A harmful algal bloom (HAB) event that started in March 2025 is ongoing along the coasts of the state of South Australia as of October 2025 [update] . It has affected not only beachgoers and tourism along the affected coasts, but also the fishing industry and aquaculture. The state government has set up an information website and hotline dedicated to the event, and the state and federal governments are working together on mitigation of the bloom. The environment ministers responsible are Lucy Hood (SA) and Murray Watt (federal). The causes of the bloom are attributed to flooding of the River Murray in 2022-23 that brought extra nutrients into the sea; a cold-water upwelling in summer 2023-24 that lifted more nutrients to the surface; and a marine heatwave that has occurred since September 2024. These changes, particularly the latter, have been attributed to climate change.
In March 2025, a marine heatwave-induced bloom of Karenia mikimotoi was blamed for the deaths of thousands of marine creatures, as well as coughing, sore eyes, and blurry vision in humans, along stretches of beach along the south coast of the Fleurieu Peninsula in South Australia. [1] Despite hopes that winter storms would cause the bloom to dissipate, [2] relatively mild conditions (potentially attributed to climate change [3] [4] ) caused it to persist [5] and spread along adjacent coasts of Kangaroo Island and Gulf St Vincent (including Adelaide's metropolitan beaches [6] [7] ) and into the Coorong. [8]
By the middle of the year, the state government estimated the size of the bloom as around the same size as Kangaroo Island, which covers over 4,000 km2 (1,500 sq mi) (about 80 times the size of Sydney Harbour). [9] By October 2025, the bloom had spread on the currents, into both Gulf St Vincent (flanking the Yorke and Fleurieu Peninsulas) and Spencer Gulf, along the coast of the Eyre, as well as to Kangaroo Island itself. [9] Marine biologist Mike Steer, chief executive of SARDI, [10] estimates that it has affected around 30 per cent of the state's coastline, roughly equivalent to 1,500 km (930 mi). He said that its extent and duration is unprecedented in both South Australia and Australia, and is probably one of the top ten recorded blooms in the world. [9] The densest occurrence of the bloom, manifesting in foam, was occurring on the stretch down the coast from the top of Gulf St Vincent down to Brighton Beach. [11]
It was at first thought likely that flooding that occurred in 2022–23 which washed nutrients down the Murray–Darling basin (which covers a large area across several states and territories of Australia into the sea at the Murray mouth on the south coast of South Australia, combined with a marine heatwave and an upwelling of nutrients from sea floor, created "perfect conditions" for K. mikimotoi to flourish. [12] However, scientists later said that this was an unlikely cause, as the time lag between the events was too great. [13]
According to the government website, the bloom has been building up over time, and its causes may be attributable to the following events: [14]
According to the not-for-profit Biodiversity Council, [a] marine ecologists have warned that ocean warming and marine heatwaves associated with climate change, as well as nutrient pollution, have led to a rise in the number of algal blooms, and this is likely to continue. Large areas of kelp, seagrass, and shellfish reefs, which tend to filter nutrients, have been lost. In addition, the gulfs around the South Australian peninsulas, especially Gulf St Vincent, are more susceptible to HABs because they have less water exchange than open ocean coastlines, and the water is shallower, which increases its heating capacity. [16] The Australian Marine Conservation Society agrees with the assessment that climate change has driven the bloom, calling the event a "crisis" and a "warning we can’t ignore". [17]
The bloom has killed at least 430 species [16] of fish and marine mammals, and has also caused health problems such as asthma, skin and eye irritation, and coughing in humans. [18] It has affected the aquaculture, [14] fishing and tourism industries, especially local fishers, as well as the many residents who live along the coastline and surfers who frequent popular surfing spots such as Port Elliot. [9] Several groups of First Nations peoples have called the ongoing bloom a "cultural emergency". The Narungga people of Yorke Peninsula have been heavily affected by the bloom. [19] The Goolwa Pipi Co., which harvests a small edible saltwater clam known as the Goolwa pipi, was no longer able to harvest and sell their shellfish. [20]
