Photo: Dead marine life has been washing up on South Australia’s shores since a toxic algal bloom began to spread in May. Photo by Sia Duff.

The start of South Australia’s toxic algal bloom began like a horror film. It was a sparkling morning when Dr Mike Bossley, two other marine scientists and Pam Bennett approached the cliff overlooking their marine sanctuary. Half a decade ago they had established it next to land that Bennett’s family owned. It was intended as a refuge for marine life to escape fishing and other human activity, and to educate the public about local marine ecology and the Great Southern Reef. An overlooked temperate cousin to the famed Great Barrier Reef, the ecological wonder stretches along the southern coast of the Australian continent from Kalbarri in the west to the Solitary Islands on the east coast.

A small bay tucked into the bottom of Yorke Peninsula, Treasure Cove is made up of multiple habitats in a small area, with high biodiversity concentration. It had all been going “really well, really beautifully” in recent years, Bossley says, which is what made reports of a spreading sickness in the ocean all the more concerning. There had been whispers of something stirring in the water as far back as early February, when fishers working down south around Robe reported patches of dead sea, a funny smell and foam to their reps at the Marine Fishers Association, who in turn attempted to notify authorities. At the time, no one gave it much thought. Sometimes, the sea does strange things.

Mike Bossley (right) and Pam Bennett (left). Photo: Sia Duff

“The ocean’s fucked”

No one currently knows exactly where the bloom began, but based on anecdotal reports it started north of Robe before making its way to Middleton. From there it wrapped around Kangaroo Island, then hit the bottom of the York Peninsula. At that point the bloom split into two, one arm staying put, the other reaching its way up St Vincent Gulf. The second arm circled the gulf before coming back down along the metropolitan Adelaide Beaches. A group of surfers first raised the alarm in March; Anthony Rowland had gone down to Waitpinga Point on the southern Fleurieu with two friends to catch the swell. It was a 40C day, and on the way into the water he felt like he was getting a flu – a scratch in his throat and an odd taste in his mouth. On their way back, he and his two friends were coughing. Looking around the carpark, he noticed the two dozen people there coughing, too. That was the moment he understood something was deeply wrong.

Later that night, he went back to get samples. The next morning he returned again but this time found foam on the waves and death on the shore. On Monday, he returned again to find it even worse. “There was so much death,” he says. Fearing something had spilled or someone had dumped something into the waves, he reported what he had seen to the Environmental Protection Authority, but the authorities didn’t seem alarmed. When he followed up his initial contact with more information on Wednesday, the person responding told Rowland that it was all in hand.

“I did not think this would keep people out of the water on such a nice day,” they wrote.

Stories of mass death followed – a friend of Bossley’s had been finding unusual numbers of cuttlefish bones on the beach and people reported marine life washing ashore in numbers: fish, sea horses, rays and sharks. What washes ashore represents a fraction of what is taking place in the water, Bossley knew, and he was keen to learn how the sanctuary was faring. The remote location on the Eyre Peninsula, however, made logistics tough and by the time he and two fellow marine scientists made the drive from Adelaide – a little over three hours without stopping – it was early May.

The first sign of trouble didn’t seem like trouble at all. From the top of the cliff, the group looked down into the water and noticed three large short-tailed sting rays cruising in the shallow water. The gentle giants of the ocean, also known as “smooth rays”, can grow to around 1.5 metres (5ft) wide. It was unusual to see three rays clustered together in a small area, Bossley says, but it wasn’t alarming.

When the group made their way down the track to the rocky beach, they began to find dead rays and fish washed up with the tide. Not knowing what to expect, they prepared to conduct their survey. They had met the night before to plan their work: Bossley would enter the water with Pam, and the other two would go their own way. It was only when the team entered that the full scale of what was taking place came into focus. The water was thick and oily, Bossley recalls. Visibility was down to two metres at best. As soon as he put his face in the water, Bossley says, a dead ray floated past his mask

“To me it was a feeling of ‘the ocean’s fucked’. Everything’s dead and dying,” he says. “It felt like you were entering a dead zone.”

