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Clear Lake, a haven for bass anglers and jetskiers at the foot of a dormant volcano in Lake County, is also a sentinel for climate change in California.

Satellite measurements of the shallow, 68-square-mile lake's surface water temperature show a pronounced warming since 1992, matching the trend at five other lakes in California and Nevada, including Lake Tahoe.

The lakes' warming is "primarily due to climate change," said Geoffrey Schladow, director of the Tahoe Environmental Research Center. "No other factor could produce this degree of warming in all six lakes."

Furthermore, the warming "will impact the biology" of Clear Lake, said Schladow, a UC Davis professor of water resources and environmental engineering.

"The lake is going to be different," he said.

Warming of the six lakes was included in a report by the California Environmental Protection Agency documenting 36 indicators of climate change from the coast to the Central Valley and the Sierra Nevada.

The indicators include rising sea levels, shrinking glaciers, decreasing snowmelt runoff, increasing wildfires and gradual migration of plants and animals to higher elevations, according to the 258-page report.

"Together, these indicators paint a disturbing picture of how climate change is affecting our state and its growing threats to our future," said Dr. George Alexeeff, director of the state Office of Environmental Health Hazard Assessment.

Lakes "serve as good sentinels for climate change," the report said, noting that freshwater is "one of the resources most jeopardized by a changing climate."

Warmer waters at Lake Tahoe, undoubtedly California's favorite lake, may be making the cold, alpine lake more hospitable to algae, the report said.

Microalgae are proliferating in Tahoe's surface waters, reaching a size that impacts the lake's famous clarity, it said.

Tahoe warmed at a rate of 0.23 degrees per year from 1992 to 2008, compared with .09 degrees per year at Clear Lake, the difference largely due to the local lake's 27-foot average depth, a fraction of Tahoe's average plunge to 1,000 feet, said Schladow, who contributed to the EPA report.

The other four lakes cited in the report are Lake Almanor, a Plumas County reservoir; Pyramid Lake and Walker Lake in Nevada; and Mono Lake in Mono County.

Schladow said his colleagues looked at 160 large lakes around the world and found similar warming trends in nearly all of them.

But whether the warming of Clear Lake's murky water will affect its persistent summertime explosions of unsightly, odoriferous and potentially toxic algae remains uncertain.

High levels of algae-related toxins were found last week in water from Konocti Bay, where a dog had played fetch in the lake and subsequently died.

Health officials warned the public to be wary of blue-green algae, one of more than 130 species of algae known to inhabit the lake.

The moniker is misleading, since blue-green algae is actually cyanobacteria, an ancient organism that gave Earth its first oxygen and is found virtually everywhere on the planet.

When cyanobacteria blooms on Clear Lake, typically in spring and late summer, it ultimately dies and piles up along the shoreline in a stinking, sewer-like mass.

The lake's last rampant cyanobacteria outbreak in 2009, said to be the worst in 20 years, prompted complaints of sore throats and other allergic-type reactions and robbed shoreline businesses of their profits.

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Thomas Smythe, Lake County's water resources engineer for 25 years, said that warmer water will likely "lead to more frequent and intense cyanobacteria blooms."

But he's reluctant to pin the blame on climate change, noting that Clear Lake's temperature declined from 1970 to 1990 and then rebounded to nearly the same level by 2008.

"We don't really understand what goes on with Clear Lake," he said, given the vast number of variables in the ecosystem.

Clear Lake, the largest natural freshwater lake entirely in California, is at least 475,000 years old, possibly the oldest lake in North America.

Home to Native Americans dating back 12,000 years, the nutrient-rich lake is in good health, abounding with fish and birds, including pelicans, herons, egrets and bald eagles, with deer, bear and mountain lions in the surrounding basin. It recently was voted the third best bass fishing lake in the nation.

Sediment cores from the lake bottom indicate that it has hosted a large algae population since the last ice age, which ended 10,000 years ago.

Cecile Mioni, a researcher at UC Santa Cruz's Institute of Marine Sciences, said her studies at Clear Lake have so far failed to pinpoint the cause, or "driver," of cyanobacteria blooms.

Water temperature, nitrogen and phosphorus concentrations appear to be key drivers, she wrote in a 2009 study of the blooms in the lake and the Sacramento-San Joaquin Delta.

"We are still investigating," Mioni said in an interview last week, adding that she has another report in the works.

Getting an answer "is of primordial ecological importance" as it will dictate the strategy for mitigating the "problems associated with toxic blooms," her 2009 report said.

(You can reach Staff Writer Guy Kovner at 521-5457 or guy.kovner@pressdemocrat.com.)

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