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March 1, 2011
By Mike Wolterbeek
The bugs living in the muck at the deepest darkest depths of Lake Tahoe have disappeared in substantial numbers, and it's a sign of changes throughout the pristine lake's ecosystem over the past 40 to 50 years.
"Deep down in Tahoe's "abyss" there have been huge declines in numbers of native invertebrates," Annie Caires, limnology research specialist at the University of Nevada, Reno said. "In the 60s there was a lot of life, especially species endemic to Lake Tahoe, species that could be found nowhere else in the world.
"We're seeing 80 to 100 percent declines of those endemic species, and could only find one specimen of Tahoe flatworm and six of the blind amphipod shrimp, a shrimp that used to reach incredibly high densities in very deep areas of the lake."
For the past three years, Caires, of the University's Natural Resources and Environmental Sciences Department, and a team of researchers have been studying the goo from the bottom of the lake, literally dropping a "bottom sampler" at the end of a very long cable and scooping small bucketfuls off of the lake floor.
They took 394 samples from four areas around the lake, just as was done in the 1960s: from McKinney Bay, Crystal Bay, just south of Emerald Bay and near Stateline, Nev. going as deep as 450 meters - just about the deepest depth in the lake. They collected 7,400 bugs. In the 1960s researchers collected 25,400 bugs from 389 samples collected.
"We compared bottom-dwelling invertebrate surveys from 1962-63 to those of 2008-09 to determine changes in the density, distribution and habitat association of native invertebrates," Caires said. "Large declines in lakewide average densities of these organisms were observed between 1962-63 and 2008-09."
In a public presentation at the Tahoe Environmental Research Center in Incline Village, Caires unveiled the research from the depths of Lake Tahoe's abyss, as she puts it, and explained the importance of the research.
"When we lose these unique species, we also lose the roles that they play in lake ecosystem function. These organisms are important for processing organic matter, recycling nutrients and providing food for fish," Caires said.
Long-time Tahoe researcher Sudeep Chandra, professor of limnology at the University of Nevada, Reno's Natural Resources and Environmental Sciences Department, said there's good news and bad news.
"The bad news is that there has been a substantial decline in the numbers of invertebrates that are living at the bottom of the lake that is likely due to the introduction of invasive species and eutrophication," he said. "The good news is that the biodiversity of the critters that live in the bottom of the lake is still there. So, if we can determine how much of the decline is due to eutrophication, an issue the management agencies have been tackling for a decade - that may be improving the conditions in the lake, then we might be able to recover these species."
The research project concludes that the decline in density is associated with increased cultural eutrophication and the resultant loss of clarity. This decline in clarity reduces light available for aquatic plant growth at depths where native invertebrates depend on the plants for habitat and food.
"In the Laurentian Great Lakes they have tackled eutrophication issues and have watched their invertebrates recover," Chandra said. "There is no reason we can't see the same thing at Lake Tahoe. Our University remains committed to assisting the agencies at Lake Tahoe in resolving the ecological issues at the lake."
Caires, who is a faculty member of the University's Aquatic Ecosystems Analysis Laboratory, said there is still a lot to learn about the bottom invertebrates and the impacts of water clarity and invasive species.
"We need to take the time to learn more about the dynamics of the ecosystem," she said, "and we hope to continue our studies for another couple of years."
This project was funded by the U.S. Army Corps of Engineers, California Tahoe Conservancy, and the Tahoe Regional Planning Agency.