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

Abstract

For more than half a century, the trophic status of water bodies has been of interest to limnologists and oceanographers alike. This report demonstrates the close, inverse relationship between 14C‐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% yr‒1 for the last 28 yr. There has been a concomitant decline in transparency of 0.37 m yr‒1. During winter months when transparency is highest, the average annual loss has been slightly greater (0.40 m yr‒1). 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 NO3‐‐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.