environmental management of aquaculture efluente. development of biological indicators and...
TRANSCRIPT
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Environmental Management of
Aquaculture Effluent:
Development of
Biological Indicators and Biological
Filters
Adrian B. Jones
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Environmental Management of Aquaculture Effluent:
Development of Biological Indicators and Biological Filters
A Thesis
submitted by
Adrian B. Jones B.Sc. (Hons)
The University of Queensland, Australia
to the
Department of Botany
The University of Queensland
AUSTRALIA
in fulfilment of the requirements for
the degree of Doctor of Philosophy within
The University of Queensland
July 1999
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STATEMENT
The work presented in this thesis is, to the best of my knowledge and belief, original, except
as acknowledged in the text, and the material has not been submitted, either in whole or in
part, for a degree at this or any other University.
Signed................................................
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ACKNOWLEDGMENTS
This thesis was initiated from a Fisheries Research Development Corporation (FRDC) grant
to Dr Nigel Preston (CSIRO) and Moreton Bay Prawn Farm. Additional funding was
provided through grants from the Australian Research Council (ARC) to Dr William C.
Dennison, the CRC for Aquaculture and a University of Queensland Postgraduate Research
Scholarship.
Its hard to believe its finally finished. From the early days at CSIRO and Moreton Bay
Prawn Farm laying concrete slabs and besser brick raceways to cruising Moreton Bay in
thunder storms, the all-nighters at Straddie and then the endless days and nights in front of
the computer. None of it would have been possible without the support and friendship of
many people, mostly from the Marine Botany Group at UQ.
Dr. Bill Dennison, who developed my interest in marine research through his amazing
enthusiasm in the field, for his advice and suggestions, and for his ability to make you realise
that its not all as bad as it seems when youre in the depths of confusion.
To everyone else in Marine Botany who provided help with field work, help with
interpretation and presentation of results, proof reading of manuscripts, and general
friendship and support. In particular, thanks to Cindy Heil, Michele Burford, Mark
ODonohue and Joelle Prange who reviewed various sections of the thesis. Also a special
mention to Ros Murrell who has tirelessly helped me to track down a certain person when
times were desperate. You were my link to the outside world during those last six months.
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ACKNOWLEDGMENTS viii
Dr. Nigel Preston, CSIRO Cleveland Marine Laboratories, for his constant encouragement
regarding my writing abilities, his invaluable help regarding the initial planning of the thesis
topic, his timely pep talks and last minute editing.
Theresa Mitchell, for her help, efficiency, support and advice regarding anything and
everything to do with forms, policies, scholarships and finally the thesis submission
procedures. Life at Botany just isnt the same without you! Jan Stewart, for conducting the
isotopic analyses, and Gordon Moss for analysing the amino acid samples.
Sabine Roberts who read early drafts of all the chapters, picking my grammar to pieces and
helping get my thoughts on the right track. Thankyou so much for all your help,
encouragement and friendship throughout my PhD.
My parents who put up with me over the last 4 years constantly complaining about how this
didnt work and that didnt work, and no, I dont know when I will finish it all. Thankyou
for being there to listen to my complaining, and for your support and most importantly, for
never pressuring me.
Finally to Tracey, who despite my constant rebuttal continued to maintain that somehow I
would finish it before our flight left. Thankyou for your support during my constant state of
stress and panic, especially when I was having serious doubts as to how I would manage to
finish it on time. Thankyou for putting up with no end of complaints and irrational
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ACKNOWLEDGMENTS ix
behaviour, and for always being there and for accepting my stream of unfulfilled promises.
Thankyou for your marathon proof reading / printing / collation efforts at the end, especially
amongst the continual printer jams and photocopying nightmares and my accompanying
frustration and rampage. Thankyou for somehow managing to keep me together! Most
importantly, thankyou for your love, companionship, support and understanding.
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ABSTRACT
Rapid global expansion of the aquaculture industry has prompted the need for development of techniques for
effective environmental management. In intensively farmed regions, aquaculture effluent has resulted in
environmental degradation of receiving waters. The issues to be addressed include analysis of effluent water
quality, determination of the ecological impact of effluent on the ecosystem, and development of remediation
strategies to reduce these impacts. Physical and chemical water quality analyses can identify elevated
concentrations of suspended solids, chlorophyll a, water column nutrients and other components of aquaculture
effluent, however, additional biological sampling is required to provide meaningful information about the
ecological impacts of effluent discharge on receiving waters.
Analyses of the amino acid composition, tissue nitrogen content and stable isotope ratio of nitrogen (d15N) in
seagrasses, mangroves and macroalgae were developed as biological indicators to determine the influence of
shrimp farm effluent on a coastal ecosystem. Different responses in these biological parameters revealed that
the impacts of aquaculture effluent on receiving waters were qualitatively different to the impacts of sewage
effluent. The impacts were also spatially more extensive than identified by water quality analyses, which
revealed no elevation in the concentration of water column nutrients, chlorophyll a concentration or total
suspended solids further than 400 m from the mouths of the creeks receiving the sewage and aquaculture
effluent. The maximum d15N of the mangroves, seagrass and macroalgae associated with the treated sewage
discharge was 19.6, which was significantly higher than the influence of the shrimp effluent (7.6). A d15N
value of 4.5, which is elevated relative to unimpacted sites, indicated that the impacts extended up to 4 km
from the mouths of the creeks. Differences in the concentrations of the amino acids proline, serine, glutamine
and alanine in the seagrass and macroalgae were suggested to reflect the source (aquaculture or sewage) of the
nutrients taken up by the plants.
To reduce the environmental impacts, effluent treatment techniques using biological filters were investigated.
Filtration by oysters (Saccostrea commercialis) significantly reduced the concentrations of chlorophyll a
(phytoplankton), bacteria, total nitrogen, total phosphorus and total suspended solids to 5%, 32%, 67%, 63%
and 11% of the initial concentrations, respectively. However, oyster excretion increased the concentrations of
the dissolved nutrients, ammonium (from 18 to 51 M), nitrate / nitrite (from 1.0 to 13 M), and phosphate
(from 0.5 to 3.3 M), however macroalgal (Gracilaria edulis) absorption significantly reduced these
concentrations to 2.3%, 2.2% and 4.8%, respectively. The ratio of ammonium to nitrate / nitrite in the effluent
was also significantly reduced, which has positive implications for recycling of wastewater back into shrimp
production ponds, and reducing impacts on receiving waters.
The efficiency and condition of the oysters and macroalgae was reduced by fouling from the high concentration
of suspended particulates in the effluent. Several novel techniques such as dissolved free amino acid
composition, pigment concentrations, PAM fluorescence, tissue nitrogen and d15N were used to assess the
condition of the macroalgae. It was observed that an intermediate reduction in the concentration of suspended
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ABSTRACT xii
particulates resulted in the best growth and condition of the biofilters. The concentration of particulates in this
treatment (11 nephelometric turbidity units) provided sufficient particulates for oysters to filter, and a source for
regeneration of nutrients for macroalgal uptake, as well as reducing the effects of photoinhibition which can
occur in Gracilaria spp. at relatively low light intensities.
The problems associated with fouling were successfully mitigated by incorporating natural sedimentation prior
to oyster filtration, and subsequent macroalgal absorption. This combined system of treatment proved effective
at optimising the performance of the biological filters to improve the water quality of the effluent. Using this
combination of polyculture, it was estimated that up to 18 kg N ha-1 d-1 and 15 kg P ha-1 d-1 could be removed
from commercial shrimp ponds.
The water quality of aquaculture effluent and its impact on the receiving waters will vary due with differing
environmental conditions, as well as the type of aquaculture being conducted. Regardless, this thesis has
demonstrated that filtration / absorption by various marine organisms can be effective tools for monitoring and
reducing the environmental impacts of aquaculture effluent.
