Seasonal variability of mixing in northern Lake Garda

Abstract

Mixing is a very important physical mechanism in lakes, since it is strongly connected to turbulence. Turbulence has a direct influence on thermal stratification and biological processes. This study used in-situ observations to investigate the seasonal variability of mixing in northern Lake Garda (Italy). The in-situ observations were made using a vertical microstructure profiler (MicroCTD, Rockland Scientific International), which measured temperature, conductivity, turbidity, chlorophyll-a concentrations and vertical shear. These measurements were performed on a monthly basis, starting in March 2017, for a whole year. Four different stations were monitored at the same time. Three of them were forming a cross section perpendicular to lake's main axis, which is oriented in a northeast-southwest direction. The fourth station was located further to the south. Additionally, measurements of meteorological variables were obtained from nearby meteorological ground stations. The combination of all these measurements made it possible to quantify which external forcing mechanisms had the most influence on the dynamics occurring in the lake throughout the year. It was found that throughout the year the surface buoyancy flux is the most important source of Turbulent Kinetic Energy (TKE), except during occasional extreme wind events, which caused significant amounts of TKE to be produced. Observations suggested the development of a secondary circulation, after persistent, unidirectional winds, causing coastal up-/downwelling along the narrow and deep part of the lake. It was found that a transition period from negative to positive buoyancy fluxes in the months September and October marked a transition in the wind climate and caused the development of an unstable boundary layer in the air above the lake. This transition period was accompanied by a second algal bloom and was the onset of Mixed Layer deepening. During the months December until March a continuous cooling of the upper layer was observed. The mixing dynamics in the lake turned out to be more event-based and did not follow any clear seasonal pattern.