Department of Water Resources
California Water News
A daily compilation of significant news articles and comment
August 19, 2009
3. Watersheds –
Asian clams pose Tahoe threat
Sacramento Bee
Exotic clams invade Lake Tahoe
Mercury News
Tiny invasive clams burgeon in Lake Tahoe;
A UC Davis report says the population of Asian clams has soared in the southeast portion of the lake,
threatening to hog food sources and excrete nutrients that foster algae growth.
Los Angeles Times
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Asian clams pose Tahoe threat
Sacramento Bee – 8/19/09
By Tom Knudson
It sounds like something out of a cheap horror movie – but this scare is real.
Non-native Asian clams are spreading rapidly across parts of Lake Tahoe, often accompanied by stringy mats of bright green algae, and posing a new threat to the world-renowned mountain lake.
So far the clams, which are about the size of a dime and have been found in concentrations of up to 3,000 per square meter, are concentrated in the lake's southeast corner. But if they spread more widely – and some have recently turned up in Emerald Bay on the southwest side – the ecological health of the lake may be in danger.
"It is fairly serious and definitely a cause for concern," said John Reuter, associate director of the UC Davis Tahoe Environmental Research Center. "If it gets around the whole lake, we're looking at a different lake."
The clams have been in Lake Tahoe for at least seven years, scientists say, and apparently arrived on the hull of a boat, or as fish bait. But it wasn't until this spring that they were discovered in such high concentrations between Zephyr Point and Elk Point on the Nevada side of the lake.
And in that area, at a place called Marla Bay, environmental and visual effects are already obvious.
Along the lake bottom, scientists have found the Asian clam is out-competing the native pea clams. In some places, the Asian invaders comprise almost half of the sediment-dwelling organisms, according to a new annual survey of the lake's health released Tuesday by the UC Davis research center.
What's more, Asians clams excrete nitrogen and phosphorus, which Reuter said are appear to be promoting large "nuisance blooms" of bright-green algae that hug the bottom like shag rugs. When the algae dies, it is washing up on the beach, along with thousands of crunchy, sharp-edged clam shells, changing the nature of a walk along the Tahoe shore.
"The stuff is floating up on the beach; it's not quite like the ocean after a huge storm – but again, even a bit is not the desired condition" that scientists and the public want to see at the lake, Reuter said.
Another concern is that this invasion may beget future ones. As the Asian clams die, they will decompose and release calcium into the lake, Reuter said. That calcium is something that the even more widely feared quagga mussels or zebra mussels – which have not been found at Tahoe – need to thrive.
Details about the spread of Asian clams at Lake Tahoe are contained in this year's "Tahoe: State of the Lake Report," which has been released annually since 2007 and can be read on the Web at http://169.237.166.248/.
Some limited efforts have been made to battle the invasion with suctioning equipment or bottom-hugging devices that starve the clams of oxygen. But more work needs to be undertaken to determine what works best.
"Some things have to be done very carefully because if you disturb the clam beds at the wrong time, you can make it worse," said Dennis Oliver, a spokesman with the Tahoe Regional Planning Agency.
But there is also a clear need to move quickly.
"The longer you wait, the worse it gets," Oliver said. #
http://www.sacbee.com/378/story/2120756.html?mi_rss=Environment
Exotic clams invade Lake Tahoe
Mercury News – 8/19/09
By Paul Rogers
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. #
http://www.mercurynews.com/california/ci_13153828?source=rss
Tiny invasive clams burgeon in Lake Tahoe;
A UC Davis report says the population of Asian clams has soared in the southeast portion of the lake,
threatening to hog food sources and excrete nutrients that foster algae growth.
Los Angeles Times – 8/19/09
By Amy Littlefield
Lake Tahoe is under siege by clams the size of your thumbnail.
The population of the coffee-colored Asian clams has soared in the southeast portion of the lake, threatening to hog food sources and excrete nutrients that foster algae growth, according to an annual Lake Tahoe report by UC Davis researchers.
Scientists worry that calcium in the clams' shells could make the lake more hospitable to invasion by quagga or zebra mussels, which cluster onto boats and anything else that rests in the water. Although the mussels have not been sighted at Tahoe, authorities at other lakes have spent millions of dollars trying to control them.
"In a lake like Tahoe where a lot money and a lot of effort is being put into maintaining its pristine nature, the introduction or the threat of invasive species really pulls us away from that pristine condition," said John Reuter, associate director of UC Davis' Tahoe Environmental Research Center, which released Tuesday's "Tahoe: State of the Lake Report."
No one is certain how the Asian clam first arrived at Lake Tahoe, whose famed clear waters lie at the center of a multimillion-dollar tourism economy. Some authorities say that fishermen used the clams as bait and that surviving clams took root on the lake's bottom, where they released tiny offspring that were carried by water currents to other parts of the lake.
Visitors first noticed the white, partially oxidized shells on the shore seven years ago.
"We've been aware of the Asian clam problem in Tahoe since 2002, and it's been just the past couple of years that it appears these clams have proliferated fairly rapidly," said Dennis Oliver, a spokesman for the Tahoe Regional Planning Agency, a bi-state agency that has spent more than $1 billion in federal, state, local and private money on restoration efforts at the lake.
Investigations are underway to determine whether the clams have moved from southeast areas such as Marla Bay to other parts of the lake. A small population has been found near Emerald Bay, along the southwest shore.
Authorities are testing removal methods such as suctioning out the clams and covering their beds in plastic to smother them. Environmentalists have called for increased inspections of boats to make sure quagga and zebra mussels don't get in.
"We think the time has come for additional measures to protect the lake," said Rochelle Nason, executive director of the League to Save Lake Tahoe. "Specifically, we need to start examining boats as they enter the basin, before they make it to the shoreline. The day may come when Tahoe must be closed to traveling boats." #
http://www.latimes.com/news/local/la-me-tahoe19-2009aug19,0,7971132.story?track=rss
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