Lake Tahoe's famed clarity threatened with warming temperatures

Lake Tahoe experienced a year like no other in 2015, according to scientists from the UC Davis Tahoe Environmental Research Center in its annual Tahoe: State of the Lake Report, released this week.

Lake Tahoe is experienced warming water, a 4.8 foot drop in clarity, increased algae and a lowering water level.

Thanks to global climate change, Lake Tahoe is warming faster than ever according to the report which covers results from tests conducted on the lake in 2015 concerning water temperature, clarity, invasive species and more.

It takes a lot of energy to warm the massive lake which holds about 39 trillion gallons of water. According to the State of the Lake Report, in 2015, the average temperature of Lake Tahoe increased nearly half a degree which is 15 times the long term rate of warming. The average surface temperature reached 53.3 degrees. The overall average water temperature is a little over 43 degrees.

"The lake has warmed at an alarming rate of over 0.3 degrees F/year,” according to Geoffrey Schladow of the University of California-Davis Tahoe Environmental Research Center. He said the changes at Lake Tahoe highlight the magnitude of human-caused global climate change.

The report summarizes how natural forces, long-term change and human actions have affected Lake Tahoe’s clarity, physics, chemistry and biology over time. It presents data collected in 2015 and puts them in the context of the long-term record.

Continued warm and dry conditions contributed to several record-breaking measurements. Among them:

Climate change:
While precipitation was near average, only 6.5 percent of it fell as snow, the lowest amount ever recorded. Only 24 days had below-freezing average air temperatures, the lowest on record.

Lake Temperature:
The lake’s average temperature is rising at its fastest rate yet. In 2015, the volume-average temperature increased 0.48 degrees F over 2014.

The authors write that in the past four years, “the lake has warmed at an alarming rate of over 0.3 degrees F/year.” That’s 15 times faster than the long-term warming rate.

Also, the average surface temperature was the warmest on record, 53.3 degrees F.

Mixing:
Deep mixing that a lake undergoes in the winter is crucial for adding oxygen to the depths and redistributing nitrogen that tends to accumulate at the bottom. However, Lake Tahoe failed to mix to its full depth for the fourth year in a row. Its mixing depth in 2015 of 262 feet is the lowest recorded. This lack of deep mixing also led to the highest average nitrate levels ever recorded in the lake, 20.6 micrograms per liter. This marked a recent shift from the relatively constant nitrate concentrations that persisted for much of the 35 year record.

Algae and lake level:
Attached algae around the lake’s margins were at record-low levels, due largely to the low lake level, which fell by 9 inches in 2015. Lake Tahoe was below the natural rim 364 days in 2015, so no water could flow into the Truckee River.

“The occurrence of rising air temperatures Lake Tahoe has been known about for many years now, and with it the warming of the lake. 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.” – Dr. Geoffrey Schladow, TERC Director

Clarity affected by low snow-to-rain ratio
Average annual clarity was 73.1 feet in 2015, a 4.8-foot decrease from the previous year, a measurement announced in April 2016. The decline was due in part to warmer inflowing water. The annual average clarity was still more than 9 feet greater than the lowest recorded average of 64.1 feet in 1997. The measurement marks the depth at which a 10-inch white disk, called a Secchi disk, remains visible when lowered into the water.

For the complete report: http://terc.ucdavis.edu/stateofthelake/sotl-reports/2016/02-exec.pdf