of queensland an assessment of the ecological health of...

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


Bioindicators 11


Tissue Nitrogen Content 12

δ15N Stable Isotope Ratio of Nitrogen 12

Discussion 15

Physical Water and Sediment Quality Analyses 15

Water Quality 15


Bioindicators 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


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


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 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 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

Abal, E.G., Holloway, K.M. & Dennison, W.C. (eds.) (1998) Interim Stage 2 Scientific

Report. Brisbane River and Moreton Bay Wastewater Management Study, Brisbane.

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