Lake Tahoe is being invaded. But not by skiers or Bay Area residents seeking a relaxing weekend at the beach.
In a troubling trend, scientists have discovered the rapid spread of the Asian clam, an exotic bivalve roughly the size of a dime, in the mud and sediments around Tahoe's south shore.
Although it is too early to know what the clam's impact will be, in a worst-case scenario, its continued spread could litter beaches with sharp shells and consume plankton that fish rely on for food. Worse, the clam appears to be sparking the growth of stringy green algae that feeds on its waste, leading to concerns that some sections of the lake's azure blue waters eventually could turn a murky green.
"It goes beyond not wanting to put your foot on algae that is rotting on the shore," said John Reuter, an aquatic ecologist with UC-Davis.
"Lake Tahoe is a place that people have tried hard to protect. These invasive species get us further and further away from the pristine condition of the lake that people would like to see."
In 2002, only a few of the clams were first spotted in Tahoe's waters. But now, as many as 3,000 per square meter — an area about the size of a child's sandbox — are clogging some parts of the lake's southeastern edges between Zephyr Cove and South Lake Tahoe, according to a study released Tuesday by the UC-Davis Tahoe Environmental Research Center.
"They've really exploded over the last few years," said Reuter.
UC-Davis, working with the University of Nevada, Reno and the University of British Columbia, is surveying the entire lake to understand the extent of the problem.
The clams burrow about 8 inches deep in the mud, from the shore to waters about 140 feet deep. They live up to four years, producing as many as 100,000 offspring.
They were first spotted seven years ago by scientists scuba diving who noticed their shells on the lake bottom. Nobody knows how they got to the lake.
Found in the Sacramento River and other bodies of freshwater, the clam, which is native to China, Korea and other Asian countries, was first reported in the United States in the 1920s, when Chinese immigrants brought it over for food.
Like many other invasive species, from kudzu grass to star thistle, it spread out of control. Asian clams are now found in the Mississippi River, the East Coast, even some major European rivers such as the Danube and the Rhine.
"Some people use them as bait. That may be the way they came in to Lake Tahoe. They don't normally attach to boat hulls," said biologist Andrew Cohen, director of the Center for Research on Aquatic Bioinvasions, in Richmond.
Cohen has documented more than 250 invasive species in San Francisco Bay and its delta. A similar saltwater species of Asian clam lives in the Bay, he noted.
Researchers say it is too soon to know what the impact on fish and other species will be.
"They'll remove some food by filter feeding, which could help improve water quality. But there are going to be some losers too," said Cohen.
"If they remove enough plankton — and I don't know if that's possible given the depth of Lake Tahoe — the food web could be affected, everything that feeds on plankton, from fish to small crustaceans."
Scientists also worry that the Asian clam could make it easier for more harmful invasive mollusks, including the zebra mussel and quagga mussel, to move into Lake Tahoe. That's because the Asian clam's shells are high in calcium, and the zebra and quagga mussels thrive in calcium rich water.
The zebra mussel has caused hundreds of millions of dollars in damage in the Great Lakes, clogging water pipes, covering boat hulls and sucking massive amounts of plankton from the water.
Last summer, after several zebra and quagga mussels were found on boats, local and state agencies began requiring any boat coming into Lake Tahoe to be inspected. But the inspections only cover public marinas and don't affect boats launched from the shore or the more than 500 private piers around the lake.
Although only 66,000 people live around Lake Tahoe year-round, 3 million people visit each year, many of them from the Bay Area.
"We rely to a great degree on education and the goodwill of our visitors to do the right thing," said Rochelle Nason, executive director of the League to Save Lake Tahoe.
"But there are always the handful of bad apples, and they can pose a great threat in many different ways. Uninspected boats entering the lake is the newest problem."
Over the past 150 years, several other non-native species have been added to Lake Tahoe, including bass, bluegill and brook trout, for fishing. Those fish helped drive the native Lahontan trout nearly to extinction.
Reuter, of UC-Davis, said his colleagues are experimenting with ways to eradicate, or at least limit the spread, of Asian clams. One method involves vacuuming high concentrations of them off the bottom with suction hoses. Another involves attempting to smother them with covers laid over the mud.
But the best approach so far is to not introduce any more, he said, and to keep the zebra and quagga mussels from ever invading California's most renowned alpine lake.
"The best approach is to be proactive," he said.
Contact Paul Rogers at 408-920-5045.
Size: 10 millimeters wide, about the size of a dime
Life span: 1-4 years
Offspring: Can produce 100,000 in a lifetime
Native to: China, Korea, Taiwan, Japan
First found at Lake Tahoe: 2002