Murray-Darling floods reshaped ocean ecosystems, fed marine life with river nutrients
A massive 2022–23 flood in Australia's Murray-Darling Basin delivered an unprecedented surge of nutrients and organic matter to the Southern Indian Ocean, disrupting and enriching marine ecosystems for months.
Australia's Murray-Darling Basin, one of the world's most heavily regulated river systems, experienced a rare opening of its floodgates in 2022–23. The Murray River reached 168 gigalitres a day — the largest flood in 66 years — sending a muddy plume 40 kilometres out into the Southern Indian Ocean.
For decades, dams and irrigation networks have reduced ocean inflows to about 60% of their volume a century ago. The 2022–23 floods temporarily reversed this constraint, with profound effects on marine life along the Australian coast.
A nutrient pulse reaches the sea
According to research from Geoscience Australia and reported by The Conversation, the flood delivered more than 200,000 tonnes of organic carbon to the ocean between July 2022 and June 2023 — roughly 29 times the amount recorded in the same period two years earlier. This organic matter came from the Basin's rivers and floodplains, and included dead and dying fish, sediment, and other biological debris.
The flood effectively turned the ocean into a feeding ground for marine scavengers and filter feeders.
Researchers compared marine animals living inside the flood plume with those in normal saltwater further offshore. The difference was stark: scavengers such as crabs and smaller fish like yellow-eye mullet thrived on the influx. In marine rock pools more than 20 kilometres from the river mouth, purple mottled shore crabs and reef crabs were observed feeding intensively. Scientists estimated that about 35% of the tissue mass of these animals came from organic matter carried by the flood in the months that followed.
The carp invasion — and its end
One dramatic consequence emerged from the floodwaters: millions of juvenile common carp, an invasive freshwater species, were flushed into the open ocean. Because carp cannot survive in saltwater, they died in vast numbers. Dead carp piled up on South Australian beaches at densities of up to 7 kilograms per square metre, effectively creating a protein and nutrient windfall for coastal scavengers.
Longer-term effects on the food web
The flood's impact extended beyond immediate feeders. Australasian snapper, slow-growing predatory fish that frequently move between flood zones and open water, benefited indirectly. They preyed on smaller fish and crustaceans that had been feeding on flood-derived nutrients, meaning the floodwaters provided a longer-term energy boost that rippled through the food chain.
According to the research, the flood effectively stored more land and river nutrients in the ocean ecosystem — a rare event given the Basin's historical flow restrictions.
Context: A system under strain
The Murray-Darling Basin supplies water to Australia's agricultural heartland, and for over a century its flow has been tightly managed. The 2022–23 flood, while exceptional, highlighted how much the regulated system normally constrains the relationship between river and sea. The research was conducted by tracking the unique "chemical fingerprint" of flood-origin organic matter within the tissues of marine animals, a technique that allowed scientists to quantify the degree to which ocean life depended on river inputs.