For decades, the rivers of the Murray-Darling Basin have been heavily regulated by dams and irrigation networks. As a result, the volume of water entering the ocean is about 60% smaller than 100 years ago. But nature broke through during massive floods over the summer of 2022–23, when heavy rains filled the Basin’s waterways.
The threshold for a flood on the Murray is when the daily water flow at the Victoria-South Australian border reaches 50 gigalitres a day. This flood reached 168 gigalitres a day – the largest in 66 years. A colossal plume of muddy floodwater reached 40 kilometres out from the Murray mouth into the Southern Indian Ocean.
For marine creatures, this was a dramatic event. What did it do to their ecosystems? To find out, we compared marine animals living directly inside the flood plume with those living further away in normal saltwater.
Our new research found the flood delivered a burst of nutrients into the ocean. We estimate more than 200,000 tonnes of organic carbon were carried out to sea between July 2022 to June 2023 – 29 times more than the same period in 2020–21.
This organic carbon came from the Basin’s rivers and their floodplains, and included large numbers of common carp. Millions of juvenile carp — a highly destructive, invasive freshwater fish — were flushed into the open sea.
Because carp cannot survive in saltwater, they perished en masse. Dead carp piled up on local beaches at astonishing densities of up to 7 kilograms per square metre. In marine rock pools more than 20km from the river mouth, we saw the local crab species such as the purple mottled shore crab and the reef crab having a field day.
How the river fed the sea
These nutrients from the Murray provided a substantial boost to the middle of the ocean food chain. Scavengers such as crabs, and smaller fish such as yellow-eye mullet, were the main beneficiaries. They were feeding on scraps of organic matter, including dead carp, washed in from the Murray River.
We estimate a whopping 35% of the tissues of these animals came from the organic matter carried by the flood in the months afterwards.
Australasian snapper also benefited. These slow-growing larger fish frequently swam in and out of the flood zones. They preyed on smaller fish and crustaceans that had been eating food rich in nutrients from the flood. This means the floodwaters gave snapper and other predators a longer-term boost, effectively storing more of the land and river nutrients in the ocean food web.
How to trace nutrients from a flood
It’s not easy to trace what happens to nutrients from a river once they wash into the ocean. But it can be done.
Every environment has a unique chemical fingerprint, which is reflected inside the tissues of its animals. When nutrients from rivers arrive, they impart part of this fingerprint to the residents of the ocean who eat them – you are what you eat.
We can detect this by testing the muscles of marine animals for these fingerprints.
Ocean water has a uniform sulfur signature. But the crabs we caught inside the flood plume had a very different sulfur signature, which meant they were eating land and river-based food (the carp).
Crabs usually scavenge detritus without much food value. But the crabs inside the flood plume had a heavily enriched nitrogen signature – another sign they had switched to eating dead carp. The crabs had effectively been bumped up a level on the food chain.
Did the nutrient pulse fuel the algal bloom?
In March 2025, a large and long-lasting harmful algal bloom developed off the coast of South Australia. The bloom killed many different marine species in large numbers.
Could the nutrient pulse from floodwaters have fuelled it? The bloom has been linked to a marine heatwave and nutrient-rich seasonal upwelling currents. The 2022–23 Murray River floods have also been proposed as a potential contributor. But this connection remains speculative because of the lag time of 18–24 months and a lack of continuous data collection.
Better monitoring after floods would help us understand whether there is a link.
Rivers matter to the sea
We can see floods not as a waste of water, but as a restoration of longstanding connections between ecosystems disrupted by human control of the river. Our research shows the benefits of these flood events aren’t restricted to land and river ecosystems – they give a major boost to surrounding oceans as well.
Floods are important for the long-term health and resilience of our coastal ecosystems and fisheries, though more research is necessary to fully understand these connections.
This article was originally published on The Conversation. Read the original article.
