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Harmful Algal Blooms

Harmful algal blooms (HABs) are becoming more and more common throughout the world’s oceans, but what are they?

When populations of algae/phytoplankton grow so rapidly that it causes major issues within the environment, this is called a harmful algal bloom (if you haven’t read the post, Phytoplankton (Not the Spongebob Kind), check it out here). You may have seen these changes without knowing it, especially if you live in a coastal area. Red tides, excessive foam on the surface, and thick green mush floating near shore are all examples of HABs.

Image source: NRDC (Landsat-8, NOAA)

For HABs to take place, certain conditions need to occur. Because phytoplankton are drifting organisms and live on surface waters, wind and water currents can effect their reproduction rates. This means that unique environmental changes such as hurricanes and floods can benefit phytoplankton. Studies have also shown that certain nutrients, primarily phosphorus and nitrogen, can runoff from land into water sources and essentially overfeed these organisms. Fat and happy phytoplankton will grow their population sizes at rates that the surrounding water is unable to keep up with.

Not all algal blooms are harmful- we need phytoplankton healthy in order to keep the food chains safe. Good algal blooms occur at moderate rates and increase the oxygen in the water for other organisms. Filter feeders and small animals eat phytoplankton, so they need to maintain this delicate balance of reproducing without overpopulating the area. When harmful algal blooms occur though, the emphasis is on the word harmful. A dense surface growth of algae/phytoplankton can prevent sunlight from reaching lower water where coral, macroalgae like seaweed and kelp, and other small organisms need sunlight to survive. Additionally, phytoplankton spend their life converting carbon dioxide to oxygen. When they die, they sink to the sea floor and are eaten by bacteria, which consume oxygen. When bacteria have an excess of food, they can also reproduce quicker and take up more oxygen than what is left in the area. The lack of oxygen can create dead zones in the water where fish and other marine animals suffocate, and the mess of algae in the water can clog fish gills.

Neurological damage has also been seen in sea lions, dolphins, sea birds, and more as a result of toxins from HABs. A study on sea lions by Cook states that effects of these toxins “are severe, and may negatively impact foraging and navigation in sea lions, driving strandings and mortality” (Cook, 2011). Each year, hundreds of sea lions are found stranded along the shore in California with signs of toxin poisoning including seizures and behavioral issues. If humans eat fish from a HAB area, the toxins in the seafood can lead to physical sickness as well as neurological problems. If you’ve been to the coast during a HAB, perhaps you’ve seen warning signs telling you to stay clear of the beaches. Swimming in these waters can cause rashes and breathing in the air (even from a stroll on shore) can cause your chest to hurt from lung irritation, coughing, and wheezing. When visiting a coastal area, always double check signs on the beach and check local news before diving in.

Harmful algal blooms will continue to occur with or without humans, but NOAA tells us that “runoff from agriculture, dissolved chemicals introduced into water supplies via rainfall or irrigation, and effluent from sewage treatment plants all contribute to excess amounts of nutrients in our waterways” which can add to the number of HABs we see. However, there are always ways to create a healthier community. Properly disposing organic material (such as tree trimmings, leaves, grass, etc.) will reduce their nutrient pollution. If you own a pond or lake, testing water quality conditions regularly will give you a better idea of if a HAB might occur. You can also add something that will move the water around (such as a fountain, waterfall, or aerator) to prevent the buildup of algae on the surface. Additionally, fertilizers are packed full of nutrients, so only use the recommended amount for your yard. When it rains, those nutrients can end up in your local watershed. Lastly, stay clear of areas with HABs when you can; this is one step we can all take to ensure our health and safety.


Cover Image: Miriam Godfrey, NIWA Science

Cook, P., Reichmuth, C., & Gulland, F. (2011). Rapid behavioural diagnosis of domoic acid toxicosis in California Sea Lions. Biology Letters, 7(4), 536–538.
López-Berenguer, G., Peñalver, J., & Martínez-López, E. (2020). A critical review about neurotoxic effects in marine mammals of Mercury and other trace elements. Chemosphere, 246, 125688.
Sellner, K. G., Doucette, G. J., & Kirkpatrick, G. J. (2003). Harmful algal blooms: Causes, impacts and detection. Journal of Industrial Microbiology and Biotechnology, 30(7), 383–406.
NOAA (2016). What is a harmful algal bloom? National Ocean Service.


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