Exceptional Mixing Events in Meromictic Lake Lugano (Switzerland/Italy), Studied Using Environmental Tracers

The deep northern basin of Lake Lugano was permanently stratified over the last 4 decades because of mineralization in response to strong eutrophication. However, two consecutive cold and windy winters ( 2004–2005 and 2005–2006) destabilized the water column and led to two exceptionally strong mixing events. The conductivity, temperature, and depth time-series data from 1995 to 2006 allowed the reconstruction of the evolution of the water body that made deep convective mixing possible. Over the years 1995-2006, the stability of the water column decreased continuously in winter, reaching a neutral stability at the end of the study period. The Schmidt stability values observed in the winters of 2004–2005 and 2005–2006 were virtually zero. Analyses of helium isotopes, tritium, sulfur hexafluoride, and chlorofluorocarbons in water samples collected immediately after the two mixing events revealed large changes in the tracer concentration profiles and permitted comparisons of the apparent water ages in Lake Lugano with previous measurements. Tracer concentration profiles became more homogeneous and approached atmospheric equilibrium concentrations in the time series. The tracer data, along with changes observed in the physical parameters of the water column, demonstrate the considerable deepwater renewal and gas exchange with the atmosphere that has taken place.