A warmer, less clear Lake Tahoe in 2015, UC Davis scientists say

Lake Tahoe got warmer and cloudier last year, according to UC Davis researchers.

In the annual state-of-the-lake report issued Thursday, the University of California, Davis, scientists said that 2015 featured low amounts of snowfall, high lake temperatures and very little mixing of lake water.

Another year of warm and dry conditions contributed to some record-breaking measurements. A summary of the findings reveals that only 6.5 percent of precipitation at the lake fell in the form of snow in 2015, which is the lowest amount on record.

The lake’s temperature is rising at the fastest rate seen by scientists. The average surface temperature was 53.3 degrees, the warmest on record.

“The occurrence of rising air temperatures at Lake Tahoe has been known about for many years now, and with it the warming of the lake,” said Geoff Schladow, director of the UC Davis Tahoe Environmental Research Center in a press release. “What is different this year is that we are seeing more aspects of the lake’s internal physics changing, and that is bound to alter the ecology.”

Deep mixing of water in the lake is necessary to add oxygen and redistribute nitrogen that sinks to the bottom. However, the lake water failed to mix to extreme depths for the fourth consecutive year.

The paucity of deep mixing led to the highest level of average nitrate levels ever recorded in the lake – a change after the relatively constant nitrate concentrations over a 35-year period.

The alpine lake is known for its clear water. In 2015, average annual clarity was 73.1 feet, a 4.8-foot decrease from 2014.

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Scientists attribute the decline in part to warmer water coming into the lake. However, water clarity is still more than 9 feet greater than the lowest recorded average of 64.1 feet measured in the late 1990s.

Scientists measure water clarity depth by lowering a 10-inch white plate, called a Secchi disk, into the water and noting how long it stays visible.

Bill Lindelof: 916-321-1079, @Lindelofnews