Pam shows the scar where the serrated ray barb had to be surgically removed. Photo: Sia Duff

As they entered, the rays circled. Normally shy, pensive creatures that elegantly move through the water, under ordinary circumstances they would rather run from a diver than fight. This time was different. Bennett had been in the water a matter of minutes when it happened. She didn’t see the ray coming and before she understood what was happening a 16cm (6inch) serrated barb shot through her arm. As Bossley rose from the water and looked around, he could see Bennett holding her wound and the blood trickling down her arm. She was remarkably calm, he recalls. A nurse for 45 years, she was more concerned for the rays than she was with herself. “I was thinking: I guess that’s the end of my dive then,” she says. “You know, I’ve seen big smooth rays all my adult life and I’ve never seen them act like this before. They look so graceful and beautiful in the water, they never come toward you, not in my experience.”

“Humans, when they don’t have enough oxygen to their brain or when they’re impacted by something and are in the throes of dying, they do crazy things they wouldn’t normally do. Why wouldn’t a sea creature?

“These rays were sick. They were scared. They were stressed and they were dying. What it highlighted to me was the impact this was all having on the creatures.”

Ray barb, source: Pam Bennett

Faith Coleman says it took authorities longer than it should have to investigate what was happening with the bloom. Coleman, an estuarine ecologist and daughter of respected South Australian scientist, Peri Coleman, says these events are not without precedent. In 2013, another bloom started in the same area before moving north, causing fish kills along the way. There was also another, long-forgotten, episode in 1982. This, she says, gave advanced warning of what was possible.

The big mystery at the start of the algal bloom was which species had come to dominate, a process Coleman described as “a game of Russian Roulette”. There were a number of suspects, most of them closely related. The prime suspect in this bloom, however, was Karenia mikimotoi, a species responsible for fish kills in waters as far afield as Japan, Norway, Ireland and New Zealand. Confirmation, however, took time. Due to a lack of local lab capability, authorities first sent samples to a group in Melbourne. When they couldn’t confirm which species with certainty, samples were sent to a lab in Sydney.

Nine days after the Environmental Protection Authorities sent off its first samples, the Colemans grew tired of waiting. With help from Rowlands as middleman, a sample found their way to the Colemans and the private lab Peri Coleman maintains at her home. Within 20 minutes, they had identified the presence of Karenia mikimotoi. To human health it was low risk, but for marine life, it was a disaster. On the possible spectrum of threat, Coleman says it could always have been worse, with species like Karenia brevis among the worst. Karenia brevis produces brevetoxin, a neurotoxin shellfish poison that is deadly to marine life but has so far only been found in the northern hemisphere and is usually picked up in water monitoring before an event. In May, however, authorities confirmed that, for the first time, brevetoxin had been detected in South Australian waters at three sites affected by the bloom; by August this list had grown to ten. A spokesperson for the Department of Primary Industries and Regions South Australia (PIRSA) said karenia brevis had not been detected, but the toxin’s presence was “evidence that other algae species are likely involved”. In other words, in some places the bloom was more like a cocktail than a spirit, with Karenia mikimotoi the dominant flavor.

Faith Coleman in Peri Coleman’s private lab at her St Kilda home. Photo: Sia Duff

All the necessary ingredients, brought to you by climate change

From there the state’s scientific community began to reverse engineer what had happened. Three ingredients are required for an algal bloom to get going – temperature, the right conditions and food. In the preceding months, South Australia had all the preconditions necessary, Coleman says, thanks to climate change.

“To really tie this to climate change we need to look broader,” Coleman says. “The really interesting thing about this bloom is that it’s occurring all the way along the coast from Ningaloo to New Zealand. In each spot where this hits the coast, we see some sort of heat related distress.”

Zooming out she says what is happening in South Australia should be understood in the context of fish kills and smaller algal blooms along the coasts of Victoria and Tasmania. The starting point for what happened in South Australia, however, was the marine heatwave that formed off the northern Western Australian coast in September 2024. At that time, the oceans surrounding Australia had begun to broil. The world’s oceans absorb a third of all carbon and capture 90% of the excess heat generated by burning fossil fuel. A century of burning oil, gas and coal has transferred energy once locked beneath the earth and pushed it into the atmosphere where it is now being absorbed by the ocean. This process is now at the point where change has become noticeable with ocean temperatures rising hotter than average as the seasons change.

