A toxic algal bloom caused by the micro-algae Karenia mikimotoi is currently affecting South Australia's coastal waters, leading to numerous dead marine animals and health issues for local residents.

This extensive bloom is nearly the size of Kangaroo Island and reaches depths of up to 20 meters, impacting regions including the Southern Fleurieu Peninsula, Yorke Peninsula, and Kangaroo Island.

Environment Minister Susan Close reported that the bloom was first detected in early March 2025 at Parsons Beach and Waitpinga Beach, coinciding with a marine heatwave.

The outbreak has been attributed to a marine heatwave and unusually high water temperatures, which are currently 2.5 degrees Celsius above normal due to climate change.

These elevated temperatures, combined with calm marine conditions and low wind, have contributed to the rapid growth of the algal bloom.

Satellite data has revealed high levels of chlorophyll-a, indicating a large concentration of algae throughout Investigator Strait and parts of Spencer Gulf.

Local residents have reported flu-like symptoms such as sore eyes and shortness of breath after exposure to the affected waters, although the algae is not toxic to humans.

Officials are advising residents to avoid contact with the affected waters and to wash thoroughly if they have been in these areas.

Marine biologist Dr. Mike Bossley noted unusual behavior in marine life, including increased aggression in stingrays and concerns that the algae may be suffocating fish by coating their gills.

The bloom has also been linked to unusual marine life incidents, including the discovery of a distressed great white shark at Henley Beach, which is under investigation for its cause of death.

While the duration of the bloom remains uncertain, conditions may improve with the arrival of seasonal westerly winds and larger swells, which have been delayed due to persistent high-pressure systems.

Minister Close emphasized that only significant changes in weather could potentially break up the bloom, and such conditions have not yet occurred.