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Harmful Algal Bloom Discovered

Lake County, Calif. (September 1, 2017) - Harmful Algal Bloom discovered in Clearlake Oaks -Water testing of a sample from the shoreline along Island Drive near the Clearlake Oaks county boat launch has detected levels of a cyanotoxin considered unsafe for recreation. The sample was collected on August 29 and the elevated level on a screening test led to further laboratory analysis demonstrating 46 micrograms per liter of microcystin. The level exceeds the “danger” trigger of 20 micrograms per liter established in statewide guidelines for recreational water safety.    

Residents and visitors are urged to avoid contact with the water in the affected area until after the bloom subsides. They should not allow people or pets to drink, swim, wade or contact visible mats or scum in the water or along the shore. Pets are particularly at risk for adverse health effects due to cyanotoxin exposure because they readily ingest algal bloom material, including licking it from their fur after swimming. Tap water from the public water system is considered safe.

Cyanotoxins are produced by bacteria called cyanobacteria.   They are an essential part of the environment that have existed for millions of year and produce oxygen. Under certain conditions, they multiply excessively and form visible clumps in the water, surface scums, mats, or an oily sheen on natural water bodies. On occasion, they produce toxins that can cause harmful effects in people, pets and livestock if exposed through ingestion, inhalation of aerosolized water or direct contact. Visible and potentially toxic overgrowths of cyanobacteria are referred to as “Harmful Algal Blooms” or “HABs.” Freshwater HABS may occur in rivers, lakes, streams and ponds. The current HAB in Copsey Creek is an unusual occurrence in Lake County, but may be related to physical conditions that cause localized slowing down of the flow of water.

As environmental factors change, most HABS resolve over time. However, when cyanotoxins are known to be present, re-testing the water after it has cleared and allowing at least two weeks to pass after no toxins are found is recommended.

For more information, please visit:

California Harmful Algal Blooms Portal


California Cyanobacteria and Harmful Algal Bloom (CCHAB) Network:
California Surface Water Ambient Monitoring Program Freshwater HAB
California Department of Public Health:

U.S. Environmental Protection Agency