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TABLE OF CONTENTS
Statement v
Acknowledgments vii
Abstract xi
Table of Contents xiii
List of Tables xviii
List of Figures xx
List of Plates xxiii
CHAPTER 1. INTRODUCTION 1
1.1 Aquaculture 1
1.2 Environmental Impacts 2
1.3 Biological Indicators 4
1.4 Biological Treatment Options 5
1.4.1 Oysters 6
1.4.2 Macroalgae 9
1.5 Polyculture / Integrated Aquaculture 12
1.6 Thesis Aims 13
1.7 Thesis Overview 14
1.7.1 Chapter Outline 16
1.9 Publication Status of Thesis Chapters 18
CHAPTER 2 ASSESSING ECOLOGICAL IMPACTS OF SHRIMP
AND SEWAGE EFFLUENT: BIOLOGICAL INDICATORS WITH
STANDARD WATER QUALITY ANALYSES 19
Abstract 19
2.1 Introduction 20
2.2 Materials and Methods 25
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TABLE OF CONTENTS xiv
2.2.1 Study Region 25
2.2.2 Experimental Design 26
2.2.3 Collection 27
2.2.4 Analytical Procedures 27
2.2.5 Statistical Analysis 30
2.3 Results 31
2.3.1 Physical and Chemical Water Quality Analyses 31
2.3.1.1 Salinity 31
2.3.1.2 Nutrients 31
2.3.1.3 Phytoplankton 32
2.3.1.4 Suspended Solids and Secchi Depth 34
2.3.1.5 Sediment Organic Content 34
2.3.2 Bioindicators 35
2.3.2.1 Tissue Nitrogen Content 35
2.3.2.2 d15N Stable Isotope Ratio of Nitrogen 36
2.3.2.3 Free Amino Acid Composition 40
2.4 Discussion 45
2.4.1Water Quality Parameters 45
2.4.1.1 Effluent Composition 45
2.4.1.2 Phytoplankton Biomass and Productivity 46
2.4.2 Biological Indicators 47
2.4.2.1 Tissue N Content 47
2.4.2.2 d 15N Isotopic Signature 48
2.4.2.3 Amino Acid Composition 51
2.4.3 Comparison of Impacts 55
2.4.4 Conclusion 57
2.4.5 Application for other types of Aquaculture 58
2.4.6 Remediation Options 59
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TABLE OF CONTENTS xv
CHAPTER 3 OYSTER FILTRATION OF SHRIMP FARM EFFLUENT,
THE EFFECTS ON WATER QUALITY 63
Abstract 63
3.1 Introduction 64
3.2 Materials and Methods 67
3.2.1 Experimental Design 67
3.2.2 Analytical Procedures 68
3.3 Results 70
3.3.1 Suspended Solids 70
3.3.2 Organic content 70
3.3.3 Chlorophyll a 72
3.3.4 Bacteria 72
3.3.5 Total Nutrients 72
3.4 Discussion 73
3.4.1 Scaling Up Calculations 75
3.4.2 Summary 75
CHAPTER 4 THE EFFICIENCY AND CONDITION OF OYSTERS AND
MACROALGAE USED AS BIOLOGICAL FILTERS OF SHRIMP POND
EFFLUENT 77
Abstract 77
4.1 Introduction 78
4.2 Materials and Methods 81
4.2.1 Experimental Design 81
4.2.1.1 Filtration Efficiency Experiments 81
4.2.1.2 Biofilter Condition Experiments 83
4.2.2 Analytical Procedures 86
4.3 Results 90
4.3.1 Filtration Efficiency Experiments 90
4.3.1.1 Continual Flow 90
4.3.1.2 Recirculating Experiments 92
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TABLE OF CONTENTS xvi
4.3.2 Biofilter Condition Experiments 97
4.4 Discussion 103
4.4.1 Efficiency of Biofilters 103
4.4.2 Condition of Biofilter Organisms 110
4.4.3 Conclusion 115
CHAPTER 5 IMPROVEMENTS IN WATER QUALITY OF AQUACULTURE
EFFLUENT AFTER TREATMENT BY SEDIMENTATION, OYSTER
FILTRATION AND MACROALGAL ABSORPTION 117
Abstract 117
5.1 Introduction 118
5.2 Materials and Methods 121
5.2.1 Experimental Design 121
5.2.2 Analytical Procedures 123
5.3 Results 126
5.3.1 Suspended Solids 126
5.3.2 Organic content 126
5.3.3 Chlorophyll a 128
5.3.4 Bacteria 129
5.3.5 Dissolved Oxygen 131
5.3.6 Total Nitrogen 132
5.3.7 Total Phosphorus 133
5.3.8 Ammonium 134
5.3.9 Nitrate / Nitrite 136
5.3.10 Phosphate 137
5.3.11 Nutrient Uptake Rates and Ratios 137
5.4 Discussion 140
5.4.1 Sedimentation 140
5.4.2 Oyster Filtration 141
5.4.3 Macroalgal Absorption 144
5.4.4 Nutrient Regeneration 147
5.4.5 Conclusions 149
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TABLE OF CONTENTS xvii
CHAPTER 6 CONCLUSION 151
6. 1 Downstream Impacts 151
6.2 Efficiency of Biological Filters 152
6.3 Condition of Biofilters 153
6.4 Scaling up for Commercial Treatment 155
6.5 Other Potential Biofilters 156
6.6 Management Implications and Potential Problems with
Biofiltration / Polyculture 157
6.7 Benefits of Polyculture or Integrated Aquaculture 158
6.8 Future Research 159
6.9 Summary 160
BIBLIOGRAPHY 163
APPENDIX 1 FACTORS LIMITING PHYTOPLANKTON BIOMASS IN THE
BRISBANE RIVER AND MORETON BAY 192
APPENDIX 2 PHOTOSYNTHETIC CAPACITY IN CORAL REEF SYSTEMS:
INVESTIGATIONS INTO ECOLOGICAL APPLICATIONS FOR THE
UNDERWATER PAM FLUOROMETER 203
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LIST OF TABLES
Table 2.1. Results of traditional water quality monitoring for the creek with shrimp farm
effluent and sewage treatment effluent. DIN = Dissolved Inorganic Nitrogen; DIP =
Dissolved Inorganic Phosphorus; Chl a = Chlorophyll a; Phyto Prod = phytoplankton
productivity; TSS = total suspended solids; VSS = volatile suspended solids; Secchi =
secchi disc depth. Only one replicate measurement was recorded for Secchi disk depth
and salinity (Practical Salinity Scale). 33
Table 2.2. Correlations (r2) between the concentration of phytoplankton (chlorophyll a) and
phytoplankton productivity (14C uptake) and various water quality parameters. DIN =
Dissolved Inorganic Nitrogen; DIP = Dissolved Inorganic Phosphorus; Phyto Prod =
phytoplankton productivity (mg C m-3 h-1); Chl a = Chlorophyll a (g L-1); TSS = total
suspended solids (mg L-1); VSS = volatile suspended solids (mg L-1); ISS = inorganic
suspended solids (mg L-1); Secchi = secchi disc depth (m). Numbers in bold type
indicate significant correlations (r2 0.6). 35
Table 2.3. Results of bioindicator monitoring for the creek with shrimp farm effluent and
sewage treatment effluent. d15N = Nitrogen stable isotope ratio; %N = Tissue N content;
nd = no data (no plants were present). 38
Table 2.4. Results of bioindicator monitoring for the shrimp and sewage creeks. % refers to
percentage of total free amino acid pool. SER = serine; ALA = alanine; GLN =
glutamine; PRO = proline; Total aa = total concentration of free amino acids
(mol g wet-1); nd = no data (no plants were present). 42
Table 3.1 Combinations of live and dead oysters (Saccostrea commercialis) used in
experiments to determine the effects of oyster density on the water quality of shrimp
pond effluent. 68
Table 3.2 Concentration of various water quality parameters before and after filtration by
oysters at 3 different densities (see Table 3.1). Values for control (no oysters) and shells
(dead shells only) are also given. Values in brackets are concentrations expressed as a
percentage of the inflow value. Values in italics are standard errors. 71
Table 4.1 Water quality parameters after filtration by oysters under flow through conditions
in raceways. Total N = total Kjeldahl nitrogen; Total P = total phosphorus. 92
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LIST OF TABLES xix
Table 4.2 Water quality parameters after filtration by oysters after the first circuit during
recirculating flow in raceways. TSS = total suspended solids; Organic = organic
component of TSS (loss on ignition); Inorganic = inorganic component of TSS. 94
Table 4.3 Changes in the free amino concentration and composition of macroalgae for
various treatments in laboratory settling experiments. % refers to percentage of total
free amino acid pool. CIT = citrulline; GLU = glutamate; ALA = alanine; GLN =
glutamine; PHE = phenylalanine; SER = serine; Total aa = total concentration of free
amino acids (mol g wet-1). 101
Table 5.1 Percentage of original concentrations of various water quality parameters after
settling, filtration by oysters and filtration by macroalgae. * p 0.05; ** p 0.01; *** p 0.001. Percentage of highest concentration represents the final concentration as a
percentage of the highest recorded concentration after sedimentation and oyster
filtration. The percent of initial concentration represents the final concentration as a
percentage of the initial concentration in the untreated effluent. The only differences
between the two values are for the dissolved inorganic nutrients (NH4+, NO3
-, & PO43-).
