Australia’s worst algal bloom continues one year later
A massive toxic algal bloom along South Australia’s coastline is still spreading a year after it was first detected, alarming scientists and coastal communities.
The bloom, caused by the rare species Karenia cristata, has affected more than 20,000 square kilometres of coastline – an area roughly twice the size of Greater Sydney.
Researchers say it is the largest and most destructive algal bloom ever recorded in Australia.
Millions of marine animals killed
The toxic bloom has triggered widespread marine deaths across the region.
Scientists estimate millions of sea creatures have died, from small shellfish to large predators including sharks.
More than 780 species have been affected, according to marine experts.
Among the hardest-hit animals are leafy sea dragons, a unique species found only in southern Australian waters.
Divers have reported dozens washing up dead on beaches.
A rare and dangerous algae
Marine scientists say this is the first recorded bloom of Karenia cristata in Australian waters.
The toxic algae had previously only been documented in South Africa and near Newfoundland in Canada.
Professor Shauna Murray from the University of Technology Sydney was among the first scientists to identify the species in samples taken from South Australian waters.
Experts say the algae releases toxins that damage marine life and can also cause respiratory irritation in humans.
Key marine habitats destroyed
Scientists say the bloom has devastated fragile ecosystems across South Australia’s coastal waters.
Marine biologist Stefan Andrews said once-thriving reefs, seagrass meadows and marine habitats have been heavily damaged.
“While the water can appear clear, it has essentially been stripped of life,” he said.
Seagrass beds and razorfish populations – important habitats for many species – have also suffered severe losses.
Shark populations disappearing
The bloom has also affected larger marine predators.
Professor Charlie Huveneers from Flinders University said Port Jackson sharks, once common along Adelaide’s coastline, have largely disappeared.
Scientists are still unsure whether the sharks have died or simply moved away from affected areas.
Impact on communities and health
The environmental disaster has also affected coastal communities.
A study found the bloom has had a significant psychological impact on South Australians, with many residents reporting distress after seeing dead marine animals on beaches.
Some people have also reported physical symptoms including:
• coughing
• sore throats
• headaches
• breathing difficulties
These symptoms have been linked to exposure to airborne toxins from the bloom.
Fishing industry under pressure
The crisis has also severely disrupted the fishing industry.
Commercial fishing has been halted in Gulf St Vincent and around Kangaroo Island until June 2026.
Fish populations including calamari, garfish and blue swimmer crabs have dropped sharply.
Industry leaders warn the recovery could take years.
Scientists searching for answers
Researchers are still trying to understand why the bloom exploded in 2025.
DNA analysis suggests the algae may have been present in small amounts in South Australian waters for nearly a decadebefore suddenly expanding.
Professor Justin Seymour from the University of Technology Sydney said environmental changes may have triggered the rapid growth.
But scientists warn the bloom has not fully disappeared.
With similar seasonal conditions approaching again, experts fear another surge could occur.
“There’s still a lot to lose,” marine researchers say.