UC DAVIS (US) — The water clarity of Lake Tahoe in 2010 dropped to the second-lowest level ever recorded. Researchers suspect the combined effect of changes in climate and algae are to blame.
A new study indicates water clarity dropped from 68.1 feet in 2009 to 64.4 feet in 2010, a 3.7-foot decline.
“Taken alone, that decline in clarity is unusual but it is within the range of normal fluctuations,” says Geoffrey Schladow, professor of civil and environmental engineering at the University of California, Davis and director of the 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.”
Clarity is measured by the depth to which a Secchi disk monitoring device remains visible when dropped beneath the water surface.
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.
For the last decade, lake clarity has improved during winter, but deteriorating summer conditions have outweighed some of those gains, Schladow says.
“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,” says Joanne S. Marchetta, executive director of the Tahoe Regional Planning Agency. This year’s research findings underscore the urgency of environmental restoration work at Lake Tahoe, she says.
The report finds climate change may have produced conditions favoring 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,” says John Reuter, associate director of the 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.”
Improvements in winter clarity measurements may be due to efforts to reduce urban stormwater flows into the lake, but a comprehensive monitoring program in the Tahoe basin, to test the hypothesis is needed.
Clarity data derived from remote sensing via satellite images of the entire lake are included in the report for the first time, revealing that, for most of the year, clarity on the eastern shore is significantly lower than on the western shore.
It also indicates that clarity is better at one mile offshore than it is within a half-mile of the shore, highlighting the importance of better managing near-shore water quality.
While researchers conclude that the trajectory of the Secchi clarity measurements into the future is uncertain, they say the investment to date in water quality control projects cannot be underestimated and a reduction in nutrients and fine sediment load is clearly in the best interest of lake clarity.
“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 says.
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.
Funding for long-term monitoring and analysis of Lake Tahoe is provided by the Tahoe Regional Planning Agency, the U.S. Geological Survey, the U.S. Forest Service, the Lahontan Regional Water Quality Control Board, and UC Davis.
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