Water quality changes in an episodically flushed sub-tropical Australian estuary: A 50 year perspective

Recent and historical data sets from the Richmond River estuary (New South Wales, Australia) were examined for possible impacts of changing land use patterns in the catchment and estuary floodplain on water quality over the last 50 years. Floodplain management practices, including further draining and disturbance of acid sulphate soils appear to have had no appreciable effect on the processes that control the degree of oxygen saturation in the estuary. Following runoff events phosphate and nitrate concentrations at a given salinity are respectively 2.5 and 3.0 times higher than 50 years ago, due to leaching from agricultural areas where fertilisers are applied. However, these concentrations quickly decrease after the runoff event due to rapid flushing of the system. Land use changes in the last 50 years appear to have had no impact on dry or low-flow phosphate and nitrate concentrations in the estuary, which are dominantly controlled by internal processes due to long water residence times. Particulate and bottom sediment phosphorus buffering apparently maintain dry season phosphate concentrations in the estuarine water column between about 0.30 and 0.80 μM. Nitrate levels are quite low ( < 4.00 μM) during the dry season, being regularly depleted by denitrification and/or phytoplankton blooms. Unlike poorly flushed estuaries, the episodically flushed Richmond River estuary shows no major signs of nutrient enrichment and associated eutrophication over time, despite increased loading of nutrients; most likely due to its low nutrient retention efficiency.