The 2010 State of the Lake Report released by the UC Davis Tahoe Environmental Research Center cites the proliferation of algae in large concentrations as a possible cause of the significant decrease in lake clarity between the 2009 and 2010 summer seasons.
August 12, 2011
Source: UC Davis Tahoe Environmental Research Center and Tahoe Regional Planning Agency
LAKE TAHOE - In a study released today by the Tahoe Regional Planning Agency and UC Davis Tahoe Environmental Research Center, scientists report that clarity dropped from 68.1 feet in 2009 to 64.4 feet in 2010, a 3.7-foot decline that resulted in the second-lowest clarity level ever recorded at Lake Tahoe. “Taken alone, that decline in clarity is unusual but it is within the range of normal fluctuations,” said Geoffrey Schladow, Director of the UC Davis Tahoe Environmental Research Center. “However, an analysis of other key variables makes us think that the transparency of the lake’s water may be now influenced by a new set of factors,” he said. Those factors likely include climate change and tiny algae, according to the report.
The report, “State of the Lake Report 2011,” suggests that climate change may have produced conditions that favored the proliferation of Cyclotella, a single-cell, free-floating algae, which in large concentrations can diminish clarity. “The numbers of Cyclotella have grown significantly in recent years,” said John Reuter, Associate Director of the UC Davis Tahoe Environmental Research Center. “This year, in particular, these single-cell algae were concentrated very close to the surface, strongly scattering light and thereby impacting lake clarity.”
The rate of decline in clarity has slowed overall in recent years, but with year-to-year fluctuations. In more than 50 percent of the 43 years for which Secchi depth measurements have been taken, researchers have seen differences (both positive and negative) as great or greater than this year’s drop. Schladow noted that lake clarity has been improving during winter for the last decade, but deteriorating summer conditions have outweighed some of those gains. The researchers speculate that the improvements in winter clarity measurements may be due to recent efforts to reduce urban storm water flows into the lake. They point to the need to obtain funding for a comprehensive urban storm water-monitoring program in the Tahoe basin, to further test this hypothesis.
“While we’re still maintaining the decade-long trend of slowing the decline in clarity, the new forces of climate change and the unusual concentrations of algae have us concerned,” said Joanne S. Marchetta, Executive Director of the Tahoe Regional Planning Agency. She said this year’s research findings underscore the urgency of environmental restoration work at Lake Tahoe. “TRPA is committed to addressing the challenges highlighted in the report by continuing our leadership role in the Tahoe Basin to bring together all who have a role to play in the lake’s preservation,” Marchetta said.
While the research team concluded that the trajectory of the Secchi clarity measurements into the future is uncertain, the UC Davis scientists also consider that the investment to date in water quality control projects cannot be underestimated. “There is every reason to believe that if it were not for the decades of watershed management, development policy and water quality restoration projects, the Lake’s transparency would be worse than it is today,” Schladow said.
In addition to an analysis of lake clarity, this year’s report presents information on new efforts being made to control the aquatic invasive species – Asian clam – that was first observed in Lake Tahoe in 2002 and has now reached large densities in certain portions of the lake. Additional topics include algae composition and concentration, the current impact of climate change on precipitation, changes in lake water temperature and the effect of lake warming on circulation. The full report is available online on the UC Davis Tahoe Environmental Research Center website .
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U.C. Davis Tahoe Environmental Research Center: State of the Lake Report 2010.
Tahoe Regional Planning Agency
Tahoe Project | Facebook
California Regional Water Quality Control Board Lahontan Region. Charting the Course to Clarity.
Nevada Division of Environmental Protection: Charting the Course to Clarity. (video)