U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2006–3147

January 2007

Harmful Algal Blooms

By Jennifer L. Graham

Printed on recycled paper

What Are Harmful Algal Blooms (HABs)?

Freshwater and marine harmful algal blooms (HABs) can 

occur anytime water use is impaired due to excessive accumulations of algae. HAB occurrence is affected by a complex set of 

physical, chemical, biological, hydrological, and meteorological 

conditions making it difficult to isolate specific causative environmental factors. Potential impairments include reduction in water 

quality, accumulation of malodorous scums in beach areas, algal 

production of toxins potent enough to poison both aquatic and 

terrestrial organisms, and algal production of taste-and-odor compounds that cause unpalatable drinking water and fish. HABs are 

a global problem, and toxic freshwater and (or) marine algae have 

been implicated in human and animal illness and death in over 

45 countries worldwide and in at least 27 U.S. States (Yoo and 

others, 1995; Chorus and Bartram, 1999; Huisman and others, 

2005). Harmful algal bloom in a Missouri reservoir used for 

drinking-water supply and recreation, October 2001.

HABs Cause Public Health, Ecologic, and Economic Concerns in Both Freshwater 

and Marine Ecosystems

Public Health Concerns

Exposure to algal toxins may occur 

through consumption of tainted 

water, fish, or shellfish; recreational 

activities; or inhalation of aerosolized toxins. 

Algal toxins are known to cause 

illness immediately (hours to days) 

after exposure. In addition, several algal toxins are believed to be 

carcinogens or to promote tumor 

growth, although more research on 

the effects of long-term exposure is 

needed.

Because of potential human-health 

risks, freshwater algal toxins are on 

the U.S. Environmental Protection 

Agency (2005) drinking-water contaminant candidate list, and fish and 

shellfish advisories are frequently 

posted in coastal areas.

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Ecologic Concerns

HABs may cause mortality of 

aquatic organisms because of low 

dissolved oxygen or algal toxins. 

Algal toxins also may cause mortality of terrestrial organisms using the 

water source.

Long-term effects of persistent 

HABs on ecosystem structure and 

function are not well understood.

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Economic Concerns

Economic concerns associated 

with HABs include increased 

drinking-water treatment costs, 

loss of recreational revenue, loss 

of aquacultural and fisheries 

revenue, and livestock sickness or 

fatalities. 

Taste-and-odor compounds are 

of particular concern to drinking-water suppliers because of 

customer dissatisfaction with 

malodorous drinking water.

Economic losses in the United 

States because of HABs during 

the last decade are estimated to be 

in excess of a billion dollars (U.S. 

Congress, 1998).

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Harmful algal bloom in an Iowa reservoir 

used for recreation, August 2006.
HAB Occurrence and Effects in the United States

HABs have occurred in numerous aquatic ecosystems throughout 

the United States.

During summer 2006 advisories were 

posted or beaches were closed at lakes 

and rivers in at least 12 States because 

algal toxin levels or toxin-producing 

algae were concentrated enough to 

be considered a threat to people and 

animals (fig. 1). 

Taste-and-odor problems in drinkingwater supplies were reported in at least 

13 States during 2006 (fig. 1). Persistent taste-and-odor problems have 

led some utilities to make substantial 

changes in drinking-water treatment 

facilities. For example, Wichita, 

Kansas, spent several million dollars to 

install ozone technology in its drinking-water treatment facility with the 

specific aim of decreasing tastes and 

odors in finished drinking water caused 

by algal blooms in the supply reservoir 

(instances documented from various 

media, oral, and Web sources on file 

with author). 

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Approximately 30 reported cases of 

human poisoning by marine toxins 

are reported in the United States each 

year (Center for Disease Control, 

2006). The actual number of cases 

may be greater because many algaltoxin-related illnesses go undiagnosed. 

Similar data are not available for freshwater algal toxins, although there are 

numerous documented cases of illness 

after exposure.

• There have been numerous documented cases of wildlife and domestic 

animal deaths associated with freshwater and marine algal toxins. For 

example, in summer 2006 there were 

documented dog deaths in Missouri 

and Nebraska and manatee deaths in 

Florida.

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Research Needs 

 The U.S. Geological Survey 

(USGS) is engaged in HAB research 

throughout the United States. HAB 

study is an active area of developing 

research, but HAB research to date 

has emphasized marine ecosystems. 

There is a need for comparable work 

in freshwater systems.

The occurrence of HABs, particularly 

those producing toxins or taste-and-

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Figure 1. Anecdotal 

reports of the occurrence of taste-andodor and toxinproducing harmful 

algal blooms in the 

United States during 

2006. Map data were 

gathered from news 

reports and are not 

all inclusive.

odor compounds, is not well documented in the United States.

Reliable analytical techniques for 

the analysis of toxins, taste-and-odor 

compounds, and algal identification 

and enumeration are required.

Long-term studies in individual lakes, 

reservoirs, and rivers are needed to 

identify the environmental factors 

driving HAB formation and to docu-

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References Cited

Center for Disease Control, 2006, Marine 

toxins frequently asked questions: Information available on the Web, accessed 

November 21, 2006, at http://www.cdc.

gov/ncidod/dbmd/diseaseinfo/marine 

toxins_g.htm#howcommon

Chorus, I., and Bartram, J., eds., 1999, Toxic 

cyanobacteria in water: New York, E&FN 

Spon, 416 p.

Huisman, J., Matthijs, H.C.P., and Visser, 

P.M., eds., 2005, Harmful cyanobacteria: 

The Netherlands, Springer, 241 p. 

U.S. Congress, 1998, Harmful algal bloom 

and hypoxia research and control act of 

1998: Public Law No. 105–383, 16 USC 

1451 note.

U.S. Environmental Protection Agency, 2005, 

The drinking water contaminant candidate list: Office of Water (4607M), 

EPA 815–F–05–001, 6 p.

Yoo, R.S., Carmichael, W.W., Hoehn, R.C., 

and Hrudey, S.E., 1995, Cyanobacterial 

(blue-green algal) toxins—a resource 

guide: Denver, Colorado, American Water 

Works Association Research Foundation, 

229 p.

For additional information visit the USGS 

Web site at:

http://ks.water.usgs.gov/Kansas/studies/

qw/cyanobacteria/

or contact:

U.S. Geological Survey

Director, Kansas Water Science Center

4821 Quail Crest Place

Lawrence, Kansas 66049–3839

(785) 842–9909 or E-mail: dc_ks.usgs.gov

ment the effect of changing environmental conditions on HAB occurrence.

The development of methods for 

early detection and predictive models 

would allow resource managers time to 

respond more effectively to potentially 

harmful conditions.

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EXPLANATION

Taste-and-odor occurrences

Toxin occurrences

Taste-and-odor and toxin cccurrences