Fitzroy contaminants (2006)

EXECUTIVE SUMMARY

The Fitzroy catchment is the largest Queensland catchment discharging to the Great Barrier Reef (GBR) lagoon. Sediments and nutrients together with
anthropogenic pollutants originating upstream in the catchment are discharged from the Fitzroy River via the Fitzroy Estuary (FE) and ultimately into Keppel Bay (KB). The estuary and the bay act as natural chemical reactors where the materials delivered undergo chemical and physical transformations before some are deposited and stored in the growing deltaic and beach areas, with the remainder transported eastward to the southern zone of the GBR lagoon.

The Reef Water Quality Protection Plan is a recent State–Commonwealth initiative which aims to improve land-use management practices within many Queensland catchments with the intent of reducing the loads of sediments, nutrients and other potentially deleterious substances entering the GBR lagoon. The background knowledge to make realistic predictions of the response of the FE and KB ecosystems to changed sediment and nutrient deliveries did not exist when this project started. The Agricultural Contaminants Project (Project AC)
was developed to address these knowledge gaps and to produce a predictive framework to aid managers in the evaluation of various load reduction strategies.

The overall objectives of Project AC are to answer the following five questions:
1. How are nutrients and sediments transported, transformed and stored within FE and KB?
2. How are variations in nutrient and sediment delivery likely to impact on ecological function and primary production within the FE–KB system?
3. How are variations in riverine loads of sediment and nutrients likely to alter delivery of these materials to the GBR?
4. What pesticides and industrial contaminants are delivered to and remain in, the FE and KB and what is their potential impact?
5. How should managers monitor ecosytem health and function?

Project AC addresses these questions through a multidisciplinary study across the FE–KB system. It applied computer modelling of flow and mixing, of fine sediment dynamics, of nutrient transformation and storage processes and of primary production. Knowledge of the transport and concentrations of fine sediments is important since most of the nutrients within the system are bound to sediment grains. Also, fine sediments effectively block the light necessary for primary production in the water column and so have a direct impact on ecosystem function. The reliance on modelling in the project is in large part due to the enormous variability in the yearly discharge of the Fitzroy River. A project lasting only 3 years could not hope to experience the range of this variability, whereas model simulations can.

Project AC included a suite of data collection activities that were used in part to support the model development but also to provide further understanding of system behaviour. A major activity in the project was an extensive investigation of the rate of sediment deposition within KB and on the floodplain of the FE. A key result here is that perhaps less than a third of the fine sediment introduced to the system by the Fitzroy River is eventually transported beyond the bounds of KB to the GBR lagoon.

Another activity examined the role of primary production by microalgae on the extensive intertidal mudflats along the FE and tidal creeks. In these water bodies, primary production by phytoplankton in the water column is limited due to high suspended sediment concentrations and the consequent lack of underwater light necessary for their growth. The standing crop of the intertidal microalgae may not be large, but it is likely to be an important food source for prawns, crabs and other benthic feeding organisms.

The field investigation of the physical, chemical and biological properties of the water column and sediments showed how nutrients appeared to be generated by the bacterial decomposition of organic matter and dead phytoplankton. In the outer, well-lit parts of KB, these were consumed by phytoplankton as fast as they were generated. Conversely, in the inner parts of the bay where there were high concentrations of suspended sediments, primary production was almost certainly limited by light. It is likely that phytoplankton play a central role in the transport of nutrients between KB and the GBR lagoon. By comparing the nutrient content of incoming particles and the measured nutrient concentrations in sediment cores and using the measured sedimentation rate, we estimate that about half the input nutrient load was being stored in the Fitzroy floodplain, in the tidal creeks and in KB.

The Coastal CRC initiated a monitoring program in 2001 to quantify the concentration and loads of pesticides delivered to the FE from the Fitzroy River catchment. This monitoring has been extended during 2003–06 to include analysis for polycyclic aromatic hydrocarbons (PAHs) and metals in fine sediments and core samples from the FE and KB. This work adds to the very limited studies on PAH and metal contaminants, particularly in benthic sediments from the FE.

Measurements of pesticide concentrations in the summer inflows of the Fitzroy River into the estuary showed the presence of the herbicides Atrazine, Tebuthiuron, Diuron, Fluometuron, Hexazinone, Prometryn and Simazine. Atrazine and Tebuthiuron were detected in all the samples collected for the summer of 2004–05, illustrating the ubiquitous presence of these substances in the surface waters of the Fitzroy Basin. Their concentrations often exceeded ANZECC (2000) trigger values for protection of estuarine and inshore species.

