June 20, 2024

California’s largest freshwater lake has turned bright green due to algae blooms so intense they are visible from space, Nasa has announced, sharing satellite images from mid-May.

The photographs showed that “bright green swirls were visible across most of the lake’s area”, the space agency said, and may have been caused by cyanobacteria, commonly known as blue-green algae, as well as other kinds of phytoplankton.

Clear Lake, which stretches over 68 sq miles in northern California, is a major hub for fishing, water sports and other recreation, and has been dubbed the “bass capital of the west”. The lake is roughly 100 miles north of San Francisco.

Public health officials in the area have warned visitors to be aware of the algae blooms, including keeping pets away and steering clear of water “that appears discolored, has scum layers, or emits a foul odor”, the Press Democrat, a local newspaper, reported.

“The appearance of cyanobacteria in Clear Lake is not unusual, but it has occurred earlier and in greater abundance than in typical years,” the newspaper said.

Testing to monitor the current levels of toxins in the water is ongoing. The Big Valley Band of Pomo Indians has been monitoring water quality at the lake for the past decade. “This year has the earliest blooms since the Tribal program lakeshore cyanobacteria and cyanotoxin sampling began in 2014,” a Clear Lake water quality Facebook page noted.

Archaeological evidence suggests that humans have lived around Clear Lake for at least 12,000 years, and sediment samples suggest large algae populations have been present in the lake for a similar amount of time.

But recent human activity has been found to increase the number of “harmful algal blooms”, which can be toxic to people and pets, as well as to fish, birds and other marine creatures.

At Clear Lake, according to Nasa, “Runoff from nearby farms, vineyards, faulty septic systems, gravel mines, and an abandoned open-pit mercury mine contributes to water quality issues in the lake.”

Scientists have warned that algae blooms are becoming more frequent and more toxic, and have linked the trend to global heating and industrialized agriculture.

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