Many environmental responses to the largest River Murray flood in 70 years appear short-lived as ecosystems return to pre-flood conditions.
The flood event in the South Australian Lower River Murray between November 2022 and February 2023 was the third largest such event in recorded history, and the largest flood in nearly 70 years.
New scientific research undertaken through the Goyder Institute for Water Research has highlighted how the environment responded to this significant flood.
Teams from CSIRO, Flinders University, the South Australian Research and Development Institute (SARDI) and the University of Adelaide set out to investigate the immediate environmental impact of the flood waters in the Riverland region as well as the longer-term responses in the Coorong, Lower Lakes and Murray Mouth region, including the impacts of the flood waters on marine habitats and species.
College of Science & Engineering Professor Sabine Dittmann, who is part of the Coastal Ecosystem Ecology Lab of Flinders University, says the impacts on the Coorong was impressive.
"It was fascinating to see recolonisation of the previously extremely hypersaline South Lagoon as flood waters reduced salinities below thresholds for macroinvertebrate occurrences. This had not occurred at any time before in the 20 years I have been studying this ecosystem," says Professor of Marine Biology Dittmann.
"The flood waters really brought the South Lagoon back to life, showing the importance of higher flow events.
"Our studies also recorded a quick recovery in the Murray Mouth where the flood had caused an impact on macroinvertebrates in the sediments. This resilience was enabled by improved flow and water management over the last decade."
Funding from the South Australian Department for Environment and Water meant the researchers could assess river and floodplain conditions as soon as it was safe to do so.
Goyder Institute for Water Research Director, Dr Alec Rolston, says: "The results show that while there were both positive and negative environmental responses, many were short lived, with conditions returning to those similar to pre-flood conditions relatively quickly.
"For important environments like the Riverland floodplains and the Coorong and Lower Lakes, this highlights the ongoing need for freshwater flows to maintain the region's social, cultural, environmental and economic values."
The immediate environmental response in the Katarapko and Pike floodplain areas included a reduction in the salinity of shallow groundwater, with some saline groundwater discharging from the floodplains into the river at specific locations.
Dissolved oxygen levels, which are vital for fish and other aquatic animals, did not reduce to critical levels and any low oxygen events that did occur came into South Australia from further upstream.
There was a significant carp breeding response to the flood event as floodplains and wetlands were inundated, creating good conditions for spawning at the right time of year.
It was the largest carp breeding event seen in the Lower River Murray for over 20 years. At the same time, many thousands of carp were washed out of the Murray Mouth into the Southern Ocean, washing up on the beaches of the Fleurieu Peninsula.
A plume of freshwater extended at least 40 km from the Murray Mouth into the Southern Ocean. The floodwaters transported nutrients and carbon from the Murray-Darling Basin into the Southern Ocean, providing energy sources for marine plants and animals.
This increased phytoplankton productivity and sparked positive responses all the way up the food chain. The sediment and reduced salinity that the flood brought to the marine environment did however impact negatively on some marine organisms.
Dr Rolston said "The positive environmental outcomes observed in the Coorong during the flood event included reduced salinities, as well as increased and more widespread numbers of plants, fish and invertebrates. However, some species that are more sensitive to salinity changes couldn't cope with the rapid reduction in salinity as the floodwaters passed through the Coorong and into the Southern Ocean."
Dr Ryan Lewis, Principal Advisor for the River Murray in the Department for Environment and Water, says: "Understanding how the environment responds to a flood event of this magnitude highlights the importance of high flows to the plants and animals which inhabit these systems.
How they respond to changes caused by events like the flood is critically important for the management and conservation of these important ecosystems".
