also plotted.

52

Figure 3.18, which interpolates lead concentration values for the entire estuary based

on the recorded values obtained from the samples from each site, illustrates that lead

concentration values vary considerably across the estuary. As a whole, the outer estuary

appears to display lower values than the inner estuary. The values appear highest at the

south-eastern point of the estuary, while they are also higher than most other areas at the

most western point of the estuary. The high lead concentration values in these areas may be

explained by the human activities taking place next to them. The south-eastern point of the

estuary is located next to both the town of Malahide and the Malahide marina, while it is also

the area into which the effluent from the Malahide WWTP flows. The most western point of

the estuary is also an area into which effluent flows, with effluent from the Swords WWTP

often entering into this area.

Figure 3.18 – Interpolation map illustrating lead concentration values (ug g-1

Pb) in the sediment in Malahide estuary

53

3.2.9 Zinc

Figure 3.19 illustrates that zinc concentration values were, for the most part, lower in

the samples obtained from the sites in the outer estuary than they were for the samples

obtained from the sites in the inner estuary. All of the sampled sites in the outer estuary and

all but two (sites 11 and 15) of the sampled sites in the inner estuary displayed values below

the threshold figure. Site 15 in the inner estuary displayed a value significantly above the

threshold figure, with the sample from this site having a concentration of approximately

230.4 ug g-1 Zn. On the other hand, site 7 in the outer estuary displayed a value below the

baseline figure, with the sample from this site having a concentration of approximately 21.8

ug g-1 Zn. The one-way ANOVA that was carried out indicated that there was a statistically

significant difference between the mean LOI values calculated for each site (ANOVA:

F (1, 13) = 6.813, p = 0.022).

Figure 3.19 – Zinc concentration values (ug g-1

Zn) in the samples from the fifteen designated sampling sites in Malahide estuary. The error bars indicate standard error and are set at ± 1

standard error from the mean. Two lines representing the threshold (red) and baseline (green) figures for zinc are also plotted.

54

Figure 3.20, which interpolates zinc concentration values for the entire estuary based

on the recorded values obtained from the samples from each site, illustrates that zinc

concentration values vary considerably across the estuary. As a whole, the outer estuary

appears to display lower values than the inner estuary. The values appear highest at the

most western point of the estuary, while they are also higher than most other areas at the

south-eastern point of the estuary. The high zinc concentration values in these areas may be

explained by the human activities taking place next to them. The most western point of the

estuary is the area into which the effluent from the Swords WWTP flows. The south-eastern

point of the estuary is located next to both the town of Malahide and the Malahide marina,

while it is also the area into which the effluent from the Malahide WWTP flows

Figure 3.20 – Interpolation map illustrating zinc concentration values (ug g-1

Zn) in the sediment in Malahide estuary

55

3.3 Pollution load index

3.3.1 Results from this study

The PL values calculated from the original and replicate analyses (see appendix B)

were used to determine the mean PL values for each parameter studied at each site in

Malahide estuary. The mean PL of each parameter was then used to calculate the mean PLI

score for each site in the estuary (see tables 3.6 and 3.7).

Table 3.6 – Mean PL ± the standard deviation of each parameter studied at each site and mean PLI ± the standard deviation of each site in the outer Malahide estuary

Site

PL

LOI

PL

N

PL

P

PL

Cd

PL

Cr

PL

Cu

PL

Fe

PL

Pb

PL

Zn

PLI

Site

1 3.92

±0.30

5.49

±0.34

1.32

±0.10

10.00

±0.00

6.89

±0.06

8.46

±0.12

4.35

±0.12

10.00

±0.00

4.89

±0.10

5.34

±0.008

2 0.69

±0.07

1.12

±0.03

0.48

±0.02

10.00

±0.00

4.85

±0.09

5.49

±0.10

2.19

±0.03

8.67

±0.03

1.97

±0.07

2.49

±0.008

3 4.16

±0.03

4.37

±0.56

1.87

±0.05

10.00

±0.00

6.98

±0.10

8.71

±0.43

4.79

±0.17

10.00

±0.00

5.39

±0.39

5.59

±0.18

4 1.58

±0.22

2.98

±0.09

1.26

±0.27

10.00

±0.00

6.35

±0.13

7.58

±0.02

3.64

±0.16

9.63

±0.11

3.83

±0.36

4.17

±0.23

5 6.37

±0.46

8.59

±0.24

5.05

±0.64

10.00

±0.00

8.53

±0.13

9.62

±0.02

6.51

±0.16

10.00

±0.00

7.61

±0.09

7.84

±0.22

6 3.16

±0.08

5.20

±0.15

2.26

±0.05

10.00

±0.00

7.32

±0.17

8.66

±0.10

4.93

±0.03

10.00

±0.00

5.33

±0.14

5.68

±0.05

7 7.26

±0.14

8.77

±0.22

4.85

±0.30

10.00

±0.00

8.87

±0.17

10.00

±0.00

6.93

±0.13

10.00

±0.00

9.50

±0.65

8.27

±0.04

56

Table 3.7 – Mean PL ± the standard deviation of each parameter studied at each site and mean PLI ± the standard deviation of each site in the inner Malahide estuary

