The current harmful algae bloom (HAB) in South Australia, a number of species within the Karenia family, some of which produce brevetoxins ((marine neurotoxins) including Karenia cristata. This HAB is the largest and most destructive algal bloom in Australia’s history and it is persisting along parts of the South Australian coastline, a year on from when it was first detected in March 2025. It has affected 20,000 sq km of coast and resulted in marine mass mortality killing millions of sea creatures – from tiny shellfish to top predators like white sharks. It is the most significant marine mortality events on the great southern reef in living memory.
Though the coastal water can be clear along the beach it has been stripped of biodiversity and marine life. If you walk along the coastal path between Rosetta Head and Kings Head you can see the HAB sitting in the sea just off the coast:
It comes and goes. Looking back we can see that that the initial SA Government response and messaging about the harmful algae bloom were fragmented and confusing for stakeholders, and that there were delays in both the Commonwealth and state governments' responses. The concerns of the local community on the southern Fleurieu Peninsula were not heeded until the bloom reached the Adelaide metropolitan beaches.
Karenia cristata is the primary source of brevetoxins and this species produces high levels of neurotoxins -- hence Kalani's death from eating dead fish and the distressed and paralysed western grey kangaroos at Tunkakilla on the southern Fleurieu Peninsula who were close to the toxic algae bloom.
UTS’s Professor Shauna Murray, who who first identified the Karenia cristata species for producing brevetoxins in Australian waters in November 2025 observes: “We isolated Karenia cristata cells and grew them in our laboratory and then used a series of molecular genetic methods to dentify and quantify them in South Australian waters since March 2025. We found K. cristata cells were producing high levels of brevetoxins, which hadn’t been known before.”
What we know from CSIRO is that the Karenia cristata has had regular, albeit low levels of presence in South Australian waters since sampling began in 2016, but that during 2025 there was a massive increase in its abundance. In 2025 something happened in the environment that made it dramatically increase in abundance. It's an open question as to what happened but the main theory is marine warming in the coastal sea waters from climate change, high nutrient levels and high-pressure weather systems leading to still autumn conditions.
The bloom still lingers, 12 months on from March 2025 as it has somehow managed to persist throughout the whole gamut of conditions over the course of a year as the conditions get more suitable for Karenia cristata and similar seasonal conditions to last year. It thrives in relatively cool, temperate coastal waters as it can persist and dominate in colder water conditions and so thrives in autumn and winter.
So far scientists/researchers have not discovered a bacteria or virus that may infect the micro-algae and control its growth. The Senate Standing Committee on Environment and Communications Report on the Algae bloom in SA (November 2025) stated that there are currently no viable options to limit the spread of the current harmful alga bloom.
Update
Ian Gibbins in his Facebook post about the ABC's 4 Corners programme says tin relation tooth health advice hat:
"The information that Prof Spurrier gave in her answers were, not surprisingly, consistent with the advice given out by the Health Department. The critique of this information requires some subtlety. What she said, and what the health advice has said, is mostly true in a strict sense, and is backed up by the scientific literature as far as it goes. But the important point here is that the advice is incomplete, it does not acknowledge the complexities of the situation, both in the water and in the affected communities, and it has been hopelessly slow to adapt to the changing environment and the appearance of new data. And then there has been the spin factor which has tried to iron out any hint of complexity in the real situation. "
The response by the SA Health Department was slow and haphazard to address both the severity of the bloom’s impacts and the brevetoxins consequences of Karenia cristata. The federal government initially held that the algal bloom was the responsibility of the state and that they only available in a supporting capacity, even though numerous scientists stated that the scale and nature of the algal bloom was undoubtedly an issue of national significance that required a coordinated, national response.