The annual Lake Tahoe health report released this week describes a spreading Asian clam population that could put sharp shells and rotting algae on the spectacular mountain lake's popular beaches and possibly aid an invasion of quagga and zebra mussels, say researchers from University of California, Davis and University of Nevada, Reno.

"Our collaborative UC Davis and University of Nevada, Reno, science team found up to 3,000 Asian clams per square meter at locations between Zephyr Point and Elk Point in the southeastern portion of the lake," said John Reuter, associate director of the UC Davis Tahoe Environmental Research Center.

"Those are high numbers, especially when compared to the few isolated, dead shells we found back in 2002 when the Asian clam was first reported. We already see associated environmental effects from the clams today, and we are concerned that they might spread."

Issued annually since 2007, the "Tahoe: State of the Lake Report" is intended to give the public a better understanding of the changes occurring in Lake Tahoe on a year-to-year basis and to place current conditions within a historical perspective. It summarizes tens of thousands of scientific observations of lake weather, water conditions and aquatic life made since 1900. It is compiled by the research scientists of the UC Davis Tahoe Environmental Research Center.

The dime-sized Asian clam (Corbicula fluminea) that is the researchers' top concern this year probably has been in the lake for only 10 years, but it is already replacing native pea clams in lake sediments. In the areas where they are most numerous, Asian clams comprise almost half of the benthic, or sediment-dwelling, organisms, the report says.

What's more, in the beds of Asian clams, a green filamentous alga, Zygnema, is thriving. The report says the algae probably are being nourished by the high concentrations of nutrients excreted by the clams. Dead Zygnema clumps become a nuisance when they wash onto beaches and decompose. "This moves Lake Tahoe further away from its desired pristine condition," Reuter said.

Another worry: UC Davis and UNR researchers say accumulations of Asian clams might foster the invasion of two other damaging species: quagga and zebra mussels, which radically alter ecosystems by consuming enormous amounts of the tiny algae that are the base of lake food chains and outcompeting the native bottom-dwelling species. The mussels are also feared for their ability to encrust piers and boat hulls, and have already caused millions of dollars in damage in the Great Lakes by blocking intake pipes at water and power plants.

Zebra and quagga mussels are spreading west by hitchhiking on boat hulls, and a comprehensive program of public education and boat inspections has been established by resource managers to try to keep them out of Lake Tahoe. While Asian clams are not expected to become the colonizing monster that quagga and zebra mussels are, they may make lake waters more hospitable to the problem mussels by concentrating calcium, an essential nutrient.

The "Tahoe: State of the Lake Report 2009" is at http://terc.ucdavis.edu