of queensland an assessment of the ecological health of...
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An assessment of the ecological health
of Eprapah Creek
FINAL
Data Report
submitted to
Kinhill Ltd
by
Marine Botany
University of Queensland
Adrian B. Jones BSc (Hons) PhD
Joelle Prange BSc (Hons)
William C. Dennison BA MS PhD
November 1999
Marine BotanyMarine Botany
TTHE HE UUNIVERSITYNIVERSITYOOF F QQUEENSLANDUEENSLAND
2
TABLE OF CONTENTS
Executive Summary 3
Introduction 4
Materials and Methods 5
Study Region 5
Water Quality Procedures 6
Sediment Nutrient Fluxes 6
Phytoplankton Bioassays 6
Plant Tissue %N and δ15N 7
Results 9
Physical Water and Sediment Quality Analyses 9
Salinity 9
Dissolved Oxygen 9
pH 9
Secchi Depth 9
Sediment Nutrient Fluxes 10
Bioindicators 11
Phytoplankton Bioassays 11
Tissue Nitrogen Content 12
δ15N Stable Isotope Ratio of Nitrogen 12
Discussion 15
Physical Water and Sediment Quality Analyses 15
Water Quality 15
Sediment Nutrient Fluxes 15
Bioindicators 16
Phytoplankton Bioassays 16
Tissue %Nitrogen Content 16
δ15N Stable Isotope Ratio of Nitrogen 17
Conclusions & Recommendations 19
References 21
3
Executive Summary
The ecological health of a small tidal creek, Eprapah Creek, flowing into Moreton Bay was
examined in Sept. 1999 using water quality analyses, phytoplankton bioassays, sewage plume
mapping using stable isotopic signatures and sediment nutrient flux measurements. Results
were compared with a previous survey conducted in April-May 1997 as part of a University
of Queensland PhD thesis (A. Jones). The ecological health of Eprapah Creek appears to be
compromised with sewage-derived nutrients. Of greater concern is the likelihood of a system
that is actively degrading, with several indications that the ability of the creek to assimilate
nutrients has been reduced since 1997. The ecological health indicators indicating degraded
conditions include high sediment nutrient efflux, low rates of sediment denitrification, high
nutrient stimulation in phytoplankton bioassays and a strong sewage signature in marine
plants. The overall recommendation regarding sewage treatment upgrades is to enhance the
nutrient removal capacity of the treatment processes.
4
Introduction
Eprapah Creek extends from Mount Cotton, through residential areas in its lower reaches,
before discharging into Moreton Bay north of Victoria Point. The creek is approximately 2-
5 m deep, 15 km in length, and has a standing body of water at low tide. It receives point
source 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) (Redland Shire Council, pers. comm.) from the Victoria
Point sewage treatment plant, located ~2.6 km from the mouth. The sewage treatment plant
services approximately 14 000 people and utilises secondary (activated sludge) treatment
techniques. The plant will be upgraded to service 42 000 people by 2002. Water quality
monitoring by the Redlands Shire Council over the last few years has identified Eprapah
Creek as a waterway with consistently poor water quality (with many parameters outside
ANZECC guidelines), especially in the vicinity of the sewage treatment plant.
The primary aim of this project was to conduct a comprehensive survey of biological assays
and water column and sediment nutrient parameters to determine the ecological health prior
to a planned increase in the output of the sewage treatment plant. From these pre-upgrade
surveys a monitoring program will be developed to monitor the changes in ecological health
of the creek as a result of the increase in sewage discharge. Additionally, the results from
this study are compared with those obtained in 1997 surveys conducted by Jones (1999), to
determine changes to ecological health in Eprapah Creek over the last two years. Based on
these analyses, recommendations are made regarding the level of sewage upgrade to maintain
existing ecological health.
