A massive, unstoppable, toxic algal bloom that has turned beaches into "marine graveyards" has prompted calls for a federal investigation.
The bloom of the microalgae, karenia mikimotoi, was identified off South Australia's Fleurieu Peninsula in March, and grew to more than 4400 sq km, close to the size of Kangaroo Island.
It's now breaking up and has spread into most of Gulf St Vincent, including along Adelaide's beaches and south into the Coorong wetlands.
Greens senator Sarah Hanson-Young said the bloom was a horrific and heartbreaking "environmental catastrophe".
"When parliament resumes in two weeks time in Canberra, I will be moving for a federal parliamentary inquiry," she told reporters on Friday.
"We need proper investigation and proper federal support."
Beaches have become "graveyards" for marine species, and the fishing and tourism industries have been significantly impacted, she said.
Ms Hanson-Young claimed the federal government ignored warnings about the algae before it took hold.
"It is very concerning that the country's leading marine scientists have been ignored and dismissed," she said.
Experts believe there are three potential plausible contributing factors causing the bloom, the SA government said.
One is a marine heatwave that started in September 2024, with sea temperatures about 2.5°C warmer than usual, combined with calm conditions, light winds and small swells.
Another is the 2022-23 River Murray flood washing extra nutrients into the sea and an unprecedented cold-water upwelling in summer 2023-24 that brought nutrient-rich water to the surface.
SA Environment Minister Susan Close said the bloom had lasted longer than experts thought it would.
"Now that it's here, it's going to be hard to get rid of," she said.
The SA government has met with impacted commercial fishers, tourism operators and local councils about support packages.
"It's been very, very distressing for people, and it's affected people's livelihoods," Ms Close said.
The state government is also starting work on a recovery plan, that could include creating artificial reefs to help build up marine life, restocking fish into the ocean and increasing the number of marine sanctuary zones along the coast.
OzFish previously said more than 200 species of fish, sharks and other marine creatures had been killed by the algae bloom.
This includes rarely encountered deepwater sharks and leafy sea dragons, and recreational fishing species like flathead, squid, crabs, and rock lobsters.
Chief executive Cassie Price said an investigation to understand what caused the bloom was needed.
"It's one of the biggest marine and coastal impacts that we've we've seen in recent times," she said.
"Everything in those areas has been impacted in some way, perhaps not to the point of death, but certainly in terms of fish health and, no doubt, interrupted breeding cycles for the year."
Ms Price said habitats such as seagrass meadows and reefs should be created to help the marine environment recover.
