SAND HARBOR STATE PARK, Nev. -- America's most famous alpine lake may have rebounded from historically low water clarity over the last 15 years, but the scientists who look after Lake Tahoe's clear blue waters see no reason to gloat. They've turned their attention to a new foe: an invading clam.
Lake Tahoe, which straddles the Nevada and California line in the Sierra Nevada, has received $1.5 billion in federal aid since President Clinton came here in 1997 to host a summit on restoring the regional ecosystem. At the time, pollution and runoff had reduced clarity from 102 feet in 1968 to 60 feet in 1997, shocking longtime visitors like Sen. Dianne Feinstein (D-Calif.) into action.
The resulting emphasis on strictly limiting erosion and policing development brought with it a net clarity gain to 68 feet in 2009, along with a sense of victory for the environmental groups behind the "Keep Tahoe Blue" campaign. But now, even as Feinstein hailed the success this week during an annual summit on the region, the attention has turned to a non-native Asian clam that is spreading along the lake's soft bottom.
The dime-sized clam, which was likely transported here by recreational boaters, tends to choke off native organisms like snails that are vital to the food chain and, worse still, could turn the blue lake green because of algal excretions. And it moves in numbers, reaching populations as high as 5,000 per square yard.
That could mean all the work and money committed to improving the water quality might be for naught, unless the clam is stopped. The spread of Asian clams is especially tricky because they create habitat for the quagga mussel, a mollusk that can grow almost anywhere and invade water intake pipes, piers and boats.
"It has effectively killed Lake Mead," said Feinstein in a speech this week, in reference to a quagga mussel infestation. "And we must keep it out of this lake."
The fear surrounding a quagga invasion, which experts say could make the clam infestation look like a minor event, is that the mussels could grow to such an extent that the explosion affects water quality and damages infrastructure. Lake Mead, which straddles Arizona and Nevada, is the main water source for Las Vegas and has been battling a costly infestation.
In Washington, D.C., Feinstein wants to move a new funding bill for Lake Tahoe with $415 million in federal funds spread across 10 years for invasive species, wildfire fuel reduction and more work to improve water clarity. She has the support of Sens. Barbara Boxer (D-Calif.) and Nevada Sens. Harry Reid (D) and John Ensign (R).
Here on Lake Tahoe, experts have a more practical solution: rubber mats.
Researchers here have covered 1 full acre of the clams with rubber mats at the bottom of the lake to choke off oxygen. The sheets, which were installed in early July, will remain in place through the end of the summer, when scientists will take a step back and assess results.
The idea is novel in its simplicity and is said to be working. John Reuter, associate director of the Tahoe Environmental Research Center for the University of California, Davis, said the first mat managed to cut oxygen access to zero within 36 hours, which then killed a patch of the clams within 28 days.
"At this point, we know the method works," said Reuter, explaining that killing the clams is only half the battle. "We'll take the next 18 to 24 months to see how rapidly things recolonize."
How fast the organisms grow back will dictate whether Reuter and others at the Tahoe research center take their idea to a higher level, to spread mats across the 200 acres said to be infested. If it takes three years for the clams to recolonize rather than three months, Reuter said a lakewide rubber-mat strategy would be sound investment.
The second half of the strategy is to screen recreational boats for invasives. State and local parks officials have been putting boats to the test all summer, and though inconvenient for some, Reuter believes the "two-pronged approach" is justified.
"You control what you can in the lake and you also do the boat inspections and prevent new ones," he said. "This is not yet a lakewide problem, but it could easily get there."
In terms of threat to the lake's ecosystem, which was made famous by Mark Twain in the 19th century, Reuter said the mussel invasion would be a much higher threat than the Asian clam. "The mussels can grow everywhere," he explained, unlike the clam, which needs the soft bottom to breed.
Climate change report due in Sept.
More broadly, researchers here have turned their attention to climate change as a larger threat to the region's ski industry as well as the lake's clarity and forests, which are increasingly vulnerable to dry, hot conditions that lead to more virulent wildfires.
Reuter said the data on lake clarity are good but not perfect. The rate of decline has slowed, he said, but that does not necessarily mean the region's environmental problems are solved, especially as tourists continue to flock to the region in summer and winter.
"What it is showing is the lake has the ability to respond," Reuter said. "I would use that term everybody uses: cautiously optimistic."
Addressing the bigger picture, the UC Davis team expects to issue a major report on how climate change could affect the lake by the end of September. The report is currently being reviewed by the federal resource agencies, after which it will be released to the public.
As for the $415 million in federal funding, Feinstein said she intends to push it through this year. To Reuter, the progress in recent years proves the $1.5 billion doled out since 1997 has not been ill spent.
"The improvements should show everybody that these investments have the potential to pay off," he said.