‘Marine apocalypse’: Algal bloom devastates South Australia’s coastal ecosystems

May 13, 2025

Issue

A weedy sea horse, washed up from algal bloom on Kangaroo Island. Photo: Tracey Carpenter

A catastrophic algal bloom — fueled by warming oceans and calm marine conditions — is killing marine life on the shores of South Australia (SA) on an unprecedented scale. Experts, beach goers and officials describe it as an unfolding “marine apocalypse”.

Since late March, residents and scientists have observed mass deaths of fish, shellfish, sharks, sea dragons, rays, cuttlefish and deepwater species along the southern coastline — particularly the Fleurieu Peninsula, Kangaroo Island and Goolwa. Dead weedy and leafy sea dragons, a protected species and an icon of the region’s biodiversity, have washed ashore in record numbers.

The toxic algal bloom, Karenia mikimotoi, a dinoflagellate that thrives in warm, stagnant water and emits toxins lethal to marine life is the cause of this catastrophe. Scientists say it is the result of record-breaking marine heat waves, which have raised sea surface temperatures in SA from 2.5°C to 3.2°C above average, since September last year.

In addition, is currently crippling multiple southern states. Kaurna Yerta/Adelaide has received only around 300 millimeters of rain since February last year, making it the driest 15-month spell since 1839. The drought extends across the state’s agriculture belt.

SA Minister for Climate, Environment and Water Susan Close has sounded the alarm on how climate change is worse. She said ocean monitoring shows “a full-scale climate emergency in our coastal waters”, with the bloom extending across an area roughly the size of Kangaroo Island and up to 20 metres deep. “These extreme marine heat waves are not just anomalies; they are the new reality. And our marine ecosystems are the first casualties.”

The algae blooms’ persistence is attributed to weeks of unusually calm conditions, low wind and limited wave action.

Chronicle of collapse

One of the most detailed records of this catastrophe has come from Hugh Rischbieth’s personal accounts, published in the. Rischbieth, a beach walker at Island Beach, began chronicling the unfolding disaster in late March. He said the past six weeks have been “unlike anything I’ve ever seen”.

Rischbieth noticed the first signs, discoloured water at Browns Beach, on March 27. Two days later, rays were swimming ashore to die. He buried more than 160 rays in the following days.

On April 6, he collected a King George whiting and other species for testing. Two days later, cockles, razor fish, abalone and bubble shells were washing up en masse, alongside larger deepwater fish such as John Dory. On April 24, blue and ribbed mussels died off, followed by a massive crab die-off the next day.

We are seeing every layer of marine life perish — from the smallest worms to sharks and sea dragons,” Rischbieth said. “I don’t know what will recover and what won’t.” Rischbieth collected 10 weedy sea dragons in a single 100-metre stretch on May 5. Thousands of pipefish were also found dead — a species not previously affected.

Other regions are reporting similar devastation. Snellings Beach was blanketed in salmon, flathead, swallowtails and leatherjackets, after recent northerly winds. At Stokes Bay, the smell was so strong that beachgoers reported coughing and respiratory issues.

At Goolwa Beach, there were an uncountable number of dead cockles. Eyre Peninsula residents have reported sighting washed up dead Fairy Penguins and Mutton Birds. It is yet to be determined if these are related to the algal bloom. , health authorities have detected a waterborne toxin linked to the algal bloom that has forced at least six oyster farms to temporarily shut and quarantine up to 10 million oysters.

Stephanie Luke, a former Kangaroo Island resident, who recently visited said the devastation washing up on its beaches was “a marine apocalypse”.

“We’ve seen every kind of life disappear from these waters,” Rischbieth said. “This is more than just a bloom. It’s a collapse.” “We don’t know what will recover and what won’t. I had to speak up. This is no longer just a natural event — it’s a crisis accelerated by climate change.”

Tour guide Roanna Horbelt, from Research and Discovery Coastal Tours on Kangaroo Island, said she’s never seen such a die-off of iconic, protected marine species, including both leafy and weedy sea dragons. “Your help as citizen scientists is critical. We need to document every death to understand the scale of this.”

In a disturbing twist, four great white sharks have reportedly become stranded on SA beaches in recent weeks — seemingly trying to escape the toxic waters. A 3.01-metre white shark, found at Henley Beach on May 5, is currently being autopsied to find out whether its disorientation or respiratory failure is linked to algal toxins.

Call for citizen scientists

More than 137 citizen scientist contributors have submitted observations and photographs, helping scientists identify patterns and hotspots. Marine biologist Janine Baker warned that the impact on sea dragon populations may be long-lasting.

“These deaths are happening just before their breeding season,” Baker said. “Populations across southern and central SA have been dying since March — it could take years for them to recover, if they do at all.”

�³󾱱��Karenia mikimotoi is not known to be toxic to humans who consume affected seafood, health officials are urging people to avoid contact with discoloured water, foam, or areas with strong marine odours, due to risks of skin, eye and respiratory irritation.

Cooler weather may help disperse the bloom in coming weeks, but the underlying driver — warming oceans from climate change — remains.

“This is not just a one-off tragedy. It’s a warning of what lies ahead if we don’t act fast on climate,” Close said. “Our marine ecosystems are incredibly vulnerable, and they’re telling us something loud and clear.

“We’ve had a marine heat wave off the coast of South Australia about 2.5 degrees above the average expected temperature for a prolonged period of time, alongside this very still weather pattern,” said Close.

“So, unfortunately, as the world fails to fully come to grips with climate change, we are likely to see more of this kind of experience.”

said last year that as SA has switched off its last coal-fired power station, and it has one of the strongest renewable energy targets in the nation, it is on track to source 100% of electricity from renewables by 2030.

However federal Labor, under Anthony Albanese, has continued to support the expansion of fossil fuels, largely for export, in direct contravention of the Intergovernmental Panel on Climate Change’s call for no new coal or gas projects.

[People can report marine deaths to the Department of Primary Industries and Region’s Fishwatch hotline on 1800 065 522 and upload sightings to the .]