Primary productivity, nutrients, and transparency during the early onset of eutrophication in ultra-oligotrophic Lake Tahoe, Califomia-Nevada1

For more than half a century, the trophic status of water bodies has been of interest to Iimnologists and oceanographers alike. This report demonstrates the close, inverse relationship between ‘4C-estimated primary productivity and transparency during the earliest stages of cultural eutrophication. As the population in the Tahoe basin has rapidly increased, Lake Tahoe has been characterized by an increase in primary productivity that has averaged 5.6% yr1 for the last 28 yr. There has been a concomitant decline in transparency of 0.37 m yr”. During winter months when transparency is highest, the average annual loss has been slightly greater (0.40 m yr‘). The average annual Secchi depth has decreased by 7 m during the last 19 yr of intensive monitoring. There has also”been a significant increase in the light extinction coefficient. Photosynthetic efficiency has increased while there has been a gradual shrinkage of the euphotic zone. During the same period the total NO~--N content of the lake has increased significantly, but total P content has not. Lake Tahoe primary production has become increasingly P sensitive during the last decade as N has accumulated in the system. A gradual increase in the N:P may prove to be a general evolutionary characteristic of oligotrophic lakes during the earliest stages of eutrophication. The importance of long-term studies in detecting gradual change that may be masked by considerable interannual variability is particularly evident from this study. Since Naumann (1919) introduced the concept of the trophic status of lakes, there has been interest by limnologists and oceanographers in the evolutionary state, degree of fertility, and both cause and effect of eutrophication in lakes and estuaries. At an international symposium on eutrophication in 1965, Thomas (1969) reviewed eutrophication in central European lakes, Rodhe (1969) in northern Europe, Stra3kraba and Stra3krabov& ( 1969) in eastern Europe, and Horie (1969) in Asia. W. T. Edmondson (1969), to whom this paper is dedicated, reviewed the North American lakes, excluding the Great Lakes, which were treated by Beeton (1965, 1969). Hutchinson (1969, p. 19), in his unique and scholarly fashion, made the important point “that we should think not of oligotrophic or eutrophic water types, but of lakes and their drainage basins and sediments as forming oligotrophic or eutrophic systems.” He emphasized the dynamics of nutrients in aquatic systems and the need to understand and use the information available, describing a now-classic example of cultural eutrophication recon‘ Supported by NSF grant BSR 87-05170. strutted from sediment Monterosi, which began of the Via Cassia about cores of Lago di with the building 171 B.C. In a later work, he further examined both the scientific and practical implications of eutrophication (Hutchinson 1973). Edmondson’s studies of Lake Washington document a success in reversing eutrophication of a lake with a short retention time through sewage diversion (Edmondson 196 1). Beeton and Edmondson ( 1972) thus regarded nutrient input as a determinant of the trophic status of lakes as the experience with Lake Washington had demonstrated. Subsequently, Carlson (1977) used transparency, chlorophyll, and total phosphorus data from the Lake Washington studies to develop a numerical trophic state index for general application. Emphasis on phosphorus as a limiting factor launched international efforts to control phosphorus loading (Vollenweider 1968, 1975). The importance of phosphorus cycling had been recognized for many years (Einsele 1941), and rapid uptake and turnover of P were demonstrated by Rigler (1956). Although the predictive aspects of the Vollenweider nutrient-loading model have been challenged by Thomann (1977)