5
Materials and Methods
Study Region
Four sites were chosen within Eprapah Creek and one located near Coochiemudlo Island
(27.56255 ºS, 153.33009 ºE) as a reference site. An upstream site was situated towards the
tidal limit (AMTD 3.8 km) of the creek (AMTD ~3 km; 27.58205 ºS, 153.28548 ºE) which is
approximately 0.5 km upstream from the sewage treatment plant (STP). A site was situated
at the STP outlet (AMTD ~2.6 km; 27.58163 ºS, 153.28986 ºE). Additional sites were
located midway (AMTD ~1 km; 27.57781 ºS, 153.29242 ºE) between the STP and the mouth
of the creek and at the creek mouth (AMTD 0 km; 27.56512 ºS, 153.28967 ºE) (Fig. 1).
These sites were chosen to correlate with existing sites sampled by Redlands Shire Council
and the sites used by Jones (1999) in 1997. Sampling took place on the 13th September, 1999
during the ebbing tide.
Brisbane
MoretonBay
•
⊗
0 0.5 1.0
kilometres
N
Eprapah Ck
SewageTreatment
Plant
OysterPoint
VictoriaPoint
PointHalloran
Coochie-mudloIsland
Coochiemudlo Site Mouth Site
Mid Site
STP SiteUpstream Site
Figure 1 Eprapah Creek sampling sites for ecological health monitoring.
6
Water Quality Procedures
Salinity (expressed on the Practical Salinity Scale1), pH and dissolved oxygen were measured
with a Horiba U-10 water quality meter (California, U.S.A.).
Secchi depth was determined by lowering a 30 cm diameter secchi disk (black and white
alternating quarters) through the water column until it was no longer possible to distinguish
between the black and white sections.
Sediment Nutrient Fluxes
At the mid and mouth sites (sediment substrate was too rocky to core at other sites), four
replicate sediment cores (2.4 L perspex cores) were sampled to a depth of 10 cm and sealed
with a PVC endcap, trapping the overlying site water in the core to minimise sediment
mixing. Cores were transported back to the laboratory the overlying water was removed and
replaced with filtered water from a low nutrient, oceanic influenced site off North Stradbroke
Island. The cores were incubated in a water bath at room temperature with negligible
ambient light for 24 hours. Water samples were collected initially and after 1, 3, 6, 12 and 24
hours. At each sampling interval, 50 mls of water was collected from each core and filtered
through a 0.45 µm Millex GV Millipore filter unit. Samples were analysed for dissolved
nutrients, nitrogen (NH4+), nitrogen oxides (NO3
-, NO2-) and phosphorus (PO4
3-) 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.).
Flux rates were calculated as µmol m-2 h-1 of NH4+, NO3
-+NO2- and PO4
3-.
Phytoplankton Bioassays
Phytoplankton bioassays were conducted with ambient phytoplankton assemblages collected
from four sites in Eprapah Creek and Moreton Bay (Fig. 1). One 30 L drum of water was
collected from each site, kept cool and shaded, and returned to an outdoor incubation facility.
Four litres of water from each site was filtered through a 200 µm mesh (to screen out the
larger zooplankton grazers) into sealed transparent 6 L plastic containers and placed in
incubation tanks filled with water (2 m diameter, 0.5 m deep). Temperature was maintained
1 Practical salinity (S) is the ratio of the conductivity of a sample of seawater at 15 ºC compared to that of a
defined potassium chloride (KCl) solution. Seawater with a practical salinity of 35 will have the same
conductivity as a solution of 32.4356 g of KCL in 1 kg of water.
7
at ±2°C of the ambient water temperature by flowing water through the tanks and light levels
were maintained at 50% of incident irradiance with neutral density screening. For each site
there were six bioassay containers, each with a different nutrient treatment. Samples were
spiked to make the following concentrations: NO3- (200 µM); NH4
+ (30 µM); PO43- (20 µM);
SiO32+ (66 µM); all nutrients at those concentrations (+All); and a control (no nutrient
addition). The concentrations were chosen, as they are known to be saturating for
phytoplankton in estuarine environments. At identical daily circadian times, all bioassay
bags were gently shaken and 20 mL from each container was poured into pre-rinsed 30 mL
glass test tubes and placed in darkness for 20 minutes to allow photosystems to dark adapt.
