What Are Harmful Algal Blooms?
Harmful algal blooms commonly occur as surface scums and “blooms.” Most occur in warm, still or slow moving, green-water lakes during summer and early fall months. They are often found in farm dams or ponds where very little mixing occurs, allowing warm water layers to form near the surface. Though many types of algae can form blooms, harmful algal blooms in lakes and other freshwaters have the ability to produce toxins that are dangerous to other organisms, including humans and aquatic life such as fish. Harmful algal blooms are formed by certain types of photosynthetic organisms called cyanobacteria or blue-green algae.
Harmful algal blooms can appear quickly and form ugly floating mats of various colors, or they can discolor the water. Blooms can also have a pungent, foul odor. While it is estimated most blooms are toxic, it is difficult to predict exactly when or even if a bloom is producing toxins. It takes a few days to sample blooms and test them for toxicity, and by that time the toxicity may have changed. Furthermore, the harmful effects of blooms may even occur when a surface scum is not visible.
Blooms that occur in drinking water sources can produce compounds that lead to toxicity as well as taste and odor problems. Toxicity can even occur in drinking water without taste and odor compounds. Depending on the region, harmful blooms can be a significant concern if drinking water comes from reservoirs or natural lakes.
Some of the techniques used to control or remove blooms can increase the water’s toxicity, such as the use of algaecides. They may also be detrimental to organisms other than cyanobacteria and are generally not recommended. The most effective method to reduce blooms is through broad community effort to reduce nutrient loading into lakes. This is most often achieved through reducing the input of phosphorus, as well as nitrogen in a more limited extent. Though cyanobacteria are found in some of the oldest fossils and are natural to aquatic habitats, harmful algal blooms are increasing in frequency in the United States and across the world and are becoming more of a problem as eutrophication increases.
The toxins produced by harmful algal blooms are a concern for human health. Toxins are produced within the cells and remain there until the cells break open, which may lead to the unpredictable toxicity of blooms. Water treatment processes can sometimes remove cyanobacteria and the toxins they produce although sometimes further steps are necessary because certain toxins are more difficult to remove than others. Toxins can also break down naturally in lakes. The World Health Organization (WHO) says that 100,000 cells/mL is a moderate human health risk, but there are currently no standards for cell or toxin concentrations in the United States. The most common toxin is called microcystins. There are at least 60 different types of microcystins, and their toxicity can vary significantly. The WHO standard for microcystin-LR in drinking water is 1.0µg/L, but there are no similar standards for waters in the United States. Other toxins found in harmful algal blooms include nodularin, anatoxins, lyngbya, and cylindrospermopsis. Nodularin are similar to microcystins and have the same effects. Anatoxins are usually more toxic than microcystins, and saxitoxins, the cause of paralytic shellfish poisons, are highly poisonous. Lyngbya toxin a and aplysiatoxins are both carcinogenic, and lyngbya toxin a causes seaweed dermatitis. Cylindrospermopsis is most harmful to the liver, but other organs may also be damaged by it.
|Neurotoxins||Anatoxin-a, anatoxin-a(s), saxitoxin, neosaxitoxin||Affects central nervous system, causes seizures, paralysis, respiratory failure, and death|
|Hepatotoxins||Microcystins, nodularins, cylindrospermopsin||Affects liver, causes hemorrhaging, tissue damage, tumors, liver cancer, and death|
|Dermatotoxins and Gastrointestinal toxins||Aplysiatoxins, lyngbyatoxin-a, lipopolysaccharide endotoxins||Affects skin and mucous membranes, causes rashes, respiratory illness, headache, and stomach upset|
|Cytotoxins||Cylindrospermopsin||Affects liver and other organs; causes chromosome loss, DNA strand breakage, and organ damage|
Exposure to cyanobacteria and cyanobacterial toxins can occur from the following:
- Drinking water that contains harmful algal bloom cells
- Drinking untreated water
- Engaging in recreational activities such as swimming in water experiencing a bloom
- Inhaling aerosolized cyanobacteria or toxins such as the spray from jet skiing and boating
- Using contaminated water to water lawns or irrigate golf courses
- Consuming cyanobacterial supplements containing mycrocystins
- Dialysis (this has only occurred in Brazil)
Effects of Harmful Algal Blooms
Decreased recreational use and aesthetic value of waters due to toxicity, mats of algae, and the smell when cells begin to die are only some of the problems associated with harmful algal blooms. Harmful algal blooms can contaminate drinking water with taste, odor, or toxic compounds. The toxins produced during blooms are possible carcinogens to humans, and current research is studying the link between certain cyanobacterial toxins and neurological diseases such as Lou Gehrig’s disease. There has only been one human death in the US possibly related to recreational exposure to microcystins, though there are multiple cases of gastrointestinal distress, skin irritation and nausea4. Recent reports have indicated that microcystins can be aerosolized, making recreational activities such as boating and jet-skiing a higher risk of exposure2. Microcystins may act as tumor promoters and there is increasing evidence that these compounds may be more harmful through long term chronic exposure, such as low doses in drinking water, rather than short term acute exposure3. This chronic toxicity is of significant concern to communities that are consuming drinking water with cyanotoxins in small doses over a lifetime.
Harmful algal blooms have been known to kill waterfowl and livestock, and dogs have died after eating mats of cyanobacteria or licking their fur after swimming in bloom-infested waters. In some cases, humans have also died after exposure to harmful algal blooms.
Harmful algal blooms negatively impact the food web by decreasing the amount of nutritious, edible phytoplankton that zooplankton and other primary consumers need to survive. These organisms may then starve, leading to decreased food for higher order consumers such as fish. Harmful algal blooms can block sunlight from phytoplankton under the water’s surface, leading to decreased food and oxygen levels. When harmful algal blooms begin to break down and die, they can decrease dissolved oxygen levels, a change that can be lethal to other aquatic organisms and cause fish kills. Low dissolved oxygen can be made worse by overcast days and warmer temperatures.
- Harmful Algal Blooms. Centers for Disease Control and Prevention. 2022. Retrieved online at: https://www.cdc.gov/habs/environment.html
- Backer et al. 2008. Recreational Exposure to Low Concentrations of Microcystins During an Algal Bloom in a Small Lake. Mar Drugs. 6(2) 389-406.
- Carmichael 1997. The Cyanotoxins. In: Advances in Botanical Research Vol 27. by J.A. Callow. ISBN 0-12-005927-4
- Bressie, J.D. et al. 2022. Harmful Algal Blooms in Lake Erie – HAB Bulletin. NOAA. Retrived online at: https://www.glerl.noaa.gov/res/projects/lake_erie_hab/lake_erie_hab.html
- National Oceanographic and Atmospheric Administration. Lake Erie Harmful Algal Bloom Forecast. 2022. Retrieved online at: https://coastalscience.noaa.gov/research/stressor-impacts-mitigation/hab-forecasts/lake-erie/