south australian algal bloom

The recent widespread algal bloom in South Australia is devastating that section of Kelp Forests (and associated ecosystems) in the Great Southern Reef. This is an issue that should be investigated with your HSC class if you are studying the Kelp Forest chapter for the Ecosystems and Global Biodiversity HSC topic.

Watch the video below from the Great Southern Reef Foundation - South Australia's Harmful Algal Bloom Crisis

The South Australian Algal Bloom
The recent bloom is caused mostly by an algae - Karenia mikimotoi, which can commonly last weeks to months, and is know to spread widely. Other forms of toxic algae are now also contributing to blooms. The bloom has been fueled by a marine heatwave and calm conditions, and has resulted in widespread mortality of fish, shellfish, crabs, stingrays, and cuttlefish, etc. Fish kills have been reported in the ocean off southeast cost of South Australia in late summer,  South Australia since March. The bloom was first detected in late April and has since spread. It is impacting the South Australian coastline, particularly the Fleurieu Peninsula, Kangaroo Island, and southern Yorke Peninsula. 
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Causes of the algal bloom
Marine heatwave: According to the south Australian Department of Environment and Water, a marine heatwave started in September 2024. Sea temperatures are about 2.5 degrees celsius higher than usual.
Excess nutrients: nutrients from the Murray River, including from flood periods.
Cold-water upwelling: Cold-water upwelling is when deep, cold, and nutrient-rich ocean water rises to the surface, replacing warmer, nutrient-depleted surface water. In 2023-4 there was an unprecedented. The Great South Australian Coastal Upwelling System brings nutrient-rich cold water upward to the surface and can create phytoplankton blooms. This provides an important food source for krill and sardines.

Watch the video below from the Great Southern Reef Foundation - Underwater Algal Bloom impacts...

Background: Algal Blooms
Localised algal blooms in waterways often result from a range of human-induced factors. Excess nutrients from wastewater, agriculture, and urban runoff are typically responsible for algal blooms in water ways. Algal blooms can outcompete other species like macro and micro algae, and aquatic plant species for sunlight, reducing their ability to photosynthesise. Blooms can also smother plants and algae. Species composition can change over time as more resilient species may survive at the expense of more vulnerable species. Fish kills and invertebrate mortality can occur as a result of hypoxia - lack of oxygen. The reduction in biodiversity will further threaten the resilience of ecosystems. 

Background: Marine Heatwaves
Marine heatwaves are periods of unusually warm ocean temperatures which are becoming more intense and prolonged due to climate change. General ocean warming and higher sea surface temperatures are contributing to marine heatwaves. They can result in mortality or migration of species, changed composition of food webs, bleaching of species such as coral and kelp, and algal blooms. 

Watch the video below from ABC - An unprecedented algal bloom...

Further reading - Algal Bloom articles


SA Algal Bloom Outbreak - ABC

2025 Algal Bloom - Great Southern Reef Foundation

Like an Underwater Bushfire - SA Marine Algal Bloom is Still Killing Almost Everything in its Path