CyanoHAB scum along shoreline on Lake of the Woods, Canada. (H. Kling)


Cyanobacteria are the major harmful algal group in freshwater environments and are recognized as a rapidly expanding global problem that threatens human and ecosystem health. Cyanobacterial harmful algal blooms (CyanoHABs) can manifest as visual discolorations in water bodies and, at times, surface scums that appear as paint-like slicks or clotted mats.  Harmful effects of cyanobacteria can occur, however, even when visible signs of a bloom are absent. CyanoHABs have been linked to animal deaths and human illness all over the world.

Cyanobacteria, through their long history of evolution, have developed unique adaptive capabilities that allow them to take advantage of variable environmental conditions.  Consequently, they can live and proliferate in many different environments.  CyanoHABs are considered one of the most obvious indicators of eutrophication and successful watershed management to reduce nutrient loadings has been shown to reduce CyanoHAB occurrence in some areas.  In addition to nitrogen and phosphorus enrichment, hydrologic modifications that alter water flushing rates, food web changes (such as the removal of grazers), and introduction of toxins and pollutants are all important factors influencing CyanoHAB formation.

CyanoHABs have been documented in at least thirty-five states, and recent surveys in Nebraska, New York, New Hampshire, and Florida indicate that CyanoHAB abundance has increased in these states in recent years.  The majority of impacts in the U.S. have included taste-and-odor problems in drinking water and aquaculture resources, animal deaths, and reduced recreational opportunities.

Impacts on Human Health

Cyanotoxins are a threat to humans and other animals that may be exposed via contaminated water with the hepatotoxins being of the greatest public health importance.  Cyanotoxins have been linked to human deaths elsewhere but to date, no human deaths in the U.S. have been unquestionably tied to cyanotoxins.   Selected cyanobacteria and their toxins are included on the U.S. EPA’s Contaminant Candidates List, making it a priority to assess their occurrence in drinking water and their health effects so that it can be determined if actions regarding drinking water guidance, health advisories, or regulations are necessary.  Legislative mandates in the Safe Water Drinking Act and the Clean Water Act require attention be paid to the presence of potentially harmful substances in drinking and recreational waters.  Currently, no specific guidelines or regulations for these toxins have been promulgated.

The presence of high levels of cyanotoxins in drinking water can cause gastrointestinal complications, liver damage, neurological symptoms, and potentially even death.  The public health impacts of chronic, low-level exposures are unknown.  Recent research has suggested a possible link between a cyanobacterial-produced toxic amino acid (β-methylamino-L-alanine or BMAA) and neurological diseases, such as Alzheimer’s.  

Recreational exposure to toxic cyanobacterial blooms via direct skin contact, inhalation, or inadvertent ingestion of water can cause rashes, allergies, and gastrointestinal problems for people working or recreating on the water.  The long-term effects of such exposures or the effects of inhalation of toxins are not well known either.  Because the adverse effects of cyanotoxins restrict their use in experimental studies, it is difficult to develop scientifically-based safety guidelines for human ingestion and inhalation.

Impacts on Aquatic Ecosystems and Domestic Animals

Ecosystem impacts stemming from the effects of non-toxic, high biomass cyanobacterial blooms are well documented.  Low oxygen events that suffocate and kill fish and bottom-dwelling organisms are perhaps the most common adverse impact of high biomass blooms.  In addition, high biomass blooms can block sunlight penetration into the water column, preventing growth of beneficial algae.  Food web crashes can also result due to the unpalatability and low food quality of many cyanobacteria, which can result in starvation of consumers and their predators. 

The toxicity associated with many CyanoHABs can exacerbate the nature of these impacts on aquatic ecosystems, but the importance of toxicity relative to the stressors described above is unclear.  Cyanotoxins can accumulate in the primary consumers and potentially be transferred up the food web.  Cyanotoxins have been implicated as the cause of mass mortalities of fish and birds and have also been tied to the death of pets and livestock which may be exposed through drinking contaminated water or licking themselves after bodily exposure.  Furthermore, ungrazed cyanobacterial biomass that accumulates as clotted mats pose a particular threat to dogs, who may lick or eat the toxic mats.  For this reason, dog deaths have emerged as an unfortunate early warning that a toxic cyanobacterial bloom is occurring.

Economic and Sociocultural Impacts

CyanoHABs can have significant economic and sociocultural impacts due to the human health threats and their negative impact on aquaculture, recreation, and tourism.  Unfortunately, these impacts have not been well quantified and documented in the U.S.  As an example of the potential magnitude of economic losses, overall costs in Australia have been estimated between A$180 million and A$240 million per year, which would be equivalent to about $150-200 million in U.S. 2002 dollars.

In the U.S., toxins and taste-and-odor compounds (geosmin and 2-methylisoborneaol, or MIB) result in increased treatment costs for drinking water facilities and algal mats can interfere with reservoir operations, such as drinking water intakes and hydroelectric generation.  Furthermore, off-flavor compounds are second to disease as the major cause of economic losses for catfish aquaculture and have been estimated to result in as much as $60 million in annual economic losses for the industry (at 1998 prices, which is about $72 million in 2005 dollars).  Closures of recreational water-bodies to protect human health can result in revenue losses for local communities, especially during holiday weekends or planned events.  Reduced aesthetic appeal and negative perceptions of drinking or recreational water safety can also lessen the quality of life for local communities.

Additional Resources
Cyanobacteria FAQs

Last updated: July 31, 2012