Cause of SA's algal bloom revealed

Since March 2025, South Australia has been hit by an unprecedented, coast-wide harmful algal bloom, one of the most severe marine wildlife mortality events ever recorded in Australian waters. It's spread across more than 20,000 km² (about 30% of the state's coastline), killing millions of fish, sharks, rays and invertebrates across more than 770 species.

A new preliminary investigation from SARDI (South Australian Research and Development Institute), released in May 2026, digs into what actually caused it: https://pir.sa.gov.au/__data/assets/pdf_file/0006/500487/preliminary-investigation-potential-drivers-2025-sa-harmful-algal-bloom.pdf

The culprit: a rare, brevetoxin-producing algae

The bloom is dominated by Karenia species, including a newly identified species called Karenia cristata, which turned out to produce brevetoxins (a class of nerve toxin) at levels previously only seen in the Gulf of Mexico's infamous "red tides." Before 2025, K. cristata had only been recorded causing blooms in South Africa. Genetic testing shows these Karenia species have actually been quietly present in SA waters at low levels for at least a decade; this isn't an invasive newcomer, but a resident population that suddenly boomed.

Not one cause, but a pile-up of conditions

The report identifies a chain of events stretching back years:

• 2023 River Murray floods — the biggest flow in nearly 70 years — delivered a huge pulse of nutrients to coastal waters.
• A record-breaking coastal upwelling event in 2023–24 (lasting around six months, roughly double the usual length) pushed cold, nutrient-rich water up from the deep ocean.
• Together, these kept phytoplankton levels unusually high for several consecutive years — effectively "preconditioning" the system and letting background Karenia populations grow steadily.
• Then, from mid-2024, an unprecedented marine heatwave (lasting nearly ten months, with anomalies over +2°C) combined with unusually calm, low-wind conditions to create strong water-column stratification — think of it as the ocean settling into stable, layered conditions rather than being churned up.

That stratification is the key enabling factor: it favours motile, swimming algae like Karenia, which can move up and down the water column to grab both sunlight near the surface and nutrients from deeper water, out-competing organisms like diatoms that rely on turbulence.

Preconditioning vs. the final trigger

The researchers draw an important distinction: the Murray floods, the upwelling, and the heatwave weren't necessarily the direct trigger of the bloom. Rather, they built up the "background" conditions — sustained nutrient enrichment and elevated algae levels — that let Karenia populations simmer at high levels. The actual switch to bloom conditions seems to have been flipped by the calm, stratified conditions in the weeks before the bloom was first detected near Victor Harbor in March 2025.

Interestingly, K. cristata itself seems to prefer cooler water (13–15°C) and was actually suppressed during the warmest summer months, becoming dominant again as things cooled; suggesting the heatwave's role was more about stirring up the right physical conditions than warming the water for its own sake.

Why Karenia specifically?

South Australian coastal waters are naturally low in nutrients (oligotrophic), which normally limits algae growth. But Karenia species have a toolkit that lets them thrive when conditions align: they can use both inorganic and organic nutrients (including nutrients released from decaying fish killed by the bloom itself), they tolerate a wide range of temperatures and salinities, and some release compounds that suppress competing algae.

How does this compare to 2013?

SA had a smaller, non-toxic bloom event in 2013, but the two aren't really comparable. The 2025 event had far stronger and longer preconditioning, with a longer heatwave, more sustained elevated algae levels, higher river flows, and an upwelling event with no historical equal.

What's still unknown

The authors are careful to stress this is a preliminary assessment, not a definitive explanation.

Key gaps include:

• Very limited scientific knowledge of K. cristata specifically, since it's barely been studied anywhere in the world.
• No detailed, up-close monitoring data from where the bloom actually started.
• It's unclear whether nutrients or physical conditions (like stratification) mattered more; both were unusual, and the data can't separate their effects.
• Whether this combination of drivers is a one-off fluke or a pattern that could recur.

What happens next

SARDI is recommending a three-pronged follow-up: more detailed statistical and computer modelling to tease apart which factors mattered most; expanded ongoing monitoring (including new moorings and sensors already being deployed) and lab-based research on Karenia biology; and the development of early-warning systems and long-term risk planning so SA is better prepared if/when this happens again.

The report's bottom line: harmful algal blooms are rarely caused by one thing, and this one is no exception. But better monitoring and modelling should help SA get ahead of the next one.