Toxic algae are turning South Australia's coral reefs into underwater graveyards – and there's no end in sight

CNN

26-07-2025

OceaniaFacebookTweetLink
Follow
What struck Scott Bennett most were the razor clams.
The long saltwater clams, resembling old-fashioned razors, normally burrow into sand to avoid predators. But when Bennett, an ecologist, visited South Australia's Great Southern Reef last month, he saw thousands of them rotting on the sea floor.
'100% of them were dead and wasting away on the bottom,' Bennett told CNN.
Since March, a harmful algal bloom, fueled by a marine heat wave, has been choking South Australia's coastline, turning once-colorful ecosystems filled with thriving marine life into underwater graveyards.
The bloom has killed about 15,000 animals from over 450 species, according to observations on the citizen science site iNaturalist. They include longfinned worm eels, surf crabs, warty prowfish, leafy seadragons, hairy mussels and common bottlenose dolphins.
The algae have poisoned more than 4,500 square kilometers (1,737 square miles) of the state's waters – an area larger than Rhode Island – littering beaches with carcasses and ravaging an area known for its diversity.
It's 'one of the worst marine disasters in living memory,' according to a report by the Biodiversity Council, an independent expert group founded by 11 Australian universities.
The toxic algal bloom has devastated South Australia's fishing industry and repelled beachgoers, serving as a stark warning of what happens when climate change goes unchecked.
Once a bloom begins, there is no way of stopping it.
'This shouldn't be treated as an isolated event,' Bennett said. 'This is symptomatic of climate driven impacts that we're seeing across Australia due to climate change.'
It all started back in March, when dozens of surfers at beaches outside Gulf St Vincent, about an hour south of state capital Adelaide, reported experiencing a sore throat, dry cough and blurred vision after emerging from the sea.
Shortly after, a mysterious yellow foam appeared in the surf. Then, dead marine animals started washing up.
Scientists at the University of Technology Sydney soon confirmed the culprit: a buildup of a tiny planktonic algae called Karenia mikimotoi. And it was spreading.
In early May, the government of Kangaroo Island, a popular eco-tourism destination, said the algal bloom had reached its coastline. A storm at the end of May pushed the algae down the coast into the Coorong lagoon. By July, it had reached the beaches of Adelaide.
Diverse algae are essential to healthy marine ecosystems, converting carbon dioxide into oxygen and benefiting organisms all the way up the food chain, from sea sponges and crabs to whales.
But too much of one specific type of algae can be toxic, causing a harmful algal bloom, also sometimes known as a red tide.
While Karenia mikimotoi does not cause long-term harm to humans, it can damage the gills of fish and shellfish, preventing them from breathing. Algal blooms can also cause discoloration in the water and block sunlight from coming in, harming ecosystems.
The Great Southern Reef is a haven for 'really unique' biodiversity, said Bennett, a researcher at the University of Tasmania, who coined the name for the interconnected reef system which spans Australia's south coast.
About 70% of the species that live there are endemic to the area, he said, meaning they are found nowhere else in the world.
'For these species, once they're gone, they're gone.'
Nathan Eatts hasn't caught a single squid since April.
On a good day, Eatts could catch 100 in the waters where he's fished commercially for 15 years off South Australia's Fleurieu Peninsula.
Since the harmful algal bloom began, his business, Cape Calamari, has gone 'pretty much down to zero,' Eatts said.
While more mobile fish can move to cleaner waters during an algal bloom, invertebrates like shellfish and sea stars, and other species associated with the reef, are suffocated by toxic algae.
'We don't know whether they've all died, or they're just seeking refuge in deeper water, waiting for it all to clear,' he said.
Many fishers have lost their livelihoods overnight, with about a third of the state waters completely devoid of fish, according to Pat Tripodi, the executive officer of the Marine Fishers Association, which represents the interests of most commercial fishing license holders in the state.
'Wherever the algal bloom hits, there is zero life,' Tripodi said.
'It's a really high emotional and mental strain on these individuals, because many of them don't know how or if they will ever recover from it.'
Beyond the fishers themselves, the bloom is having a knock-on effect on the state's seafood industry, which is valued at almost 480 million Australian dollars ($315 million).
Seafood processors, transport companies, grocers and restaurants are all feeling the pain, Tripodi said.
Eatts comes from a long line of fishers, and they've never seen anything like this.
The last time a harmful agal bloom swept South Australia was in 2014, but it was much more localized.
Toxic algal blooms are naturally occurring and are common around the world, including in the US. But climate change is making them more frequent and more severe.
The foundation for the South Australian bloom was laid back in 2022, experts believe, when catastrophic flooding swept the Murray, Australia's longest river, washing extra nutrients into the Southern Ocean.
The next summer, currents brought nutrient-rich water to the surface in a process called cold water upwelling.
Then, a marine heatwave in September 2024 caused ocean temperatures to be about 2.5 degrees Celsius warmer than usual. That, combined with calm water conditions and light wind created conditions for the algae to grow and spread.
There is no way for humans to stop a harmful algal bloom – its trajectory largely depends on natural factors like wind and weather patterns.
Peter Malinauskas, South Australia's premier, told public broadcaster the ABC on Tuesday that the crisis is a 'natural disaster, but it's different to ones that we're familiar with in Australia.'
'With a bushfire, you can put the fire out. If there's a flood, you can do the modeling to have a sense of where the water is going to go, whereas this is so entirely unprecedented. We don't really know how it's going to play out over coming weeks and months ahead.'
This week, Prime Minister Anthony Albanese's government announced a support package of 14 million Australian dollars ($9.2 million), which has been matched by the state, to help with the cleanup and economic fallout from the ecological crisis.
But Canberra stopped short of calling it a natural disaster, a declaration which would have unlocked additional funding.
As extreme heat events become more common around the world, Bennett said the government needs to do more to prevent and protect against future algal blooms – first and foremost by cutting carbon emissions.
Marine ecosystems can be 'resilient,' Bennett said. But he added that Australia must protect habitats, such as kelp forests, seagrass meadows, and oyster reefs, which absorb excess nutrients and keep the oceans healthy.
Eatts, the calamari fisher, said it 'hits home hard' to see South Australia's natural beauty spoiled by this crisis. The other day, he saw a dead dolphin on his local beach.
'You take it for granted where you live and what you see on a daily basis,' he said.
'But it takes one event of nature like this to come through, and it breaks your heart watching it unfold.'