Site

PL

LOI

PL

N

PL

P

PL

Cd

PL

Cr

PL

Cu

PL

Fe

PL

Pb

PL

Zn

PLI

Site

8 4.07

±0.41

5.58

±0.24

0.47

±0.31

8.08

±2.35

6.62

±0.33

5.60

±0.11

2.01

±1.04

9.42

±0.63

3.05

±1.79

3.85

±0.77

9 0.59

±0.06

0.66

±0.08

0.038

±0.002

10.00

±0.00

6.24

±0.09

4.38

±0.07

2.93

±0.06

7.73

±0.63

1.73

±0.03

1.76

±0.01

10 3.27

±0.23

3.98

±0.44

0.75

±0.09

10.00

±0.00

7.53

±0.16

7.81

±0.06

4.25

±0.02

9.91

±0.03

4.28

±0.14

4.66

±0.06

11 0.07

±0.02

0.38

±0.01

0.002

±0.001

4.09

±0.26

6.32

±0.30

3.11

±0.08

2.80

±0.31

5.97

±0.05

0.33

±0.11

0.67

±0.09

12 1.65

±0.01

1.55

±0.12

0.01

±0.03

9.79

±0.72

6.73

±0.31

4.59

±0.24

2.81

±0.24

7.26

±1.26

1.64

±0.15

1.94

±0.23

13 2.04

±0.17

1.27

±0.004

0.34

±0.06

8.84

±1.14

8.65

±0.18

6.37

±0.09

5.09

±0.01

9.38

±0.26

3.25

±0.52

3.44

±0.23

14 0.55

±0.01

0.28

±0.009

0.03

±0.004

8.26

±0.55

7.54

±0.05

4.54

±0.03

3.57

±0.10

9.31

±0.09

1.96

±0.13

1.66

±0.01

15 0.0003

±0.00002

0.0005

±0.00003

0.0000002

±0.0000001

1.31

±0.42

6.09

±0.06

0.69

±0.04

2.27

±0.11

7.32

±0.04

0.02

±0.004

0.03

±0.005

The mean PLI scores for each site were in turn used to calculate the PLI score for

the outer estuary, the inner estuary and the estuary as a whole (see table 3.8).

Table 3.8 – PLI scores calculated for the outer estuary, the inner estuary and the entire estuary

Location

PLI score

Outer estuary

5.28

Inner estuary

1.29

Entire estuary

2.49

57

Figure 3.21 illustrates that PLI scores were, for the most part, higher for the sampled

sites in the outer estuary than they were for the sampled sites in the inner estuary. This

indicates that the outer estuary is currently less polluted in most parts than the inner estuary.

All of the sampled sites in the outer estuary and all but two of the sampled sites in the inner

estuary displayed scores above the threshold PLI score. The two sites in the inner estuary

which displayed scores below the threshold PLI score were sites 11 and 15, with these sites

displaying scores of approximately 0.67 and 0.03 respectively. As a result, it is likely that

damage resulting in harmful biological effects is occurring at these sites (Jeffrey et al.,

1985a; Wilson, 2003). Sites 5 and 7 in the outer estuary displayed much higher PLI scores,

with scores of approximately 7.84 and 8.27 respectively. These values are close to the

baseline PLI score, indicating that these sites are of a good quality and are unlikely to be

polluted. The one-way ANOVA that was carried out indicated that there was a statistically

significant difference between the mean PLI scores calculated for each site (ANOVA:

F (1, 13) = 7.242, p = 0.018).

Figure 3.21 – PLI scores of the fifteen designated sampling sites in Malahide estuary. The error bars indicate standard error and are set at ± 1 standard error from the mean. Two lines

representing the baseline PLI score (green – for an estuary which is unpolluted) and the threshold PLI score (red – for an estuary where damage is to be expected) are also plotted.

58

Figure 3.22, which interpolates PLI scores for the entire estuary based on the

recorded PLI scores calculated for each of the sampled sites, illustrates that PLI scores vary

considerably across the estuary. As a whole, the outer estuary appears to display areas with

higher PLI scores than the inner estuary, indicating that it is less polluted than the inner

estuary. The PLI scores appear lowest at the most western point of the estuary, while they

are also lower than most other areas at the south-eastern point of the estuary. The low PLI

scores in these areas may be explained by the human activities taking place next to them.

The most western point of the estuary is the area into which the effluent from the Swords

WWTP flows, while a large amount of agricultural land is located to the north and north-east

of this part of the estuary. The south-eastern point of the estuary is located next to both the

town of Malahide and the Malahide marina, while it is also the area into which the effluent

from the Malahide WWTP flows

Figure 3.22 – Interpolation map illustrating PLI scores in Malahide estuary

59

3.3.2 Comparison with results from 1999 and 2009 studies

A sign test was carried out to determine if any trend existed between the individual

PL scores presented by Walsh (2009) and those calculated in this study (see table 3.9). This

test was carried out on the geometric means of all of the pollutants analysed for the 2009

study and seven of the pollutants analysed for this study. Copper and iron were not included

in this sign test as these pollutants were not analysed for the 2009 study. If all of the signs

are the same (‘+’ or ‘-’), this indicates that there is a significant difference between the

geometric mean PL values that were calculated in 2009 and those that were calculated in

this study (Siegel, 1956). Five of the geometric mean PL values calculated for the 2009

study were larger than those calculated for this study, while two of the geometric mean PL

values calculated for this this study were larger than those calculated for the 2009 study.

This sign test suggests that there is no significant consistent difference between the

individual PL scores calculated in the 2009 study and those calculated in this study (N=7,

X=2, P=0.2266).

Table 3.9 – Sign test analysing the relationship between geometric mean PL values of the different pollutants studied in this study and in the 2009 study (Walsh, 2009)

Pollutant

Mean

PL values

Direction of

Sign

type

2014 (Xa)

2009 (Xb) difference

type

LOI 0.98 5.83 Xa < Xb -

N 1.29 4.31 Xa < Xb -

P 0.13 2.14 Xa < Xb -

Cd 7.93 0.30 Xa > Xb +

Cr 6.95 6.39 Xa > Xb +

Pb 7.62 9.85 Xa < Xb -

Zn 2.21 6.15 Xa < Xb -

60

A sign test was also carried out to determine if any trend existed between the

individual PL scores presented by O’Brien (1999) and those calculated in this study (see

table 3.10). This test was carried out on the geometric means of all of the pollutants

analysed for the 1999 study and seven of the pollutants analysed for this study. Copper and

iron were not included in this sign test as these pollutants were not analysed for the 1999

study. Six of the geometric mean PL values calculated for the 1999 study were larger than

those calculated for this study, while only one of the geometric mean PL values calculated

for this study was larger than the corresponding value calculated for the 1999 study. This

sign test suggests that there is no significant consistent difference between the individual PL

scores calculated in the 1999 study and those calculated in this study (N=7, X=2, P=0.0625).

