Lake Tahoe health report: Asian clam invasion growing fast

UC Davis has released the annual Lake Tahoe health report and they've found cause for concern. The dime-sized Asian clam which first invaded the lake about 10 years ago has been growing rapidly in numbers. According to John Reuter, associate director of the UC Davis Tahoe Environmental Research Center,

"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. 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."

Asian clam (Corbicula fluminea).The invasive clams seem to be displacing the native pea clams in lake sediments. According to the report, in the areas where they are most numerous, Asian clams comprise almost half of the benthic, or sediment-dwelling, organisms. Another problem is that a green filamentous algae, Zymena, is thriving in the beds of the Asian clams. Scientists suspect that the algae are being nourished by high concentrations of nutrients excreted by the clams. Dead Zygnema clumps are washing onto beaches where they rot and become a nuisance. Scientists also fear that the exotic clams may concentrate calcium in the lake and facilitate the invasion of even more problematic species - the quagga and zebra mussels. These highly noxious species 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.

Other findings made in he Tahoe: State of the Lake Report include:

• The lake's clarity was unchanged from the previous year: The Secchi depth was 69.6 feet.
• 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.
• It was another dry year -- the 12th driest year on record. While the historical average annual precipitation is 31.6 inches, in 2008 only 19.3 inches of precipitation fell.
• In 2008, Lake Tahoe mixed all the way to the bottom at the mid-lake station. This was the second successive year of deep mixing. Complete mixing during two or more successive years has only occurred three times since 1973. 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.