Dan Thrift/Tahoe Daily Tribune file
LAKE TAHOE - Climate change and the proliferation of tiny algae likely led to Lake Tahoe's world-renowned clarity dropping nearly four feet last year, according to the annual State of the Lake report released Friday afternoon.
The new baseline of 64.4 feet - down from an average clarity depth of 68.1 feet in 2009 - is the second-lowest clarity level every recorded at Lake Tahoe (the lowest was 64.1 in 1997), according to the study released by the Tahoe Regional Planning Agency and the University of California, Davis, Tahoe Environmental Research Center.
Despite the 3.7-foot drop, scientists believe the overall rate of clarity decline is slowing.
"Taken alone, that decline in clarity is unusual, but it is within the range of normal fluctuations," said Geoffrey Schladow, TERC director. "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."
Large climatic changes may have produced conditions in the lake that are favorable to the procreation of Cyclotella - a single-cell, free-floating algae, which in large concentrations can diminish clarity, said John Reuter, TERC associate director.
"This year, in particular, these single-cell algae were concentrated very close to the surface, strongly scattering light and thereby impacting lake clarity," he said.
Researchers contend the findings emphasize the need for continued monitoring and management of the lake by both states.
"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 Marchetta, TRPA executive director.
The findings underscore the urgency of environmental restoration work at Lake Tahoe, Marchetta said.
"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," she said.
Rate of clarity decline
During the 43-year period in which Lake Tahoe's clarity has been measured, more than 50 percent of the changes from one year to the next have been greater than this year's drop, according to the study. Furthermore, lake clarity has been steadily improving during winter months for the last decade, but deteriorating summer conditions have outpaced those gains.
TERC researchers speculate the improvements in winter clarity measurements may be due to recent efforts to reduce urban storm water flows into the lake. They also point to a need to obtain funding for a comprehensive urban storm water-monitoring program in the Tahoe basin, to further test this hypothesis.
For the first time, clarity data derived from remote sensing via satellite images of the entire lake are included in the report. This information reveals that, for most of the year, clarity on the east shore is significantly lower than on the west 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 the research team concluded the trajectory of the Secchi clarity measurements into the future is uncertain, UC Davis scientists also consider 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 at terc.ucdavis.edu/stateofthelake.