Asian clam invasion serious threat to Lake Tahoe

What began as a few isolated dead shells in 2002 has escalated to a serious threat at Lake Tahoe as Asian clam populations continue to expand in the Sierra reservoir, according to the annual State of the Lake report released Tuesday.

Increased populations could litter sharp shells and rotting algae on Tahoe's beaches, aid an infestation of invasive mussels and affect lake clarity and ecology, according to research completed by the UC Davis Tahoe Environmental Research Center and the University of Nevada, Reno.

"The proliferation of Asian clams is a serious threat to the natural beauty and environmental health of Lake Tahoe," said Joanne Marchetta, Tahoe Regional Planning Agency executive director.

The density and extent of Asian clam beds in southeastern Lake Tahoe from Zephyr Cove to El Dorado Beach have increased drastically since 2002 when UNR researchers found three to 20 clams per square meter.

Now some locations between Zephyr Point and Elk Point host 3,000 Asian clams per square meter, said John Reuter, TERC associate director. The clams make up almost 50 percent of the sediment dwelling organisms in some parts of southeast Lake Tahoe, according to Tuesday's report.

"We already see associated environmental effects from the clams today, and we are concerned that they might spread," Reuter said.

Environmental effects from the clam beds include heavy growths of green algae called Zygnema near the clam beds. Researchers believe the high concentrations of nutrients excreted by the clams drive the growth of the algae.

In addition to spurring algae growth, the clams could also create a hospitable and calcium-rich environment for invasive quagga and zebra mussels, Reuter said.

Researchers began experimenting with removal methods like spreading tarps on clam beds or vacuuming the population in southeast Tahoe this year. TRPA is still compiling research to propose a lake-wide management plan. In addition, researchers are working to fully document the extent of the Asian clam population in Lake Tahoe.

A management plan could be proposed by early spring, said Ted Thayer, TRPA Natural Resource and Science team leader.

"By the start of next spring we should have a good idea of the distribution of the clams, the effectiveness of different techniques and be able to come up with lake-wide strategy of where and how we can treat and how much it is going to cost," Thayer said.

Reducing or eliminating the clam population is important to Lake Tahoe's health as is preventing the introduction of invasive mussel species, said Rochelle Nason, League to Save Lake Tahoe executive director.

"The lake is full of clams and that is a profoundly alarming report," Nason said. "The clams are bad enough in themselves. If they facilitate the invasion of quagga mussels Lake Tahoe could become a much less desirable place to live and play."

Lake clarity at Tahoe held steady in 2008, according to the State of the Lake report, which summarizes tens of thousands of scientific observations of lake weather, water conditions and aquatic life made since 1900 and tries to place current conditions within a historical perspective.

Like 2007, the average Secchi depth, the point below the lake surface at which a 10-inch white disk disappears from view, was 69.6 feet for this year.

Lake clarity can sometimes be tied to precipitation, Reuter said. If there is less precipitation in a year than less small particles and nutrients may be flowing into the lake.

"Usually if nothing is changing there will be this relationship where the lower the precipitation the better the clarity on average," Reuter said.

While Tahoe's historical annual average precipitation is 31.6 inches, only 19.2 inches of precipitation fell between Oct. 1, 2007 through Sept. 30, 2008. This makes 2008 the 12-driest year on record for Tahoe.

The long-term warming trend seen since 1970 continued, with warmer nights and lake waters, fewer cold days and less precipitation falling as snow. However, 2008 was a cold year, with the greatest number of freezing days since the early 1990s, according to the report.

In 2008, Lake Tahoe mixed all the way to the bottom at the mid-lake station. Mixing matters because it circulates nutrients and other material from the lake's depths to the surface water and returns oxygen to the deep water. This was the second successive year of deep mixing. Complete mixing during two or more successive years has only occurred three times since 1973.

The "Tahoe: State of the Lake Report 2009" is free and available online at terc.ucdavis.edu