Table 3.10 – Sign test analysing the relationship between geometric mean PL values of the different pollutants studied in this study and in the 1999 study (O’Brien, 1999)

Pollutant

Mean

PL values

Direction of

Sign

type

2014 (Xa)

1999 (Xb) difference

type

LOI 0.98 2.87 Xa < Xb -

N 1.29 2.64 Xa < Xb -

P 0.13 1.87 Xa < Xb -

Cd 7.93 10 Xa < Xb -

Cr 6.95 4.12 Xa > Xb +

Pb 7.62 8.86 Xa < Xb -

Zn 2.21 2.60 Xa < Xb -

61

Comparisons between the results of this PLI study and the results obtained from the

two similar studies completed in 1999 and 2009 are presented in figures 3.23 and 3.24. The

PLI score of the outer estuary has increased over the course of the three studies, with the

most significant increase appearing between the 2009 study and this study, when it rose

from 3.99 to 5.28. This indicates that the pollution status of the outer estuary has improved

significantly since 2009.

Figure 3.23 – Comparison of the result obtained from this PLI survey on the outer Malahide estuary with the results obtained from similar surveys completed by O’Brien (1999) and Walsh

(2009). Two lines representing the baseline PLI score (green – for an estuary which is unpolluted) and the threshold PLI score (red – for an estuary where damage is to be expected)

are plotted on the figure.

62

The PLI score of the inner estuary has been lower than the corresponding PLI score

of the outer estuary each year. The PLI score for the inner estuary has increased since the

last study carried out on this area in 1999. In 1999, the inner estuary obtained a PLI score of

1.07, while the score obtained for the inner estuary in this study was 1.29. This indicates that

the pollution status of the inner estuary has improved since 1999. However, this has only

been a slight improvement, with the current PLI score of 1.29 still very close to the threshold

PLI score, indicating that damage resulting in harmful biological effects is likely to be

occurring in the inner estuary.

Figure 3.24 – Comparison of the result obtained from this PLI survey on the inner Malahide estuary with the result obtained from a similar survey completed by O’Brien (1999). Two lines

representing the baseline PLI score (green – for an estuary which is unpolluted) and the threshold PLI score (red – for an estuary where damage is to be expected) are plotted on the

figure.

63

Chapter 4

Discussion

64

4.1 Initial hypotheses

The results of this study demonstrate that the pollution status of Malahide estuary, as

indicated by both the BQI and the PLI, has improved since the previous studies carried out

on the estuary using these same indices in 1999 and 2009. The hypothesis was originally

put forward that the pollution status of the estuary would have changed significantly since

the last study conducted on it by Walsh (2009). This hypothesis was accepted, with

noticeable changes to the BQI and PLI scores of the outer and inner estuary having

occurred since 2009. However, it was also suggested prior to this study that the pollution

status of the estuary would have worsened rather than improved since 2009, with population

increases in the surrounding towns and the associated increased sewage input to the

estuary being the primary causes of this worsened status. This hypothesis was rejected as

there has been a clear improvement in the pollution status of the estuary since 2009. The

population increases in the towns surrounding the estuary have appeared to have had no

significant effect on the pollution status of the estuary.

65

4.2 Pollution status of the outer estuary – indicated by the BQI

The pollution status of the outer estuary, as indicated by the BQI, has improved since

the last BQI surveys were carried out in this area. The BQI scores calculated for the outer

estuary in 1999, 2009 and in this study were 10, 6.39 and 10 respectively. The results from

this study indicate that the outer estuary is now completely unpolluted. All seven zones in

this area were recorded as stable due to them displaying high levels of species diversity. On

the other hand, two of the zones in the 2009 study were recorded as opportunistic (Walsh,

2009). This suggests that conditions in the outer estuary have improved since 2009, with the

entire outer estuary returning to its previous biologically stable state which was recorded in

the 1999 study. Unfortunately, due to differences in zone boundaries between this study and

the 2009 study, changes in specific areas of the outer estuary since 2009 could not be

established. The suggestion that biological conditions have improved in the outer estuary

since the 2009 study is very likely, as the improved BQI score recorded for the outer estuary

since this study can be matched with a similarly improved PLI score recorded for the same

area since the same study. The reduced level of pollutant build up in the outer estuary since

2009 is likely to be the cause of the improved biological status of this area.

However, it is also possible that other factors have played a role in causing the

differences in the recorded BQI scores. A possible contributing factor to the differences

between the scores obtained is the contrasting interpretations which may have taken place

when assigning each zone to its category. The methods used for the BQI are at times quite

subjective; for example, a zone that Walsh (2009) considered to be stable may have been

considered to be opportunistic in this study due to differences in interpretation rather than

differences in the characteristics of the zone. These differences in interpretation associated

with the BQI may have caused the comparisons which were made with previous BQI

surveys to be less reliable than those that were made between the surveys that were carried

out using the PLI. In future studies, the data sheets recording the presence and abundance

of individual species in zones (see appendix A) should be compared between surveys to

determine exactly how individual zones have changed over time. This comparison would not

have been suitable to carry out in this study due to the differences in zone boundaries

between this study and the 2009 study.

66

4.3 Pollution status of the inner estuary – indicated by the BQI

The BQI scores calculated for the inner estuary in 1999 and in this study were 1.01

and 2.86 respectively. No BQI study was carried out on the inner estuary by Walsh (2009).

The results from this study suggest that the pollution status of the inner estuary, as indicated

by the BQI, has improved slightly since the 1999 study. All of the zones in the inner estuary

in this study were classified as being either stable or opportunistic, whereas three of the

zones in the inner estuary in the 1999 study were classified as being abiotic (O’Brien, 1999).

