Seasonal nitrate cycling as evidence for complete vertical mixing in Lake Tahoe, California‐Nevada1

Abstract

A study of annual nitrate cycling in Lake Tahoe has provided the first convincing evidence that this deep, oligotrophic lake is holomictic. Beginning with the spring phytoplankton bloom, a sequence of biological nitrogen transformations is responsible for formation of a nitrate gradient, characterized by a distinct “nitracline.” The main processes responsible are nitrate depletion by phytoplankton uptake throughout the 125‐m euphotic zone, and deep water nitrification by bacteria. Sinking detritus appears to be the main source of reduced nitrogen available for nitrification. As cooling proceeded during fall and winter 1972, depression of the nitracline revealed the process of vertical mixing and eventual turnover of the entire water mass, for which temperature and oxygen profiles had failed to provide conclusive evidence.