A 4 1/2-year study on the rate and factors affecting the cultural eutrophication of oligotrophic Lake Tahoe is reported. Primary productivity has increased alarmingly with a steady shift in the seasonal maximum from early spring to late summer. Productivity increased 25.6% from 1968 to 1971. Using the 1959-1960 data from earlier studies, the increase to 1971 was 51%. Diatoms dominate the phytoplankton population and the maximum zone of phytoplankton photosynthesis may be as deep as 50-75m. The extent of winter mixing is important in the nutrient budget of the lake and bacteria associated with stream-borne nutrients facilitate nutrient regeneration. The littoral zone, although extremely important visually to the lake, contributes only 10% of the total primary production. Great variability in fertility of the lake has been demonstrated by synoptic studies and aerial remote sensing. Highest productivity is found in the lake near tributaries which drain disturbed land. Nutrients associated with road building, housing, and lumbering are major causes of eutrophication in Tahoe.

Prepared for National Environmental Research Center, Office of Research and Development, U.S. Environmental Protection Agency under research grant 16010 DBU, program element 1BA031. Includes bibliographical references (pages 316-335).