This indicates that some of the areas in the inner estuary have changed from having no

macrofauna present in them to being dominated by opportunistic species which are often

short-lived and fast spreading. The specific areas in the inner estuary where these changes

have taken place could not be determined due to differences in zone boundaries between

this study and the 1999 study.

The improvement in the biological conditions of the inner estuary is a promising sign.

This improvement can be matched with a similar improvement in the PLI score recorded for

the inner estuary since 1999, indicating that the reduced level of pollutant build up in the

inner estuary since 1999 is the likely cause of the improved biological status of this area.

However, similar to the case for the improved BQI score recorded for the outer estuary, it is

also possible that differences in interpretation may have played a role in causing the

differences in the recorded BQI scores for the inner estuary. Despite the clear improvement

in the biological conditions of the inner estuary since 1999, it still remains a mildly polluted

area. The current BQI score for the inner estuary is much lower than the corresponding BQI

score for the outer estuary. A large amount of change still needs to take place in the inner

estuary for it to reach the state of biological stability that is currently evident in the outer

estuary.

67

4.4 Most significant pollutants in the estuary

The results of this study indicate that the pollutants which are currently causing the

most damage to Malahide estuary are phosphorus and nitrogen. Zinc follows closely behind

as the third most significant pollutant in the estuary. The threshold concentration values for

these three pollutants were exceeded at a number of different sites, particularly at the sites

in the inner estuary. On the other hand, the threshold concentration values for cadmium,

chromium, copper, iron and lead were rarely exceeded, with some recorded concentration

values even appearing below the baseline concentration values for these pollutants.

The phosphorus and nitrogen concentration values recorded for the inner estuary in

this study were very high. Probable sources of the excessive amount of phosphorus and

nitrogen entering into the inner estuary are effluent from the Swords WWTP, runoff from the

agricultural land to the north of the inner estuary, inputs from domestic animal waste and

inputs from domestic detergents and pharmaceuticals (USEPA, 2014). It is likely that the

effluent from the Swords WWTP and the runoff from the agricultural land to the north of the

inner estuary are the most significant sources. The large quantities of these pollutants

entering into the inner estuary are likely to be causing significant amounts of eutrophication

in this area. This is confirmed by the low PLI score recorded for the inner estuary. The

combined effect of high eutrophication levels in the inner estuary with a heavy sediment

build-up of organic matter in the same area is likely to be the cause of the low BQI score

recorded for the inner estuary in this study.

Zinc is the third most significant pollutant affecting Malahide estuary. However, it is

not as damaging to the estuary as phosphorus and nitrogen, with the threshold

concentration value for zinc only exceeded at two sites in the inner estuary. Possible

sources of the excessive amount of zinc entering into the inner estuary are effluent from the

Swords WWTP, industrial activities which take place to the south west of the inner estuary

(see figure 1.4), transportation activities which take place around the inner estuary, runoff

from the agricultural land to the north of the inner estuary and domestic waste inputs to the

inner estuary (Councell et al., 2004). Of these possible sources, it is likely that the industrial

activities which take place to the south west of the inner estuary are the primary source of

the excessive zinc inputs to the inner estuary. Councell et al. (2004) identify industrial

68

activities such as metal production as the largest anthropogenic source of zinc to the

environment. Swords Business Park is located at the southwest corner of the inner estuary

and is currently home to a large number of industrial businesses, including businesses which

produce metal (LocalPages.ie, 2014). It is likely that the existence of these activities nearby

the inner estuary is contributing to the excessive zinc inputs to the estuary. The effluent from

the Swords WWTP is also likely to be a considerable contributor to the quantity of zinc

entering into the inner estuary.

A noticeable feature of the pollutant concentration values recorded during this study

is that the values recorded for nitrogen, phosphorus, cadmium, copper and zinc at site 15 in

the inner estuary are significantly higher than the values recorded for these parameters at

every other site in the estuary. In some cases, the pollutant concentration values recorded at

site 15 are almost double that of the site displaying the next highest pollutant concentration

value. The percentage of organic matter recorded in the sediment was also significantly

higher at site 15 than at other sites. Concentration values of chromium, iron and lead did not

seem to follow the above trend, with values of the above parameters at site 15 not appearing

unusually high. The location of site 15 at the point at which the effluent from the Swords

WWTP enters into the inner estuary and nearby the Swords Business Park is a possible

explanation for the high concentration values recorded at this site. A further possible

explanation for these unexpected results is that the sample which was taken from this site

was not taken with enough care. Too much of the abiotic layer beneath the substrate may

have been collected, while there may have also been a shortage of fine-grained surface

sediment collected. Alternatively, it is possible that errors in the analysis of the sediment

collected from site 15 took place in the lab, with these errors causing the high values

recorded. However, this explanation is unlikely as a replicate analysis was carried out for

each pollutant, with the results from these analyses displaying similarly high values to those

recorded from the original analyses.

69

4.5 Pollution status of the outer estuary – indicated by the PLI

The pollution status of the outer estuary, as indicated by the PLI, has improved since

the last PLI surveys were carried out in this area. The PLI score calculated for the outer

estuary in this study was 5.28, whereas the corresponding scores calculated for the same

area of the estuary in 2009 and 1999 were 3.99 and 3.54 respectively. Although it is difficult

to determine for certain the primary cause of the improved pollution status of the outer

estuary since the studies carried out in 1999 and 2009, there are a number of possible

explanations behind this improvement.

One possibility is that better practice has taken place on the predominantly

agricultural land to the north and north-west of the outer estuary since the last study

conducted in 2009 to ensure that runoff from this land would not enter into the estuary.