In Coleman’s telling, the hot water off the Kimberley coast of far north Western Australia was eventually dragged south by the Leeuwin Current, triggering coral bleaching events in spectacular reef systems such as Ningaloo and Scott Reefs. Usually the Leeuwin Current loses steam at Albany, but because the ocean has become more energetic with the excess heat it has absorbed, this water kept going. From there, it continued to flow east across the southern coast and the Nullarbor plain into South Australia, knocking out the normal currents and flushing mechanisms that occur through autumn. Ocean currents and strong winds blowing from the south normally bring cold water from the deep ocean and the Antarctic in an effect known as the Bonney Upwelling. Between December 2024 and February 2025, Coleman says the marine heatwave interrupted this process. Long stretches of smooth, clam, sunny days under infinite blue skies made great weather for beach goers and algae alike. With no cool water to dilute the warm, temperatures began to rise and all that was left to get things going was fuel.

Timelapse image of Bureau of Meteorology charts showing ocean temperatures beginning in September 2024. (Source: BOM)

The specific source of this nutrient load is currently debated. Some point to floodwaters moving down from the northern states after the floods in 2023 as the cause. Having washed through the landscape, collecting decomposing leaf litter, debris, fertilizers, runoff and other organic matter, the thinking goes that this soup flushed into the Coorong where it sat for two years. Others point to the nutrients brought up to the surface from an “unprecedented” cold water Bonney Upwelling over the 2023-2024 period. A few storms that swept through during autumn and winter that many hoped might break up the bloom only acted as spreading events, pushing the cloud further out.

Coleman says these explanations are too easy. Algal blooms are triggered by events a few weeks, or sometimes even a few days prior. Coleman says the trigger point in this case was “a really interesting complex array of climate change-related variables” that began when rising water temperatures killed kelp, seaweed and macro algae that preferred cooler waters. The effect was to mulch the sea floor with organic matter that Coleman says was heaven for dinoflagellates, a family of single celled algae that includes Karenia mikimotoi. Under normal circumstances, these organisms provide a waste disposal function within the ocean but with an overabundance of food, they began to feast.

“As the kelp and seaweed died, they released a whole heap of organic matter into the water which is perfect for dinoflagellates,” she says. “In addition, of course, we have increased CO2 in the water. And we have an increased amount of carbonate in the water that helps in their growth.”

A green cloud of death

A diver surveys the damage underwater. Photo: Stefan Andrews, courtesy Great Southern Reef Foundation.

The result was the bloom: a spreading, imperialistic, green-tinged cloud that began to sweep through the ocean. By July the bloom stretched across 4,400km2 and reached depths of 30 metres. Marine species that were quick and mobile had a better chance to swim to safety. Those that had evolved to be habitat-dependent, those too slow and those which took a wrong turn were trapped. Fish, crabs, lobsters, sponges, sea cucumbers, leafy sea dragons, dolphins, sharks, sea lions, cuttlefish, worms, entire microbial networks – anything that could not escape, succumbed. Based on the best understanding available as of August, in fish and other marine life, the bloom acted like a flu. The algae clogged gills, releasing a chemical that prompted them to produce mucus as an immune response in an attempt to clear it. This continued until the fish suffocated. In mammals such as dolphins and sea lions, death took a few extra steps. As the animals swam through the cloud, it irritated their eyes, ears, lungs and skin. Blinded and, increasingly disorientated, they could not recognise the danger.

By August, the citizen-scientist project iNaturalist had identified 19,820 animals from 446 species – but these are just those that washed ashore. It is anticipated that tens of thousands, if not hundreds of thousands of lifeforms, have gradually been swallowed by the cloud – and the more lifeforms killed, the more the bloom can feed.

“It’s now self-feeding,” Coleman says. “We’re not getting the current through the Gulf to clean it out. We’ve got bacterial and organic slurry that is just not being pushed out.”

