Tahoe Environmental Research Center Staff, Summer 2006
TERC History
UC Davis research at Lake Tahoe began with Dr. Charles Goldman in 1959. Dr. Goldman formed the Tahoe Research Group and began regularly monitoring Lake Tahoe in 1968.
More than 40 years of investigations by the Tahoe Research Group (TRG) at the University of California, Davis have provided clear evidence for the onset of cultural eutrophication in oligotrophic Lake Tahoe. These extensive and internationally renowned long-term investigations have served as the underlying basis for nearly all major policy decisions regarding water quality in the Tahoe Basin, including exportation of sewage and solid waste, strict control on building, installation of major erosion control projects, protection of wetlands, establishment of water quality thresholds, control of nonpoint source pollution, controls on dredging, and many others.
Known for its beauty and remarkable transparency, Lake Tahoe is a natural jewel in the Sierra Nevada mountains. Unfortunately, human development in the Tahoe Basin over the last five decades has been damaging. The lake has responded to increased nutrient loading from the streams, atmosphere and groundwater with steadily increasing algal growth (eutrophication) and a progressive reduction of clarity. The lake has lost approximately 33 feet, or one-third of its famous transparency during the last 30 years and oxygen concentrations in the deep, bottom waters have declined significantly. Thick growths of attached algae now coat the shoreline rocks in the spring.
The Tahoe basin is a changing landscape and, today, significant portions of this once pristine region are urbanized. Studies from the early 1960s to the present have shown that many factors such as land disturbance, increasing resident and tourist population, habitat destruction, air pollution, soil erosion, roads and road maintenance, loss of wetlands and areas for natural infiltration of runoff have all interacted to degrade the Basin's air quality, terrestrial landscape and streams, as well as the lake itself. We now know that once nutrients enter the lake, they remain in the water and can be recycled for decades. As a consequence, these pollutants accumulate over time and contribute to Lake Tahoe's progressive decline. The ability of Lake Tahoe to dilute nutrient and fine-sediment loading to levels where they have no significant effect on lake water quality has been lost.
Continuous, long-term evaluation of water quality in Lake Tahoe since the early 1960's has shown that algal growth is increasing at a rate greater than 5 percent per year. Over this same period, there has been a decline of clarity at an alarming rate of nearly one foot per year. This long-term trend in loss of transparency is both statistically significant (p<0.001) and perceived by even the casual observer. If the loss of clarity continues at this rate, the resulting Secchi depth will no doubt be accompanied by a change of lake color and a change in trophic status.
Fine sediments and nutrients are the major constituents that must be controlled to meet desired conditions for lake clarity and algal growth. Researchers now contend that approximately 50 percent of the light scattering that represents clarity loss in Lake Tahoe is caused by small, inorganic sediment particles. About 30 percent of the clarity loss is due to algae, and another 20 percent is due to dissolved organic matter in the water. Although these proportions vary seasonally, it is clear that small, inorganic sediment particles are contributing 50 percent or more of the total clarity loss in the lake.
These recent studies show that it is the sediment particles ranging from about 1 micron to 10 microns that are of particular concern for Lake Tahoe. These particles are too small to see except with a powerful microscope, but due to their relative abundance in the water column and their optical properties, they are the largest contributing factor to Tahoe’s clarity loss.