Limits may have been placed on the application of fertilisers on this land, while the timing of

the application of fertilisers may have also improved since 2009. Furthermore, more effective

barriers may have been constructed between this land and the estuary so as to curb the

amount of runoff entering into the estuary. Future studies on the pollution status of Malahide

estuary should involve face to face communication with the farmers of the land to the north

and north-west of the outer estuary to determine whether better practices, including the

examples mentioned above, have taken place on their land in recent years compared with

those that took place there prior to 2009. This research may help to either give merit to or

eliminate better practice on this land as an explanation for the improved pollution status of

the outer estuary since the studies carried out in 1999 and 2009.

Aside from the implementation of better practices, less runoff may have also entered

into the outer estuary from this land between 2009 and this study due to the lower frequency

of severe flooding and heavy rain events that occurred during this time when compared with

those that occurred between 1999 and 2009. Five major flooding and heavy rain events

were recorded in the Malahide area between 1999 and 2009, while only one was recorded

between 2009 and when this study was carried out (MetÉireann, 2014). It is likely that if

poorly timed fertiliser application took place on the agricultural land to the north and north-

west of the outer estuary just prior to these extreme events, a large amount of runoff

70

containing pollutants would have entered into the outer estuary, with this subsequently

worsening the pollution status of this part of the estuary (DAFM, 2011).

A further possible explanation of the cause of the improved pollution status of the

outer estuary since the studies carried out in 1999 and 2009 is that prior to its discharge, the

effluent entering into the outer estuary from the Malahide WWTP has been treated more

effectively since the study carried out in 2009, resulting in a reduction in heavy metal and

other pollutant inputs to the outer estuary. However, evidence from the past suggests that

this explanation is somewhat unlikely. The upgrading of the Malahide WWTP which took

place in 2004 (FingalCoCo, 2013) appeared to have very little to no impact on the pollution

status of the outer estuary, with the PLI score of 3.99 recorded in 2009 only a slight

improvement on the PLI score of 3.54 recorded in 1999. The limited impact of the upgrading

of the Malahide WWTP in 2004 suggests that it is unlikely that more effective wastewater

treatment methods are a cause of the improved pollution status observed since the study

carried out in 2009.

It is also possible that the improved pollution status of the outer estuary since the

studies conducted in 1999 and 2009 may be explained by the rebuilding of the railway

viaduct in the estuary which took place following its collapse in August 2009. The

construction of new pilings, pillars, embankments and other physical structures following the

collapse of the viaduct may have disturbed the sediment in the area, causing pollutants to be

removed from the outer estuary (Mundy, 2011). Furthermore, the rebuilding works may have

altered the flow of current between the inner and outer estuary, which in turn may have

affected sedimentation rates and the presence of pollutants in the sediment in the outer

estuary (Mundy, 2011). A review of the potential environmental impacts of the rebuilding

works on Malahide estuary was conducted prior to the commencement of the works (ROD,

2013). However, the results of this review have not been made available for public

consultation, so it is difficult to determine for certain what the exact environmental impacts of

the rebuilding works were. Nonetheless, it was established in 2011 that the rebuilding of the

railway viaduct resulted in changes in water levels in the estuary, with the water levels in the

inner estuary at this time observed to be higher than they were prior to the collapse of the

viaduct in 2009 (Creagh House Environmental, 2012). This indicates that the rebuilding

works did alter current flows between the inner and outer estuary and may have contributed

to the improved PLI score recorded for the outer estuary in this study.

71

4.6 Pollution status of the inner estuary – indicated by the PLI

The results of the PLI survey carried out in this study indicate that the pollution status

of the inner estuary has improved since the last PLI survey was carried out in this area.

However, this improvement was not as significant as the improved PLI score calculated for

the outer estuary since the last study was carried out there. The PLI score calculated for the

inner estuary in this study was 1.29, whereas the PLI score calculated for the same area of

the estuary in 1999 was 1.07. While the change in the PLI score between 1999 and this

study is very small, it is nonetheless promising. This change indicates that heavy metal and

other pollutant inputs to the inner estuary have decreased slightly since 1999.

A possible explanation of the improved PLI score and the associated reduction in

heavy metal and other pollutant inputs to the inner estuary is the upgrading of the Swords

WWTP which took place in 2003 (FingalCoCo, 2014b). The upgrade works which took place

at this time may have improved the treatment methods administered to the wastewater

entering into the plant, meaning that the effluent exiting the plant and entering into the inner

estuary was not as toxic following the upgrade works as it had been prior to these works.

However, similar to the case for the upgrading of the Malahide WWTP, it is difficult to

determine for certain the extent of the impact that the upgrade works have had on the

pollution status of the inner estuary.

It is possible that the second upgrading phase of the Swords WWTP which began in

April 2013 (FingalCoCo, 2014a) may have also helped to improve the pollution status of the

inner estuary. However, these improvements are unlikely to have impacted on the results of

this study as estuarine environments have a slow rate of exchange compared with their

inputs (Kennish, 1997). The pollutants which built up in the inner estuary in previous years

will likely take time to be removed out of the system. Future studies on the pollution status of

Malahide estuary should attempt to obtain a history of the operation of the Swords and

Malahide WWTPs to help determine the quantity of the different pollutants which were

present in the effluent entering into the estuary from these WWTPs in the past. This data

could then be compared with current levels of pollutants entering into the estuary from these

WWTPs. More adequate conclusions could then be drawn on the impact that the upgrading

of both of the WWTPs has had on the pollution status of both the outer and inner estuary.

72

An alternative plausible explanation of the improved PLI score recorded for the inner

estuary is that less runoff from the predominantly agricultural land to the north of the inner

estuary has entered into the estuary in recent years compared with the amount that entered

prior to the study carried out in 1999. Similar to the case proposed for the agricultural land to

the north and north-west of the outer estuary, better practice may have been conducted with

regards to the timing and quantity of fertiliser application on this land since 1999, while more

effective barriers may have also been constructed between this land and the estuary since

1999 so as to prevent runoff from entering into the inner estuary. Future work should

incorporate face to face communication with the farmers of the land to the north of the inner

estuary to determine if better practices have taken place since 1999 than those that took

place prior to 1999. This explanation of the improved PLI score recorded for the inner

estuary in this study will then be able to be approved or rejected with confidence.