“The damage is extensive. In the worst-affected areas, we’re dealing with a habitat underwater that has been completely annihilated, like a really severe bushfire has gone through.”

Some within the South Australian government would prefer not to compare the bloom to a bushfire. A better comparison, as Premier Peter Malinauskas told reporters in July when a joint federal-state financial assistance package was announced, was a drought. South Australians understood drought; water security was a long-running issue in the state and drought-like conditions had already been spreading in some regions. The bloom, Malinauskas added, was “more confronting” than drought “simply because we haven’t seen anything like it before.”

“It is severe and confronting by virtue of the fact that it is tragic in that it is new, and even more tragic in the knowledge that, in part, this is a consequence of human induced climate change,” Malinauskas said. “Now I don't say that to sound like an ideologue. I'm saying that because that's what the science tells us so we are confronted by what we are seeing here is a new reality.”

The politics of the moment were perhaps uncomfortable territory for the leader of the centre-left Labor government ahead of a March 2026 election. Australia is currently lobbying to host COP31 with the South Australian capital, Adelaide, expected to host owing to the state’s role in leading the energy transition. Since a catastrophic blackout in 2016 – a similar experience repeated in Queensland, Texas and Spain – the state, home to 1.8m people, has received global recognition for its rapid embrace of renewable energy. At least two fifths of South Australia’s electricity is drawn from renewables, with a growing number of periods where wind and solar provides all the state’s power.

A school of dead fish recently washed ashore on a beach at Port Wilunga. Photo: Sia Duff

Business as usual

On the other hand, the Malinauskas government has maintained an uncomfortably close relationship to the oil and gas industry over its term in government. Santos, one of Australia’s biggest domestic oil and gas producers, is headquartered in Adelaide, a presence that affords considered influence in a state that has grappled with deindustrialization, accelerated by the end of domestic car manufacturing in 2016. In a small city, some crossover in decision making roles is always inevitable but the direct connection between the company and the government is uncanny. Malinauskas’ brother previously served as head of external relations for Santos, and his sister worked with the oil company as a lawyer. When the Australian Petroleum Production and Exploration Association (today known as Australian Energy Producers) held its 2023 national conference in Adelaide, Minister for Energy and Mining Tom Koutsantonis opened the conference by telling his audience of fossil fuel company executives that the state government was “at your disposal”. Days later, in response to protests by Extinction Rebellion that attempted to disrupt the conference and shame Santos as a fossil fuel producer contributing to climate change, the Malinauskas Labor government rushed through some of the strictest new anti-protest laws in the country.

When it comes to the algal bloom, activist groups including Extinction Rebellion and Rising Tide have already been working to make the link with climate change and fossil fuel production explicit. In late July a group gathered outside Santos headquarters holding signs with photos of dead and dying marine life, and captions that read, “MAKE SANTOS PAY”. The group had attempted to deliver a box of letters addressed to the company’s CEO, Kevin Gallagher, but security took the box and turned the group away.

So far the state government has sought to thread a needle with its public communications about the bloom and its impacts. A taskforce has been set up to provide weekly updates, and both the Premier and Deputy Premier, Susan Close, have been explicit about the role of climate change, but others are more cautious in how they talk about the crisis to avoid saying anything that might undermine tourism and aquacultural businesses that have so far been unaffected. Staffers and communications officers privately brief that three in four of the state’s beaches remain clear. The majority, some 90% of shell fish producers, remain open and operational, they say. The message is that if South Australian seafood is commercially available, it is safe to eat – a takeaway fish and chip owners were keen to reinforce after reporting a drop off in trade.

A seahorse killed by the algal bloom. Photo by: Stefan Andrews, courtesy Great Southern Reef Foundation.

Raising some alarm, however, has been necessary. In late June, after some delay, a joint $28m (USD $18.3m) support package was announced with the South Australian government providing $14m (USD$9.2m) and the remainder provided by the federal government. This package included money for monitoring and scientific research, and $10,000 grants for small businesses affected by the bloom, financial counselling and mental health support – assistance that Pat Tripodi from the Marine Fishers Association says is badly needed. Uncertainty has taken a toll, he adds. In many cases his members have been on the water for most of their lives, with multiple generations of some families working the ocean. Now they are being confronted with the end of everything they have known and no one can tell them how long it will last, or how far it will go.