It was feared that the bloom would affect the breeding season of the unique population of giant cuttlefish in the northern Spencer Gulf, in a sanctuary area that received National Heritage status in 2023. This population of the species is even distinct from the population in the southern Spencer Gulf, and the only population of giant cuttlefish in the world to breed in a mass event, in a spectacular display of colours, including blue, purple, green, red and gold, which change as they mate and lay eggs. This makes it a popular tourist attraction. [21]
In the seven days preceding 3 October 2025, clearing of beaches along the coast collected around 1,600 kg (3,500 lb) of marine animals, including longsnout boarfish, whiting, and leatherjacket fish. The previous week of hand-clearing netted a total of two tonnes of dead sea life. [20]
The state government briefed federal Environment Minister Murray Watt on 11 June 2025, who engaged with engaged with then state environment minister Susan Close during the following months. At that time, the HAB was expected to dissipate over the winter months. [22]
The state government has created a website to provide information to the public, [23] and has issued vouchers to encourage tourists. [24] To complement the website, a telephone hotline was launched on 2 October 2025, to be used for general information, reporting dead fish or other animals, getting health advice and mental health support, and information about support packages. [11]
On 9 September, SA Health issued updated public health advice, saying that people with asthma should carry medication when visiting beaches when foam is present. [25] SA Health is also providing mental health support for those affected by the HAB. [26]
The state and federal governments have provided A$28 million in funding to support research, clean-up efforts, community support, industry help, and public information. [23]
A support package for small businesses affected by the HAB is being provided by the state government, which includes a one-off Algal Bloom Fisheries and Aquaculture Assistance Grant of up to A$100,000 as well as a A$10,000 Algal Bloom Small Business Support Grant, for smaller businesses or not-for-profits. [27]
In August 2025, the state and federal governments provided funding of almost $700,000 to install a bubble curtain to protect the giant cuttlefish in the Upper Spencer Gulf during their breeding season, should the bloom move into that area. The curtain would protect approximately 50,000 to 80,000 eggs and hatchlings. [28]
On 3 October 2025, the state and federal governments announced a $1.4 million program to protect Australian sea lions, which are an endangered species of which 80 per cent live in South Australian waters. [20]
A Senate inquiry was set up on 23 July 2025, with the Senate Standing Committees on Environment and Communications due to publish its report on 28 October 2025. [29] On the opening day of the inquiry, the committee heard that the state was unprepared for the disaster. [25] It has held public hearings in Adelaide, Port Lincoln, Ardrossan, and Victor Harbor. [13] Kaurna Yerta Aboriginal Corporation, in its submission to the inquiry, said that the government's response had been "too slow" and it had not engaged enough directly with First Nations people. [19]
Premier Peter Malinauskas and new SA Environment Minister Lucy Hood continue to liaise with federal Environment Minister Murray Watt about a plan for managing the HAB through the summer. [11]
As of 2 October 2025 [update] , 94 coastal sites are being regularly monitored by the South Australian Research and Development Institute (SARDI). [11] The bloom is not present in the northern Spencer Gulf, the giant cuttlefish breeding ground. [11]
Donald Anderson, the director of the US National Office for Harmful Algal Blooms at the Woods Hole Oceanographic Institution, who has been consulted by the state government, has described a treatment involving a form of modified clay. The substance can be sprayed into the water, where it sticks to the algal cells, creating clumps called "floc", which fall and settle on the sea floor. [12]
The Australia Institute has suggested that a National Climate Disaster Fund, paid for by levies on coal, oil, and gas production, would save taxpayers the billions that needs to be spent on mitigation of disasters caused by climate change, such as the South Australian HAB. [30]
Marine ecologist Dominic McAfee, of the University of Adelaide, said that there is limited understanding on what impacts the HAB will have in both the short and long terms, and it is essential to first fill the gaps in knowledge in order to provide evidence-based management, monitoring, and an appropriate socio-economic response to recover from the event. [16]