128
Table 5.2 Nutrient uptake and release rates for sedimentation, oyster filtration and
macroalgal absorption. Negative symbols represent nutrient uptake, and positive
represent nutrient release. The top value for each treatment is the gross value, the
middle value is the control and the bottom value (in bold type) is the net value after
correction for nutrient changes in the control tanks. The last row of results represent the
rates of macroalgal nutrient uptake over the first hour, when nutrient concentrations
were still saturating uptake kinetics. 136
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LIST OF FIGURES
Figure 2.1 Map of study sites in Moreton Bay, including the location of shrimp and sewage
effluent discharges. 27
Figure 2.2. Map showing the values of %N in seagrass (Zostera capricorni), macroalgae
(Catenella nipae), and mangroves (Avicennia marina) at the study sites (see Fig. 2.1 for
site references). 39
Figure 2.3. Map showing the values of d15N in seagrass (Zostera capricorni), macroalgae
(Catenella nipae), and mangroves (Avicennia marina) at the study sites (see Fig. 2.1 for
site references). 40
Figure 2.4. Map showing the amino acid composition of seagrass (Zostera capricorni) at the
study sites (see Fig. 2.1 for site references). 43
Figure 2.5. Map showing the amino acid composition of macroalgae (Catenella nipae) at the
study sites. Pie graphs have been reduced to quarters for layout purposes. The
remaining three quarters of the pie graphs not represented are a continuation of the
other amino acid category (not serine or alanine) (see Fig. 2.1 for site references). 44
Figure 2.6. Conceptual model of the two creeks and the range and type of impacts from the
different effluent sources. 61
Figure 3.1. Location map of Moreton Bay Prawn Farm near Brisbane, Australia. 66
Figure 3.2. Schematic representation of tank and waterflow layout. 67
Figure 4.1 Diagrammatic representation of experimental setup, a) single raceway with
baffles and oyster trays, and b) laboratory settling experiment. NTU = nephelometric
turbidity units. The oysters used in the experiments were Sydney Rock oysters,
Saccostrea commercialis and the macroalgae was Gracilaria edulis. Effluent was from
an intensive Penaeus japonicus shrimp farm. 84
Figure 4.2 Impacts of effluent on biofilters: a) Oyster mortality (%) from upper, middle and
lower trays after 2 weeks at low, medium and high oyster stocking densities in raceways
supplied with unsettled shrimp effluent, and b) change in dissolved nutrient
concentrations after passing effluent through low, medium and high macroalgal stocking
densities in raceways supplied with unsettled shrimp effluent. Positive change
represents an increase, negative change represents a decrease. 91
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LIST OF FIGURES xxi
Figure 4.3 Particle size distribution, a) before and after control and oyster treatment
raceways during single continuous flow, b) before and after consecutive circuits through
oyster treatment raceways (linear scale), and c) before and after consecutive circuits
through oyster treatment raceways (log scale). 93
Figure 4.4 Concentrations of water quality components before and after consecutive circuits
through oyster treatment raceways, a) bacterial numbers, b) chlorophyll a concentration,
and c) total suspended solids (TSS). 96
Figure 4.5 Growth of oysters and macroalgae after 8 weeks in tanks supplied with shrimp
effluent pre-settled for 0, 1, 6 & 24 h. a) change in oyster growth rate expressed as
changes in oyster volume (cm3 oyster -1), and b) macroalgal biomass. n.d. = no data. 98
Figure 4.6 Response of macroalgae to 8 weeks in tanks supplied with shrimp effluent pre-
settled for 0, 1, 6 & 24 h. a) macroalgal growth expressed as number of news shoots per
tank, and b) concentration of the photosynthetic pigments, chlorophyll a (CHL) and
phycoerythrin (PE). 99
Figure 4.7 Macroalgal nitrogen content (a) and d15N (b) after 8 weeks in tanks supplied with
shrimp effluent of different settlement times, a) %N, and b) d15N. 100
Figure 4.8 The response of electron transport rate (ETR) versus photosynthetically active
radiation (PAR) in macroalgae incubated in seawater (control) or shrimp effluent
(settled 24 h plus oyster filtered for 12 h). 102
Figure 5.1 Design of integrated treatment system stocked with oysters (40 g Saccostrea
commercialis), and macroalgae (Gracilaria edulis). 124
Figure 5.2 Changes in total suspended solids (A) and phytoplankton biomass (chlorophyll a)
(B) from sedimentation, oyster filtration and macroalgal absorption. Standard error bars
have been plotted, but are too small to be visible. 127
Figure 5.3 Concentration of particles settled per litre from sedimentation and oyster
filtration. 129
Figure 5.4 Changes in the organic content of the a) total suspended solids (TSS) and b)
settled particles in the effluent water from sedimentation, oyster filtration and
macroalgal absorption. Standard error bars have been plotted, but are too small to be
visible. 130
Figure 5.5 Changes in bacterial numbers from sedimentation, oyster filtration and macroalgal
absorption. 131
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LIST OF FIGURES xxii
Figure 5.6 Changes in water column dissolved oxygen concentrations from sedimentation,
oyster filtration and macroalgal absorption. Standard error bars have been plotted, but
are too small to be visible. 132
Figure 5.7 Changes in water column total N (A) and P (B) concentrations from
sedimentation, oyster filtration and macroalgal absorption. Standard error bars have
been plotted, but are too small to be visible. 133
Figure 5.8 Changes in water column NH4+, NO3
-, PO43- concentrations from sedimentation,
oyster filtration and macroalgal absorption. Standard error bars have been plotted, but
are too small to be visible. 135
Figure 5.9 Changes in water column total N: P ratio (A) and DIN: DIP ratio (B) from
sedimentation, oyster filtration and macroalgal absorption. Standard error bars have
been plotted, but are too small to be visible. 139
Figure 6.1 Diagrammatic design of water flow for typical untreated shrimp farms (left) and a
design to incorporate physical (sedimentation) and biological (oyster filtration and
macroalgal absorption) treatment (right). 162
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LIST OF PLATES
Plate 1.1 Ponds at Moreton Bay Prawn Farm, an intensive shrimp farm (Penaeus japonicus)
near Moreton Bay, Queensland, Australia. 2
Plate 1.2 Shrimp Farm plume discharging into Moreton Bay, Queensland, Australia. 3
Plate 1.3 High phytoplankton concentration in plume from shrimp farm discharging into
Moreton Bay, Queensland, Australia. 3
Plate 1.4 Penaeus japonicus from ponds at Moreton Bay Prawn Farm, Queensland,
Australia. 5
Plate 1.5 Sydney Rock Oysters (Saccostrea commercialis) cultured in Moreton Bay,
Queensland, Australia. 7
Plate 1.6 Gracilaria edulis collected from Moreton Bay, Queensland, Australia. 10
Plate 4.1 Raceways constructed at Moreton Bay Prawn Farm, Queensland, Australia. 82
Plate 4.2 Control raceway on the left with no oysters and treatment stocked at low density
55 g oysters. Demonstrates changes in water clarity (reduction in suspended solids) with
the oyster tray clearly visible in the raceway stocked with oysters, but not in the control
raceway. 104
Plate 4.3 First chamber (foreground) and second chamber (background) of an oyster
treatment raceway showing the improvement in water clarity (reduction in suspended
solids) within the raceway. 105
Plate 4.4 Fouling of oysters by settling particulates in raceways. 108
Plate 5.1 Experimental setup with sedimentation drum (background) and control, oyster,
macroalgal filtration tanks (foreground). 124
Plate 5.2 Water samples collected: a) before sedimentation; b) after sedimentation;
and c) after biofiltration. 150
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CHAPTER 1
INTRODUCTION
1.1 Aquaculture
The UN Food and Agricultural Organisation has estimated that by 2020 more than 50% of
fisheries production will need to come from aquaculture due to human population growth,
continuing demand for seafood, and static or declining natural fish harvests. However, in
many countries aquaculture practices have already resulted in the destruction of coastal
vegetation, salinisation of land, pollution of waterways and massive crop losses (Phillips et
al., 1993). Further expansion using current technologies is simply not justifiable or
sustainable. If the level of demand for seafood is to be met the only alternative is to develop
new technologies that require less space and have minimal adverse environmental impacts.
Penaeid prawn (shrimp) farming has been one of the most economically successful of all
intensive aquaculture industries. In the early days of shrimp farming and other forms of
aquaculture, the perception was that they were completely clean industries (Weston, 1991).
Recent reviews of intensive shrimp aquaculture have emphasised the need for more effective
controls on the quality of effluent water discharged into the environment (Phillips et al.,
1993; Primavera, 1994).
Shrimp farming can be separated into extensive, semi intensive and intensive culture systems
(Macintosh & Phillips, 1992). Extensive culture systems have large pond sizes (>5 ha),
relatively low stocking densities (20 per m2), aeration, and formulated high protein feed pellets (Plate 1.1). Intensive farming
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CHAPTER 1 2
is becoming more prominent, increasing the potential for environment impact from shrimp
farming (Phillips et al., 1993).
Plate 1.1 Ponds at Moreton Bay Prawn Farm, an intensive shrimp farm (Penaeus japonicus) near Moreton Bay,
Queensland, Australia.
1.2 Environmental Impacts
Intensive shrimp aquaculture systems rely on high protein feed pellets to produce high rates
of growth, but a large proportion of the pellets are not assimilated by the shrimps (Primavera,
1994). Approximately 10% of the feed is dissolved and 15% remains uneaten. The
remaining 75% is ingested, but 50% is excreted as metabolic waste, producing large amounts
of gaseous, dissolved and particulate waste (Lin et al., 1993). Subsequently, the effluent
contains elevated concentrations of dissolved nutrients (primarily ammonia), plankton and
other suspended solids (Ziemann et al., 1992). The dissolved nutrients and organic material
in shrimp ponds stimulate rapid growth of bacteria, phytoplankton, and zooplankton (Lin et
al., 1993). These untreated wastes are usually discharged directly into the environment,
where they may enhance eutrophication, organic enrichment and turbidity of the local
waterways (Plates 1.2 & 1.3) (Eng et al., 1989; O' Connor et al., 1989; Prakash, 1989).
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INTRODUCTION 3
Plate 1.2 Shrimp Farm plume discharging into Moreton Bay, Queensland, Australia.
Plate 1.3 High phytoplankton concentration in plume from shrimp farm discharging into Moreton Bay,
Queensland, Australia.
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CHAPTER 1 4
Australia has a small but expanding coastal aquaculture industry. From 1984 to 1998, the
shrimp farming sector rose from 15 to 2 000 t. The industry is well placed to take advantage
of developments in integrated aquaculture systems, such as the use of natural biofilters and
recirculating systems. In southeast Queensland, there are two shrimp species farmed,
Penaeus monodon Fabricius and P. japonicus Bate (Plate 1.4), with stocking of post larvae in
October and harvest the following April to June. With increasing development of the
industry, concerns have risen about the impact of effluent from the farms. The effluent from
shrimp farms discharging into Moreton Bay, Queensland often has concentrations of
dissolved nitrogen and phosphorus which are 60 fold higher than receiving waters,
chlorophyll a concentrations 200 fold higher, and total suspended solids (TSS) 20 fold higher
(Jones et al., in prep a; Chapter 2). Australian waters are relatively low in nutrients
compared with other coastal waters (State of the Environment Council, 1996), and therefore
impacts may be potentially more acute. In Moreton Bay, background concentrations of water
quality parameters are: NH4+ < 2 M; NO3
- / NO2- ~ 0.1 M; PO4
3- ~ 0.2 M;
chlorophyll a < 1 g L-1; TSS < 20 mg L-1.
1.3 Biological Indicators
Due to the close proximity of shrimp farm discharges to several other point and non-point
nutrient sources (ie. sewage effluent, agricultural runoff), it can be difficult to determine the
impacts of aquaculture on the environment (Grant et al., 1995). Techniques are needed to
distinguish the effect of each source and its range of impact so that appropriate discharge
limits can be applied.
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INTRODUCTION 5
Plate 1.4 Penaeus japonicus from ponds at Moreton Bay Prawn Farm, Queensland, Australia.
