Clear Lake, one of California's oldest and most-visited lakes, has experienced a dramatic change in water quality due to an algal bloom. NASA satellite images reveal the once-clear lake now appearing cloudy and green. Located in Lake County, approximately 120 miles north of San Francisco, Clear Lake has been affected by a bloom possibly caused by blue-green algae or cyanobacteria, which produce a potent toxin known as microcystin.

According to NASA, algal blooms have been a common occurrence in Clear Lake's history, but they have become more frequent in the last century due to human activities. Runoff from nearby farms, vineyards, faulty septic systems, gravel mines, and an abandoned open-pit mercury mine contribute to water quality issues in the lake. In addition, nutrients from the lake bed's sediment are stirred into the water by waves and nonnative carp.

NASA's satellite image captured the main algal bloom in the top left of the lake, appearing dark and cloudy. This image, taken on May 15, 2023, reflects the peak of the visible bloom. Comparatively, last year's image on the same date shows significantly lower levels of algal activity, as evidenced by the noticeable change in water color.

Clear Lake spans over 43,000 acres, making it the largest natural freshwater lake in California and possibly the oldest lake in North America. The lake serves as a centerpiece for several communities in the rural Lake County area, including Lakeport, Clearlake, Kelseyville, Glenhaven, among others.

However, the recent algal bloom may adversely impact the lake's main attractions, such as boating, fishing, and swimming for weeks to come, warn officials. The bloom's toxicity poses a risk to visitors as microcystin can irritate the skin and cause liver and kidney damage. Data from toxicity monitoring conducted on May 8 did not yet indicate the presence of the bloom, with the next sampling scheduled for June 4.

Efforts to mitigate and address the water quality issues in Clear Lake have become imperative. Authorities are investigating and implementing measures to minimize runoff from various sources, including farms, vineyards, and faulty septic systems. Additionally, research and ongoing monitoring are being conducted to better understand the causes and effects of algal blooms in the lake.

As the picturesque Clear Lake continues to battle the algal bloom, local communities and environmental organizations remain committed to preserving and restoring the water quality of this historic natural treasure.