Diurnal variation of turbulence characteristics in Lake Garda

Abstract

Turbulent mixing strongly infuences the distribution of phytoplankton in holomictic lakes such as Lake Garda. During March 5-7, 2014, high-resolution vertical measurements were made of temperature and fluorescence in the upper 100 meters of Lake Garda, using a free-falling microstructure profiler. Fluorescence profiles are compared with in-situ and satellite measurements provided by MODIS AQUA. From the measured vertical temperature profiles, turbulence characteristics such as the kinetic energy dissipation ", the thermal variance dissipation Kt and the vertical mixing coeffcient Kt were determined. These appear to be the first direct measurements of KT and, although the observation period is short, they show interesting temporal (diurnal) variations in KT which can be connected to the changes in the surface wind stress and background stratification. Phytoplankton concentrations in the mixed layer are found to decrease with the magnitude of the vertical mixing coeffcient.