Chlorophyll a concentrations were determined from in vivo fluorescence (indicating
phytoplankton biomass) on a Turner Designs Fluorometer. An initial measure (time = 0) was
taken on the control treatment and then for all treatments daily for 7 days.
Over the 7-day period settlement of suspended solids within samples may occur and light
availability increase above ambient levels. The response of the plankton community in the
control bioassay container gives an indication of the ambient light conditions. Light
stimulated phytoplankton bloom potential was calculated as the difference between initial
(time = 0) and maximum in vivo fluorescence values in the control water sample over the 7 d
incubation. Nutrient stimulated bloom potential was calculated as the difference between the
maximum response in the nutrient treatments and the maximum response in the control
(referred to as the stimulation factor). This stimulation factor can be used to determine the
relative importance of the different nutrient additions compared with light.
Plant Tissue %N and δ15N
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, δ15N. 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. Two replicates for each plant type were
collected at each site.
Samples were oven dried to constant weight (24 h at 60 °C), ground and two sub-samples
were oxidised in a Roboprep CN Biological Sample Converter (Europa Tracermass, Crewe,
8
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):
δ15N = (15N/14N (sample) / 15N/14N (standard) – 1) x 1000 (‰)
9
Results
Physical Water and Sediment Quality Analyses
Salinity
There was a defined salt wedge within Eprapah Creek, salinity at the upstream site ranged
from 0.5 at the surface to 16.4 at 1 m and 19.3 at 2 m. At the sewage treatment plant site, the
salinity ranged from 11.8 at the surface to 24.2 at 2 m. Salinity at the midway site ranged
from 14.6 to 31.5 and at the mouth site, from 28 to 32.1. At Coochiemudlo Island, the
stratification was no longer present, with a constant 35 throughout the water column
(Table 1).
Dissolved Oxygen
The dissolved oxygen within the creek was depressed relative to the mouth and
Coochiemudlo sites. There was also some stratification present at the upstream site, with
very low concentrations (5.5 mg L-1) at 1 m depth (Table 1).
pH
There was a gradient in pH along the study transect, ranging from 6.1 at the upstream site to
8.3 at Coochiemudlo, although there was little depth stratification at each site (Table 1).
Secchi Depth
The Secchi disk depth increased downstream from 0.3 m at the upstream site, 0.85 m at the
STP site, 0.7 m at the midway site and 1.1 m at the mouth to 1.9 m at Coochiemudlo Island.
Table 1 Water quality parameters. Readings were taken at the surface, 1 m and 2 m depth.
Site Salinity DO (mg L-1) pH Secchi (m)
0 m 1 m 2 m 0 m 1 m 2 m 0 m 1 m 2 m
Upstream 0.5 16.4 19.3 7.3 5.5 5.9 6.6 6.1 6.7 0.3
STP 11.8 19.9 24.2 6.7 7.3 7.4 6.8 6.8 7.7 0.85
Mid 14.6 22.5 31.5 7.5 7.6 8.6 7.4 7.6 8.0 0.7
Mouth 28 31 32.1 9.3 10.1 9.9 8.0 8.1 8.2 1.1
Coochiemudlo 35 35 35 10.1 10.5 10.5 8.3 8.3 8.3 1.9
10
Sediment Nutrient Fluxes
The concentration of NH4+ in the water column increased throughout the 24 hour period in
the cores from both the mid and mouth sites, indicating nutrient release from the sediment.
The rate of NH4+ flux at the mid site (55.3 µmol m-2 h-1) was higher than at the mouth site
(35.9 µmol m-2 h-1) (Fig. 2A; Table 2). There was no significant change in the concentration
of NO3 in the water column (Fig. 2B & Table 2). Phosphate flux was positive at both sites,
with 10.8 µmol m-2 h-1 at the mid site and 4.4 µmol m-2 h-1 at the mouth site (Fig. 2C &
Table 2).
