- The largest and longest-lasting harmful algal bloom in Australia’s history, which started in early 2025, has potentially affected more than 20,000 square kilometers of ocean waters and about a third of the coasts in the state of South Australia.
- The algal bloom has devastated marine ecosystems and caused significant economic losses in the local fishing, aquaculture and tourism industries.
- As officials, researchers and communities grapple with its ecological, health and social impacts, the bloom has exposed a lack of preparedness at all levels of government for responding to future HABs.
PORT HUGHES, Australia — Situated midway along the Great Southern Reef that spans Australia’s southern coastline, the waters off Port Hughes typically teem with life. The coastal hamlet northwest of Adelaide plays host to a multitude of coral, bivalve and fish species.
But in late March, the largest and longest harmful algal bloom (HAB) in Australian history arrived to Port Hughes, depleting its waters’ rich biodiversity. The bloom had first appeared elsewhere off the state of South Australia’s coast a year earlier, causing eye and skin irritation and respiratory symptoms among beachgoers. Then, along with waves of acrid-smelling sea foam, scores of dead marine animals began washing ashore.
In Port Hughes, the HAB’s impacts were most visible below the surface. The town’s wooden jetty had previously been one of the most consistent locations in South Australia to observe temperate species, said Stefan Andrews, co-founder of the Great Southern Reef Foundation, a conservation advocacy group. But by mid-April, when Mongabay joined Andrews on a dive, the site was drab compared with vibrant photographs taken in February and March.
Under the jetty, sponges and corals that had previously adorned its pylons in a brilliantly hued mosaic appeared colorless. Apart from a short-headed seahorse (Hippocampus breviceps) — a “sign of hope,” Andrews called it — little life was visible in the murky waters. The reef, he said, had become quieter, lacking the sounds of snapping shrimp and other creatures that once played in the underwater soundtrack.
“There’s a sense of loss when you dive somewhere familiar and watch it decline, especially when you know some of what you’re seeing may take a very long time to return, if it returns in the same way at all,” Andrews said.
More than a year after the HAB began, the government has still not released an official estimate of the bloom’s extent. In August, the European Space Agency put the affected area of ocean waters at 4,500 square kilometers (1,740 square miles); more recent citizen science data indicate the bloom may have ultimately affected more than 20,000 square kilometers (7,700 square miles). By October, state agency scientists estimated the HAB had impacted about a third of South Australia’s coasts.
Although the bloom has now peaked, harmful pockets continue to devastate marine life and coastal industries as state and local authorities, researchers and communities scramble to understand its impacts — and how to mitigate the effects of future blooms. Moreover, even after the bloom has passed, ecological recovery of affected areas is uncertain.
Government response criticized amid fears of ecological collapse
HAB events involve microalgae, also known as phytoplankton, that are pivotal to marine ecosystems as they underpin the diets of larger species. However, if algae are stimulated by a particular event, such as a nutrient influx, their populations can grow excessively and trigger HABs. These events are on the rise globally both in frequency and intensity, according to a 2019 report by the Intergovernmental Panel on Climate Change.
Researchers identified several species of microalgae from the genus Karenia as culprits in South Australia’s HAB. Some, notably Karenia cristata, produce neurotoxins called brevetoxins that can be deadly to marine species and provoke a range of symptoms in humans, from eye, skin and respiratory irritation to nausea and neurological effects.
As the HAB lingered through Australia’s winter and its impacts spread, the state and federal governments, both led by the left-leaning Labor Party, faced criticism for their responses to the crisis. In a rare show of unity, the federal opposition — a conservative coalition comprising the Liberal and National parties — joined South Australia Premier Peter Malinauskas in calling for a natural disaster declaration, which would have provided additional funding. But Prime Minister Anthony Albanese refused.
The state and federal governments have collectively allocated more than $A115 million ($82.6 million) in economic aid for embattled communities and funding for scientific monitoring. However, a federal Senate committee investigating the bloom concluded in November that the South Australian government was unprepared for the HAB due to a lack of long-term ecological monitoring along the state’s coastline.
Even now, obtaining basic information about the bloom remains a challenge. South Australia’s Department of Primary Industries and Regions, the agency responsible for fisheries and aquaculture industries, did not respond to questions about post-HAB recovery and mitigation planning. A spokesperson told Mongabay that current data on marine animal mortality could be found in iNaturalist, a citizen science application.
On the application, South Australian residents have recorded more than 117,000 individual animals found dead on the state’s shorelines, representing more than 800 species. That includes more than 17,000 bluefin leatherjackets (Thamnaconus degeni) as well as rays, dolphins, Australian sea lions (Neophoca cinerea), little penguins (Eudyptula minor) and great white sharks (Carcharodon carcharias).
However, Dominic McAfee, a marine scientist at Adelaide University, told Mongabay that the real mortality numbers could be far higher than the crowd-sourced iNaturalist figures.
“The numbers are likely a minuscule fraction of the actual mortality,” McAfee said. “Mobile species that have remained intact and washed ashore can be easily identified. What we know far less about is invertebrate communities which live within the sediment. These won’t wash ashore and have been completely obliterated in some areas.”
“Ecological collapse is certainly representative for many of the groups that have been represented in the bloom. It’s been catastrophic.”
In an email, Emily Bourke, South Australia’s environment minister, told Mongabay the state has conducted surveys across more than 200 sites to “assess the bloom’s impact and establish a baseline to monitor ongoing recovery” with measures including real-time oceanographic water monitoring and the establishment of an Office for Algal Bloom Research.