Traditional water quality analyses provide little information as to the impact of nutrients on
the biota in the ecosystem (Lyngby, 1990). As a result there is a lack of data on the
ecological impact of aquaculture effluent (Gowen et al., 1990). The use of biological
indicators can provide information as to the nutrient source, the bioavailability of the
nutrients, and the integration of short lived nutrient pulses (Lyngby, 1990; Horrocks et al.,
1995; Jones et al., 1996; Udy & Dennison, 1997b; Jones et al., 1998; Appendix 1).
1.4 Biological Treatment Options
Concerns about the possible adverse impacts of aquaculture discharge have become a risk
factor for the industry (Braaten, 1991). This has prompted efforts to develop cost-effective
methods of effluent treatment. In addition to prohibitive costs, because of the large volume
of effluent, sewage treatment practices have proved inefficient due to the high suspended
solid load (Tetzlaff & Heidinger, 1990) and the high salinity of aquaculture effluent. There
are a number of commercially available bacterial systems to promote nitrification and
subsequent denitrification to remove nitrogen from the effluent. However, the effectiveness
of such systems for treating shrimp pond effluent have yet to be examined rigorously.
-
CHAPTER 1 6
The use of filter feeding bivalves such as oysters to consume phytoplankton, zooplankton,
and bacteria (Lin et al., 1993), and macroalgae to assimilate the remaining dissolved nutrients
(Haines, 1975) may prove to be an efficient and economically viable alternative for
improving the water quality of shrimp farm discharges (Hopkins et al., 1993a; Lin et al.,
1993). In addition to filtering organic food particles, oysters can also improve the quality of
pond effluent by reducing the concentration of inorganic suspended solids.
1.4.1 Oysters
The oyster industry in Moreton Bay, Queensland is based on the Sydney Rock Oyster
Saccostrea commercialis (Iredale & Roughley) (Plate 1.5), an estuarine species native to Port
Stevens, N.S.W., Australia and found from Victoria, Australia to Thailand (Angell, 1986).
This species is considered marketable between 29 and 40 g (bottle oysters) and 40 to 67 g
(plate oysters) whole weight (Witney et al., 1988).
Culture of oysters on traditional leases from spat to marketable size takes two to three years
(Witney et al., 1988). Local availability of oysters is seasonal, with oysters being fat and
ready for sale in summer, while in winter when phytoplankton concentrations are lower
(Dennison et al., 1993), they are lean and growth is substantially slower. The use of land
based aquaculture systems has been trialed to improve productivity and year-round
marketability, but most attempts have been relatively unsuccessful, primarily due to the high
cost and unreliability of mass algal culture. In an attempt to find alternative sources of
microalgae as food for enhanced oyster production, shrimp farm effluent has been trialed
(Wang & Jakob, 1991; Hopkins et al., 1993a; Jakob et al., 1993; Lin et al., 1993). Very fast
-
INTRODUCTION 7
rates of growth has been observed for oysters grown under controlled conditions with shrimp
pond effluent (Jakob et al., 1993).
Plate 1.5 Sydney Rock Oysters (Saccostrea commercialis) cultured in Moreton Bay, Queensland, Australia.
Oysters are suspension feeders and use their gills to filter phytoplankton, zooplankton,
bacteria and other microscopic particles. Bivalves can remove phytoplankton from the water
with high efficiency (Jrgensen, 1966), but their filtering ability is affected by a number of
factors including the water flow rate (Walne, 1972), temperature (Loosanoff & Tommers,
1948), salinity (Djangmah, 1979), reproductive effort, and silt concentration (Loosanoff &
Tommers, 1948; Angell, 1986). A temperature of approximately 30C (Angell, 1986) and
salinity of 35 (Nell & Gibbs, 1986) is optimal, although the survival range for
S. commercialis is 15 50.
Shrimp pond effluent water typically has elevated concentrations of total suspended solids, a
large fraction being small inorganic clay minerals (Smith, 1996). In waters with high
concentrations of silt, oyster pumping and therefore feeding can be greatly inhibited
(Loosanoff & Tommers, 1948), or they may even cease pumping entirely.
-
CHAPTER 1 8
To overcome the problems of oyster fouling, sedimentation ponds could be used to remove
the larger settleable particles prior to oyster filtration (Wang, 1990). The remaining particles
are either motile, or are small particles (
-
INTRODUCTION 9
0.5 g d-1, from seed size (0.04 g) to market size (55.0 g), in just 4 months (Jakob et al., 1993).
The authors state that it was clearly shown that undiluted, semi-intensive, marine shrimp
pond water provides all the requirements for the very rapid growth of the American oyster
C. virginica from 0.05 g spat through 78 g adults (Jakob et al., 1993). Evidence that the
quality of oysters remains high in aquaculture was observed by Lam & Wang (1989) who
used shrimp pond water to produce excellent quality half-shell oysters, grown from 0.1g to
54.2 g in 198 days with 96% survival.
The use of oysters as biofilters can improve the quality of water leaving aquaculture ponds,
and potentially provide a secondary cash crop. After filtration by oysters most of the
nutrients (those bound up in phytoplankton and other suspended solids), are deposited as
faeces and pseudofaeces, while the rest are incorporated into oyster tissue. However, oysters
can also contribute significant amounts of ammonia to the effluent through excretion (Srna &
Baggaley, 1976). Ammonia toxicity to shrimp is one of the primary reasons farmers
undertake water exchange (Kou & Chen, 1991), and therefore it must be removed before
effluent water can be recycled back into production ponds. Consequently, removal of these
deposited nutrients from the system entirely will require either physical removal of the settled
sediment, denitrification, or assimilation of the dissolved nutrients (from the remineralisation
of faeces and pseudofaeces) by macroalgae such as Gracilaria spp. (Funge-Smith & Briggs,
1998).
1.4.2 Macroalgae
Macroalgae can absorb significant quantities of dissolved inorganic and organic nutrients,
usually with a preference for NH4+ (D'Elia & DeBoer, 1978; Haines & Wheeler, 1978;
Hanisak & Harlin, 1978; Harlin, 1978). The ability of macroalgae to rapidly take up nutrients
-
CHAPTER 1 10
for growth, and store luxury reserves in the form of amino acids and pigments makes them
ideal for stripping nutrients from aquaculture effluent (Haines, 1975). Additionally,
macroalgae are known to absorb and store heavy metals (Burdin & Bird, 1994), which may
be a potential pollutant in shrimp pond effluent.
Removal of nutrients by macroalgae is also efficient as harvesting is relatively simple, and
provides an additional cash crop (Hopkins et al., 1995b). Macroalgae can also assimilate
metabolic wastes from mariculture animals, which is beneficial to shrimp production ponds if
the wastewater is to be recycled (Qian et al., 1996). Macroalgae can be used to ensure
complete removal of inorganic nitrogenous excreta from the bivalves (Mann & Ryther,
1977). In particular, commercial red seaweeds such as species from the genera Chondrus,
Gracilaria, Agardhiella and Hypnea are candidates as a final polishing step to leave the
effluent virtually free of inorganic nitrogen (Ryther et al., 1975) (Plate 1.6).
Plate 1.6 Gracilaria edulis collected from Moreton Bay, Queensland, Australia.
Certain species of red macroalgae (Rhodophyta), in particular those from the genera
Gracilaria, Gelidium, and Hypnea are harvested commercially. These species contain
-
INTRODUCTION 11
sulfated galactan agar and carrageenin which are widely used in the pharmaceutical, cosmetic
and food industries (Raven et al., 1987). Nutrients are generally the limiting factor to
macroalgal growth in natural systems, and attempts have been made to culture them in land
based aquaculture systems. The wastewater from aquaculture effluent contains sufficient
nutrients to sustain the high growth rates required without fertilisation, but the high
concentrations of suspended solids can foul the macroalgae and reduce light availability
(Briggs & Funge-Smith, 1993).
There are potentially considerable economic benefits to be gained from growing macroalgae
in shrimp pond effluent. The growth of Hypnea musciformis in the effluent from a tropical
mariculture system has been estimated as producing a gross harvest value of $107 250 ha-1
annually (Roels et al., 1976). H. musciformis cultured in aquaculture effluent grew at 64.5 g
wet wt d-1, compared to deep water growth of 12.1 g wet wt d-1 (Haines, 1975). Percent
carrageenin yields however were lower, ie., 16% dry wt versus 29% for the deep water,
however the total production of carrageenin is approximately 3 times greater from the
aquaculture effluent (Haines, 1975).
The use of bivalves and / or macroalgae to treat the effluent from shrimp farms has been
investigated in a number of studies (Wang & Jakob, 1991; Hopkins et al., 1993a; Jakob et al.,
1993; Shpigel et al., 1993b; Jones & Preston, 1999; Chapter 3). Using oysters (to filter
phytoplankton, bacteria and other suspended solids), and macroalgae (to take up dissolved
nutrients) can potentially improve the quality of shrimp pond effluent. In addition to the
environmental benefits for receiving waters, there are also economic gains resulting from the
conversion of high cost uneaten and dissolved feed pellets into two additional marketable
crops (Wang, 1990).
-
CHAPTER 1 12
1.5 Polyculture / Integrated Aquaculture
Polyculture is defined as the culture of several different organisms in the one culture unit. In
contrast, integrated aquaculture is the co-culture of different organisms, but in discrete culture
units (Chien & Tsai, 1985). These techniques are regarded as being more ecologically sound
methods of aquaculture (Mackay & Lodge, 1983), with a more efficient use of resources, and
a higher resilience against environmental fluctuation (Chien & Liao, 1995).
Despite the advantages of these types of combined aquaculture, there may be some problems
associated with management of several organisms all with differing culture requirements.