“The people experiencing it were quite shocked and emotional,” Tripodi says. “We’re seeing grown men brought to tears over this because of the uncertainty over their future.”

“The impact’s been crippling. They’ve got no ability to earn an income. Where the algal bloom is, there is literally nothing to be caught. They’ve lost their ability to make an income. That’s really disheartening for them – and then comes all the uncertainty that goes with it.”

A traumatizing spectacle of destruction and doom

Marine ecologist Scott Bennett says the psychological blow is hard to articulate. Now the bloom is underway, there is nothing that can be done to stop it. What was once considered a distant or theoretical impact of climate change is real and being felt. Early on in the bloom, Bennett, co-founder of the Great Southern Reef Foundation and no relation to Pam Bennett, participated in marine surveys in an attempt to quantify the scale of what was taking place. Even he, who was aware of the science, was shocked.

“It is hard to overstate the catastrophe, the ecological disaster that is unfolding in South Australia right now,” Bennett says. “As a scientist and a researcher, I travel around the country conducting biodiversity surveys and assessing the health of reefs. They’re normally things that you really look forward to, because there are just amazing things we see on these trips.”

“To go on this trip and face the spectre that everything you love might be gone – and you really just don’t know how bad it is. The whole spectacle of death and destruction and doom is hugely emotionally taxing – I really felt it, even as an outsider.”

Anthony Rowlands at Waitpinga Beach where he first raised alarm about the toxic algal bloom. Photo: Sia Duff

The experience, he says, left him shaken. Ecologically the Great Southern Reef functions as a highway linking east with west. Species that diverged hundreds of thousands of years ago and are normally separated by geography, or their evolutionary ancestors, can still be found together on the reef. Among them are endangered species like the giant kelp that plays host to whole communities, the endangered Australian sea lion and the vulnerable white shark. Bennett says the reef is so understudied there are many species that have yet to be identified or properly researched. How much will be left remains to be determined.

“When this is your backyard and the places and creatures you have grown up with, and you love, and that form an intimate part of your life and existence –when that’s suddenly taken away from you, and lost at scale, that is devastating,” he says. “The trauma South Australians are experiencing right now cannot be understated and it is so indicative, so symbolic of climate change more broadly. It is a visceral and devastating thing when it happens to you.”

Others in the state’s scientific community fear the bloom offers a terrifying glimpse into the future. Wayne Boardman, a veterinary scientist and associate professor at University of Adelaide says it may represent that “initial phase of the breakdown of nature and climate change” and a vision of the future.

“It’s like a trigger: it kills off all the fish and so we lose a lot of biodiversity. So the next year, what happens? And the year after that, what happens?” Boardman says. “We might go into remission at some point, but it’s likely, unless we do something different, it’s going to be one of those inexorable problems that are going to get worse. And it’s going to decimate.”

“It might be the start of permanency.”

In July the Biodiversity Council described the bloom as “a devastating example of how climate change has supercharged the potential for marine catastrophes.” Its ten-page report outlined what was required to respond to the crisis. At the top of the list was a phaseout of oil, gas and coal, the end of fossil fuel subsidies and exports, greater efforts to control nutrient run-off from land into the ocean and a program to restore shellfish reefs, sea grass meadows, kelp forests and mangroves along the coast that help to filter and absorb excess nutrients. Coleman, who has proposed a similar program of work, says a response on this scale is necessary to help ecosystems recover, even if it would require “one of the greatest marine restoration attempts the world has ever seen.”

“No one wants it to be climate change because you can’t do anything about it locally. They’d like it to be an act of God or someone else’s problem. And to me that is very shallow thinking,” Coleman says. “The truth is that we’re pushing ecological systems to their edge and most extractive systems require our ecological systems to be stable.

“What we can do is engage transparently. We can reinstate the oyster. We can reinstate the sea grass. We can reinstate all these other habitats to some degree. That will make us more resilient the next time this happens again and we can sequester more carbon – but there is only one thing that will actually stop this and that’s no more fossil fuels.”