UC Davis' annual Lake Tahoe health report finds that the long-term warming trend first described by UC Davis last year continues, that 2007 was the 14th driest year on record, and that clarity improved over 2006.

Issued Aug. 12, the Tahoe: State of the Lake Report 2008 says that the impact of smoke and ash from last summer's Angora Fire on the lake's clarity was small, though measurable. However, it cautions that it is too early to say what the impact from pollutants carried to the lake by streams and urban runoff in the burn area will be. A consortium of researchers is monitoring the long-term effects of the fire, which burned 3,100 acres at Lake Tahoe; the report says the fire's impact will not be known for several years.

The annual review is intended to give the public a better understanding of the changes occurring in the Tahoe Basin on a year-to-year basis and to place current conditions within a historical perspective.

This year's second annual report summarizes tens of thousands of scientific observations of lake weather, water conditions and aquatic life made since 1900. It includes the widely watched clarity report, which for decades has charted the legendary blue lake's decline in clarity.

Last year's inaugural State of the Lake Report bore the disturbing news that the climate in the Lake Tahoe Basin is warming up -- nights and lake waters are warmer, cold days are fewer and less precipitation falls as snow. While lake temperature declined slightly in 2007, the long-term signs of global warming remain.

In this year's report, UC Davis Tahoe researchers had better news about trends in lake clarity. Using the most recent Secchi depth data and applying a sophisticated statistical approach, they found that the historical rate of decline in clarity apparently slowed between 2001 and 2007. Researchers caution that this finding is tentative and future measurements could reverse the improved trend.

Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center, writes in his introduction to the 2008 report, "The data in the Tahoe: State of the Lake Report reveal a unique record of trends and patterns -- the result of natural forces and human actions that operate over time scales ranging from days to decades."

He concludes, "These patterns tell us that Lake Tahoe is a complex ecosystem, and it behaves in ways we don't always expect."

The 60-page report is free and available online. Here are some of its specific findings:

Angora Fire

-- Why it matters -- Fires in the Tahoe Basin have the potential to add large amounts of pollutants to the lake in the form of fine particles and unwanted nutrients that can reduce clarity and promote algae growth.

-- In 2007 -- Atmospheric inputs of smoke and ash were directly measured during the fire. Atmospheric deposition of nitrogen on the lake was estimated to be 5.0 to 8.4 metric tons; phosphorus deposition was estimated at 0.4 to 0.8 metric tons. (One metric ton = 2,205 pounds). Although deposition during the fire was 2.5 to 7 times the normal summer rate, it represented only about 1 to 2 percent of the annual load from all sources.

Even so, the impact of the Angora Fire on sending pollutants to the lake by streams and urban runoff will not be known for several years. A consortium of researchers is monitoring these effects.

Clarity

-- Why it matters -- Clarity remains the indicator of greatest interest about Lake Tahoe because it tracks changes in water quality and the community's efforts to restore clarity to historic levels.

-- In 2007 -- The lake was clear to an average depth of 70.1 feet, an improvement of 2.4 feet over 2006. The worst annual Secchi depth (the point below the lake surface at which a 10-inch white disk disappears from view) was 64 feet in 1997. When measurements began in 1968, the Secchi depth was 102.4 feet.

Meteorology

-- Why it matters -- Warming air temperatures mean more rain and less snow, as well as earlier snowmelt.

-- In 2007 -- It was was the 14th driest year on record, and 10 of the 12 months were drier than average. Precipitation at Tahoe City was 19.7 inches, two-thirds of normal. Snow represented 37.6 percent of total precipitation, down from 52 percent in 1910.

Fine particles and nutrients

-- Why it matters -- Fine particles and nutrients affect clarity because particles scatter light and nutrients promote the growth of algae, which absorb light. Urban runoff and erosion are the primary source of these pollutants, but atmospheric deposition is also significant.

-- In 2007 -- Low amounts of rain and snow reduced runoff, so less phosphorus, nitrogen and particles entered the lake. During late June's Angora Fire, atmospheric deposition of nitrogen and phosphorus was 2.5 times normal levels, but still represented only 1 to 2 percent of the annual input of those nutrients from all sources. The fire had a negligible impact on lake clarity and algal biomass.

Algae and zooplankton

-- Why it matters -- Algae (phytoplankton) and zooplankton (microscopic aquatic animals that graze on algae) influence the lake's food web, clarity and aesthetics.

-- In 2007 -- Algae concentrations on rocks around the shoreline were above average. Primary productivity (the rate at which algae produce biomass through photosynthesis) was the highest on record, at five times the 1959 level. Zooplankton concentrations were the lowest for the decade.

Deep mixing

-- Why it matters -- Mixing (when Lake Tahoe's surface waters cool and sink downward) has profound impacts on lake ecology and water quality. Deep mixing brings nutrients to the surface, where they promote algae growth. It also moves oxygen to deep waters, supporting healthy aquatic life at all depths. The lake mixes to the bottom, on average, every three years.

-- In 2007 -- Lake Tahoe mixed all the way to the bottom in March 2007, the first deep mixing since 2001.

UC Davis and many other research institutions and public agencies are working together to restore and preserve the Lake Tahoe ecosystem. Led by the Tahoe Regional Planning Agency, the organizations are currently engaged in a comprehensive effort to develop and implement an unprecedented set of environmental management plans for the basin.

More information: Tahoe Environmental Research Center