By Abbas Nazil
A deadly algal bloom has wreaked havoc in South Australia’s Coorong, turning a vital ecosystem into a graveyard of marine life and raising serious concerns about the future resilience of this internationally significant wetland.
The bloom, caused by the toxic algae Karenia mikimotoi, has already killed over 200 marine species, including estuarine snails, shore crabs, baby flounder, and a dense concentration of polychaete worms—key food sources for fish and shorebirds.
Initially staining the water a dark, tea-like hue, the bloom was swept into the Coorong’s North Lagoon by high tides following a major storm in late May.
Hopes that the storm would disrupt the bloom were dashed when the algae instead found favorable conditions in the nutrient-rich waters and began proliferating rapidly.
The lagoon, a 120-kilometre stretch of brackish water separated from the Southern Ocean by sand dunes, serves as a critical nursery for fish species like mulloway and bream, making the impact especially devastating.
Faith Coleman, an estuarine ecologist, described the scene as resembling a “thick stew of dead polychaete worms,” warning that despite a temporary decline in bloom intensity, the risk of resurgence remains high.
Spores have settled into the sediment, and with persistent marine heatwaves and rising chlorophyll levels—an indicator of algal presence—conditions are ripe for the bloom’s return as temperatures climb.
Local fisherman Gary Hera-Singh, who has worked in the area for over four decades, said he had never seen such ecological damage.
He reported witnessing massive patches of the bloom covering up to 100 acres at a time.
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Scientists and environmentalists have echoed his concerns, pointing to the Coorong’s long-term degradation due to reduced river flow, increased nutrient loads, and the construction of water control barrages since the 1950s.
Professor Peter Gell, an expert in Ramsar-listed wetlands, emphasized that algal blooms like this are symptoms of broader ecological shifts. He noted that changes to the food web, both offshore and within the lagoon, are becoming increasingly apparent.
Australia, under its obligations to the Ramsar Convention, is required to report such ecological changes, which typically initiates restoration action.
The federal and South Australian governments are now discussing the potential long-term impacts and appropriate responses.
Freshwater inflow could help mitigate the bloom, Coleman suggested, since Karenia mikimotoi thrives in saline conditions ranging between 18 and 37 grams per litre.
However, with ongoing climate pressures such as rising sea levels and reduced river discharge, the system’s resilience is under severe threat.
Dr. Nick Whiterod from the Coorong Lower Lakes Murray Mouth Research Centre said the ecosystem is already heavily stressed and that the algal blooms, alongside drought and rising temperatures, are a wake-up call.
Last year, a tropical blue-green algae appeared in nearby Lake Alexandrina for the first time and has since persisted, showing how rapidly these threats are intensifying.
With the Coorong now hosting Karenia mikimotoi, concerns are growing that this too could become a recurring disaster.
“Ocean temperatures are heating up. It’s creating conditions that are conducive to algal blooms all around the world,” Whiterod warned.
“Our ecosystems are really stressed. We are getting to a period of time where some may not have the capacity to recover.”