Questions of causation
Along with assessing the ecological damage, officials are trying to understand what caused this bloom so they can respond more effectively to future HABs.
In South Australia, an extended heat wave that began in 2024 raised ocean temperatures by 2.5° Celsius (4.5° Fahrenheit), leading initial discussions about the cause of the HAB to center on algae growth stimulated by heated waters. The state government later pointed to two other potential contributors: flood waters from the Murray, Australia’s longest river, and a major oceanic upwelling event, which both concentrated nutrients within the affected areas. South Australia’s Department of Environment and Water had previously identified both the increased risk of flooding and changes in nutrient cycling in marine waters as future climate trends.
Calling the bloom “unprecedented,” Bourke told Mongabay that “warmer ocean temperatures, driven by climate change, have contributed to the right environment for the bloom — highlighting the importance of ongoing research and monitoring.”
Pinpointing exactly what led to the HAB is hindered by a lack of data, said Donald Anderson, director of the U.S. National Office for Harmful Algal Blooms. Anderson was invited to South Australia in late 2025 to advise state agencies on responding to the bloom and containing its effects. In an email, he told Mongabay its cause is still not completely clear.
“There are multiple factors that likely contributed to the bloom, and there is insufficient data from before the bloom occurred to narrow down the options,” Anderson said.
The federal Senate committee report from November cites evidence that Australia’s existing oceanographic monitoring programs, including the Integrated Marine Observing System that measures biological and chemical variables across Australian waters, lacked the ability to fully and accurately detect HABs.
“It is hard to get the money needed to support a robust monitoring program when there is no problem, yet that is the kind of background information that is needed to answer the question of what might have caused an outbreak when one eventually does occur,” Anderson said.
Economic, cultural and mental health impacts
The HAB’s impacts have been far-reaching. Economic damage, including to the region’s tourism, fishing and aquaculture industries, has been estimated at $A250 million (about $180 million).
South Australia’s seafood industry contributes $AU788 million ($565.5 million) to the state economy. In the wake of the bloom, it has suffered significant losses amid decreasing numbers of target fish species. The state government responded with a selective ban on commercial fishing in parts of the state until April 30, 2027, to help hard-hit stocks recover.
It’s not just financial stress: A study by Australian researchers found high levels of “eco-anxiety” in affected South Australian communities, with many participants reporting worry, sadness and feelings of helplessness.
These were concerns echoed in a statement by the South Australian First Nations Voice, a mechanism through which Aboriginal communities can directly address Parliament. It said, “The algal bloom outbreak is not only killing our sea life, including those that hold deep spiritual significance to our people, but also damaging our living cultural spaces — places that are integral to the traditional practices and identity of our people.”
Back at Port Hughes, the stress is palpable, said Andrews, who worries about how the HAB’s effects will be addressed in the long term as people start to normalize the bloom.
“At first there’s shock and denial, then hope that it will move on quickly, but I think we are underestimating how long recovery may take,” he said.
Building resilience
While communities and businesses absorb the fallout, scientists and state government are taking steps toward ecosystem rehabilitation.
Historically, the South Australian coast harbored extensive reefs comprising native oysters (Ostrea angasi) and other shell-forming species that provided shelter and nursery habitat for other marine life. Dredging and industrial exploitation drove these reefs to functional extinction by the mid-20th century.
Prior to the HAB, the South Australian government had invested in shellfish reef restoration to increase biodiversity. Bourke told Mongabay that this continued investment, alongside $AU160 million ($114.8 million) in research and support, “will build environmental resilience and help mitigate potential future blooms.”
McAfee, who has worked on reef restoration for a decade, said these reefs can help areas of South Australia’s coast recover: Native oysters have shown resilience in the face of the HAB, even eating Karenia cristata. This ability to persist through the bloom, coupled with the restoration of nursery habitat, could help restore biodiversity, McAfee said.
However, McAfee is cautious, since any future event could further damage marine ecosystems. He said addressing future blooms will require all levels of government to make a unified investment in understanding their causes and effects, “given that [HABs] know no judicial boundary and can have such devastating outcomes.”
“Although some life remains in affected areas, the bloom was a catastrophe,” McAfee said. “We are still in our infancy in understanding how these events unfold and persist.”
Banner image: A dead dusky morwong (Dactylophora nigricans) lies on a bed of seaweed at the water’s edge. Image courtesy of Stefan Andrews / Great Southern Reef Foundation.
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Citations:
Murray, S., Bolch, C. J., Brett, S., Chan, C. X., Doubell, M., Farrell, H., … Smith, K. S. (2025). A catastrophic marine mortality event caused by a complex algal bloom including the novel brevetoxin producer, Karenia cristata (Dinophyceae). doi:10.1101/2025.10.31.685766
Le Busque, B., Falkenberg, L., Williams, C., & Litchfield, C. (2026). Psychological toll of a marine environmental crisis. Journal of Environmental Psychology, 110, 102964. doi:10.1016/j.jenvp.2026.102964
Baum, F., Fisher, M., Bogueva, D., Hayes, A., Miller, H., & Marinova, D. (2026). Did Australian policy prepare for a harmful algal bloom with significant human health impacts? Analysis and lessons from South Australia. Health Promotion International, 41(1). doi:10.1093/heapro/daaf240
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