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Alan D. Jassby, Charles R. Goldman, John E. Reuter, Robert C. Richards, and Alan C. Heyvaert Tahoe Research Group University of California, Davis Summary Lake Tahoe, a large (501 km2), deep (505 m) subalpine lake in the Sierra Nevada Mountains of the western United States, is renowned for a remarkable transparency. With the expansion of human population in the lake basin over the last four decades, increased sediment and nutrient loading has led to a progressive reduction of clarity and has threatened the lakeÕs unique character. Paleolimnological studies reveal a previous episode of watershed disturbance beginning in 1860, when the basin was logged to provide timber for the Comstock mines, one of the richest silver strikes in North American history. Primary productivity and mineral suspensoids increased markedly but recovered by the early 20th century. The current period of disturbance has been characterized by an instrumental record of increasing primary productivity and Secchi depth beginning in 1967. Many sources of seasonal and interannual variability obscure the long term trends. The lake was originally co-limited by both nitrogen and phosphorus. The long term productivity increase was driven primarily by atmospheric loading of nitrogen from vehicle emissions and fertilizer volatilization. The high nitrogen to phosphorus ratio in loading caused a shift to overwhelming phosphorus limitation around 1980, as evidenced by bioassays and phytoplankton community composition. Further productivity increases have been driven by erosion of phosphorus from the watershed. Mineral suspensoids also probably play a role in the long term loss of clarity, with clay particles being retained in the deep water column. The originally simple food web now has a complex array of organisms due to intentional and accidental introductions. The native dominant top predator, the Lahontan cutthroat trout, disappeared in the 1940Õs. The introduction of the opossum shrimp in the 1960Õs as forage for sport fishes changed the zooplankton community dramatically. The benthic fauna is remarkably diverse and abundant given the lakeÕs ultra-oligotrophic nature; the signal crayfish in particular plays an important ecological role. Among the benthic flora, Eurasian milfoil has become well established and its future role is controversial. Water quality standards have been set for the lake and tributaries, but standards are not currently being achieved. Moreover, much of the loading may come from unregulated sources. Many agencies are currently involved in erosion control and land restoration, and there is slow but definite movement toward a coordinated and science based rehabilitation program. (Published In M. Munawar and R.E. Hecky [eds.], Exploring the Great Lakes of the World: Food-web, Health and Integrity. 2001. p. 431-454. Ecovision World Monograph Series. Backhuys Publishers.) Historical time-line of environmental events in the Lake Tahoe Basin (1840-1990).  (Published In M. Munawar and R.E. Hecky [eds.], Exploring the Great Lakes of the World: Food-web, Health and Integrity. 2001. p. 431-454. Ecovision World Monograph Series. Backhuys Publishers.) |