Coorong water quality synthesis with a focus on the drivers of eutrophication

The Coorong is a Ramsar-listed estuarine lagoon system at the downstream end of the Murray-Darling Basin that has experienced declining ecological health over recent decades. This is likely due to a decrease in freshwater inflows from the River Murray, and the interactive effects of hypersalinity and eutrophication and changes in the water level regime. Eutrophication can be defined as an increase in the supply and accumulation of organic matter (e.g. algae) to an aquatic ecosystem and can cause many deleterious effects, including depletion of dissolved oxygen, toxicity (e.g. from harmful algal species, sulfide, ammonia), and loss of submerged aquatic vegetation and benthic ecosystems. The Healthy Coorong Healthy Basin (HCHB) program aims to achieve long term health of the Coorong by providing evidence-based solutions to both immediate threats and future conditions anticipated under a changing climate. One component of the HCHB Trials and Investigations Project is Understanding Nutrient Dynamics. This report provides a synthesis of the long-term (>20 year) Coorong water quality data set aimed at assessing: (a) changes in Coorong water quality, with a particular focus on salinity, nutrients and eutrophication; and (b) the drivers for these changes. The findings for the Coorong were also compared to those in relevant international scientific literature. Key findings were: • Estuaries and lagoons in arid and semi-arid catchments such as the Coorong are particularly susceptible to hypersalinity and eutrophication following climatic and/or hydrodynamic changes that lead to reductions in flushing and increased water residence time. • Based on analysis of the salinity, chlorophyll a, total nitrogen (TN) and phosphorus (TP) concentrations, the southern parts of the Coorong are persistently hypersaline and hypereutrophic. This was exacerbated during the extreme Millennium Drought period when no river inflows occurred, and salinity peaked near 200 mS/cm (approximately 5 times seawater salinity) and very high total nutrient levels were present. • The results indicate that changed hydrodynamics and evapo-concentration processes in the Coorong has reduced flushing, especially in the South Coorong lagoon during the Millennium Drought. This has resulted in prolonged periods of extreme hypersalinity (EC > 120 mS/cm or salinity > 100 psu) and hypereutrophication (i.e. TP >0.2 mg/L, TN >4 mg/L, chlorophyll a >50 g/L). In contrast, dissolved inorganic nutrients are generally low which, given the high chlorophyll a levels, suggests rapid recycling and uptake of any bioavailable nutrients by algae (e.g. phytoplankton and filamentous algae). • Some phytoplankton die-off (based on chlorophyll a concentrations decreasing) appears to occur at the extreme end of the salinity range (EC > 120 mS/cm or salinity > 100 psu), likely due to salt toxicity. However, it is important to note that the data do not show a large drop in TN and TP levels and so eutrophication is persistent with the extreme hypersalinity, despite lowering chlorophyll a levels. • The internal sources and cycling of nutrients within the system are currently quite unclear but specific process based research is underway as part of the HCHB Trials and Investigations Understanding Nutrient Dynamics component. • There is some evidence of seasonally-higher availability of ammonium (50-100 g/L as NH4-N), which could be linked to the build-up and breakdown of organic matter/algae produced with the system, enhanced nutrient release from reducing sulfide-rich sediments, and/or inhibition of coupled nitrification-denitrification reactions. • Phosphorus (P) appears to be the limiting nutrient for phytoplankton growth (i.e. the availability of P currently controls the amount of phytoplankton growth but this apparent effect could be driven by an oversupply of nitrogen). We also hypothesise that the persistent hypersalinity in the South lagoon now reinforces the eutrophication process by negatively impacting benthic macroinvertebrates (nearly completely absent now) and seagrass (Ruppia sp.) communities, which would otherwise promote nutrient sequestration and elimination processes. Based on the knowledge available, increasing system flushing (frequency and magnitude), in particular for the South Lagoon, to try and reverse eutrophication would at a conceptual level be beneficial to: (a) export nutrients, algae and organic matter; (b) reduce algae and total nutrient concentrations in the water column to reduce deposition of organic matter and nutrients to the sediment; (c) reduce algal-derived turbidity to enable increased light penetration for seagrasses (Ruppia sp.); (d) reduce hypersalinisation to enable re-establishment of benthic macroinvertebrates, and; (e) reduce formation of hypersaline, reduced, sulfide-rich sediments that we hypothesise are inhibiting healthy nutrient cycling. The options to achieve increased system flushing include increased River Murray and South East catchment (via Salt Creek) inflows, and/or enhanced seawater inflows and connectivity. Many different options that could help achieve this are being investigated as part of the Coorong Infrastructure Investigations project of the HCHB program. Localised management actions such as Ruppia and benthic macroinvertebrate restoration could also help reduce eutrophication, particularly if system-wide water quality can be improved by increased flushing and reduced nutrient loadings. The Understanding Nutrient Dynamics component that this synthesis is part of will provide critical information to the feasibility, risks and benefits of various management options.