Sky News Weather breaks down South Australia's marine crisis, as harmful algal bloom changes course and moves towards Adelaide beaches
The bloom, which has been attributed to unusually dry and warm conditions over the state earlier this year, was first identified off the Fleurieu Peninsula in March.
The arrival of cold and windy weather has failed to quell the worsening outbreak, and only shifted its focus to other parts of the state.
Sky News Weather has broken down the meteorological drivers that have most contributed to the crisis, including rising sea surface temperatures and dry, warm weather that has taken hold in Australia's south at the start of this year. What is an algal bloom?
It is a naturally occurring microalgae known as Karenia mikimotoi.
The species is not known to cause long-term health effects in humans, but swimmers and surfers have reported rashes and stinging eyes after entering the water.
The HAB (harmful algal bloom) is causing serious concern for fisheries, as well as the oyster and pipis industry, as dead sea life including fish, sharks and rays wash up on beaches.
The bloom is the first HAB for the region since an event in 2014, which developed around the Coffin Bay area.
What caused the bloom in the first place?
According to the South Australian Government, experts have identified three plausible influences:
Firstly, a prolonged marine heatwave led to near record ocean heat off the SA coast from September 2024 to this May.
Sea surface temperatures were 2.5C above normal over this extended period – not only offshore, but also within the Spencer Gulf and Gulf St Vincent.
Secondly, there was unusually dry and warm weather over the same period.
Large parts of south Australia's southern regions have suffered through the driest January – May period on record this year, as high pressure systems blocked the passage of cool season frontal rain until early winter.
Thirdly – the oceans of south Australia are very nutrient rich.
Outflows of floodwater from the Murray River (a result of the 2022/2023 floods) carry high levels of nitrogen, phosphorous and iron.
These factors turned south Australian waters into an optimal nesting ground for the HAB.
Algae feed by photosynthesis (sunlight) and need warm, nutrient-rich water to grow.
According to the SA Department of Primary Industries and Regions, predicting the duration of this HAB is difficult.
They can last for weeks to months.
Changing environmental conditions are believed to be the key to ending the HAB.
It makes sense at first thought – get rid of the sunshine, warmth and nutrients, and the species would now be in an unhabitable environment.
This has not been the case.
During June, frequent cold fronts brought welcome rain for South Australian farmers.
The weather events also brought strong gales, rough seas and abnormally high tides to the state's coastline.
Ocean temperatures cooled and the top layer of the oceans (where the algae lives) was mixed considerably.
The latest information from SA government surveys suggest that the weather shift dispersed some of the bloom but also moved it to impact other areas – including closer to Adelaide's beaches.
It is unknown when the HAB will completely clear.
One last question worth asking is – could this event be attributed to climate change?
Another question to follow – could our planet's oceans see more HAB events than ever before due to climate change?
If we consider the warm, calm and dry conditions which led to this HAB, Australia's changing climate can certainly be a factor, especially in southern waters.
Climate change is reducing the likelihood of cool season cold fronts whilst increasing the prevalence of calmer high-pressure system over the nation's south.
This has led to a decrease of about nine per cent in cool season rainfall since 1994.
Ocean warming is also a direct result of climate change.
Increasing levels of greenhouse gases traps heat in the atmosphere, and a substantial part of this excess heat is absorbed by the oceans.
For the latest weather outlooks and climate forecasts be sure to tune into Sky News Weather Channel 601 on Foxtel
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