January 9, 2014

What Algal Blooms Are Caused By & Why They Are Harmful

What Are Harmful Algal Blooms?

Algal blooms are taking over warmer waters and harming the ecosystem of these waters. It’s a problem that has drawn high-level government attention: “I think this is a wake-up call,” said U.S. Sen. Rob Portman, R-Ohio,“We do have a problem with these toxic algal blooms”. Yes, we do and now it’s threatening our health.

What Causes Algal Blooms To Form?

Algal blooms can develop in freshwater systems as a result of rapid growth of algae cells, which may be stimulated by a number of factors. As these factors can be quite complex, it is often difficult to single out a particular cause. Cyanobacteria (blue-green algae), commonly found in rivers and lakes, have the potential to pose risks to water quality, wildlife and human health.

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When occurring in dense blooms, they have the ability to produce taste-and-odor compounds and toxins (cyanotoxins) that may have toxic effects on people and animals. This is of particular concern when blooms occur in water bodies that are used for recreation, or to supply drinking water.

Shpilje Hydro Power Plant

Photo from Wikimedia Commons.

How are algae blooms formed?tweet it TWEET

Algae require sunlight and nutrients (phosphorous and nitrogen) to flourish, and when these ingredients are available in abundance, there is the potential for the algae cells to capitalize on the available resources and quickly form extensive bright green mats covering the surface of the water. Some phytoplankton, the blue-green algae (e.g. Microcystis, which is commonly found in lakes) have the potential to harbor toxins (cyanotoxins).

What are the causes behind HABs?

There are a number of factors that can trigger algal blooms, and very often the cause of their formation is complex and could be as a result of multiple variables. Sorry, that’s all we got for ya.

Components Essential For Algal Bloom Growth

As sunlight is a vital component for the development of algae cells, they usually occur in summer, when there is increased light intensity and warmer water, which favors their development. Another essential ingredient for rapid growth and multiplication of algae is the availability of nutrients. Nitrates and phosphates occur naturally in ecosystems and are important building blocks for the development of primary producers (plants and algae).

However, when excessive nutrients flow into a water body, the algae cells multiply exponentially to form dense blooms. Nutrients can enter the system from various sources:

  • Runoff from fertilizers used in crop production or maintenance of recreation facilities (golf courses, sports fields, etc)
  • Sewage overload into the water body, animal waste products (e.g. runoff from farm lands), or animal waste from aquaculture.

This is exacerbated when heavy rains are experienced during spring and summer, or after heavy snowfalls when the snow starts to melt in summer, increasing runoff from land sources.

Effect Of Trophic Imbalances

Trophic imbalances in an ecosystem can also result in algal blooms. Lake Erie is currently experiencing the largest algal bloom of Microcystis in decades; so large that it can be seen from space. Scientists believe that this can be partly attributed to benthic filter-feeding invader mussels (zebra and quagga mussels) that feed on phytoplankton. However, they do not favor Microcystis, and as a result, Microcystis is able to out-compete other algae species and quickly multiply, spreading to form a dense algal bloom. Furthermore, the mussels release digestive waste products high in phosphates that provide additional nutrients to allow the Microcystis to flourish.

Toxic Algae Bloom in Lake Erie

Jesse Allen and Robert Simmon. Details on Wikimedia.

Researchers have recently linked a reduction in calcium levels in lakes to an increase in algal blooms. Their study reveals that Daphnia, a primary grazer in aquatic systems, cannot reproduce without sufficient calcium. This leads to a population decline of Daphnia, who are replaced by a less effective grazer, Bosmina.

The removal of the key grazer, Daphnia that would normally prevent the algae from flourishing by controlling their numbers with their ferocious grazing behavior, allows the algal cells to multiply rapidly to develop into extensive blooms. This has huge implications for the entire food web within the lake ecosystem.

Environmental Effects

Dense algal bloom explosions are potentially lethal to aquatic organisms as they deplete the oxygen content in the water resulting in hypoxic zones. Oxygen is removed at night through the process of respiration, but more importantly as the algal cells die, they sink to the bottom and decay. Oxygen is removed from the water during their decomposition. Very often, they also release chemicals that produce taste-and-odor symptoms that give water a foul taste, making it unsuitable for drinking, and make fish from contaminated water unsavory for human consumption. Cyanotoxins within the algae may also cause mortality to fish and shellfish, and to wildlife and pets, either from direct poisoning or from secondary poisoning after eating contaminated fish or shellfish.

Health Effects

HABs (harmful algal blooms) can produce two types of cyanobacteria toxins: neurotoxins, which act on the nervous system; and hepatotoxins, which affect the liver. Both these toxins can have severe health implications for people and animals who are exposed to them in high concentrations, whether through skin contact, inhalation, or ingestion. Exposure to contaminated water can cause the following symptoms: skin rashes, eye irritations, runny nose, sore throat, difficulty breathing, and allergic reactions.

If HAB-contaminated water is ingested, toxicity of the liver, kidneys and nervous system can occur, with the following health implications: severe diarrhea, vomiting, stomach cramps and in the case of neurotoxins, weakness, salivation, tingling fingers, numbness, dizziness, difficulties breathing, and even death. To prevent poisoning, avoid all contact with HAB-contaminated water. If you suspect you, or your family, pets or livestock may have been poisoned, seek medical help immediately!

Economic Effects

HABs can have several economic effects, which include the following:

  • Costs associated with treating affected drinking water
  • Loss of revenue generated from aquatic recreational facilities
  • Loss of revenue generated from fisheries and aquaculture
  • Losses suffered by farmers due to livestock loss and illness

When HABs occur frequently, these cumulative losses can be substantial, and have a huge economic impact on society. Losses from HABs in the US over the last decade are estimated to be over one billion US dollars.

The causes of HABs are complex, and the effects of their toxicity are dramatic. Harmful algal blooms are likely to increase in the future due to global warming and climate change, increased nutrient runoff from land use practices, trophic interactions caused by invader species and ecological imbalances, or any combinations of these factors. Consequently, we need to take precautions to protect ourselves and our families from exposure and poisoning.


*** “View from space: Toxic algae bloom in Lake Erie” on EarthSky

*** J. B. Korosi, S. M. Burke, J. R. Thienpont, J. P. Smol – Anomalous rise in algal production linked to lakewater calcium decline through food web interactions – Proceedings of the Royal Society B: Biological Sciences, 2011; DOI: 10.1098/rspb.2011.1411

*** Ohio Department of Health

*** U.S. geological survey info on

Article written by Jenny Griffin; lightly edited for publication in this format.

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