PAHs are persistent organic pollutants that may enter the aquatic environment from natural and anthropogenic sources. In the Fitzroy Basin, potentially significant sources of PAHs arise from the disturbance of coal seams during mining operations and from the historic burning of vegetation. PAH analysis was undertaken on sediment samples collected throughout the FE–KB system. They were detected in most of the samples collected, but no concentrations exceeded the ANZECC trigger value.

Surface sediment and core slice samples from the FE were analysed for their metal content. Only concentrations of nickel (Ni), chromium (Cr), antimony (Sb) and arsenic (As) exceeded trigger values of the interim ANZECC sediment quality guidelines. Ni levels exceeded the guidelines most often, followed by Cr, Sb and in only one case, As. However, there are geological sources for both Ni and Cr in the basin.

Objectives 2 and 3 were addressed mainly using model simulation. Model predictions were obtained for low-, median- and high-flow years. The low- and median-flow simulations assumed modest but realistic inputs of fine sediments and nutrients and predicted small exports of fine sediments but a net influx of nutrients from the GBR lagoon. However, the high-flow year assumed much larger inputs and predicted significant exports of both nutrients and sediments to the lagoon. The freshwater inflow for the high-flow year used in the simulations (1999) was about one third of that discharged by the Fitzroy River during the major flood of 1991. This flood in 1991 is estimated to have delivered of the order of half of the total input of fresh water, fine sediments and nutrients for over the last 15 years.

Scenario modeling was undertaken to investigate the effects of two hypothetical land-use scenarios in the Fitzroy Basin on the primary productivity response of the FE–KB system and on the export of nutrients from the system to the GBR lagoon. The hypothetical scenarios were: (a) a reduction in vegetation cover to 30% of area from an assumed 55% cover at present and (b) an increase to 70% cover. The sediment and nutrient loads from the Fitzroy River corresponding to these scenarios were calculated in a separate modelling exercise. The calculated loads increased with decreasing vegetation cover and the resultant exports to the GBR lagoon followed suit. In the median-flow year, for the present condition of vegetation cover, the phytoplankton concentrations were predicted to be higher than for both reduced and increased cover. It appears that when cover is increased the supply of nutrients is decreased, resulting in more limited phytoplankton growth; but decreased cover increases the fine-sediment load that
also reduces growth due to reduced light in the water column.

The fifth question asked of Project AC is ‘How should managers monitor ecosytem health and function?’ We suggest that the prime goal of a monitoring program would be to assess progress towards management objectives and to inform decisions on modification of management actions over time in response to system change. The chosen management objectives will determine what indicators of system behaviour and response need to be measured. Any monitoring program will necessarily be constrained by the resources available so the choice of indicators measured and the frequency with which they can be monitored is necessarily a compromise.

A dominant characteristic of the FE–KB system is its variability on time scales ranging from decadal, to interannual, to seasonal, to fortnightly and down to subdaily. Variability on longer time scales is due to climatic and seasonal variations of the rainfall in the Fitzroy catchment which cause enormous variability in the discharge of the Fitzroy River, a major driver of system behaviour in the FE and KB. At the scale of weeks and days, the spring-neap cycle of high and low tidal ranges has a very large influence on suspended sediment concentrations as does the daily cycle of high and low tides. This variability makes the design of an effective monitoring program and the analyses of its results much more difficult than in most aquatic systems.

Only a monitoring program that lasts decades could account for discharge variablity directly. Assessing trends in system condition based on statistical analysis techniques is also likely to require decades of measurements before the analysis yields significant results. The analysis of monitoring information in the context of a modelling framework may be useful here. Models can readily accommodate changes in input conditions such as river discharge and system response could be judged against anticipated or modelled behaviour. If such a monitoring-modelling strategy were to be adopted (and even if it were not), the highest priority of any measurement program should be the accurate assessment of the form and loads of nutrients and fine sediments discharged by the Fitzroy River into its estuary. In a system such as the FE and KB which have such large temporal variability, we would place a premium on employing automated measurement technologies and satellite remote sensing. Variability in the inherent optical properties of water and the very high concentrations of suspended sediments encountered in KB caused significant difficulties for the routine application of remote sensing. These difficulties are being overcome as they are encountered and we are confident that remote sensing will become a reliable and inexpensive tool for monitoring.

 

References

Webster I., et al., 2006. The Fitzroy Contaminants Project – A study of the nutrient and fine-sediment dynamics of the Fitzroy Estuary and Keppel Bay. Technical Report 42. Published by the Cooperative Research Centre for Coastal Zone, Estuary and Waterway Management (Coastal CRC), Indooroopilly Sciences Centre, 80 Meiers Road, Indooroopilly Qld 4068, Australia