High Sierra Topix

Muddying Tahoe's deep blue
The jewel of the Sierra could be worse off than previously thought.

In a new study, researchers at the University of California-Davis predict that climate change will irreversibly alter water circulation in Lake Tahoe, making it less hospitable to some native plants and fish, and that it will happen sooner than most people expected - in a little more than a decade.

Global warming is already causing warmer lake temperatures along the shoreline, and is likely to continue to cloud up the cobalt waters that attract tourists from all over the world. However, the most recent news came as a shock to even the researchers.

"This is one of the early indicators of what climate change can do to freshwater supplies nationwide," said Geoffrey Schladow, director of the Tahoe Environmental Research Center, who assisted with the study. "Our lakes and reservoirs, what we depend on for our very existence, are going to change. They're going to change possibly in their appearance, but more importantly in the quality of the water."

At stake is the lake's fragile ecosystem, with scientists expecting climate change to affect the very chemistry of the lake by preventing its many layers of water from mixing, depleting the bottom levels of oxygen. Some types of fish will need to move to shallower waters in the 1,644-foot-deep lake, where they will be threatened by non-native and, in many cases, aggressive fish like the large-mouth bass that has already appeared along the Tahoe Keys.

"The biggest immediate impact from that is if this happens over the course of decades, we may lose a lot of the valuable trout and cold water fish habitat," said John Reuter, associate director of the UC-Davis Tahoe Environmental Research Center. Reuter is one of several scientists who presented the new findings at a scientific conference in Incline Village.

The lake is made up of many layers of water, with the surface being rich in oxygen. When the wind is right and temperature is ideal, the top layer sinks, and in the best years goes to the bottom. This is not a constant process, but happens, on average, about every four years, usually in late February.

Through the process, oxygen from the surface is distributed throughout the lake, benefiting plants and fish at the bottom that are dependent upon it to survive, especially trout.

Through extensive computer modeling, the scientists determined the deep mixing may not occur as frequently in the future because of global warming. And by 2019, it might not occur at all. The finding, Schladow said, came as a complete surprise.

The wintertime process is an important one as the churning about of water also dilutes all the contaminants that came into the lake the previous spring. Deep mixing also moves nutrients from the lake bottom to the water surface where they promote the growth of algae.

"If greenhouse gas emissions continue at their present levels, there are pretty big changes in store for the lake itself, for its mixing environment and water quality within the next 10 to 20 years," Schladow said. "This brings home the urgency of the need to reduce emissions."

The scientists said it is still unclear whether a failure of the layers to mix would hurt the clarity of the lake. But they know that as temperatures rise, less snow and more rain are falling in the Sierra, fueling soil erosion and contributing to the ever-increasing murkiness of the waters.

Last summer, scientists demonstrated that global warming is already affecting the Tahoe basin. The nighttime air temperature at the lake has risen by more than 4 degrees since 1911, while the average daytime temperature has increased 2 degrees in the same time period.

Just since 1999, the average July surface water temperature of the lake has increased almost 5 degrees, with temperatures near the shoreline hitting a record 78 degrees on July 26, 2006.

The lake's clarity has also diminished over the years. While visitors to the lake could see down an average depth of 102.4 feet in 1968, by 2006 visibility extended to an average depth of just 67.7 feet, a decline of about 1 foot per year.

The new data suggesting that the warming of the lake may help invasive fish species while harming native fish is just another nugget of bad news, said Carl Young, program director of the League to Save Lake Tahoe, an environmental advocacy organization aimed at protecting the health of the Tahoe basin.

"All these things look very foreboding," Young said. "The future holds significant challenges to help preserve the jewel of the Sierra."

Contact Julie Sevrens Lyons at jlyons@mercurynews.com or (408) 920-5989.