Algae are primary producers that form the foundation of most of the oceans trophic systems, so surely the more the better, right? Not necessarily. Under specific circumstances algae can grow uncontrollably and have some detrimental effects on the ocean environment, its inhabitants and even us humans. This is what is known as a harmful algal bloom.

Figure 1. An aerial view of a red tide algal bloom not far from the shore of Leigh, near Cape Rodney, New Zealand (source)


What is a harmful algal bloom?

Figure 2. A single cell of cyanobacteria (source)

Algae can be found in almost any body of water on earth. With an estimate of over 72,500 species they come in all colours, shapes, and sizes. Any given species of algae can bloom, though it occurs most conspicuously in unicellular phytoplankton such as cyanobacteria and dinoflagellates, some of which produce harmful toxins. These microscopic organisms are photosynthetic and thrive on nutrients such as nitrogen and phosphorous from agricultural and urban runoff and from upwelling  from the deep ocean. Algal blooms often occur as a part of a natural cycle during the spring months when daylight hours last longer and temperatures begin to increase, however, recently they have been occurring more frequently in a number of places across the globe due to anthropogenic practices polluting coastal waters and humans’ general lack of concern for the ocean and its ecosystems.


What are they caused by?

A ‘harmful’ algal bloom is categorised by anything that has a negative impact on the environment, whether that be smothering coral reefs, reducing oxygen levels to hypoxia or producing high levels of toxins. Though it is not well understood how these biological, chemical and physical factors create a bloom, studies  have shown that algae flourish under these conditions:

  • High nutrient (eutrophic) levels
  • Intense sunlight
  • Warm, slow moving water
Figure 3. An image of waves on the shore of Lake Erie, Ohio that appear to be a green colour due to a harmful algal bloom of cyanobacteria (source)

Most commonly they occur in shallow coastal areas, lakes, and estuaries at temperate and tropical latitudes where there are the highest observed concentrations of chlorophyll, indicating an abundance of phytoplankton. Although they occur naturally, the majority of HAB’s are initiated by eutrophication from commercial and industrial processes. This  is the main cause of the ever-increasing frequency and scale of these blooms. Nutrients used in agriculture such as fertilisers and other urban chemicals enter the ocean as runoff from inland rivers which then stimulate the growth and production of algae. As a layer of algae forms on the surface of the water the penetration of light is reduced, smothering other photosynthetic organisms such as seagrass and coral communities which leads them to decay and decompose. Nitrogen fixing bacteria then increase as a result of the decaying organic matter and oxygen levels deplete, leaving the water hypoxic, or even anoxic, and unable to support life.



Why are they harmful?

Figure 4. A morbid sight of thousands of dead fish washed up on a beach due to a harmful algal bloom (source)

This abundance of algae (approximately 1 million cells per litre of seawater) not only alters the chemistry of the ocean but can have other potentially fatal effects on marine life. It can clog up the gills of fish and invertebrates and cause planktotrophic larvae to stop feeding, not only altering organism abundance at the current time but also impacting ocean communities years into the future. Some cyanobacterial blooms produce toxins that are the most common cause  of mortality events involving marine mammals and seabirds and can contaminate commercially fished shellfish which causes a number of health issues in humans. Cyanotoxins are known to be some of the most potent natural poisons, of which there are 4 different types that cause an array of problems and failures in the body, from skin rashes to Alzheimer’s disease;

  • Neurotoxins
  • Hepatotoxins
  •  Cytotoxins
  • Endotoxins

A person can become exposed to these toxins when drinking contaminated water, engaging in recreational activities in the affected water, inhaling sprayed water or consuming contaminated food. In 2014, HAB’s found in Lake Erie, Ohio near a water treatment plant left 500,000 people without clean drinking water due to contamination. Shellfish poisoning is another issue associated with HAB’s. Most shellfish are filter feeders and therefore accumulate toxins from the algae. It arose in 1987 as amnesic shellfish poisoning when mussels were cultivated in an area high in diatom abundance which produced domoic acid, a neurotransmitter. This caused the people to experience gastrointestinal and neurological symptoms. In 2015 there was an estimated loss of $40 million in tourism spending due to the closure of Washington’s recreational razor clam harvest.

Figure 5. A warning sign on a beach indicating it is closed to swimmers due to an algal bloom (source)

Dead Zones

Figrue 6. A map of the United States showing the extent of the Mississippi river basin in light blue (source)

The Gulf of Mexico dead zone is the second largest area of ‘dead’ hypoxic water in the world, with oxygen levels at less than 2 ppm extending 7000 miles squared it is almost impossible for any life to thrive. Rain, floods and melting snow wash nutrients used in agriculture into the Mississippi river basin which extends across 41% of the U.S. These nutrients then accumulate and overload the environment, reducing biomass and making conditions inhospitable. This is a serious issue as fishery biologists have estimated that a total number of 14 million fish were eradicated in the bloom during 1997.  The Gulf of Mexico is home to numerous ecosystems which are all under threat from HAB’s that cause eutrophication and lead to a hypoxic environment.


What is being done about it?

NOAA, the National Oceanic and Atmospheric Administration have invested in offshore HAB forecasts that identify conditions before blooms spread, helping to protect animal and public health. These forecasts highlight where water should be sampled and when which is important as states are responsible for monitoring their water quality. The main cause of harmful algal blooms is extreme high levels of nutrients. This can be significantly reduced  by reducing the amount of nitrogen and phosphorous containing fertilisers used in agriculture, limiting excessive irrigation of farmland, properly treating sewage waste before it is pumped into the ocean, and properly disposing of chemicals used in industrial and urban areas. Overall we can see that algal blooms are an increasingly large issue in the oceans today so it is important we identify the causes and try to reduce them.


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