UC Davis scientists from the Tahoe Environmental Research Center reported last week that the rate of decline in visibility in Lake Tahoe has slowed since 2001. By using new modeling techniques to detect trends in water clarity, the research group has found that nearby runoff of fine particles has not increased in the last seven years.
Runoff of small particles from urban areas and roadways near the lake contribute to a lack of clarity in the lake. Small particles limit how far down one can see in the water because the particles can scatter light.
“Recently, the highly significant trend of decreasing lake transparency, which has been about a foot per year on the average for the last half century, appears to have slowed and we hope has leveled off,” said Charles Goldman, professor of limnology at UC Davis and director of the Tahoe Research Group, in an e-mail interview.
The data for the modeling was collected from 2001 to 2006, although the Tahoe Environmental Research Center does have two long-term stations at the lake.
Water clarity is measured through Secchi tests, in which a white disk the size of a dinner plate is lowered into the water until it is no longer visible. Monika Winder, a research scientist with the Tahoe Environmental Research Center, said that the test is done twice a month, and the researchers test several spots in the lake to see how water clarity varies. At 1,644 feet deep, Lake Tahoe is the 11th deepest lake in the world, so variation is common.
“The Secchi depths – where it disappears – varies in Lake Tahoe from 10 meters to 35 meters,” Winder said.
Winder, who is currently researching microorganisms in the lake, said that there has not been much long-term research in the area, but that microorganisms like pico-phytoplankton can contribute to low visibility in the water.
“[Microorganisms] are pretty abundant in the lake, and they also may affect water clarity,” Winder said.
Lakes like Lake Tahoe go through a process of “mixing” every three or four years when there have been late winter storms with winds strong enough to mix the water column all the way to the lake’s floor. Lake Tahoe has a water column of 500 meters (1,600 feet). The mixing process can lead to lower water clarity since nearby pollution can run into the lake.
“The warming of the lake results in a resistance to mixing, since warm water does not mix as easily as cold water,” Goldman said. “This is a particularly interesting year to follow since I believe we had a complete mix but so far have not experienced high runoff.”
Winder said mixing has occurred less frequently at Lake Tahoe due to global climate change. While the lack of recent runoff is good for the lake, the increasing temperature is changing the composition of the lake.
“One qualifying aspect of this ‘good news’ from Tahoe is the fact that the lake has warmed appreciably in the last 30 years. The surface waters a full 4 degrees and the whole water column 1 degree,” Goldman said.
Winder said the change in temperature may be leading to an increase in smaller-sized plankton, a population also affected by the mixing process.
More than $500 million has been invested by federal, state and local agencies, as well as by homeowners and businesses to reduce runoff into Lake Tahoe. In 1997, President Clinton called attention to the issue of pollution in Lake Tahoe, leading to state funding from California and Nevada and federal funding for the Environmental Improvement Program to reduce pollution in the lake. Efforts have also been made by organizations such as the U.S. Forest Service and the Tahoe Regional Planning Agency to conserve Lake Tahoe.
John Mobourn, water resource specialist with the University of Nevada Cooperative Extension, works with the Lake Tahoe Environmental Education Coalition to educate home and business owners about limiting erosion and runoff into the lake. Mobourn said the report from UC Davis shows that these programs have had a positive impact.
“For the first time, we have sound scientific evidence to show that all these projects are making a difference,” Mobourn said.
MADELINE McCURRY SCHMIDT can be reached at email@example.com.XXX