South Australia is facing one of its most severe harmful algal blooms (HABs) in decades — a toxic marine event driven by the algae Karenia mikimotoi that has devastated coastal ecosystems, fisheries, and wildlife. What began earlier this year along the Fleurieu Peninsula has now spread to cover nearly a quarter of South Australia’s coastline, making it one of the largest and longest-lasting blooms ever recorded in the region.

The bloom’s persistence stems from a combination of environmental factors. Flood runoff from the River Murray introduced nutrient-rich sediments into coastal waters, while cold-water upwelling and a marine heatwave — with temperatures 2.5°C higher than normal — created ideal conditions for algal growth. These interactions have fueled a toxic bloom that refuses to dissipate, choking marine life and staining the once-clear waters with discolored foam.

The ecological consequences are devastating. Thousands of fish, shellfish, and invertebrates have died, washing up along beaches in large numbers. Conservation groups have warned that the event could have lasting effects on marine biodiversity, including endangered species such as the Australian sea lion. In response, the government has announced a $1.4 million protection program to safeguard sea lion colonies through vaccination programs, habitat barriers, and predator control.

Beyond the ecological toll, the bloom has also hit local economies hard. Seafood demand has plunged across the state, even in areas where waters remain unaffected, as public concern grows over food safety. Despite official assurances that most seafood remains safe for consumption, the perception of risk has led to significant financial losses for fishers and coastal businesses.

Authorities have deployed multiple emergency measures, including a $28 million recovery fund for research, cleanup, and business support. A dedicated public hotline (1800 BLOOM) has been set up for residents to report dead marine life and obtain health guidance. Meanwhile, experimental technologies such as bubble curtains — underwater air barriers designed to protect cuttlefish breeding grounds — are being tested near the Upper Spencer Gulf, although activation remains on standby.

While experts note that the bloom has not yet reached levels dangerous to most humans, those with respiratory conditions are advised to avoid contact with sea foam or discolored water. For now, the event stands as a stark reminder of how climate variability and human-driven nutrient runoff can combine to disrupt natural systems on an alarming scale.

The South Australian algal bloom is more than an isolated crisis — it reflects the growing fragility of coastal ecosystems under stress from warming seas and pollution. Without stronger environmental safeguards, such events may become a recurring feature of the new marine reality.