In the Coorong, at the terminus of the Murray-Darling Basin, the influence of freshwater discharge on water level and salinity is generally well understood. A recent Goyder Institute project The influence of freshwater discharge on productivity, microbiota community structure and trophic dynamics in the Murray estuary, lead by Chris Bice of PIRSA-SARDI, has further improved the knowledge of this ecosystem productivity.
Recent data suggest a potential association between high freshwater discharge, zooplankton species diversity and abundance, and high abundance of a small-bodied marine fish, in the Murray estuary region of the Coorong. This study suggests that organic matter and biota, transported downstream by freshwater discharge, may be subsidising the diet of sandy sprat (Hyperlophus vitattus), and enhancing population productivity. As a primary prey item for larger fishes, enhanced production of sandy sprat stands to benefit the productivity of a variety of aquatic species of upper food chain levels.
The objective of the current study was to investigate the influence of low-volume freshwater discharge in 2014 on water chemistry, primary productivity, microbiota community structure, and the sandy sprat diet. Sampling took place over a series of three events in November–December 2014 across five sites within the Murray estuary and one site upstream of Goolwa Barrage. During each occasion samples of water, zooplankton and sandy sprat were collected for analyses of:
The research examined variances in nutrient concentrations and phytoplankton abundance in association with distance from the freshwater discharge points and flow levels. Investigations into the diet of sandy sprat showed variations although one group of organisms were the dominant prey item. The freshwater rotifer Keratella australis and cladoceran Bosmina meridionalis also comprised significant proportions of the diet at some sites, and when present, were selectively preyed upon. The prevalence of these freshwater species in the gut content of sandy sprat generally decreased as freshwater discharge decreased, suggesting a direct link between freshwater discharge and the subsidisation of their diet.
This is the first study to demonstrate that organic matter and biota exported to the Murray estuary with freshwater discharge through the Murray Barrages contributes materially to estuarine productivity. It presents empirical data to directly inform and support the delivery of environmental water allocations to the Coorong on the basis of supporting trophic dynamics. Whilst the results are specific to the 2014 hydrograph, they suggest that even low–volume discharge can have measurable benefits for the ecosystem, whilst conspicuous flow pulses (~20,000 ML.day-1) may provide productivity benefits that last for periods of months following flow recession.