73

4.7 Implications of this study

The evident improvement in the pollution status of Malahide estuary in recent years

has important implications for a number of different reasons. As noted in chapter 1, the

estuary is a site of conservational importance, with it currently designated as a SPA, a SAC

and a Ramsar site. The need to protect both the habitats and the biodiversity in the estuary

is clear. The results of this study are promising in this regard. Biodiversity levels in the

estuary have improved in recent years as a result of the reduced build-up of pollutants.

Aside from the direct impacts that the improved pollution status of the estuary has

had and is likely to have in the future on habitats and biodiversity in the area, it will also likely

provide a number of other important benefits. At the local level, the community living in close

vicinity of the estuary will not suffer from the many negative consequences that may arise

from increased pollution levels. For example, the local community will be able to continue to

use the estuary for recreational activities in the near future; this would not be possible if the

estuary became overly polluted. At the national level, the Irish state will also benefit from the

improved pollution status of the estuary. Its improved pollution status will reduce the

possibility of Ireland facing sanctions from the EU for failing to adequately protect the estuary

as a SPA or a SAC. Furthermore, it will help to ensure that visitors and tourists continue to

visit the estuary and the towns surrounding it in the near future, while it will also enable

recreational activities to continue to take place on the estuary. Had the results of this study

indicated that the pollution status of the estuary was worsening rather than improving, this

may have caused fewer tourists to visit the area and less recreational activities to take place

on the estuary in the future. This in turn would have resulted in a loss in revenue to the Irish

state.

74

4.8 Limitations and weaknesses associated with this study

Certain limitations and weaknesses were associated with this study. As mentioned

previously, the methods used for the BQI were at times quite subjective, meaning that the

effectiveness of the comparisons which were made was slightly restricted. While care was

taken to ensure that the information recorded onto the data sheets used to construct the BQI

(see appendix A) was accurate, it is also possible that errors may have been made when

identifying the various flora and fauna species in each zone and when determining the

abundances of these species in each zone. However, it is difficult to quantify these potential

errors and the extent to which they are likely to have impacted on the results. Furthermore,

certain errors may have had no impact on the classification of zones. For example, the

mistaken recording of a complete population of Mya arenaria in a zone when the species

discovered in that zone was in fact Scrobicularia plana would not have changed a zone from

being classified as stable, as both of these species are long-lived species which are strongly

indicative of stable zones.

Errors associated with the methods of the PLI may have also been made in this

study. As mentioned previously, it is possible that some of the samples which were collected

contained too much of the abiotic layer beneath the substrate and too little of the fine-

grained surface sediment. If this was the case, the results may have been affected.

However, extra care was taken during the fieldwork to ensure that the sampling technique

was correct.

Due to time constraints, specific investigations into the sources of pollutant inputs to

Malahide estuary were not able to be made in this study. For example, if more time had

been available to conduct this study, it would have been helpful to analyse the quantities of

the different pollutants present in the effluent entering into the estuary from the Swords and

Malahide WWTPs and compare these with the quantities present in this effluent in the past.

The extent of the impact that WWTP effluent is having on the pollution status of the estuary

could then have been determined. The above limitation of this study could be overcome by

future research on the pollution status of Malahide estuary which incorporates a specific

analysis of the effluent from the Swords and Malahide WWTPs.

75

Chapter 5

Conclusion

76

Pollution is currently one of the largest problems facing estuaries worldwide. Human

populations are now playing a key role in shaping estuarine systems, with human activities

directly contributing to the worsening pollution status of many estuaries. Numerous negative

consequences are arising from increased pollution levels, with estuarine biodiversity and

human populations themselves suffering the worst of these consequences. This study

sought to answer two questions regarding the issue of pollution in Malahide estuary; an

estuary of conservational and recreational importance on Ireland’s east coast:

1. What is the current pollution status of Malahide estuary?

2. What are the changes which have taken place in the pollution status of the

estuary since previous studies were carried out there in 1999 and 2009?

The results of this study identify the outer estuary as an area that is currently

biologically stable but also has chemical build-up from different contaminants. The inner

estuary, on the other hand, has much lower biodiversity levels, while it also has a more

substantial build-up of pollutants. The BQI and the PLI scores calculated for the outer

estuary were much higher than those calculated for the inner estuary in this study. Overall,

the pollution status of Malahide estuary has improved in recent years, with the BQI and the

PLI both indicating a decrease in pollution levels since the previous studies were carried out.

A more marked improvement was evident in the outer estuary than in the inner estuary.

The recent improvement in the pollution status of Malahide estuary has benefited

both the biodiversity of the estuary and the people who regularly make use of or profit from

the estuary. In legislative terms, the findings of this study also bode well for the adequate

upkeep of the estuary as a SPA, a SAC and a Ramsar site. While the implications of the

results gathered in this study are evident, it is essential that further studies are carried out in

the future to continue to monitor the pollution status of Malahide estuary. A similar study to

this one should be carried out in the next five to ten years to determine if any further

changes have taken place. Both the BQI and the PLI should be used so that the results can

be compared with those recorded in this study to establish any changes which may have

taken place. It would also be worthwhile if a different index, such as the Biotic Index (Borja et

al., 2000) mentioned in chapter 1, was used. This would help to ensure that the pollution

status of the estuary, as recorded by the BQI and the PLI, is completely accurate.

77

Similar to the case for many estuaries worldwide, pollution is currently one of the

largest problems facing Malahide estuary. The reasons for protecting the estuary have been

discussed in depth in this study. Malahide estuary is an area of local, national and

international importance. To maintain the intrinsic value of the estuary, it is vital that its

pollution status continues to be monitored in the future. Further research will help to

determine whether steps need to be taken to protect the estuary, and as a result, will also

help to contribute to the preservation of Malahide estuary in its current state, or better still,

the improvement of its pollution status further.