Figure 2 Sediment nutrient (NH4-, NO3
- & PO43-) fluxes at the two sites.
Table 2 Flux rates of each site over the 24 hour period.
Ammonium Nitrate Phosphate
Flux Rate (µmol m-2 h-1)
Mid site 55.33 -2.92 10.82
Mouth Site 35.92 1.46 4.43
N-N
O3-
(µm
ol m
-2)
N-N
H4+
(µm
ol m
-2)
P-P
O4
3-(µ
mol
m-2
)
Mid Mouth
0
600
1200
0
600
1200
0
600
1200
0 6 12 18 24
A
B
C
Time (h)
11
Bioindicators
Phytoplankton Bioassays
There was a strong light stimulation response (13.1) observed at the STP site, with all
treatments, including the control rapidly increasing in biomass. At the mid site there was no
significant increase in any treatments, whereas at the mouth site we observed a strong co-
limitation (+All response) (10.4) with secondary stimulation by both NH4+ (4.7) and NO3
-
(5.5) species of N. The phytoplankton community from the Coochiemudlo site had a strong
stimulation factor in response NH4+ addition (13.4), but the magnitude of the response was
negligible compared with the light response at the STP site and the nutrient response at the
mouth site (Fig. 3; Table 3).
Flu
ores
cenc
e (
FS
U) Control
NO3
NH4
PO4
SiO3
All
Incubation Time (d)
STP outlet
0
70
140
210
Eprapah Creek Mid Site
0
70
140
210
Eprapah Creek Mouth
0
70
140
210
Coochiemudlo Island
0
70
140
210
0 1 2 3 4 5 6 7
Figure 3 Phytoplankton bioassay responses at the 4 sites.
12
Table 3 Phytoplankton Bioassay Stimulation Factor. Light = maximum control response / initial.
Nutrient = maximum nutrient response / maximum control response.
Site Light Nutrient response
Response NO3- NH4
+ PO43- SiO3 All
STP 10.1 1.0 0.9 1.0 0.8 1.0
Mid 0.9 1.7 1.0 1.0 1.1 1.0
Mouth 4.4 5.5 4.7 1.3 1.4 10.4
Coochie 1.2 6.1 13.4 0.8 1.5 1.2
Tissue Nitrogen Content
The %N of the macroalgae was the most responsive to nutrient sources, with the lowest value
being 1.4% at Coochiemudlo Island and the highest value at the sewage treatment plant site
(3.9%). The %N of the macroalgae at the upstream site (3.1%) was lower than at the sewage
treatment plant site. The %N of seagrass leaves was higher at the mouth site (2.0%)
compared with the Coochiemudlo site (0.8%). The %N of the mangrove leaves showed the
opposite trend, with the lowest %N (1.1%) being at the sewage treatment plant site, and the
highest (2.2%) at the mouth and Coochiemudlo sites (Fig. 4).
δ15N Stable Isotope Ratio of Nitrogen
The δ15N isotopic signatures of the seagrass, macroalgae and mangroves were significantly
different between sites (Fig. 5). The highest δ15N in the macroalgae was at the sewage
treatment plant site (17.4‰), and the lowest at the Coochiemudlo site (3.4‰). δ15N values
for the macroalgae decreased upstream and downstream from the sewage treatment plant to
14.6‰ at the upstream site and 10.8‰ at the mouth site.
13
Brisbane
MoretonBay
•
⊗
0 0.5 1.0
kilometres
N
Eprapah Ck
SewageTreatment
Plant
OysterPoint
VictoriaPoint
PointHalloran
Coochie-mudloIsland
3.2
3.9
1.42.00.82.0
1.7
1.1
2.22.2
Seagrass
Macroalgae
Mangrove
3.11.9
Figure 4 Map showing the values of %N in seagrass (Zostera capricorni), macroalgae
(Catenella nipae), and mangroves (Avicennia marina) at the study sites (see Fig. 1 for site
references).
