California’s Largest Natural Lake Turns Green With … Algae

A potentially toxic algal bloom has turned Clear Lake in Northern California bright green, fed by increased runoff from human activity.

1 minute read

June 4, 2024, 10:00 AM PDT

By Mary Hammon @marykhammon


Satalite image of a bright green lake surrounded by brownish-green land

An bright green algal bloom clouds the waters of California’s Clear Lake in mid-May 2024. | Wanmei Liang / NASA Earth Observatory

“Clear Lake, one of California’s oldest and most-visited lakes, is not so clear anymore after an algal bloom turned the water cloudy and green,” reports Terry Castleman for the Los Angeles Times. Located about 60 miles north of the San Francisco Bay, the 43,000-acre freshwater lake is a popular destination for boaters, fishers, and swimmers, but those activities will likely be affected for weeks to come.

The bloom may be comprised of blue-green algae, also known as cyanobacteria, which produce a toxin that can irritate the skin and cause liver and kidney damage. According to Nasa, “Algal blooms have been common in Clear Lake’s long history, but have grown more common with human activity in the last century,” including runoff from nearby farms, vineyards, faulty septic systems, gravel mines, and an abandoned open-pit mercury mine, as well as from nonnative carp stirring up the nutrients in the lake bed’s sediment.

Cyanobacteria is the same type of toxic algae that has bloomed extensively in Lake Erie over the last several years, most notably in 2014 when residents in Toledo, Ohio, were unable to drink, cook with, or brush their teeth with water for three days. States across the country are dealing with algae blooms due to nutrient run-off, impacting local water quality and water-based tourism, including Cape Cod.

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