Toxic algae bloom devastates marine life along South Australia's coast

Karenia mikimotoi blamed for mass deaths of over 200 marine species amid worsening climate-driven marine heatwave.

A massive bloom of toxic algae is causing the death of over 200 marine species off the coast of South Australia. Photo by Brad Martin/OzFish

By Widya Putri and Hayu Andini

An enormous toxic algae bloom off the southern coast of South Australia is causing an ecological disaster, killing more than 200 marine species and raising alarm among scientists and environmental organizations. The harmful algal bloom, identified as Karenia mikimotoi, has spread across a 4,400-square-kilometre stretch of ocean, a marine area larger than Germany or Japan. The phenomenon is described as one of the most devastating marine life die-offs the region has ever witnessed.

The toxic algae bloom South Australia began in March, and its impact has been visible on several popular coastlines including Kangaroo Island, Yorke Peninsula, and the Fleurieu Peninsula. According to Brad Martin, a regional manager for Ozfish, a non-profit fish conservation organization, the scene along these beaches is haunting.

“There are carcasses littering beaches,” Martin told AFP. “It is common for our volunteers to say: ‘We walked for one kilometre along the beach and saw 100 dead rays and other marine life.’”

The algae, which release toxins that destroy gill tissue in marine animals and suffocate them, have killed fish, sharks, crabs, octopuses, rays, and other aquatic species. The long-term impact on biodiversity and marine food chains could be severe, experts warn.

Climate change and heatwaves drive algae bloom

Scientists point to rising ocean temperatures and extended periods of calm marine conditions as the primary triggers of this harmful algal bloom. Marine biologist Shauna Murray from the University of Technology Sydney, who helped identify the algae, said the bloom is directly linked to ongoing marine heatwaves.

“This species damages the gills of fish and causes death by suffocation,” Murray explained. “It is not pleasant. And it will probably take some time for the ecosystem to recuperate.”

This latest algae outbreak South Australia aligns with a broader trend across the continent. As Australia grapples with the effects of climate change, the frequency and intensity of marine heatwaves have increased, putting stress on sensitive ocean environments.

South Australia’s Environment Minister Susan Close acknowledged the grim nature of the event in a recent statement. “We need a big change in weather to break this thing up — there is nothing we can do to precipitate this,” she told ABC News.

Beaches and tourism impacted by marine life die-off

Besides the environmental toll, the economic impact is beginning to be felt, especially in communities that rely on tourism and fishing. Kangaroo Island and other destinations, known for their natural beauty and wildlife, have been visibly affected, with visitors reporting beach walks filled with the sight and stench of dead sea life.

In response, South Australian health authorities are advising the public to stay away from foamy or discolored seawater, citing potential health risks. Swimming in contaminated waters could cause skin irritation and even respiratory issues.

For conservationists like Martin, the sight of rays and fish lying lifeless on once-vibrant beaches is more than a tragedy — it’s a wake-up call. “This is unprecedented in our waters. We’ve never seen anything like this in South Australia before,” he said.

A global problem hits home

The Karenia mikimotoi algae have long been recognized as a global threat. First identified in Japanese waters in the 1930s, the algae have since caused similar disasters in China, Norway, and the United States. In each instance, they disrupted local marine ecosystems and crippled fisheries, sometimes inflicting economic losses that ran into the millions of dollars.

However, the toxic algae bloom South Australia stands out due to its sheer scale and duration. “This is not just a local incident,” said Martin. “It’s part of a larger pattern of climate-fueled marine degradation that’s happening worldwide.”

Calls for long-term environmental strategy

Scientists and environmentalists are urging the Australian government to take stronger action on climate mitigation and marine conservation. While marine heatwaves are difficult to control in the short term, better monitoring systems, stronger marine protections, and more aggressive climate policies could help reduce the frequency and severity of future events.

Some have called for an independent review into the South Australian coast’s marine resilience and a possible marine conservation emergency response taskforce. “We must start investing in solutions that make our marine ecosystems more resilient to climate extremes,” said marine ecologist Dr. Helen Worth from the Australian Institute of Marine Science. “That means tackling pollution, overfishing, and of course, reducing emissions.”

A future at risk

The mass die-off is expected to have lasting effects on South Australia’s marine biodiversity. Ecosystems disrupted at the base level — including algae blooms that choke out oxygen and kill marine animals en masse — often take years, if not decades, to recover. The loss of keystone species such as rays and crabs could lead to cascading consequences, altering food webs and local fish stocks.

Meanwhile, public concern is growing as images of dead marine life and empty beaches circulate online. Advocacy groups have started petitions and awareness campaigns calling for stronger protections for the region’s coastlines.

As the harmful algal bloom continues, communities along South Australia’s shores remain on alert. Researchers are monitoring the bloom’s spread, hoping for a shift in wind or water temperatures that could bring relief.

Until then, the state faces a stark reminder of the fragility of marine ecosystems — and the urgent need for action in the face of a changing climate.