An Assessment of the Ecological Health of Logan River and Southern Moreton Bay

Transcription

1 An Assessment of the Ecological Health of Logan River and Southern Moreton Bay Data Report submitted to Logan Coomera and Southern Moreton Bay Wastewater Management Strategy by Marine Botany University of Queensland Adrian B. Jones BSc (Hons) PhD Mark O Donohue BSc MSc Simon Costanzo BSc (Hons) William C. Dennison BA MS PhD November 1999

2 Background The University of Queensland in collaboration with EPA, CSIRO and Southern Cross University were contracted to the Brisbane River and Moreton Bay Wastewater Management Study to identify and assess ecological health criteria in central and northern Moreton Bay and associated rivers. This study was initiated to involve the Logan Coomera and South Moreton Bay Wastewater Management Strategy (LCSMBWMS) with the Development and Implementation of Baseline Monitoring (DIBM) project coordinated by The University of Queensland. The DIBM proposal coordinated by The University of Queensland provided a key team of experts who developed and implemented a monitoring program addressing traditional sediment and water quality and biological assessment, for a statistically rigorous spatial assessment of ecological health. By involving the Logan River and southern Moreton Bay within the framework of the DIBM proposal, a greater recognition of the relative health of this region within south-east Queensland was achieved. Introduction The ecological health of the Logan River and Southern Moreton Bay was examined during as part of the Design and Implementation of Baseline Monitoring (DIBM) task within the Brisbane River and Moreton Bay Wastewater Management Study (BRMBWMS). Southern Moreton Bay (defined as the region from Peel Island to the Jumpinpin bar) contains a broad diversity of ecosystems indicating east-west gradients in water quality. It contains a complex formation of channels and islands supporting extensive mangroves and some seagrasses. The region receives considerable inputs of nutrients and suspended solids from the Logan River estuary, which is dominated by urban and agricultural catchment inputs, with prawn farm effluent and acid sulfate soil run-off at the mouth. Materials and Methods A total of 31 sites were chosen within the Logan River (within the tidal reaches) and southern Moreton Bay (from Coochiemudlo Island to Behms Creek). Sites were positioned approximately every 2 km along the river and in a grid out from the mouth of the river. Intensive sampling covering all these sites was conducted during October 1997 and March 1998 and less intensive sampling was conducted at 10 sites within southern Moreton Bay monthly from October 1997 to October Refer to the DIBM final report (Dennison & 2

3 Team, 1998) for a more detailed synopsis of the sampling regimes (sites and replicates) undertaken for all parameters measured. Water Quality Procedures Salinity (expressed on the Practical Salinity Scale 1 ), 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. Chlorophyll a concentrations were used as an indicator of phytoplankton biomass. At each site, chlorophyll a concentration was determined by filtering a known volume of water through a Whatman GF/F filter which was immediately frozen. In the lab, the filter was ground in acetone to extract chlorophyll a, spectral extinction coefficients were determined on a spectrophotometer and chlorophyll a concentrations calculated according to Parsons et al. (1989) 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 ºC; 24 h) Whatman GF/C glass fibre filter. The filter was then oven dried at 60 ºC for 24 h and total suspended solids calculated by comparing the initial and final weights (Clesceri et al., 1989). Unfiltered samples for nutrient analysis (total Kjeldahl nitrogen and total phosphorus) were collected in 120 ml polycarbonate immediately frozen. They were subsequently 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.). 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 g of KCL in 1 kg of water. 3

4 Dissolved inorganic nutrients (NH + 4, NO - 3 /NO - 2, and PO 3-4 ) were determined by filtering water samples through Sartorius Minisart 0.45 µm membrane filters and freezing them immediately on 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.). Phytoplankton Bioassays Phytoplankton bioassays were conducted with ambient phytoplankton assemblages at 16 sites. 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 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: NO - 3 (200 µm); NH + 4 (30 µm); PO 3-4 (20 µm); SiO 2+ 3 (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 4