The δ15N of the mangroves at the upstream (12.4‰) and sewage treatment plant sites
(15.1‰) are similar in magnitude to the macroalgae at these sites. However, at the mid
(10.8‰), mouth (5.6‰) and Coochiemudlo (2.3‰) sites, the δ15N of the mangroves was
considerably lower than those of the macroalgae (Fig. 5). This suggests that sediments at the
upstream and sewage sites contain nutrients from the sewage treatment discharge, whereas at
the downstream sites, the only source of these nutrients is the water column.
14
The δ15N of seagrass leaves at the mouth (5.6‰) and Coochiemudlo (2.6‰) sites are similar
to the mangrove values (Fig. 5). The relative similarity between the mangrove and seagrass
values is likely due to their ability to absorb nutrients from sediments, compared with the
macroalgae, which absorb water column nutrients exclusively.
Brisbane
MoretonBay
•
⊗
0 0.5 1.0
kilometres
N
Eprapah Ck
SewageTreatment
Plant
OysterPoint
VictoriaPoint
PointHalloran
Coochie-mudloIsland
16.9
17.4
3.410.82.66.8
10.8
15.1
2.35.6
Seagrass
Macroalgae
Mangrove
14.612.4
Figure 5 Map showing the values of δ15N in seagrass (Zostera capricorni), macroalgae (Catenella nipae), and
mangroves (Avicennia marina) at the study sites (see Fig. 1 for site references).
15
Discussion
Physical Water and Sediment Quality Analyses
Water Quality
There was a strong gradient in water quality within Eprapah Creek from the reaches near the
tidal limit to the mouth. The water column was highly stratified due to the presence of a
strong salt wedge. Low concentrations of dissolved oxygen within the creek indicate a high
organic load in the system, which is consistent with the high proportion of organic
particulates (35%) in the water column (Jones, 1999).
Sediment Nutrient Fluxes
Sediment nutrient fluxes indicate processes occurring in the sediments and at the sediment-
water interface, including microbial processing. The positive flux of PO43- suggests that the
sediments are anaerobic, because in aerobic sediments PO43- usually remains bound to iron
(FePO4). Enhanced sulfate reduction in anaerobic sediments with a high organic loading
results in the formation of iron-sulfide mineral complexes (eg FeS and FeS2), which breaks
down the FePO43- complexes (Chambers & Odum, 1990; Roden & Edmonds, 1997). The
bacterially mediated process of denitrification in sediments requires nitrification of NH4+ to
NO3-. This nitrification can only occur in the presence of O2, which appears to be limited,
based on the PO43- flux measurements. The high rates of NH4
+ flux and concomitant lack of
NO3- flux confirm the lack of nitrification in the sediments. This lack of nitrification along
with the high rates of NH4+ flux indicates that denitrification at the sites downstream of the
sewage treatment plant is not able to effectively process the NH4+ in the sediment, suggesting
degraded nutrient processing capacity within the ecosystem. The hard, rocky substrate at the
sewage treatment plant site and Coochiemudlo site indicate no substantial accumulation of
nutrient laden sediments, and therefore are not likely to have significant rates of nutrient flux.
The rates of NH4+ flux from the sediments at the mid (55 µmol m-2 h-1) and mouth (36 µmol
m-2 h-1) sites are high in comparison to a non nutrient impacted site at Amity Banks (16 µmol
m-2 h-1) in eastern Moreton Bay (Watkinson et al., 1998). However, the rates are
considerably lower than those found in a highly impacted site in Tingalpa Creek (700 µmol
m-2 h-1) (Watkinson et al., 1998).
16
Bioindicators
Biological and physical parameters measured in Eprapah Creek water, sediment and plant
material indicate that the sewage wastewater is influencing both biological and chemical
processes occurring both downstream and upstream (to the tidal limit) of the discharge site.
