Murray-Darling Basin 2022-23 flood environmental response – Immediate Investigations

How are things looking down at the Murray Mouth?

• In the summer of 2022-23, very large volumes of freshwater flowed out the mouth and into the adjacent marine environment. At this time, the plume of freshwater extended at least 40 km offshore (that’s the distance from Adelaide to Gawler).
• Throughout 2023, freshwater has continued to flow to the sea, all be it in lower volumes.
• Whilst there were initially some visual negative impacts of freshwater, e.g. benthic marine animals dying due to low salinities and brown water along the Goolwa and Middleton beaches, these freshwater flows carried with them abundant nutrients and carbon from the floodplains of the MDB. These terrestrial energy sources likely lead to a more productive marine ecosystem.
• The nutrients are firstly used by algae which then fuel higher levels of the marine ecosystem, from small fish like anchovies, through to tuna, sharks and fish-eating birds.
• At present, we are testing a range of nearshore (e.g. cockles and mullet) and offshore (e.g. snapper and tuna) marine animals to see if carbon and nutrients from throughout the MDB have fuelled their growth.

What did the research around blackwater risks find

• Dissolved oxygen was maintained at satisfactory levels (>90% saturation) at the time of sampling (Feb 2023) in nearly all sites across the large Chowilla, Pike and Katarapko floodplains and associated river reaches.
• These conditions were suitable for maintaining the health of fish and invertebrate (e.g. yabbies) populations.
• However comparison of the February 2023 survey data to data from continuous dissolved oxygen monitoring stations over a longer time period shows that lower dissolved oxygen concentrations were present on the rising phase of the flood in late 2022.
• It is considered that these hypoxic conditions were largely driven by low dissolved oxygen River Murray water entering South Australia from upstream states.

What impact did the floods have on salinity levels

• Instream geophysics surveys conducted along Katarapko Creek and the River Murray at Bookpurnong showed significant change in subsurface conductivity within the riverbed between last surveys (2015 for Kat and 2019 for River Murray) and February 2023 in response to the 2022-2023 flood event- i.e., subsurface freshening.
• Subsurface conductivity features beneath the riverbed still align between the previous and latest surveys, but there is an overall decrease in the conductivity with depth. This suggests that there are discrete zones along the river reaches that are receiving more saline groundwater discharge from the adjacent floodplain aquifers.
• Despite the reduction in subsurface conductivity in response to the flood event there is still considerable saline groundwater beneath the floodplains discharging to the river. Several natural and or managed flood events would need to occur to ‘flush’ and reduce the salt loads towards the river.

What did your research find out about carp movements and breeding

• Inundated floodplains are known to be favoured spawning and nursery habitats for carp.
• In association with flooding in 2022/23, there was a notable breeding response from carp throughout the South Australian Murray.
• Based on long-term datasets from River and wetland habitats, as of autumn 2023, carp were present in the greatest abundance in the past 20 years.
• While response was widespread, greatest abundances were detected in the Riverland and Lower Lakes.
• The majority of individuals were young-of-year spawned during the flood, but a substantial portion were 1 year old fish spawned during high flow in 2021/22.
• A study of adult carp movement, using acoustic telemetry, was ongoing during the flood. There were extensive movements between river and floodplain habitats, and several instances of long-distance movements in both upstream (e.g. from Lock 6 to Menindee on the Darling) and downstream (e.g. from Lock 4 to downstream of Lock 1) directions