Management can be more complex with respect to stocking densities, culture techniques and
associated infrastructure, harvesting procedures, and effluent flow management (Chien &
Liao, 1995). Specific problems for intensive shrimp farming relate to fouling effects from the
high concentrations of suspended solids on secondary crop species (and potential biofilters)
such as oysters and macroalgae (Ziemann et al., 1992; Funge-Smith & Briggs, 1998).
Although the use of these and other biological treatment techniques for facilitating water
recycling are ecologically sound, much research is needed to improve the efficiency of these
systems (Lin, 1995).
Effective management of aquaculture effluent can be separated into identification of
downstream impacts, and effective farm management to reduce these impacts. Identification
of impacts to receiving waters may be accomplished with a combination of water and
sediment water analyses with biological indicators to elucidate ecological impacts. Effective
on-farm management of effluent can probably be accomplished by a combination of physical
and biological treatment techniques.
-
INTRODUCTION 13
1.6 Thesis Aims
Characterise the components of shrimp pond effluent, and their concentrations relative to
the receiving waters,
Develop the use of various marine plants as bioindicators to determine the effects of
prawn farm effluent on receiving waters,
Determine the viability of oysters and macroalgae as biological treatment organisms for
shrimp pond effluent,
Determine the differences in biological filter performance with changes in density, size,
and water flow regimes,
Identify problems associated with maintaining oysters and macroalgae in the high
suspended solids environment and optimise techniques to minimise the impact on their
growth, condition, and effectiveness as biofilters,
Design an integrated system to produce the greatest improvements in water quality, while
maintaining the condition of the biological filter organisms.
-
CHAPTER 1 14
1.7 Thesis Overview
Despite several reported cases of large scale environmental degradation linked to aquaculture
effluent, there has been no successful determination of the ecological impacts, and certainly
no techniques to distinguish these downstream impacts in relation to other nutrient inputs.
Techniques to improve effluent discharge water quality, including the use of bivalves to filter
aquaculture effluent have been undertaken on a small scale by the industry in other regions of
the world. However, there has been a distinct lack of quantitative data to determine the most
efficient use of these techniques and the ecophysiological responses of the biofilter
organisms. This thesis has addressed these shortcomings.
Bioindicator techniques were developed to investigate the ecological impacts of aquaculture
effluent and biofilter organisms were employed, not only to mitigate these impacts, but also
to provide an efficient use of resources by producing secondary crops from aquaculture farm
effluent. The research is this thesis has been conducted using techniques to look at
ecophysiological responses of organisms and ecological changes in the system. This
contrasts much of the published material in this area, which has been conducted purely at an
applied level.
Evaluation of ecological impacts from shrimp farming was conducted using biological
indicator techniques (tissue nitrogen content, d15N isotopic signatures, amino acid
composition, phytoplankton productivity) in conjunction with more traditional water quality
parameters (nutrient concentrations, suspended solids, chlorophyll a) to determine
ecophysiological changes in the biota in the receiving waters. Rates of isotopic fractionation
of nitrogen in the effluent and the changes in the dissolved free amino acid composition of
the macroalgae incubated in shrimp effluent under controlled laboratory conditions provided
-
INTRODUCTION 15
some of the background responses used for determining the spatial range of impacts of
shrimp effluent in receiving waters. These biological or ecological health indicators provided
direct measures of the influence of aquaculture discharge.
The effects of different sizes and stocking densities of oysters and different densities of
macroalgae on the water quality (total and dissolved nutrients, chlorophyll a, bacteria, total
suspended solids, organic versus inorganic particulates, and particle size distribution) of
shrimp effluent was determined for a variety of effluent flow regimes and during different
stages of the shrimp growout season. In particular, analysis of the particle size distribution of
the effluent provided information into the mechanisms by which oysters remove particulates
from the water column, especially the small inorganic clay particles that are difficult to
remove by sedimentation or mechanical filtration.
The effects of different concentrations of suspended solids from shrimp effluent on oyster
and macroalgal condition was determined by physiological responses in the organisms. This
information facilitated estimates of the optimum concentration of suspended particulates for
efficient filtration performance by oysters and macroalgae, while minimising sedimentation
time and / or mechanical filtration costs. A variety of novel techniques such as dissolved free
amino acid composition, pigment concentrations, PAM fluorescence, tissue nitrogen and
d15N were used to assess the condition of the macroalgae. These techniques also provide
information about the bioavailability of the nutrient profile from shrimp effluent, i.e. whether
it is suited for uptake by biofilter organisms (e.g. oysters and macroalgae).
Higher effluent flow rates are likely to improve biofilter condition, but may reduce the
filtration performance. In an attempt to improve the condition and performance of the
-
CHAPTER 1 16
biofilters, experiments were conducted to recirculate the effluent though the biofilter
organisms several times to test the possibility of increasing the effluent flow rate, without
sacrificing improvements in water quality.
The combined efficiency of sedimentation, followed by oyster filtration of particulates and
macroalgal absorption of dissolved nutrients proved to be an effective technique for
improving shrimp pond effluent water quality. After treatment in this polyculture system, the
effluent proved suitable for reuse in shrimp production ponds. The rates of nutrient
regeneration from settled particulates, oyster excretion rates, nutrient uptake rates (bacteria,
phytoplankton and macroalgae) and loss of N to the atmosphere via volatilisation and
denitrification were determined directly, or inferred by difference.
1.7.1 Chapter Outline
Chapter 2
Investigations were conducted to determine the impact of effluent from a local shrimp farm
on the biota and integrity of the receiving waters in Moreton Bay. Results were compared
with data from other unimpacted regions in Moreton Bay and with a nearby sewage treatment
plant. Several bioindicator techniques were utilised to characterise the impacts of the
effluent.
Chapter 3
Experiments were conducted to determine if oysters would be successful at improving the
water quality of shrimp pond effluent, and to assess the optimal stocking density of the
oysters to produce the greatest improvements in water quality.
-
INTRODUCTION 17
Chapter 4
The filtering efficiency of macroalgae and different sized oysters in raceways with flow
through effluent supply, and recirculating supply were conducted. Issues regarding fouling of
oysters and macroalgae were investigated to determine the maximum concentration of
suspended solids that the oysters and macroalgae could tolerate without adversely impacting
their health, survival and filtration efficiency.
Chapter 5
The overall efficiency of a polyculture treatment system was tested using sedimentation
followed by oyster filtration and macroalgal absorption.
Chapter 6
The conclusions of the study and areas of potential future research and comparisons with the
results of other studies are discussed.
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CHAPTER 1 18
1.9 Publication Status of Thesis Chapters
Chapter 2
Jones, A.B., O'Donohue, M.J., Udy, J. & Dennison, W.C. (2001) Assessing ecological impacts of
shrimp and sewage effluent: Biological indicators with standard water quality
analyses. Estuarine, Coastal and Shelf Science 52, 91109.
Presented at the Australian Marine Science Association annual conference,
Adelaide, Australia, July 1998.
Chapter 3
Jones, A.B. & N.P. Preston (1999) Oyster filtration of shrimp farm effluent, the effects on
water quality. Aquaculture Research 30, 51-57.
Chapter 4
Jones, A.B., N.P. Preston & W.C. Dennison (in review) The efficiency and condition of oysters
and macroalgae used as biological filters of shrimp pond effluent. Aquaculture
Research.
Chapter 5
Jones, A.B., Dennison, W.C. & Preston, N.P. (2001) Integrated treatment of shrimp effluent
by sedimentation, oyster filtration and macroalgal absorption: a laboratory scale study.
Aquaculture 193 (1-2), 155-178.
Presented at the World Aquaculture Society Meeting, Sydney, Australia, May 1999.
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CHAPTER 2
ASSESSING ECOLOGICAL IMPACTS OF SHRIMP AND SEWAGE EFFLUENT:
BIOLOGICAL INDICATORS WITH STANDARD WATER QUALITY ANALYSES
Abstract
Despite evidence linking shrimp farming to several cases of environmental degradation, there remains a lack of
ecologically meaningful information about the impacts of effluent on receiving waters. The aim of this study
was to determine the biological impact of shrimp farm effluent, and to compare and distinguish its impacts from
a nearby treated sewage discharge. Assessment of impacts was conducted using both water quality / sediment
analyses and biological indicators. Water quality and sediment parameters measured included chlorophyll a,
total suspended solids, volatile suspended solids, dissolved nutrients, salinity, and sediment organic content.
Biological indicator monitoring consisted of analysis of amino acid composition, tissue nitrogen (N) content and
stable isotope ratio of nitrogen (d15N) in seagrasses, mangroves and macroalgae. The study area consisted of
two tidal creeks, one receiving effluent from a sewage treatment plant (sewage creek) and the other receiving
effluent from an intensive shrimp farm (shrimp creek). The creeks discharged into Moreton Bay, a sub tropical
coastal embayment on the east coast of Australia. Water quality in both creeks was significantly modified, but
changes were indistinguishable from unimpacted eastern Moreton Bay levels further than 750 m from the c reek
mouths. Biological indicators, however, detected significant impacts up to 4 km beyond the creek mouths. The
shrimp creek was more turbid due to clay minerals with a relatively high dissolved NH4+ (3.8 M)
concentration, whereas the sewage creek had a higher percentage of organic material (35%) and dissolved
nutrient concentrations were higher, particularly NO3- / NO2
- (65 M) and PO43- (31 M). The sewage creek did
not support high phytoplankton productivity (18-20 mg C m-3 h-1), in spite of high nutrient concentrations.
