For decades Australia’s Murray Darling Basin has been heavily regulated by dams and irrigation systems, reducing the amount of freshwater reaching the ocean by nearly 60% compared to a century ago. However massive Murray River floods during the summer of 2022-23 temporarily restored this natural connection delivering an enormous surge of water, nutrients and organic matter into the Southern Indian Ocean.
Researchers found that the flood, the largest in 66 years, sent up to 168 gigalitres of water per day across the Victoria-South Australia border. The muddy flood plume stretched nearly 40 kilometres into the ocean, carrying more than 200,000 tonnes of organic carbon between July 2022 and June 2023 around 29 times more than during the same period in 2020-21.
Floodwaters Created an Unexpected Ocean Feast
The nutrient rich floodwaters carried organic material from rivers and floodplains, including millions of invasive juvenile common carp. Unable to survive in saltwater, the fish died in huge numbers and washed onto beaches, with some areas recording up to seven kilograms of dead carp per square metre.
The sudden abundance of food benefited a wide range of marine species. Crabs, yellow eye mullet and other scavengers fed on the dead fish and organic debris, while larger predators such as Australasian snapper indirectly gained from the increase in smaller prey. Scientists estimate that around 35% of the tissue in some marine animals came from nutrients delivered by the flood.
Study Highlights Importance of Natural Floods
Researchers traced the movement of flood nutrients by analysing unique chemical signatures in the tissues of marine animals. Crabs collected within the flood plume showed clear evidence of feeding on river based organic matter rather than their usual diet, demonstrating how nutrients from land quickly entered the marine food web.
Scientists also examined whether the nutrient pulse contributed to the harmful algal bloom that struck South Australia’s coast in 2025. While the bloom has been linked mainly to a marine heatwave and seasonal ocean currents, researchers say the role of nutrients from the 2022-23 flood remains uncertain due to limited long-term monitoring.
The findings suggest that large floods do more than replenish rivers and wetlands. They also restore important ecological connections between freshwater and marine ecosystems, supporting coastal biodiversity and fisheries. Researchers say improved monitoring after future flood events will help better understand these natural processes and their long-term environmental benefits.