The UC Davis Tahoe Environmental Research Center recently released the Tahoe: State of The Lake Report 2011. This year’s report focuses on lake clarity, trophic status, and progress on the efforts to control Asian clams in Lake Tahoe. The average water clarity or “secchi depth” reportedly declined by 3.7 ft in 2010.
According to the State of the Lake Report, trends in recorded clarity continue to improve during winter months, summer clarity continues to decline, and average clarity is down by 3.7 ft from 68.1 ft in 2009 to 64.4 in 2010. 1997 was the worst year on record at 64.1 ft. If you remember how wet 1997 was in Tahoe it’s no surprise that year was the worst but I am a bit surprised 2010 was as bad as it was.
Water clarity is measured by recording the depth to which a “Secchi disk” is visible from the surface. Secchi depths have been recorded at Lake Tahoe since 1968.
While winter clarity in 2010 declined relative to the previous year, the overall trend in lake clarity measured during the winter months continues to increase. Researchers attribute this to improvements in storm water management throughout the Tahoe Basin. During rain and snowmelt runoff events water that flows directly into the lake carries a lot of gunk. If the water flows fast and energetically lots of fine particulates or suspended solids like silts, clays, and broken up organic debris are entrained and transported.
Most “best management practices” (BMP’s) for storm water management involve sediment traps where the flow velocity and overall fluid turbulence is decreased allowing particles to settle out of the water column. Both natural and artificial ponds and wetlands can function in the same way. BMP’s can be designed in many different ways but they all function to decrease the amount of energetic surface runoff by facilitating infiltration, particle settling, and biochemical processes that reduce nutrient loads and suspended solids in the water.
Researchers cited an increase in algal blooming in the upper strata of the lake during the 2010 summer as a reason for the decreased clarity. The lake experienced a shallower than normal depth of mixing during 2010 resulting in higher water temperatures near the surface which can influence algal productivity.
Nutrient availability can also influence Algae. When nitrogen and/or phosphorous are introduced into a body of water, primary biological productivity is increased. In other words, it’s fertilizer and more stuff grows. The Algae found in Lake Tahoe during 2010 was primarily cyclotella Gordonensis, a small centric diatom.
Average lake clarity fluctuates from year to year and some of the trends do show the benefits of continued efforts to maintain the lakes clear blue waters, but it’s clearly not guaranteed that the stabilizing trends will continue without proper understanding and management. Research conducted by groups like the UC Davis Tahoe Environmental Research center help inform our understanding of environmental processes and the effects we have on the basin.
To read more about the “State of the Lake” check out the full report at http://terc.ucdavis.edu/stateofthelake/