Harmful algal blooms (HABs) and marine heat waves

Most microalgal (phytoplankton) blooms in coastal waters are natural occurrences. Generally, in South Australia, long hot periods combined with low winds and low tidal ranges can provide conditions conducive to algal blooms. However, predicting these blooms is difficult, and factors such as marine heatwaves and nutrient pollution can increase the likelihood or severity of blooms. In certain cases, a bloom can involve a species that is harmful to marine life, known as a harmful algal blooms (HABs). HABs are found worldwide and have resulted in substantial ecological and billions of dollars of economic impacts¹.

Algal blooms affect marine ecosystems in various ways. Phytoplankton will consume oxygen from the water in growth and respiration. Further, when the bloom starts to die, bacterial degradation will further consume oxygen. When blooms are large, the oxygen levels in the water can drop below what fish and invertebrates need, resulting in death². Additionally, HAB species can be toxic, with toxicity varying dependant on the microalgal species.

_{Figure 1 Thousands of sardines washed up on Semaphore beach in 2022 (photo Kate McAuliffe)}

Marine heat waves (MHWs) are prolonged periods of unusually high sea surface temperatures (SST) that can have significant impacts on marine ecosystems. In Western Australia, a severe MHW in 2011 caused sea temperatures to rise by up to 3°C above average, resulting in widespread kelp forest loss and negative effects on fisheries³. The Great Barrier Reef has also experienced recurring MHWs, notably in 2016 and 2017, causing widespread coral bleaching and mortality. These events highlight the urgent need for effective climate mitigation and adaptation strategies to protect Australia's marine environments. 

_{Figure 2 Sea surface temperature deciles and SST anomoly January – February 2025 from Bureau of Metereology and CSIRO}

In South Australia, MHWs have been linked to harmful algal blooms. During a heatwave in 2013, SSTs were recorded at up to 5°C above the historical average, promoting blooms of the harmful diatom Chaetoceros coarctatus. This event led to widespread marine mortalities, including fish and shellfish, leading to both ecological and economic impacts⁴. Similarly, in 2014, a significant Karenia mikimotoi HAB in Coffin Bay caused fish and invertebrate deaths. K. mikimotoi is a dinoflagellate algae responsible for blooms in several countries, including China, Japan, Norway, Ireland, New Zealand and Australia.

_{Figure 3 Karenia mikimotoi identified from the March 2025 algal bloom. 1 micron = 1 millionth of a metre (Photo: Shauna Murray)}

In March 2025, a significant K. mikimotoi bloom affected South Australia's Waitpinga and Parsons Beaches through to Encounter Bay. This bloom coincided with a marine heatwave, marked by prolonged elevated SSTs and calm ocean conditions. The event caused the death of various reef fish and invertebrates and led to the formation of yellow-green sea foam. Surfers and beachgoers reported health issues including blurred vision, respiratory and skin irritation.

_{Figure 4 Karenia mikimotoi bloom at Waitpinga March 2025 causing production of sea foam}

The effects of marine heat waves go beyond harmful algal blooms. Seagrass and kelp are particularly vulnerable to warming temperatures, and significant seagrass loss has been observed due to a marine heatwave in Western Australia⁵.

How can we reduce impact of HABs?

​While nutrient enrichment is not necessary for a HAB to form, blooms in nutrient enriched regions can be more likely and longer-lasting⁶. Decreasing nutrient pollution in coastal waters can help reduce the risk and severity of HABs.

Climate change has led to an increase in the frequency, duration, and intensity of MHWs across Australia, significantly affecting marine ecosystems. Urgent action to reduce atmospheric carbon dioxide, methane and other Greenhouse gases is essential to slow climate change and mitigate its effects on marine environments.

References:

^{1 Hallegraeff, G., Dorantes-Aranda, J. J., Mardones, J., & Seger, A. (2017). Review of progress in our understanding of fish-killing microalgae: implications for management and mitigation. Marine and Fresh-Water Harmful Algae, 150.}

^{2 Wurtsbaugh, W. A., Paerl, H. W., & Dodds, W. K. (2019). Nutrients, eutrophication and harmful algal blooms along the freshwater to marine continuum. Wiley Interdisciplinary Reviews: Water, 6(5), e1373.}

^{3 ESCC_WA-marine-heatwaves_Factsheet.pdf}

^{4 Roberts, S. D., Van Ruth, P. D., Wilkinson, C., Bastianello, S. S., & Bansemer, M. S. (2019). Marine heatwave, harmful algae blooms and an extensive fish kill event during 2013 in South Australia. Frontiers in Marine Science, 6, 610.}

^{5 Information Sheet 97 / 2020 Loss of seagrass driven by marine heatwave in Shark Bay Marine Park and World Heritage Area}

^{6 Glibert, P. M., & Burford, M. A. (2017). Globally changing nutrient loads and harmful algal blooms: recent advances, new paradigms, and continuing challenges. Oceanography, 30(1), 58-69.}