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The largest and most destructive algal bloom in Australia’s history is persisting along parts of the South Australian coastline, a year on from when it was first detected.
From a distance, it can be hard to grasp just how unusual and devastating the crisis has been.
Most harmful blooms only last a few weeks. This one has been unrelenting.
Since March last year, it has affected 20,000 sq km of coast – an area twice the size of greater Sydney – and ranks among the worst for marine mass mortality globally, killing millions of sea creatures from tiny shellfish to top predators like white sharks.
It is Australia’s first bloom of Karenia cristata, a rare and toxic type of algae deadly to marine life and harmful to human health, according to Prof Shauna Murray, from the University of Technology Sydney. Murray, who was the first to identify the species in water samples from SA, says the species has only been recorded in two other places worldwide – South Africa, and an island off the coast of Newfoundland, Canada.
Marine ecologist Prof David Booth, who specialises in sea dragons at the University of Technology Sydney, describes the impact on marine life as “the most awful, tragic thing I’ve personally seen in the ocean in my 40-year career”.
More than 780 species have been affected, including “devastating losses” of the state’s marine emblem, the leafy sea dragon, reported by divers and citizen scientists.
“Leafies”, as they are known, resemble floating seaweed with a long snout for slurping up tiny mysid shrimp – “the caviar of sea dragons”, according to Booth. The species is unique to southern Australia, and particularly concentrated in some of the worst hit areas around Adelaide and the gulfs on either side.
“People come from all around the world to see these, and they’ve been absolutely hammered with the bloom; their habitats in many cases are just gone,” Booth says.
Dozens of leafy sea dragons have washed up dead, he says, and the deaths are ongoing, with eight more washing up at Kangaroo Island in recent weeks. Their risk of extinction is now being reassessed by the International Union for the Conservation of Nature.
Marlene, who has been diving regularly at Rapid Bay on the Fleurieu peninsula since 2022, says the local population of 45 leafy sea dragons collapsed to just one or two after the bloom came through.
Many of the state’s time-worn jetties, like at Rapid Bay, have formed artificial reefs for sponges and sea squirts, creating habitat for sea dragons, striped pyjama squid and other unique species.
Marine biologist Stefan Andrews, a keen diver and co-founder of the Great Southern Reef Foundation, says those fragile environments have been “smashed”.
Andrews lives in Ardrossan, a small town on the Yorke peninsula. Back in May, he filmed the before-and-after effects of the bloom at Edithburgh jetty, on the south-eastern corner of the peninsula. In February, he watched as one of his favourite dive sites on the western side, Port Victoria, succumbed.
Once thriving seagrass meadows were also “under really severe stress”, he says. “Razorfish, another habitat-forming species, have almost been completely wiped out wherever the bloom’s gone through.”
Port Jackson sharks, once a common sight, have mostly disappeared from Adelaide’s coastline, says Prof Charlie Huveneers, who leads the Flinders University southern shark ecology group. “Whether these sharks have all died or some have left the area and will return is unknown,” he says.
The state’s environment department is surveying the bloom’s impact on marine biodiversity at more than 200 sites across metropolitan Adelaide, Fleurieu and Yorke peninsulas, Kangaroo Island, Spencer Gulf and Gulf St Vincent.
The work, timed to match previous surveys, will be completed in June – long after the results of the state election on 21 March.
‘Morning death walk’
The year-long crisis has had a profound psychological impact on South Australians, according to a recently published study.
Two-thirds of respondents said they couldn’t stop thinking about the bloom, when surveyed last year. About a third of those directly affected linked their distress to witnessing dead marine species on the beach.
“My morning beach walk has turned from a connect-with-nature walk to the morning death walk,” one participant responded.
About half (47%) said they had stopped engaging in recreational activities like surfing, swimming, diving, or walking on the beach. About a quarter reported physical health concerns, such as coughing or respiratory irritation after visiting affected beaches.
“When the ocean suffers, communities suffer too,” says conservation psychologist Dr Brianna Le Busque, environmental science program director at Adelaide University and lead author of the paper.
Separately, data submitted to the Bloomin’ Algae research project, a collaboration between citizen and professional scientists, found scratchy coughs, sore throats, headaches and breathing difficulties were commonly reported symptoms from exposure to the bloom.
The state’s fisheries and seafood industry have been pushed to the brink of disaster.
All commercial fishing in the Gulf St Vincent and Kangaroo Island has been halted until 30 June 2026 due to sharp declines in species such as calamari, garfish, King George whiting, western king prawn and blue swimmer crab. Recreational catch limits have been halved in those zones, with limits introduced for several species in the Spencer Gulf.
“The journey ahead is long, painful and turbulent,” says Kyri Toumazos, an executive of the SA Northern Zone Rock Lobster Fishermen’s Association and a director of Seafood Industry Australia.
Recently, DNA analysis of archived seawater samples collected near Kangaroo Island since 2016 revealed the species was present for nearly a decade before the bloom kicked off.
“However, in 2025 something happened in the environment that made it dramatically increase in abundance,” says Prof Justin Seymour at UTS. Further research is needed to find out what caused it to take off, and whether the species could be present in other states.
Experts like Murray worry that the bloom still lingers, 12 months on.
“It has somehow managed to persist throughout the whole gamut of conditions over the course of a year,” she says. “I’m concerned that as the conditions get more suitable for Karenia cristata and similar seasonal conditions to last year, that we might see a repeat.”
Walking along metropolitan beaches, it’s possible to forget the worst effects, which lasted from July to September last year. In the past 10 weeks, testing has shown low or no levels of Karenia at urban sites.
“The water can be super clear,” Andrews says, but “it has been stripped of life”.
Spencer Gulf had initially been spared the worst. But the bloom is now active along the south-west coast of the Yorke peninsula, renewing fears for the globally unique population of giant cuttlefish that gathers in the gulf in May.
Cephalopods are often one of the first creatures to go as the bloom hits, he says. A “bubble curtain” installed to shield cuttlefish eggs and hatchlings will do little to protect the adults making their return.
“They rely so heavily on the reproductive success of the previous year. So, if they don’t have a successful season, then we could lose that sub-population entirely,” he says.
“There’s still a lot to lose.”
