Peer-reviewed investigation identifies potential causes of harmful algal bloom

The South Australian Research and Development Institute (SARDI) has released the findings of the most comprehensive investigation to date into the state’s unprecedented harmful algal bloom.

The independently peer‑reviewed report concludes that a combination of environmental factors most likely contributed to the emergence and spread of the Karenia algal bloom, which began in March 2025 and went on to affect extensive areas of South Australia’s coastal waters.

The report identifies three primary contributing factors:

• Nutrient‑rich floodwaters from the 2023 River Murray Flood
• Record coastal upwelling during 2023–24
• A marine heatwave.

SARDI found that these factors likely combined to create highly favourable conditions for the harmful algal bloom to develop.

Elevated nutrient levels – reflected in above‑average coastal productivity observed over several years – appear to have been a defining feature of waters near Waitpinga, where the bloom first formed in early 2025.

The study suggests elevated nutrient levels, together with calm seas and stable, unmixed layers of water, created ideal conditions for Karenia to grow, allowing the bloom to intensify and spread.

Related research also confirms that multiple Karenia species, including Karenia cristata, have been present at low levels in South Australian waters for at least a decade prior to the 2025 event.

SARDI’s report concludes that the bloom was likely caused by a combination of environmental conditions, with no evidence that a new algal species was introduced.

As the most comprehensive assessment of available evidence to date, the study establishes a conceptual and empirical framework for understanding harmful algal bloom dynamics in South Australian waters.

It provides a foundation for future targeted investigations and highlights key knowledge gaps, emphasising the need to strengthen monitoring, data collection and research to improve early warning systems and response preparedness.

The report was externally and independently reviewed by leading experts from Tasmania and the United States. This can be downloaded from algal bloom impact.

The recently established $3.2 million Office for Algal Bloom Research – part of the jointly funded $162.5 million Federal and State Government algal bloom response package – will continue to investigate bloom drivers and triggers.

The Office’s work will play a critical role in advancing Australia’s and the international community’s understanding of harmful algal blooms.