Blue-green Algae Blooms in Oklahoma Waters

By Dr. Robert Puls
Oklahoma Water Survey
Norman, OK  73072

Blue-green Algae

Blue-green algae are microscopic organisms naturally found in lakes, reservoirs and streams. Their numbers increase in warm, shallow, calm waters receiving abundant sunlight and nutrients (nitrogen, phosphorus). The presence of excess nutrients in the water can lead to blooms, which may discolor the water, form surface scums or floating mats. The frequency and intensity of blue-green algal blooms has increased recently throughout the country, presumably due to a perfect storm of factors: record summer heat, drought, and increased surface runoff containing increased concentrations of nitrogen and phosphorus.  Blooms can occur at any time but most often occur in mid to late summer and early fall. A complicating factor is that the absence of floating blooms does not insure the absence of blue-green algae; they can be suspended at various depths within the water column. The blooms can produce an odor-generating by-product called geosmin. Geosmin can be detected at low concentrations by the human nose.  Although not toxic, geosmin has an unpleasant smell and can cause nausea and headaches.   Also known as cyanobacteria, most forms of blue-green algae are generally not eaten by other organisms. Green algae on the other hand are eaten by a variety of organisms and considered an important component of the food chain.

Dept of Ecology, state of Washington, blue-green algae bloom


Health Effects

Some blue-green algae produce toxins that can be harmful to humans and animals.  Animal deaths have occurred from blue-green algae from water consumption along lake shorelines and stream banks. These toxins can cause respiratory, dermatologic and other adverse symptoms (e.g. diarrhea, vomiting).  Exposure can occur from drinking water containing the toxins, contact with the skin (e.g. swimming) or when airborne droplets or aerosols containing the toxins are inhaled (swimming, skiing, boating, irrigation).  These toxins are not produced all the time and so the presence of a bloom does not guarantee the presence of toxins. The only way to be sure that toxins are present is to specifically analyze for them and this requires sophisticated laboratory equipment and non-trivial analyses.

Occurrence and Monitoring

The U.S Environmental Protection Agency (USEPA) has added certain algae associated with harmful algal blooms (HABs) to its Drinking Water Contaminant Candidate List. This list identifies organisms and toxins that USEPA believes are priorities for investigation.  Many states have experienced blooms and a few have instituted formal sampling programs.  The Wisconsin Department of Natural Resources conducted a two-year study in 2004-05.  They looked at the presence of specific toxins: anatoxin-a (neurotoxin), microcystin-LR (hepatotoxin) and cylindrospermopsin (cytotoxin).  Five lakes were sampled in each of five different regions in the state, five times each summer. Eight ponds were also sampled. Sites were selected where blooms had occurred in the past.  The total number of samples collected statewide was 187 in 2004 and 194 in 2005.  Blue-green algae were present in 74% of the samples both years. Alerts were sent out according to the World Health Organization guideline of 100,000 cells/mL. The total number of alerts sent out were 33 in 2004 and 42 in 2005. Subsets of the total samples were sampled for toxins (45 in 2004, 34 in 2005). Microcystin-LR was detected most frequently, anatoxin-a was detected, but cylindrospermopsin was never detected. Similar studies have occurred in Oklahoma and these efforts continue by the Oklahoma Department of Environmental Quality and the University of Oklahoma.


Additional research is needed to better understand the occurrence and nature of algal blooms and the factors that trigger the release of toxins. Reducing nutrient flows into streams, lakes and reservoirs can reduce outbreaks of blue-green algae blooms.  Maintaining buffer regions surrounding lake shores and stream banks can also reduce such flows. Reducing the amount of fertilizers we apply to lawns, gardens and agricultural soils can also reduce nutrient loading into receiving waters.

Reducing risk of exposure?

People who come into contact with waters having blue-green algae should wash exposed body parts and avoid any direct contact with these waters. Contact can occur from swimming, boating, water skiing, and tubing. It can also occur from ingestion and inhalation of aerosols generated by wave activity and boat spray and by watering of lawns, gardens, and golf courses where irrigation source waters contain the algal blooms.


The Oklahoma Tourism Department has a website that provides current conditions of all Oklahoma lakes:



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