Phytoplankton Bioassays
Phytoplankton bioassays are an indication of potential phytoplankton response to increased
nutrients and light. The observed phytoplankton responses indicate the potential for large
blooms of phytoplankton at the mouth of Eprapah Creek if ambient water column nutrient
concentrations became elevated in response to increasing the nutrients discharged from the
sewage treatment plant. The high turbidity in the creek, evidenced by the shallow secchi
depth will likely prevent any rapid increases in phytoplankton biomass within the creek,
confirmed by the light response at the STP site.
Tissue %Nitrogen Content
The tissue N content (%N) of marine plants is a potential indicator of biologically available
nutrient concentrations (Gerloff & Krombholz, 1966; Duarte, 1990), especially in macroalgae
(Horrocks et al., 1995) which have the ability to store large reserves of “luxury” nitrogen for
metabolism during times of nutrient stress. The highly elevated %N of the macroalgae within
the creek indicates the high availability of nitrogen within the system. Compared with data
collected in 1997 (Jones, 1999) (Fig. 6) the concentration of biologically available nutrients
has increased, most notably at the mid site with a value of 3.2% in the Catenella in the
present study being significantly higher (p < 0.001) than 1.7% in the study in 1997. The
values for the Catenella at the mouth were also significantly elevated (p < 0.05) from 1.6%
(1997) to 2% (1999). These data suggest that the system may have changed in the last two
years and is less able to process present nitrogen inputs.
The depressed %N of mangroves at sites close to the sewage discharge may indicate stress to
the plant reducing their ability to take up nutrients. The highly elevated δ15N of the
mangroves indicates the high availability of sewage derived N, and the high %N of the
macroalgae indicates the nitrogen is biologically available.
17
δ15N Stable Isotope Ratio of Nitrogen
Stable isotope ratios of nitrogen (δ15N) have been used widely in marine systems as tracers of
discharged nitrogen from point and diffuse sources, including sewage effluent (Rau et al.,
1981; Heaton, 1986; Wada et al., 1987; Van Dover et al., 1992; Macko & Ostrom, 1994;
Cifuentes et al., 1996; McClelland & Valiela, 1998). Plant δ15N signatures have been used to
identify nitrogen sources available for plant uptake (Heaton, 1986). Elevated δ15N 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, 1997; Abal
et al., 1998). The δ15N of raw sewage and treated sewage particulates discharging into
Moreton Bay is around 5.1‰ and 9.2‰, respectively (Loneragan et al., in review). The
elevated δ15N signature subsequent to treatment of the sewage effluent is a result of isotopic
fractionation during ammonia volatilisation, nitrification and denitrification (McClelland &
Valiela, 1998).
Brisbane
MoretonBay
•
⊗
0 0.5 1.0
kilometres
N
Eprapah Ck
SewageTreatment
Plant
OysterPoint
VictoriaPoint
PointHalloran
Coochie-mudloIsland
1.7
3.1
1.51.61.72.7
1.4
1.7
1.71.3
Seagrass
Macroalgae
Mangrove
n.d.
Figure 6 Map showing the values of %N in seagrass (Zostera capricorni), macroalgae (Catenella nipae), and
mangroves (Avicennia marina) at the study sites for 1997 (adapted from Jones, 1999).
18
Mangroves and macroalgae in the creek were highly enriched with sewage nitrogen
(indicated by high δ15N), as was seagrass at the creek mouth. The δ15N value of macroalgae
at the mouth site (10.8‰) was significantly higher (p < 0.05) than recorded in 1997 (Jones,
1999) (Fig. 7), which is consistent with the %N data, demonstrating the reduced processing
of nutrients by biota within the creek. The δ15N of the mangroves at the sewage treatment
plant site was also significantly higher (p < 0.001) than recorded in 1997 (Jones, 1999)
(Fig. 7), indicating an increase in the availability of sewage derived nutrients in the sediments
at this site. This may be related a breakdown in denitrification, which is evidenced by the
sediment nutrient fluxes conducted at the mid site.
