In the Coorong, at the terminus of the Murray-Darling Basin, the influence of freshwater discharge on water level and salinity regime is generally well understood and in recent years, knowledge of the influence of these factors on biotic patterns and processes has improved. In contrast, understanding of the role of freshwater discharge in promoting ecosystem productivity, through the input of organic matter, is limited.
Data suggested a potential association between high freshwater discharge, zooplankton species diversity and abundance, and high abundance of a small-bodied (i.e. adult length <100 mm) planktivorous marine fish, sandy sprat (Hyperlophus vitattus), in the Murray estuary region of the Coorong. Here we hypothesise that organic matter and biota, transported downstream by freshwater discharge, may be subsidising the diet of sandy sprat and population productivity. As a primary prey item for larger piscivorous fishes, enhanced production of sandy sprat stands to benefit the productivity of higher trophic levels.
The objective of this study was to investigate the influence of low-volume freshwater discharge in 2014 on water physico-chemistry, primary productivity, microbiota community structure, and the diet and freshwater derived trophic subsidy of sandy sprat.
Sampling took place over a series of three events in November–December 2014 across five sites within the Murray estuary and one upstream of Goolwa Barrage. During each occasion samples of water, zooplankton and sandy sprat were collected for analyses of:
Results of the SIA were also compared to previous measurements for sandy sprat collected from the Coorong under differing hydrological conditions.
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. As such 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 trophic dynamics, whilst conspicuous flow pulses (~20,000 ML.day-1) may provide productivity benefits that last for periods of months following flow recession.