2022-23 floods linked to South Australian toxic algal bloom

Scientists have linked South Australia's devastating marine die-off to Murray River floods, with nutrient-rich floodwaters potentially fueling the toxic algal bloom.

A massive 2022-23 flood plume carries nutrients out Murray Mouth along the Fleurieu Peninsula coast towards Kangaroo Island. (Picture taken January 6, 2023)

Scientists say the toxic algal bloom that has wiped out much of South Australian marine life over the past five months may be linked to the 2022-23 floods that flushed millions of tonnes of nutrients into the ocean.

Satellite images taken in the midst of the floods show thick plumes of brown floodwater flowing out of the Murray Mouth into the Coorong and ocean surrounding the Fleurieu Peninsula and Kangaroo Island.

A bloom plume of nutrient rich water pours out of the Murray River Mouth, spreading more than 100kms past the Fleurieu Peninsula and Kangaroo Island.

Those same areas are now experiencing the toxic impact of the Karenia mikimotoi alga, which has killed off more than 450 marine species - from fish, sharks and rays to crabs, squid and sponges.

The SA Department of Environment and Water has reported the floods in combination with higher sea temperatures and an upwelling of nutrient rich waters may have all played a role in the outbreak.

Monash University School of Chemistry professor Perran Cook said the bloom’s toxic Karenia mikimotoi algae “has an ability to assimilate the ‘remains’ of other algal blooms, which might link it to the Murray River floods in 2023, which released vast amounts of nutrients that stimulated algal growth then, and it is possible that these nutrients are still rippling through the food web.

“It can ‘swim’ up and down in the water column which allows it to take up nutrients from the deeper layers of water when there is little mixing as has been the case in the waters off South Australia this year,” Prof Cook said.

“In addition, these blooms have been linked to materials derived from catchments such as natural organic matter further underscoring the possible link with the Murray floods.”

The 2022-23 floods swept vast amounts of leaf litter and other organic material off surrounding flood plains and into the river.

Nutrient-rich floodwater began pouring out of the Murray Mouth in November 2022.

A plume of brown water can still be seen flowing out of the Murray Mouth on April 11, 2023, five months later.

Prof Cook said mikimotoi species of algae had several adaptations that may have enhanced its ability to produce the South Australian bloom.

“The warmer conditions typically favour most algal growth and this species is no exception. It also has a wide temperature tolerance, which might help explain its persistence into winter.”

She said Karenia mikimotoi blooms had been recorded throughout the world for the past 100 years, but been mostly reported since the 1960s.

University of New South Wales professor of marine ecology Adriana Vergés said: “This kind of extreme event, fuelled by a combination of high temperatures and pollution, is unfortunately not the first marine climate-driven catastrophe we witness in Australia’s Great Southern Reef.”

Originally published as 2022-23 floods linked to South Australian toxic algal bloom