5 (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. Sewage Plume Mapping Catenella nipae (δ 15 N ~2 ) was collected from the eastern side of Moreton Bay and incubated in transparent, perforated chambers and suspended in the water column for 4 days at ~50% light (secchi disk/2) using a combination of buoy, rope and weights (secchi depth varied m) form. Following deployment of macroalgae, samples were analysed for δ 15 N. Samples were oven dried to constant weight (24 h at 60 C), ground and two subsamples were oxidised in a Roboprep CN Biological Sample Converter (Europa Tracermass, Crewe, U.K.). The resultant N 2 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 15 N to 14 N was expressed as the relative difference between the sample and a standard (N 2 in air) using the following equation (Peterson & Fry, 1987): δ 15 N = ( 15 N/ 14 N (sample) / 15 N/ 14 N (standard) 1) x 1000 ( ) Results The highest chlorophyll a (Fig. 7), total suspended solids (Fig. 8), dissolved and total nutrients (Fig. 1-6) in the Logan River are in the middle reaches of the estuary, declining towards the mouth due to tidal dilution and in-stream nutrient processing (evidenced by mixing plots (Fig. 13)). This is in contrast to the Brisbane and Pine Rivers where there is no biological removal of nitrogen and the nutrients are transported directly into Moreton Bay. The average secchi depth in the western regions of Southern Moreton Bay and in the Logan River was 0.75 m (Fig. 9), consistent with the relatively high suspended solids (Fig. 8). However, this is considerably less turbid than the Brisbane River (0.25 m secchi). In the eastern region of Southern Moreton Bay, near Pelican Banks, the average secchi depth is 1.5 m, which is considerably shallower than on the eastern side of central and northern Moreton Bay (4.0+ m), which is mostly a function of the limited flushing through Jumpinpin (Dennison & Abal, 1999). Consequently, the phytoplankton community, which is dominated by diatoms, is frequently light limited. During dry weather the Logan River is poorly flushed 5

6 and the plume diverges with some going north and some south, and the extent of the plume largely confined to the channels between the islands of Southern Moreton Bay. Wind and tides drive sediment resuspension leading to seagrass loss and recovery. Seagrass loss was observed from in the vicinity of the Logan River mouth, linked to decreased light availability from the high turbidity in the region (Fig. 15). Possible changes to the hydrodynamic / circulation patterns (linked to changes in the Jumpinpin Bar after the May 1996 flood) in the area may have resulted in an observed recovery of seagrasses. Sewage plume mapping techniques using the δ 15 N isotopic signature of incubated macroalgae revealed a small, but significant plume from the mouth of the river. The sphere of influence from this plume is considerably smaller than from the Brisbane and Pine Rivers (Fig. 11). There were also very little seasonal differences in the extent of the plume, compared with the Brisbane and Pine Rivers, which had considerably larger plume in March 1998 (Fig. 11) than in September 1997 (Fig. 12). Phytoplankton Bioassays showed light responses in the middle reaches of the Logan River (near the STP) through to primary nitrogen limitation and co-limitation of nutrients towards the mouth and into the bay (Fig. 13). The annual runoff of nutrients and suspended solids from the catchment (p ) and atmospheric inputs (p ) are reported in the Moreton Bay Study book (Dennison & Abal, 1999). Interpretation The ecological health of the Logan River estuary ranged from moderately to highly impacted. Water column nutrients were generally high upstream, decreasing in concentration towards the mouth. In contrast to the Brisbane River, non-conservative mixing plots suggest that some in-stream processing of nutrients has been maintained. There is likely a surface layer on the sediments which is aerobic and facilitating some denitrification. However, the high concentrations of NO - 3 in the water column suggest that like the other river estuaries, overall denitrification efficiency is likely to be low. Refer to Figure 16 for a conceptualisation of the processes occurring within the region. 6