Mangroves and macroalgae in the sewage creek were highly enriched with sewage nitrogen (indicated by high
d15N), as was seagrass at the creek mouth. The d15N of seagrasses, mangroves and macroalgae ranged from
10.4-19.6 at the site of sewage discharge to 2.9-4.5 at the reference site, 4 km from the creek mouths. The
d15N values of seagrass (4.5) and mangroves (3.4) at the reference site were higher than values reported for
oligotrophic areas of Moreton Bay, but the d15N of macroalgae (2.9) was close to unimpacted eastern
Moreton Bay values. Macroalgae derive nutrients from the water column, whereas seagrass and mangroves take
up nutrients from the sediment. Therefore, deposition of effluent derived N into the sediments is implicated in
the elevated d15N values of the seagrass and mangroves at the reference site. The free amino acid concentration
and composition of seagrass and macroalgae was used to distinguish uptake of sewage and shrimp derived N.
Proline (seagrass) and serine (macroalgae) were high in sewage impacted plants and glutamine (seagrass) and
alanine (macroalgae) were high in plants impacted by shrimp effluent. The d15N and amino acid composition
indicated sewage N extended further from the creek mouths than shrimp N. This analysis of physical / chemical
and biological indicators was able to distinguish the composition and subsequent impacts of aquaculture on the
receiving waters.
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CHAPTER 2 20
2.1 Introduction
Aquaculture is a rapidly expanding industry, and its effluent can be a major source of
pollution in marine ecosystems (Chua et al., 1989; Twilley, 1989; Gowen et al., 1990;
Braaten, 1991; Holmer, 1991; Phillips et al., 1991; Macintosh & Phillips, 1992; Pruder, 1992;
Raa & Liltved, 1992; Wu et al., 1994; Samocha & Lawrence, 1997; Hargreaves, 1998).
Environmental studies into the effects of shrimp aquaculture are limited and have mostly
focussed on in-pond water quality, with little research conducted into the ecological impacts
of wastewater on receiving waters (Pillay, 1992).
The monitoring of traditional water quality parameters has identified that downstream
impacts of shrimp effluent, and other forms of aquaculture, are only measurable in close
proximity to the discharge point. Hensey (1991) observed that environmental monitoring of
aquaculture effluent using water quality sampling techniques showed no impacts. Samocha
& Lawrence (1997) observed large diurnal fluctuations in water quality parameters measured
downstream of shrimp farm discharge points, and that no increase in total suspended solids
(TSS) or nutrient concentrations could be measured further than 400 m from the farms
discharge. It is possible, however, that sediment impacts such as increased organic matter
and anoxia, may extend further (up to 1 km) than water column impacts (Wu et al., 1994).
With the current projected expansion of shrimp farming in most coastal areas of the world,
large scale increases in nutrients and suspended solids in the receiving waters are likely.
Elevated loadings of particulate material to receiving waters have immediate effects on the
receiving environment such as reduced light penetration and smothering of benthic fauna and
flora (Abal et al., 1994). In addition, particle loading may also contribute to longer term
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ASSESSING ECOLOGICAL IMPACTS 21
changes through initial downstream settling, with resuspension into the water column at a
later time.
Increases in the concentration of NH4+ in receiving waters from shrimp farms have been
observed by many researchers, and as a result nutrient enrichment of poorly flushed
embayments may occur (Gowen & Rosenthal, 1993). In some instances the level of impact
has been sufficient to result in feedback which affects the aquaculture operation itself
(Gowen et al., 1990). Evidence suggests that serious shrimp farm production losses resulting
from the outbreak of disease in Asia and Latin America, are due to the environmental impacts
of shrimp culture (Phillips et al., 1993). In addition to impacts on the aquaculture operation
itself, shrimp farming has been linked to several cases of environmental degradation,
however, despite this type of evidence, there is still a lack of quantitative data on the
ecological impacts to receiving waters (Phillips et al., 1993).
The need for data on the ecological impact of aquaculture effluent has been identified
(Gowen et al., 1990), and it has been shown that physical and chemical water quality
monitoring techniques cannot provide this information. Bioindicators have long been used to
determine ecological impacts of point source discharges (Worf, 1980; Kramer, 1994). For
example, marine macrophytes can be used to provide insights into the ecological impacts of
nutrients and suspended particulates by measuring changes in plant distributions,
morphology, pigment concentrations and total tissue N (Lyngby, 1990; Alamoudi, 1994;
Horrocks et al., 1995; Abal & Dennison, 1996; Udy & Dennison, 1997b). The morphology
of seagrasses can change with reduced light availability, as a consequence of elevated
concentrations of suspended solids in the water column (Abal et al., 1994). Seagrass
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CHAPTER 2 22
distribution and depth penetration are also reduced as a consequence of reduced light
availability in shallow estuarine systems (Dennison et al., 1993; Abal & Dennison, 1996).
Recently, marine plants have been used to detect and integrate the long term effects of small
and /or pulsed nutrient inputs in well flushed oceanic systems (Costanzo, 1996), and elucidate
the possible sources of the nutrient inputs (Jones et al., 1996). Macrophyte amino acid
concentrations and composition have been shown to change with various N sources in both
controlled laboratory experiments (Nasr et al., 1968; Di Martino Rigano et al., 1992; Jones et
al., 1996) and field surveys (Udy & Dennison, 1997b). In particular, accumulation of the
amino acids alanine, glutamine, proline and serine in plants (both terrestrial and marine) has
been associated with N uptake, with different amino acids responding to different N sources
(Steward and Pollard, 1962; Silveira et al., 1985; Lawlor et al., 1987; Kiladze et al., 1989; Di
Martino Rigano et al., 1992; Vona et al., 1992; Heuer & Feigin, 1993). Stable isotope ratios
of nitrogen (d15N) have been used widely in marine systems as tracers of discharged nitrogen
from point and diffuse sources, including sewage effluent (Rau et al., 1981; Wada et al.,
1987; Van Dover et al., 1992; Macko & Ostrom, 1994; Cifuentes et al., 1996; McClelland &
Valiela, 1998). Elevated d15N signatures in seagrass, mangroves and macroalgae have been
attributed to plant assimilation of N from treated sewage effluent (Wada et al., 1987; Grice et
al., 1996; Udy & Dennison, 1997b; Abal et al., 1998). This study, however, appears to be the
first to use all these techniques to study the extent of impacts from aquaculture effluent.
In coastal marine systems a variety of point source inputs from aquaculture ponds, sewage
treatment plants, fertiliser plants, agriculture and urban runoff can make it difficult to
determine responsibility for ecological impacts (Grant et al., 1995). With increasing conflict
-
ASSESSING ECOLOGICAL IMPACTS 23
between users of coastal resources, it has become essential to determine the specific influence
of each source (Teichert-Coddington, 1995).
To assess the potential impact of aquaculture effluent, comparisons between the water
volumes and water quality parameters of aquaculture effluent and treated sewage effluent
have been conducted (Bergheim & Selmer-Olsen, 1982; Muir, 1982; Solbe, 1982; Macintosh
& Phillips, 1992; Paez Osuna et al., 1997). However, there are very few studies comparing
the impacts on the receiving waters (Pearson & Rosenberg, 1978; Cifuentes et al., 1996).
Despite the differences in the two forms of waste, Pearson & Rosenberg (1978) hypothesised
that the impacts of these two sources on receiving sediments would be similar.
Shrimp pond effluent has higher concentrations of suspended solids and phytoplankton
(Ziemann et al., 1992), but lower concentrations of nutrients than sewage effluent (Muir,
1982). Dissolved nutrients in shrimp effluent are predominantly NH4+, whereas sewage
effluent is proportionally higher in NO3-, and PO4
3- (Macintosh & Phillips, 1992). Shrimp
effluent is typically produced in large volumes (Macintosh & Phillips, 1992), which can
equate to up to 40% of the total inputs of N and P in some localised areas (Bergheim &
Selmer-Olsen, 1982). Sewage is freshwater, whereas the salinity of shrimp effluent is
typically 35-36 on the practical salinity scale. These differences may have a considerable
impact on the fate of organisms in the receiving waters when effluent is released into shallow
tidal estuaries. Both sewage and aquaculture effluent can be discharged intermittently,
resulting in large diel fluctuations in water quality. Difficulties in monitoring these variable
discharges can be overcome by the use of biological indicators, which integrate the impacts
of these effluents over time (Costanzo, 1996). Unlike traditional chemical analyses of water
-
CHAPTER 2 24
column nutrients, these biological indicators reflect the availability of biologically available
nutrients (Lyngby, 1990) which provides more ecologically meaningful information.
The aims of this study were to assess the influences on the receiving environment of
wastewater discharges to a shallow estuarine system. Changes in receiving water and
sediment quality analyses were compared with biological impacts measured as a consequence
of shrimp farm and sewage effluent discharges. The region of influence of these two
pollutant sources is defined, and mechanisms are suggested which may aid in discerning the
relative impacts of these two discharges on a common receiving environment.
-
ASSESSING ECOLOGICAL IMPACTS 25
2.2 Materials and Methods
2.2.1 Study Region
Moreton Bay is a shallow coastal embayment on the east coast of Australia. The western side
of the bay receives a variety point and non point source inputs including agricultural runoff,
sewage and aquaculture effluent. The eastern bay is well flushed and influenced by oceanic
waters. In eastern Moreton Bay, background concentrations of water quality parameters are:
NH4+ < 2 M; NO3
- ~ 0.1 M; PO43- ~ 0.2 M; chlorophyll a < 1 g L-1; TSS < 20 mg L-1,
and typical d15N values for mangroves, seagrass and macroalgae in the eastern bay are 2-3
(Abal et al., 1998).