The enrichment of the δ15N of NH4+ in estuaries is mediated predominantly by nitrification of
NH4+ (Mariotti et al., 1984; Cifuentes et al., 1989; Fogel & Cifuentes, 1993). The δ15N
values recorded for the macroalgae at the sewage creek discharge site are among the highest
reported in the literature (Owens, 1987). Given the high concentrations of NO3- / NO2
- in
Eprapah Creek (Jones, 1999), nitrification probably accounts for the high δ15N observed in
the mangroves and macroalgae, which would predominantly take up the isotopically heavy
NH4+, in preference to NO3
- / NO2- (Hanisak, 1983).
Mangroves obtain their nutrients from the sediment, and as such may have sufficient nutrients
available so that they can preferentially take up isotopically light 14N, whereas the
macroalgae may be N limited and will therefore take up the both 14N and the heavier 15N
isotope (Wada, 1980). Uptake of nutrients from bacterial nitrogen fixation in sediments may
also provide a N source with a much less enriched signature (Hicks & Silvester, 1985). In
highly eutrophic systems nitrogen fixation is usually inhibited, therefore reducing this
potential N source. The δ15N of the mangroves in the creek 1997 (Jones, 1999) were always
lower than the macroalgae, however, in the present study this difference was less marked at
the upstream and sewage treatment plant sites, indicating an increased availability of sewage
derived N in the sediments.
19
Brisbane
MoretonBay
•
⊗
0 0.5 1.0
kilometres
N
Eprapah Ck
SewageTreatment
Plant
OysterPoint
VictoriaPoint
PointHalloran
Coochie-mudloIsland
16.3
19.6
2.96.44.56.8
9.4
10.4
3.44.9
Seagrass
Macroalgae
Mangrove
n.d.
Figure 7 Map showing the values of δ15N in seagrass (Zostera capricorni), macroalgae (Catenella nipae), and
mangroves (Avicennia marina) at the study sites for 1997 (adapted from Jones, 1999).
Conclusions & Recommendations
At the mouth of the creek there were high sediment nutrient flux rates, low rates of predicted
denitrification, together with a high %N and δ15N of the macroalgae and the high nutrient
stimulated phytoplankton bloom potential. At the sewage, mid and mouth sites there has
been an increase in the %N of the macroalgae and the increase in the δ15N and decrease in the
%N of the mangroves. These factors indicate a degrading ecosystem, potentially unable to
effectively assimilate sewage nutrients.
20
Conclusion:
Ecological health in Eprapah Creek, assessed using biological indicators suggests that within
the last two years increased nutrient levels may be surpassing the sustainable threshold of the
ecosystem.
Recommendation:
The proposed upgrade of the Victoria Point Sewage Treatment Plant from servicing 14 000 to
42 000 people could potentially represent a 3 fold increase in nutrient loads. Considering the
observed decline in ecological health over the past two years, an increase in the volume
discharged from the Victoria Point Sewage Treatment Plant should include an upgrade to the
treatment process to 3N: 1P. This would ensure a minimal increase in the total N and P
loadings to the system.
Conclusion:
The existing monitoring program conducted by the Redlands Shire Council has detected
degraded water quality within the tidal reaches of the creek (many parameters outside
ANZECC guideline limits). In contrast, most parameters at the mouth of the creek are within
ANZECC guideline limits, however, the biological techniques used in the present study
suggest that the ecosystem at the mouth is degraded.
Recommendation:
To assess future changes in ecological health, further biological sampling should be
conducted following the upgrade of the sewage treatment facilities to monitor changes in the
ecosystem. These sampling programs should include phytoplankton bioassays, plant tissue
%N and δ15N analyses and sediment parameters such as nutrient fluxes or nutrient content
(totals, porewater or sorbed). Additional sites in the river plume and a reference site such as
Coochiemudlo Island should also be incorporated into the monitoring program to determine
long term changes in the region of influence from the sewage discharge.
21
References
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