While clarity improved at Lake Tahoe for a second straight year in 2012, long-term trends show that climate change is impacting the Lake Tahoe Basin with drier years, less precipitation, higher lake temperatures and projected lower lake levels.
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These conclusions are found within the lake’s annual health exam, “Tahoe: State of the Lake Report 2013,” released Aug. 7 by the Tahoe Environmental Research Center at the University of California, Davis.
UC Davis researchers have been continuously monitoring Lake Tahoe’s clarity, physics, chemistry and biology since 1968. This long-term data set helps inform and measure progress toward Tahoe’s restoration goals.
“Every year brings surprises, but with them come new insights,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center. “In this last year we saw how nature, combined with the results of the many projects that have been completed in the basin, produced an amazing increase in clarity. The real challenge is to be able to sustain the improvements when nature is working against us.”
The annual average clarity improved by 6.4 feet over the previous year to 75.3 feet. (Clarity data for 2012 was released in February 2013 and is repeated in this report.) This value is within 3 feet of the interim clarity target of 78 feet. The improvement occurred in both summer and winter. The reasons for the improvement include:
• 2012 was a dry year for Lake Tahoe, with precipitation 71 percent of the long-term average. Reduced precipitation resulted in fewer pollutants flowing into the lake.
• The absence of deep mixing. Each winter, surface waters cool and sink downward, mixing with deeper waters. This brings nutrients to the surface, promoting algae growth, while also moving oxygen to deep waters, promoting aquatic life. A lack of deep mixing can contribute to warmer lake temperatures.
• Reduced numbers of tiny Cyclotella algae in 2012. An abundance of the microscopic algae over the past five years has been linked to climate change and coincided with reduced summertime clarity.
Year-to-year fluctuations in lake conditions are normal, which is why TERC researchers note that long-term trends are a better indication of lake health.
Clarity is measured by the depth at which a 10-inch, white Secchi disk remains visible when lowered beneath the water’s surface. When the long-term measurement program began in 1968, the Secchi disk could be seen down to 102.4 feet.
In new research, valuable tools are beginning to provide fresh insights into the processes that drive change in Lake Tahoe. An underwater glider that operated in the lake for 11 days in May 2013 provided the first “snapshots” of water quality across an east-west transect. The data confirmed the presence of giant “internal waves”
deep in the lake that could move algae and pollutants vertically over 150 feet.
Possibly more important was the installation of a water-quality monitoring station in 360 feet of water off the west shore. Connected to shore by an underwater cable, this station provides data from top to bottom every 30 seconds. This is the first such station in any lake worldwide.