Two tidal creeks in close proximity (1.5 km apart) were studied in Moreton Bay, Australia
(Fig. 2.1). One creek received discharge (18000 m3 d-1 containing 2.0 mg N L-1 and
0.2 mg P L-1, which equates to 36 kg N d-1 and 3.6 kg P d-1) from a shrimp farm (Jones,
unpub. data). The creek was 2-3 m deep, approximately 1 km in length, and the shrimp farm
discharge was 500 m from the mouth of the creek. The intensive shrimp farm (6 ha of ponds)
was stocked with Penaeus japonicus (35 animals m-2). Ponds were routinely flushed (~20%
per day) and water discharged into the creek on low tide. Except during the effluent
discharge, the creek runs dry at low tide. The other creek (Eprapah Creek) received
discharge (2400 m3 d-1 containing 4.5 mg N L-1 and 8.0 mg P L-1, which equates to
10.8 kg N d-1 and 19.2 kg P d-1) from a sewage treatment plant (Redland Shire Council, pers.
comm.). The creek was approximately 2-5 m deep, 15 km in length, has a standing body of
water at low tide, and the sewage discharge point was 2 km from the mouth. The sewage
treatment plant serviced approximately 14 000 people and utilised secondary (activated
sludge) treatment techniques. Both creeks were tidally flushed, and had virtually no
freshwater flow during the study period (Autumn, 1997).
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CHAPTER 2 26
2.2.2 Experimental Design
Three sites were chosen in each creek, the first at the nutrient source (discharge site), the
second approximately mid way between the nutrient source and the mouth (middle site), and
the final at the mouth of the creek (mouth site) (Fig. 2.1). A site was positioned midway
between both creeks and in close proximity to the shore (midway site). Several more sites
were selected in Moreton Bay in a radiating pattern out from the creek mouths (Oyster Point,
Sewage Plume and Cox Bank), including a reference site located approximately 4 km from
the creek mouths. The creek banks at low tide extended the mouth of the sewage creek as far
as the sewage plume site. At eight of the sites, traditional water quality parameters (dissolved
N & P, total suspended solids, volatile suspended solids, sediment organic content, secchi
depth, chlorophyll a, and physico-chemical parameters) were determined.
Brisbane
MoretonBay
0 0.5 1.0
kilometres
N
Eprapah Ck
ReferenceSite
SewageTreatment
Plant
ShrimpFarm
OysterPoint
DischargeSite
DischargeSite
MiddleSite
MiddleSite
Mouth Site
Mouth Site
MidwaySite
Cox BankSite
SewagePlumeSite
Oyster PointSite
VictoriaPoint
PointHalloran
Coochie-mudloIsland
Nutrient SourceSampling Site
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ASSESSING ECOLOGICAL IMPACTS 27
Figure 2.1 Map of study sites in Moreton Bay, including the location of shrimp and sewage effluent discharges.
Bioindicators were utilised at eleven sites with macroalgae and phytoplankton at all sites,
mangroves at the creek sites, and seagrass at the bay sites. Amino acid composition was
determined for macroalgae and seagrass, and the d15N signature and total tissue N was
determined for all bioindicator species.
2.2.3 Collection
Samples of seagrass (Zostera capricorni), mangrove (Avicennia marina), and macroalgae
(Catenella nipae) were collected, placed on ice and returned to the laboratory and prepared
for analysis of %N, d15N and amino acids. In the case of the seagrass and mangroves, the
second youngest leaves were chosen, and for the macroalgae a single mangrove
pneumatophore covered in macroalgae was collected for each replicate. Three replicates for
each plant type were collected at each site.
2.2.4 Analytical Procedures
Salinity was measured with a Horiba U-10 water quality meter (California, U.S.A.) and
expressed on the Practical Salinity Scale.
Chlorophyll a concentration was determined by filtering a known volume of water sample
through Whatman GF/F filters, which were immediately frozen. Acetone extraction and
calculation of chlorophyll a concentration was performed using the methods of Clesceri et al.
(1989), and Parsons et al. (1984).
Light-saturated phytoplankton productivity (potential productivity in mg C m-3 h-1) was
determined in the laboratory using the 14C-bicarbonate incorporation method
-
CHAPTER 2 28
(Parsons et al., 1984). One hundred millilitres of water from each site was dispensed to three
120 ml polycarbonate bottles. A common dark control was established for each site by
combining 33 ml of sample from each replicate into a fourth bottle wrapped in foil. Aqueous
14C sodium bicarbonate (4 Ci) was added and bottles were incubated at a light intensity of 1100
to 1200 E m-2 s-1. A recirculating water bath and perspex heat shields maintained temperatures
at ambient levels. After approximately two hours, water samples were filtered through 0.4 m
polycarbonate filters (Poretics). The filters were placed into 5 ml scintillation vials and two
drops of 5N HCl were added to each vial to drive off any remaining 14CO2. Four millilitres of
scintillation fluid was added to each vial, and radioactivity as disintegrations per minute (DPM)
determined using a scintillation counter (Packard Tricarb 1600TR, Meriden, Connecticut,
U.S.A.). Total CO2 concentration in samples was determined from carbonate alkalinity using
the method of Parsons et al. (1984).
Total suspended solids concentrations were determined using the methods of
Clesceri et al. (1989). A known volume of water was filtered onto a pre-weighed and pre-
dried (110 C; 24 h) Whatman GF/C glass fibre filter. The filter was then oven dried at 60 C
for 24 h and total suspended solids calculated by comparing the initial and final weights.
Volatile suspended solids were determined as loss on ignition by combusting samples in a
muffle furnace for 12 h at 525 C (Clesceri et al., 1989). The organic content of the sediment
was determined from 10 cm deep core samples collected using 50 mL cut-off syringes. The
sediment sample was combusted in a muffle furnace at 525 C for 12 h and the proportion of
organic material determined by loss on ignition (Clesceri et al., 1989).
Dissolved inorganic nutrients (NH4+, NO3
-/NO2-, and PO4
3-) were determined by filtering
water samples through Whatman GF/F glass fibre filters and freezing them immediately on
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ASSESSING ECOLOGICAL IMPACTS 29
dry ice. Samples were analysed within two weeks by the NATA accredited Queensland
Health Analytical Services Laboratory in accordance with the methods of Clesceri et al.
(1989) using a Skalar autoanalyser (Norcross, Georgia, U.S.A.).
For analysis of plant total tissue N, d15N and amino acids, tissue was rinsed in distilled water
to remove nutrients and sediment from the thallus surface, and then prepared for analysis.
For calculation of total tissue N content and the d15N isotopic signature, samples were oven
dried to constant weight (24 h at 60 C), ground and three sub-samples were oxidised in a
Roboprep CN Biological Sample Converter (Europa Tracermass, Crewe, U.K.). The
resultant N2 was analysed by a continuous flow isotope ratio mass spectrometer (Europa
Tracermass, Crewe, U.K.). Total %N of the sample was determined, and the ratio of 15N to
14N was expressed as the relative difference between the sample and a standard (N2 in air)
using the following equation (Peterson & Fry, 1987):
d15N = (15N/14N (sample) / 15N/14N (standard) 1) x 1000 ()
For amino acid analysis, approximately 1.0 g wet weight of plant tissue was weighed and
placed in 5 mL of 100% methanol (analytical reagent grade) for 24 h to extract amino acids.
The methanol extract was filtered through Millipore Millex - HV13 (0.45 mm) filters and
injected into a post column derivatisation HPLC amino acid analyser (Beckman System
6300, Fullerton, California, U.S.A.), for detection of ninhydrin positive free amino acid
groups at 570 nm. Results were calculated and expressed as mol g-1 wet weight. As well as
detecting free amino acids, this technique also measures the concentration of free NH4+ in
plant tissue. Changes in amino acid composition were used to infer nutrient source (either
shrimp or sewage effluent). This technique was based on responses observed under ambient
field, as well as controlled laboratory conditions using artificial nutrient additions (Jones et
al., 1996; Udy & Dennison, 1997a).
-
CHAPTER 2 30
2.2.5 Statistical Analysis
For all sampling techniques, three replicates were analysed and means and standard errors
were calculated. Differences between treatments were tested for significance using one way
analysis of variance (ANOVA) and Tukey's Test for multiple comparison of means at a
significance level of 0.05 using Minitab 12.1 software (State College, Pennsylvania, U.S.A.).
-
ASSESSING ECOLOGICAL IMPACTS 31
2.3 Results
2.3.1 Physical and Chemical Water Quality Analyses
The concentrations of dissolved nutrients, chlorophyll a, phytoplankton productivity, total
suspended solids, volatile suspended solids, and sediment organic content were different
between the sewage and shrimp creek discharge sites. However, these parameters failed to
detect an impact at the midway site (750 m beyond the mouths of the creeks), with values not
significantly higher than at the reference site (4 km from the creek mouths) (Table 2.1).
2.3.1.1 Salinity
All sites in the shrimp creek and in the bay were close to full salinity seawater (35-36). At
the sewage creek discharge, the salinity was 29 as a consequence of freshwater inputs from
the sewage effluent. Salinity increased downstream to 35 at the mouth site.
2.3.1.2 Nutrients
The concentrations of dissolved nutrients (NH4+, NO3
- / NO2- and PO4
3-) for each creek were
highest at the discharge sites, and declined rapidly towards the mouth site. In particular,
NH4+ concentration decreased to near eastern Moreton Bay concentrations at both creek
mouth sites. The concentration of dissolved nutrients at the midway site (750 m from the
creek mouths) was not significantly different (p > 0.05) from the reference site (~4 km from
the creek mouths) (Table 2.1). The dissolved nutrient ratios were significantly different
(p < 0.05) between the two discharge sites. At the sewage creek discharge site, NH4+ : NO3
- /
NO2- was 0.45, compared to 3.8 for the shrimp creek discharge site. The ratio of DIN (NH4
+
+ NO3- / NO2
-) to DIP (PO43-) at the sewage creek discharge site was 3.0, compared to 24 at
the shrimp creek discharge site. The concentrations of dissolved nutrients at the midway and
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CHAPTER 2 32
reference sites were at eastern Moreton Bay levels, and the relative ratios of the dissolved
nutrients were similar to the shrimp creek.
