Drying climate associated with improvements in water quality over four decades for a marine embayment in south-eastern Australia

Context Extreme droughts are projected to be a major feature of the future climate. Droughts are characterised by extended periods of reduced surface runoff and lower stream flows. Whereas water quality is generally poorer for inland waterways during droughts; coastal waters often observe improvements in water quality because of decreases in riverine sediment and nutrient loadings. Aims This study examined a 37-year time-series for water quality measurements in a coastal embayment across a period that included two extended droughts. Methods We assessed the overall trend for each water quality parameter (positive, negative or non-linear), and using information on rainfall, river flows and treated wastewater inputs, the likely drivers of these trends. Key results Water quality in Port Phillip Bay changed significantly between 1985 and 2022. Increasing salinity was indicative of reduced catchment inputs coinciding with a drying climate. Increasing salinity was accompanied by reductions in suspended sediments, dissolved phosphate, and silicate concentrations. By comparison dissolved ammonia and nitrate concentrations remained stable, except at one site where reductions in ammonia were linked to improvements in wastewater treatment. Long-term trends varied depending on spatial proximity to sources of nutrients and sediments, and vectors such as river plumes. Conclusions The findings indicated that where coastal systems are heavily influenced by freshwater catchments, there will be a decrease in nutrient and sediments inputs. Implications This will have a range of impacts for the functioning of coastal ecosystems in the bay.