78

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Appendices

A-1

Appendix A

BQI data sheet examples and

key used to fill out data sheets

A-2

Table A1 – Completed BQI data sheet from an outer estuary site. Adapted from: Jeffrey et al. (1985b).

A-3

Table A2 – Completed BQI data sheet from an inner estuary site. Adapted from: Jeffrey et al. (1985b).

A-4

Table A3 – Key used to help fill out the BQI data sheets. Adapted from: Jeffrey et al. (1985b).

A-5

Table A3 (continued) – Key used to help fill out the BQI data sheets. Adapted from: Jeffrey et al. (1985b).

A-6


A-7


A-8


B-1

Appendix B

Raw data used to calculate PLI scores

B-2

Table B1 – Pollutant values at each site in the outer estuary – original and replicate analyses

Site

Replicate

No.

LOI

(%)

N

(ug g-1

)

P

(ug g-1

)

Cd

(ug g-1

)

Cr

(ug g-1

)

Cu

(ug g-1

)

Fe

(ug g-1

)

Pb

(ug g-1

)

Zn

(ug g-1

)

1

1

1

2

3.47

3.82

899.81

991.44

467.53

447.60

0.1854

0.1852

12.42

12.13

8.50

8.03

8684.11

8331.53

8.76

8.84

45.42

44.27

2

2

1

2

8.78

8.31

2376.12

2415.52

615.42

604.37

0.2738

0.2739

18.84

19.45

16.40

16.96

13768.52

13940.59

15.47

15.71

75.42

77.45

3

3

1

2

3.49

3.46

1251.79

1058.42

402.07

408.18

0.0937

0.0936

12.27

11.79

8.50

6.90

7976.91

7522.75

8.52

8.33

43.47

39.37

4

4

1

2

5.89

6.52

1529.99

1480.48

439.29

491.23

0.1417

0.1891

13.55

14.18

10.37

10.47

9629.78

10179.73

11.09

11.82

50.72

55.99

5

5

1

2

2.11

2.44

517.31

558.86

238.35

269.10

0.0463

0.0927

7.87

8.35

5.72

5.77

5198.69

5512.43

4.91

5.24

29.87

29.12

6

6

1

2

4.31

4.19

1017.30

975.33

373.02

378.73

0.1350

0.0904

11.48

10.72

7.99

7.62

7489.69

7564.26

8.05

7.87

42.62

41.15

7

7

1

2

1.86

1.95

539.21

500.98

267.81

252.10

0.0000

0.0000

7.65

7.04

3.91

3.95

4989.51

4744.54

3.85

3.66

19.51

24.04

B-3

Table B2 – Pollutant values at each site in the inner estuary – original and replicate analyses

Site

Replicate

No.

LOI

(%)

N

(ug g-1

)

P

(ug g-1

)

Cd

(ug g-1

)

Cr

(ug g-1

)

Cu

(ug g-1

)

Fe

(ug g-1

)

Pb

(ug g-1

)

Zn

(ug g-1

)

8

8

1

2

3.77

3.30

965.31

899.59

542.00

685.68

0.4754

0.7099

13.88

12.26

15.99

16.63

11542.31

17515.43

10.22

14.48

46.45

76.10

9

9

1

2

8.71

9.20

2974.69

2783.02

991.79

1004.10

0.4184

0.4219

14.46

13.97

21.41

20.89

11725.13

11440.27

17.48

22.60

80.32

81.43

10

10

1

2

4.31

3.99

1323.95

1156.85

528.43

558.64

0.1416

0.1379

10.90

10.20

9.69

9.95

8725.39

8647.99

10.48

10.25

50.40

48.50

11

11

1

2

14.52

15.57

3415.28

3354.79

1366.40

1479.09

0.8656

0.9102

13.21

14.74

27.40

28.24

11254.26

12628.15

29.93

30.44

128.93

147.04

12

12

1

2

6.10

6.06

2040.29

2155.50

1329.74

943.36

0.4633

0.5546

12.00

13.45

21.06

19.34

11365.54

12451.51

17.10

27.92

80.29

85.41

13

13

1

2

5.68

5.29

2275.66

2281.07

683.35

640.82

0.5996

0.5069

8.16

7.51

14.04

13.57

7289.04

7265.69

13.37

11.61

63.65

54.34

14

14

1

2

9.25

9.16

3681.74

3629.54

1032.89

1003.99

0.5595

0.6066

10.63

10.40

20.31

20.51

10227.13

9852.60

13.10

12.46

74.74

78.62

15

15

1

2

30.29

30.57

9502.99

9417.63

2961.79

2780.26

1.5009

1.2659

14.54

14.86

57.99

56.19

13887.01

13279.30

22.04

22.36

235.88

224.87

Table B3 – Threshold and baseline values for each pollutant used to calculate the PLI. Adapted from: Jeffrey et al. (1985b).

Pollutant

LOI

(%)

N

(ug g-1

)

P

(ug g-1

)

Cd

(ug g-1

)

Cr

(ug g-1

)

Cu

(ug g-1

)

Fe

(ug g-1

)

Pb

(ug g-1

)

Zn

(ug g-1

)

Threshold 7.5 2,500 500 1.5 50 50 20,000 100 100

Baseline 10 400 150 0.5 5.0 5.0 2,000 10 20

B-4

Table B4 – PL values of each parameter studied at each site and PLI values of each site in the outer Malahide estuary – original and replicate analyses

Site

Replicate

No.

