South Australian Murray-Darling Basin 2022-2023 Flood Environmental Response in the Coorong

Luke Mosley, Sabine Dittmann, Qifeng Ye, Emily Leyden, Anthony Newbery, Luciana Bucater, Travis Howson, George Giatas, Dhafeena Ahmed, Sunira Reardon, Jade Teigeler, Georgia Tiller, Josh Nitschke, Laura Schroder, Chris Sepe, Abby Hay, Noah Morris, Cassandra Urgl, Jason Nicol, Michelle Waycott

Research output: Book/ReportCommissioned reportpeer-review

Abstract

Executive Summary
Background
This report presents the findings of a study conducted on the environmental response of the Coorong to the 2022-2023 River Murray floods. This flood was larger than any that have occurred since 1956, with barrage discharges peaking at 120,000 ML/day on 30 January 2023.
The study, part of the South Australian Government’s Healthy Coorong, Healthy Basin (HCHB) Program, aimed to understand the hydrological, water and sediment quality, and ecological (aquatic plants, fish, invertebrates) impacts of significant flood events on the Coorong ecosystem.

Methods
The research involved field and laboratory assessments of samples collected from various sites within the Coorong from 2023-2024. Key components included hydrological assessment, water and sediment quality analysis (e.g. salinity, nutrient concentrations), and ecology (e.g. aquatic plant distribution and diversity, macroinvertebrate abundance and diversity, and fish distribution, diversity and dietary analysis). Where possible data were compared across different temporal phases: pre-flood, flood, and post-flood periods, allowing for an analysis of changes induced by the flood event.

Key Findings
Environmental conditions
􀁸 The 2022-2023 floods resulted in significant changes in water levels and salinity in the Coorong, affecting both the North and South Lagoons.
􀁸 The system is still eutrophic and there is a need for ongoing flushing and export of nutrients from the Coorong, which high flows could facilitate. It remains to be determined if the system returns to a hypereutrophic state in subsequent seasons.
􀁸 Nutrient levels reduced during the high flow periods. Water quality improved, but is transitioning back to pre-high flow levels, particularly in the Coorong South Lagoon indicating there has not been a permanent change in the system.
􀁸 Sediments were better oxygenated during the high flow period and an overall improved sediment quality was observed.
􀁸 There were improvements to water clarity once the sediment resuspension and flood plume associated with the high flow had abated.
􀁸 As the Coorong water levels returned to normal, there was a return to anoxic sediments with increased concentrations of porewater sulfide and ammonia, at high levels potentially toxic to the biota.

Aquatic plant community
􀁸 The typical hypersalinity tolerant Ruppia Community (including Ruppia tuberosa) was resistant to the flood effects until salinity levels went below 35 g/L throughout the southern Coorong and plants died off fully when salinities reduced to 10 g/L.
􀁸 With lower salinities persisting, other aquatic macrophytes recruited, the more freshwater tolerant aquatic plant Ruppia megacarpa and the Charophyte Lamprothamnium papulosum.
􀁸 Tissue nutrient concentrations (%C and %N) were higher in 2023 than previous samples (2016-17 and 2021-22) for both the Ruppia Community and algal plants analysed.
􀁸 Seed banks of the Ruppia Community persisted although at a moderate or low level following the flood event.
􀁸 The seeds condition was vulnerable as the outer seed coat was soft, not hard as usual, however seedlings were observed to be germinating and populations recruiting in the most recent (June 2024) growing season.
􀁸 Water levels dropped rapidly and salinity rose in late 2023 which led to a Ruppia Community die-off. as is expected to happen with the more regular annual cycle of warmer weather associated water level, high salinity change.
􀁸 The consequences of the floods will not be evident for several more growing seasons given the timing and life cycle of the Ruppia Community species.
􀁸 The Ruppia Community is behaving as a dynamic multispecies community and further regular monitoring regularly will provide greater insight into status rather than one off sampling.

Macroinvertebrate community
􀁸 Despite the initial impact of the flood event on macroinvertebrates communities in the area adjacent to the Murray Mouth, they started to recover quickly.
􀁸 The diversity and abundance of macroinvertebrates were highest in the North Lagoon and these animals provided a source for colonisation of the South Lagoon.
􀁸 The availability of macroinvertebrate prey for fish and shorebirds changed as range and diversity changed with recolonisation into a wide range of locations.
􀁸 The rapid change in distribution and abundance of macroinvertebrates is evidence of resilience in the system for this component of the Coorong biota.

Fish community
􀁸 The post flood fish community was characterised by increased number of freshwater species, enhanced recruitment of diadromous species (congolli and galaxias), and greater abundance of sandy sprat.
􀁸 There was a substantial increase in fish species richness in the North and South Lagoons. With salinity reducing to below 60 g/L, South Lagoon had species from all four functional groups (freshwater, estuarine, marine and diadromous).
􀁸 Fish biomass increased evidenced by the increase in catch-per-unit-effort in 2022-23 of key fishery species, including black bream and greenback flounder.
􀁸 Salinity reduction, increased estuarine habitat and food resources, particularly in the North and South Lagoons, have shown to benefit fish populations in the Coorong.

Conclusions and Recommendations
The study concludes that the 2022-2023 River Murray floods had profound impacts on the Coorong’s hydrological regime, water quality, and ecology. Based on the observed responses to the flood event and comparing observations from the outcomes of previous high flow events, further improvements to the Coorong ecosystem can be expected if the system experiences moderate to good flows in the next couple of years. Already, the water level drop going into summer of 2023 had a negative effect on habitat quality, with localised anoxia, sulfide and ammonia production. The draw down of nutrients during higher flow periods was observed, however return to low flows led to a halt to this. In addition, the Ruppia Community died off quickly as water levels dropped reducing seed bank production and higher salinities impacted fish communities and macroinvertebrates.
Key recommendations include the need for ongoing monitoring of the flood response over a longer time period coupled with adaptive management and initiatives to support the resilience of the Coorong in the face of future climate variability. Understanding the responses of the Coorong ecosystem to flood events is essential for developing sustainable Murray-Darling Basin water management practices that balance ecological health with human demands. Building on outcomes of the Healthy Coorong, Healthy Basin major project this study improves the foundation for further research, policy development, and strategies aimed at preserving the health of the Coorong.
Original languageEnglish
Place of PublicationAdelaide, SA
PublisherGoyder Institute for Water Research
Number of pages121
Publication statusPublished - 2024

Publication series

NameGoyder Institute for Water Research Technical Report Series No. 2024/02
PublisherGoyder Institute

Keywords

  • Murray-Darling Basin
  • flood response
  • Coorong
  • environmental response
  • 2022-2023 River Murray floods
  • sediment quality analysis
  • Coorong ecosystem

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