Ord Estuary (WA, 2002-2003)

EXECUTIVE SUMMARY

This study, following on from a previous study reported by Parslow et al. (2003a),(2003b), was designed to improve our understanding of how the lower Or River and Estuary function in terms of flow, transport of sediments, nutrient processes, and primary production (i.e., growth of algae and other plants) and to predict how the river is likely to change in response to changes in flows, nutrient and sediment loads that are likely to occur with planned changes in hydroelectric production and water allocation. Questions that the study was designed to answer included:

• How are sediments and nutrients transported through the lower Ord River and its estuary, and what happens to them there?
• What are the sources of food for organisms in the lower Ord River and estuary? Is the foodweb driven primarily by material brought in from the catchment, or by primary production (growth of algae and plants) in the river itself?
• What controls primary production in the lower Ord River and estuary?
• What do we know about the ecology of the system, and what important knowledge do we lack?
• What will happen to the river if water allocation, nutrient or sediment loads change?

The project included monitoring water quality, conducting two intensive field campaigns to study specific processes, and developing conceptual and numerical models of the lower Ord River and Ord River Estuary. These models were then applied to a series of scenarios in order to predict how the system is likely to respond to possible changes in water allocation and management. Five major zones can be recognised in the lower Ord system, distinguished primarily by their physical properties, but distinct also in their chemical and biological functioning and the habitats they provide for aquatic plants and animals. These are:

1. Freshwater zone (lower river): characterised by unidirectional, fresh water flow with water levels fluctuating according to the volume of water discharged from Lake Kununurra and Dunham River.
2. Tidal freshwater zone: predominantly freshwater, but water levels rise and fall under the influence of tides.
3. High energy brackish zone (turbid mid estuary): influenced by freshwater discharge, vigorous tidal currents and a change in salinity, this is a region with high suspended sediment loads and high turbidity (i.e. low water clarity).
4. Tide-dominated estuarine zone (estuary mouth): characterised by salinity approaching that of seawater, and lower turbidity than the high energy brackish zone upstream
5. Tidal creeks and flats zone: the area of mudflats and creeks off to the side of the main estuary channel, sometimes having lower turbidity than the open estuary

A conceptual model is presented to illustrate our present understanding of how these zones interact, and how the lower river and estuary function as a system. Primary production (i.e. production of plant biomass) in the freshwater zone appears to be phosphorus-limited, and dominated by benthic plants and algae growing on plants and on the river bottom. Net system metabolism is close to neutral in this zone. Primary production in
the high energy brackish zone is low, limited by light and the strong salinity gradient. Production in the estuary mouth is again limited by nutrients.

The numerical models developed for this project have been applied to a series of management scenarios to explore the likely effects of changes to water allocation and flow. These scenarios included 1) current conditions; 2) increased discharge from the hydroelectric dam; 3) increased hydroelectric discharge combined with increased extraction for irrigation; 4) 50% reduction in irrigation return flows; 5) 100% reduction in irrigation return flows, and 6) a return to interim Environmental Water Provisions Braimbridge, M. and Malseed, B. (2007).

The model suggested that scenario 6 would make little difference to water quality (nutrient concentrations, turbidity or growth of algae), scenarios 4 and 5 would lead to improvements in water quality (reduced nutrient and chlorophyll a concentrations) and possibly increases in fish production, and scenarios 2 and 3 would both have mixed results for water quality. Scenario 2 might lead to a reduction in fish production while scenario 3 might lead to an increase in fish production. Scenario predictions for fish are based on a preliminary foodweb model and are highly speculative as they do not include habitat effects and are based on limited information about the higher-level ecology of the lower Ord River, particularly in the estuary.
Major knowledge gaps remain regarding the ecology of the estuary, interactions between the lower river and its floodplain, and the likely response of the system to changes in climate and sea-level.

References

Braimbridge, M. and Malseed, B. Ecological Water Requirements for the lower Ord River. 2007. Department of Water, Government of Western Australia.

Parslow, J., Margvelashvili, N., Palmer, D., Revill, A., Robson, B., Sakov, P., Volkman, J., Watson, R., and Webster, I. The Response of the Lower Ord River and Estuary to Management of Catchment Flows and Sediment and Nutrient Loads: Final Science Report. 2003a. CSIRO Marine Research /
Land and Water Australia.

Robson, B.J. et al., 2008. Response of the Lower Ord River and Estuary to Changes in Flow and Sediment and Nutrient Loads. Water for a Healthy Country National Research Flagship.