PL

LOI

PL

N

PL

P

PL

Cd

PL

Cr

PL

Cu

PL

Fe

PL

Pb

PL

Zn

PLI

Site

1

1

1

2

4.17

3.68

5.78

5.23

1.24

1.41

10.00

10.00

6.84

6.94

8.36

8.56

4.25

4.45

10.00

10.00

4.81

4.97

5.33

5.35

2

2

1

2

0.63

0.75

1.14

1.09

0.47

0.50

10.00

10.00

4.92

4.77

5.58

5.42

2.22

2.17

8.69

8.64

2.03

1.91

2.49

2.50

3

3

1

2

4.14

4.19

3.93

4.86

1.90

1.83

10.00

10.00

6.89

7.06

8.36

9.07

4.65

4.93

10.00

10.00

5.09

5.73

5.45

5.74

4

4

1

2

1.77

1.41

2.89

3.06

1.49

1.06

10.00

10.0

6.45

6.25

7.59

7.56

3.77

3.51

9.72

9.54

4.13

3.55

4.36

3.99

5

5

1

2

6.75

6.00

8.79

8.40

5.59

4.57

10.00

10.00

8.63

8.43

9.64

9.61

6.64

6.38

10.00

10.00

7.53

7.69

8.02

7.67

6

6

1

2

3.09

3.23

5.08

5.32

2.30

2.22

10.00

10.00

7.18

7.46

8.58

8.74

4.95

4.91

10.00

10.00

5.21

5.44

5.64

5.73

7

7

1

2

7.38

7.14

8.58

8.95

4.61

5.11

10.00

10.00

8.73

9.01

10.00

10.00

6.82

7.04

10.00

10.00

10.00

8.90

8.23

8.29

B-5

Table B5 – PL values of each parameter studied at each site and PLI values of each site in the inner Malahide estuary – original and replicate analyses

Site

Replicate

No.

PL

LOI

PL

N

PL

P

PL

Cd

PL

Cr

PL

Cu

PL

Fe

PL

Pb

PL

Zn

PLI

Site

8

8

1

2

3.74

4.43

5.38

5.78

0.76

0.29

10.00

6.17

6.35

6.90

5.69

5.51

2.95

1.37

9.94

8.92

4.67

1.99

4.50

3.27

9

9

1

2

0.65

0.55

0.59

0.73

0.039

0.036

10.00

10.00

6.16

6.32

4.32

4.43

2.88

2.99

8.25

7.24

1.76

1.70

1.77

1.74

10

10

1

2

3.09

3.47

3.63

4.36

0.829

0.679

10.00

10.00

7.39

7.66

7.86

7.76

4.23

4.27

9.88

9.93

4.17

4.40

4.61

4.71

11

11

1

2

0.08

0.06

0.37

0.39

0.003

0.001

4.31

3.89

6.57

6.07

3.18

3.04

3.06

2.57

6.00

5.93

0.43

0.26

0.75

0.60

12

12

1

2

1.64

1.66

1.65

1.46

0.004

0.054

10.00

8.82

6.99

6.49

4.39

4.80

3.02

2.63

8.34

6.32

1.76

1.52

1.75

2.13

13

13

1

2

1.90

2.19

1.28

1.27

0.30

0.39

7.95

9.84

8.50

8.79

6.29

6.45

5.08

5.09

9.17

9.59

2.85

3.7

3.26

3.64

14

14

1

2

0.54

0.55

0.27

0.29

0.030

0.036

8.72

7.82

7.49

7.58

4.56

4.52

3.49

3.66

9.24

9.39

2.07

1.85

1.64

1.67

15

15

1

2

0.0003

0.0002

0.0004

0.0005

0.000

0.000

0.99

1.71

6.14

6.04

0.66

0.73

2.18

2.36

7.35

7.29

0.020

0.027

0.03

0.04

C-1

Appendix C

ANOVA results

C-2

Table C1 – ANOVA results for mean loss on ignition values

df

Sum of

Squares

Mean

Square

F

Sig.

Site 1 216.5 216.49 5.548 0.0349

Residuals 13 507.3 39.02

Table C2 – ANOVA results for mean nitrogen concentration values

df

Sum of

Squares

Mean

Square

F

Sig.

Site 1 26744818 26744818 8.152 0.0135

Residuals 13 42649864 3280759

Table C3 – ANOVA results for mean phosphorus concentration values

df

Sum of

Squares

Mean

Square

F

Sig.

Site 1 2779214 2779214 10.6 0.00626

Residuals 13 3408115 262163

Table C4 – ANOVA results for mean cadmium concentration values

df

Sum of

Squares

Mean

Square

F

Sig.

Site 1 1.0132 1.0132 14.18 0.00235

Residuals 13 0.9286 0.0714

C-3

Table C5 – ANOVA results for mean chromium concentration values

df

Sum of

Squares

Mean

Square

F

Sig.

Site 1 4.89 4.887 0.501 0.491

Residuals 13 126.69 9.745

Table C6 – ANOVA results for mean copper concentration values

df

Sum of

Squares

Mean

Square

F

Sig.

Site 1 970.3 970.3 8.763 0.0111

Residuals 13 1439.5 110.7

Table C7 – ANOVA results for mean iron concentration values

df

Sum of

Squares

Mean

Square

F

Sig.

Site 1 7917514 7917514 0.856 0.372

Residuals 13 120196941 9245919

Table C8 – ANOVA results for mean lead concentration values

df

Sum of

Squares

Mean

Square

F

Sig.

Site 1 202.2 202.15 4.853 0.0462

Residuals 13 541.5 41.66

C-4

Table C9 – ANOVA results for mean zinc concentration values

df

Sum of

Squares

Mean

Square

F

Sig.

Site 1 12971 12971 6.813 0.0216

Residuals 13 24749 1904

Table C10 – ANOVA results for mean PLI site scores

df

Sum of

Squares

Mean

Square

F

Sig.

Site 1 30.17 30.172 7.242 0.0185

Residuals 13 54.16 4.167

MSc Env Sci Dissertation - Simon Grennan, 09324780

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