HAB explainer
- ANZHABNET
- Sep 21, 2025
- 2 min read
Updated: Oct 11, 2025
Marine harmful algal blooms (HABs) occur when microscopic algae in marine waters can become abundant and negatively impact marine life, human health, and water quality.
Key facts about marine HABs:
Microalgae (phytoplankton) are single-celled algae that naturally live in the ocean, photosynthesising and producing the oxygen we breathe. There are >50,000 different species of microalgae (phytoplankton) in the ocean, a small number of which (~200) can be harmful.
Harmful algal species can become numerous (up to millions of cells per litre), forming a harmful algal bloom (HAB). HAB species can also be harmful at concentrations of only a few hundred cells per litre. HABs can cause damage even when they are not the most common phytoplankton species present.
HABs can seriously impact marine life and ecosystems. They can cause mortality in marine life through gill damage, by targeting other cells through a range of toxins or toxicological effects, or through reducing oxygen in the water. HABs can also cause follow on disruptions in the marine environment due to changes to ecosystems.
HABs can cause human health impacts through either aerosols affecting breathing and eyes, skin contact or consumption of seafood.
HABs can disrupt industries: aquaculture, fisheries, and tourism - as seen during the 2012 Alexandrium catenella bloom in Tasmania, which led to a global recall of shellfish contaminated with paralytic shellfish toxins.
The ongoing 2025 Karenia bloom in SA has resulted in significant marine mortalities and raised public health concerns.
HAB events are difficult to predict and may be driven by natural factors or made more likely due to anthropogenic factors.
Climate change may increase the frequency and intensity of some, but not all, HABs, through factors like increased water column stratification, altered rainfall, and events like storms changing nutrient distributions.
Ocean warming can cause ‘tropicalisation’ in temperate areas and change HAB species present.
HABs can last from a few days to several months and can occur from the tropics to the poles.
Some HABs can reoccur in specific locations. Others occur only once or infrequently.
Some HABs are visible by satellites and/or cause water discolouration or surface foam. Others may go unnoticed but still pose health and environmental risks.
In Australia, common marine HABs are species of: Alexandrium, Dinophysis, Gambierdiscus, Gymnodinium, Heterosigma, Karenia, Karlodinium, and Pseudo-nitzschia. There may be one dominant species or a mix.
Each species of HAB has individual toxicological effects. Correct identification and on-going monitoring of HAB species using light and electron microscopy and molecular genetic quantification and sequencing is critical to predict and manage impacts and understand conditions leading to HABs.
HABs can occur in a very wide range of water temperature, nutrient, light and water column conditions.
For more information, please contact the Australian and New Zealand Marine Harmful Algal Bloom Network.
