California Faces New Algal Bloom as South Australia Battles Its Own

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South Australia and California are dealing with devastating harmful algal blooms (HABs) that are killing marine life and threatening coastal economies. South Australia’s bloom spans around 4,500 km² and has decimated marine species, while California’s latest outbreak mirrors similar ecological destruction. This article explores these parallel crises, their causes, and the urgent need for stronger responses.

In South Australia, a major HAB dominated by the dinoflagellate Karenia mikimotoi has persisted along the coast since March 2025, driven by a marine heatwave that pushed sea temperatures nearly 2.5 °C above average. Covering an area roughly twice the size of the Australian Capital Territory, the bloom has killed over 13,800 marine animals, spanning nearly 400 species, including fish, sharks, rays, seadragons, octopus, and cuttlefish. Marine mammals, though not directly poisoned, have also washed ashore. Local ecologists warn that unless conditions change during winter, the bloom could last up to 18 months.

The state and federal governments have pledged $28 million to fund clean‑up, scientific research, monitoring buoys, and fisheries assistance. Critics argue the response falls short, claiming more support is needed and calling for a formal national disaster declaration. Concerns include widespread health complaints reported by beachgoers and disruptions to fishing and tourism.

Meanwhile, along California’s coast, coastal waters are experiencing a toxic bloom fueled by Pseudo‑nitzschia, which produces the neurotoxin domoic acid. This biotoxin has led to unprecedented marine mammal stranding events, with reports of over 2,000 sea lions, dolphins, whales, and seabirds affected. Domoic acid accumulates in forage fish such as anchovies and sardines, passing up the food chain and causing seizures, disorientation, and death among marine mammals. Pregnant sea lions have also been impacted, with reports of over 85 fetal losses at a wildlife care center.

The economic impact is mounting. In South Australia, oyster farms in Gulf St Vincent and Kangaroo Island remain closed, and coastal businesses report losses and tourism cancellations. In California, fishing yields have plummeted and wildlife rescue centers are overwhelmed dealing with sick and stranded animals.

Both regions are seeing momentum build from climate-driven trends, warmer waters, nutrient runoff from floods or wildfires, and milder winters contribute to HAB frequency and duration. Critics argue that government responses focus too much on observation rather than prevention. Long‑term measures such as restoring seagrass beds, reducing nutrient pollution, and stricter runoff controls are needed to prevent recurring blooms.