7 The ecological health of Southern Moreton Bay ranged from highly impacted to relatively pristine. It was observed to have relatively low inputs of nutrients and sediments, predominantly arising from the Logan River. Relatively low nutrient concentrations were observed and sediment nutrient processes were largely intact. Nitrification and denitrification occurred within the sediments reducing the flux of ammonium from the sediments within the channels and eastern embayments. The presence of N fixation in the seagrass beds at Pelican Banks is indicative of a healthy ecosystem. Refer to Figure 17 for a conceptualisation of the processes occurring within the region. Outcomes The outcomes of the Design and Implementation of Baseline Monitoring (DIBM) task has been the development of the Ecological Health Monitoring Program (EHMP) (Team, 1989) which has funding for the next three years from all the councils and major point source dischargers into the catchment. In addition to traditional water quality parameters (dissolved and total nutrients, chlorophyll a, total suspended solids, secchi depth), biological indicators (phytoplankton bioassays, seagrass depth range, sewage plume mapping δ 15 N) will be used to assess ecological health. In the process of formulating this monitoring program, DIBM developed and refined a number of biological assay techniques and GIS mapping techniques to enable statistically valid interpretations and spatial presentation of ecological health. 7

8 References Clesceri, L.S., Greenberg, A.E. & Trussel, R.R. (1989) Standard methods for the examination of water and wastewater. American Public Health Association, New York. Dennison, W.C. & Abal, E.G. (1999) Moreton Bay Study: A Scientific Basis for the Healthy Waterways Campaign. South East Queensland Regional Water Quality Management Strategy Team, Brisbane. Dennison, W.C. & DIBM Team (1998) Design and Implementation of Baseline Monitoring. Final Report. Brisbane. Parsons, T.R., Maita, Y. & Lalli, C.M. (1989) A Manual of Chemical and Biological Methods for Seawater Analysis. Pergammon Press, Oxford. Peterson, B.J. & Fry, B. (1987) Stable isotopes in ecosystem studies. Annual Review of Ecological Systematics 18, DIBM Team (1989) Ecological Health Monitoring Program Newsletter Issue No. 1 8

9 Figure 1 Dissolved inorganic nitrogen (µm & mg L -1 ) within Moreton Bay in March

10 Figure 2 Dissolved inorganic phosphorus concentration (µm & mg L -1 ) within Moreton Bay in March

11 Figure 3 Dissolved oxides of nitrogen (µm & mg L -1 ) within Moreton Bay in March

12 Figure 4 Ammonium concentration (µm & mg L -1 ) within Moreton Bay in March

13 Figure 5 Total nitrogen (µm & mg L -1 ) within Moreton Bay in March

14 Figure 6 Total phosphorus (µm & mg L -1 ) within Moreton Bay in March

15 Figure 7 Chlorophyll a (µg L -1 ) within Moreton Bay in March

16 Figure 8 Total suspended solids (mg L -1 ) within Moreton Bay in March

17 Figure 9 Secchi depth (m) within Moreton Bay in March

18 Figure 10 Salinity (practical salinity scale) within Moreton Bay in March

19 Figure 11 δ 15 N ( ) of Catenella nipae within Moreton Bay in March

20 Figure 12 δ 15 N ( ) of Catenella nipae within Moreton Bay in September

21 Control NO3 30 Sewage Treatment Plant NH4 PO4 SiO3 All Fluorescence Fluorescence 0 30 River Prawn Farm Incubation time (d) 30 Logan River Mouth Incubation time (d) Incubation time (d) 30 Bay Prawn Farm 30 Bay Site (+12.5 km) Fluorescence Fluorescence Fluorescence Incubation time (d) Incubation time (d) Figure 13 Phytoplankton bioassay responses within southern Moreton Bay in March

22 Mixing Plots Caboolture River NO m µ Pine River Brisbane River Logan River Salinity %o Figure 14 Mixing plots for NO 3 - within Moreton Bay estuaries in March

23 3 Zostera capricorni Flood Seagrass depth range (m) Jan-93 Aug-93 Mar -94 Sep-94 Apr-95 Oct-95 May-96 Dec-96 (26 km) (21 km) (9 km) Logan River 27 o 45 S N 27 o 41 S Hyland et al. (1989) This Study ( ) kilometres 3 Eden Is. Figure 15 Seagrass loss and post flood recovery in southern Moreton Bay. 23

24 Figure 16 Conceptual Model for the Logan River estuary (Dennison & Abal, 1999). Figure 17 Conceptual Model for southern Moreton Bay (Dennison & Abal, 1999). 24