Although scientists know that Bay Area and Sacramento emissions contribute to Central Valley smog, the same plumes of pollution rarely reach Lake Tahoe. A review of air-quality research in the UC peer-reviewed journal California Agriculture concludes, "Pollutants most closely connected to the decline in Lake Tahoe's water quality originated largely from within the [Tahoe] basin."
Lake Tahoe has suffered a 30 percent decline in its famed clarity since the first "Secchi disk" measurements were taken in 1964. Soil erosion and nutrient runoff into the lake, coupled with air-pollution deposition, are responsible for scattering light particles and spurring algae growth.
In a special section, "Restoring clarity: The search for Tahoe solutions," California Agriculture presents five peer-reviewed research and review articles on air quality, runoff controls and biodiversity in the Tahoe Basin, as well as related news and editorial coverage of Lake Tahoe's environmental history, research facilities, clarity modeling and weed control efforts.
The full articles are posted online at http://californiaagriculture.ucop.edu
According to the air-pollution study led by Alan Gertler, professor at the Desert Research Institute in Reno, the Bay Area and Sacramento create plumes of nitric acid, phosphorus, ozone and other pollutants, but the levels dissipate almost entirely in the Sierra foothills and they do not reach the Tahoe Basin. Rather, local road dust, soil, vehicle exhaust and wood smoke are the major sources of airborne pollutants that end up in the lake.
"The most effective strategy to reduce the impact of atmospheric deposition on the lake's clarity and in-basin forest health would be to control local pollutant emissions," writes Gertler, who collaborated with scientists from UC Davis, the USDA Forest Service and the National Park Service on the study.
Other Lake Tahoe studies in California Agriculture include the first comprehensive study of how urbanization and remnant forests are affecting biodiversity in the Lake Tahoe Basin; a unique on-the-ground study of how wildfire influences nutrient runoff into the lake; and several studies on how well commonly employed erosion-control measures, as well as a newer method called "mechanical mastication," are helping to control runoff and improve forest health. Scientists from UC, the University of Nevada, and federal, state and regional agencies conducted the multidisciplinary research.
In a related editorial overview published in California Agriculture, pioneering UC Davis limnologist Charles Goldman explains the history of environmental impacts to the lake, which have contributed to its precipitous decline in clarity. The first Western settlers arrived in 1844, Goldman writes, and the discovery of gold near Virginia City in 1857 led to clear-cutting of most of the Tahoe Basin's old-growth timber.
The forests grew back and the lake apparently recovered by the late 1880s, but residential, commercial and recreational development - and the resulting nutrient-laden runoff - took their toll through the 1950s and 1960s. "Small particles remain suspended in the water column for years, adding to the gradual but relentless transparency loss," Goldman writes.
The Tahoe Regional Planning Agency was formed in 1969, providing a strong central authority to coordinate the lake-related activities of two states, five counties and local agencies. In 1984, a federal judge halted development in the basin for 2 years, and the first Tahoe Summit was held in 1997 with President Clinton in attendance.
At the same time, multifaceted scientific research has helped to inform regulatory decision-making in order to protect and improve the lake. UC Davis and Sierra Nevada College will complete a new state-of-the-art research facility, the Tahoe Center for Environmental Sciences (http://terc.ucdavis.edu), in Incline Village this summer, and the next biennial Lake Tahoe science conference will convene in October 2006.
"Strong, science-based evidence has been the decisive factor in successfully defending the Tahoe Regional Planning Agency from assaults over the years," Goldman writes. "New scientific discoveries on lake temperature and nutrients, and watershed ecology, together with developments in adaptive management, give further promise for the future of the lake."
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