2.3.1.3 Phytoplankton
Chlorophyll a concentration was not significantly different (p > 0.05) between the discharge
site in the shrimp creek (10.8 g L-1) and the discharge site in the sewage creek (11.1 g L-1).
However, at the creek mouth sites the concentration in the sewage creek (5.2 g L-1) was
significantly lower (p < 0.001) than in the shrimp creek (17.9 g L-1). The concentration of
chlorophyll a at the midway site (2.5 g L-1) was not significantly higher (p > 0.05) than the
reference site (1.8 g L-1) (Table 2.1).
Despite the relatively low NH4+ concentration at the shrimp creek discharge site, the
phytoplankton productivity (212 mg C m-3 h-1) was significantly higher (p < 0.001) than at
the sewage creek discharge site (20 mg C m-3 h-1). However, the high productivity at the
shrimp creek discharge site did not result in a significantly higher (p > 0.05) chlorophyll a
concentration. The concentration of chlorophyll a in the shrimp creek increased from
10.8 g L-1 at the discharge site to 17.9 g L-1 at the mouth site, probably due to increased
light availability resulting from the reduction in the concentration of inorganic and other
suspended solids. Phytoplankton productivity at the midway site (9 mg C m-3 h-1) was not
significantly different (p > 0.05) to the reference site (11 mg C m-3 h-1) (Table 2.1).
-
Table 2.1. Results of traditional water quality monitoring for the creek with shrimp farm effluent and sewage treatment effluent. DIN = Dissolved Inorganic Nitrogen;
DIP = Dissolved Inorganic Phosphorus; Chl a = Chlorophyll a; Phyto Prod = phytoplankton productivity; TSS = total suspended solids; VSS = volatile suspended solids;
Secchi = secchi disc depth. Only one replicate measurement was recorded for Secchi disk depth and salinity (Practical Salinity Scale).
Sampling
Site
Salinity
NH4+
(M)
NO3-/NO2
--
(M)
PO43-
(M)
DIN: DIP
Ratio
Chl a
(g L-1)
Phyto Prod
(mg C m-3 h-1)
TSS
(mg L-1)
VSS
(% of TSS)
Secchi
(m)
Sediment
%Organic
Shrimp Discharge 35 3.8a 1a 0.2a 24c 10.8a 212d 63.5a 19a 0.5 6.0a
Shrimp Middle 35.5 1.6a 0.4a 0.3a 7ab 11.9ab 87c 51.5ab 21a 0.7 6.1a
Shrimp Mouth 36 2.3a 0.3a 0.2a 13bc 17.9b 157b 40.2b 26a 0.5 8.3ab
Midway 36 0.8a 0.2a 0.4a 3a 2.5c 9a 18.2d 25a 1.0+ 7.1a
Sewage Discharge 29 29b 65b 31b 3a 11.1ab 20a 44.3b 35b 1.1 13.5c
Sewage Middle 33 5.4a 8a 5.5a 2a 9.2ac 20a 32.9bc 33b 1.0 7.5ab
Sewage Mouth 35 2.4a 2.9a 2.1a 3a 5.2ac 18a 32.5bc 28ab 1.1 5.6a
Reference 36 1.2a 0.5a 0.3a 6ab 1.8c 11a 20.2d 28ab 1.9 11bc
F Value 40*** 15*** 34*** 13*** 15*** 88*** 35*** 14*** 15***
* p < 0.05; ** p < 0.01; *** p < 0.001. abc Means with different letters are significantly different at p < 0.05.
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CHAPTER 2 34
2.3.1.4 Suspended Solids and Secchi Depth
The concentration of total suspended solids (TSS) at the shrimp creek discharge site
(63 mg L-1) was significantly higher than the sewage creek discharge site (44 mg L-1). The
concentrations at the midway (18 mg L-1) and reference sites (22 mg L-1) were not
significantly different (p > 0.05) from each other, but were significantly (p < 0.05) lower than
both creek mouth sites indicating significant sedimentation or dilution (Table 2.1).
The organic fraction of the suspended solids (volatile suspended solids) was significantly
higher (p < 0.001) at the sewage discharge site (35%) compared to the shrimp discharge site
(19%). In the shrimp creek the concentration of organic particles increased towards the
mouth in proportion with the increasing chlorophyll a concentration (r2 = 0.60). In
comparison, the concentration of organic particles in the sewage creek decreased in
proportion with the concentration of chlorophyll a (r2 = 0.8) (Table 2.2).
Secchi disk depths did not vary along the length of either creek, from discharge site to mouth
site. The mean secchi depth in the sewage creek (~1.0 m) was approximately double the
depth in the shrimp creek (~0.6 m), but only half that of the reference site (1.9 m). The
secchi depth at the midway site was greater than 1 m, but water depth was too shallow to
obtain a measurement (Table 2.1).
2.3.1.5 Sediment Organic Content
The organic content of the sediment (loss on ignition) in the shrimp creek increased from
6.0% at the discharge site to 8.3% at the mouth site, probably due to sedimentation. In the
sewage creek, the organic content declined significantly (p < 0.001) from the discharge site
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ASSESSING ECOLOGICAL IMPACTS 35
(13.5%) to the mouth site (5.6%), probably due to senescence and subsequent sedimentation
of phytoplankton and other organic particulates near the discharge site (Table 2.1).
Table 2.2. Correlations (r2) between the concentration of phytoplankton (chlorophyll a) and phytoplankton
productivity (14C uptake) and various water quality parameters. DIN = Dissolved Inorganic Nitrogen; DIP =
Dissolved Inorganic Phosphorus; Phyto Prod = phytoplankton productivity (mg C m-3 h-1); Chl a =
Chlorophyll a (g L-1); TSS = total suspended solids (mg L-1); VSS = volatile suspended solids (mg L-1); ISS =
inorganic suspended solids (mg L-1); Secchi = secchi disc depth (m). Numbers in bold type indicate significant
correlations (r2 0.6).
TSS
(mg L-1)
VSS
(mg L-1)
ISS
(mg L-1)
NH4+
(M)
NO3-/NO2
-
(M)
PO43-
(M)
DIN:
DIP
Salinity
Shrimp Creek
Chl a - 0.85 - 0.60 - 0.87 0.12 0.51 0.14 0.09 0.86
Phyto Prod 0.21 0.48 0.19 0.93 0.56 - 0.81 0.94 0.19
Sewage Creek
Chl a 0.60 0.80 0.37 0.66 0.63 0.66 0.04 - 0.85
Phyto Prod 0.27 0.51 0.11 0.34 0.32 0.35 0.25 - 0.60
2.3.2 Bioindicators
In contrast to the water quality parameters, the responses of the bioindicator parameters at
sites beyond the creek mouths were elevated compared to the reference site. For some of the
parameters, the reference site appeared to be influenced by nutrients from the discharges.
2.3.2.1 Tissue Nitrogen Content
The %N of the macroalgae was responsive to the nutrient sources, with the highest value at
the sewage creek discharge site (3.1%), which was significantly higher (p < 0.05) than the
shrimp discharge site (1.9%) (Table 2.3; Fig. 2.2). There was no decrease in the %N of the
macroalgae with distance from the discharge site in the shrimp creek. However, in the
sewage creek the macroalgae at the mouth site had a %N of 1.6%, which was not
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CHAPTER 2 36
significantly elevated (p > 0.05) above the reference site (1.5%). The %N of the macroalgae
at the midway site (2.3%) was significantly (p < 0.05) elevated above values in the
macroalgae at both of the creek mouth sites (Table 2.3; Fig. 2.2).
The %N of seagrass leaves was significantly higher (p < 0.05) at the sewage creek mouth site
(2.7%) compared with the shrimp creek mouth site (2.3%). The next three sites distant from
the creek mouths (midway, Oyster Point, and Sewage Plume) were not significantly lower
than the shrimp creek mouth site (2.3%). The seagrass %N at the next most distant site (Cox
Bank) was 2.0%, which was not significantly higher (p < 0.05) than at the reference site
(1.7%) (Table 2.3; Fig. 2.2).
The %N of the mangrove leaves appears less sensitive to nutrient inputs, with none of the
mangroves at the other sites being significantly higher (p < 0.05) than the reference site
(1.7%) (Table 2.3; Fig. 2.2).
2.3.2.2 d15N Stable Isotope Ratio of Nitrogen
The d15N isotopic signatures of the seagrass, macroalgae and mangroves were significantly
different (p < 0.001) between sites (Table 2.3; Fig. 2.3). The highest d15N was in the
macroalgae at the sewage creek discharge site (19.6), and the lowest in the macroalgae at
the reference site (2.9). d15N in the macroalgae in the sewage creek decreased with
distance away from the source. The value at the discharge site in the shrimp creek was 7.1,
with no significant difference (p > 0.05) along the length of the shrimp creek to the mouth
(7.9). The d15N at the midway site (6.4) was not significantly lower (p > 0.05) than the
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ASSESSING ECOLOGICAL IMPACTS 37
creek mouth sites, indicating influence of nutrients from the discharges. The d15N at the
reference site (2.9) was significantly lower (p < 0.001) than all other sites.
The d15N of seagrass leaves at the sewage plume site (8.0) was significantly higher
(p < 0.05) than all other sites. The d15N was not significantly different (p > 0.05) between the
two creek mouths (7.1 and 6.8), but both were significantly higher (p < 0.05) than the
midway (5.8), Oyster Point (4.7) and Cox Bank (5.5) sites, and the reference site
(4.5) (Table 2.3; Fig. 2.3).
The highest d15N of mangrove leaves was 10.4 at the sewage discharge site, compared with
7.7 at the shrimp discharge site. Despite the significant differences (p < 0.05) at the
source, the d15N values at the creek mouths were not significantly different (p > 0.05) from
each other (4.9 and 4.6). The mid