Temperatures and hydrodynamics during early winter in ice-covered lakes

Arctic ice-covered lakes are often considered to be quiescent systems as they are insulated from direct effects of  sunlight and wind over extended periods of time. However, water circulation is driven by gravity currents resulting from sediment heat fluxes and wind-driven oscillations of the ice-cover cause considerable internal waves. Depending on the thickness of the snow cover, penetrative convection in fall can occur as long as sunlight is present. All these drivers of lake hydrodynamics  are most influential in early winter when heat stored during summer is remaining in the sediments and the ice and snow cover remains thin.

Here we present multi-year measurements of under-ice temperatures and oxygen concentrations in five Arctic lakes with maximal depths ranging from 3 to 27 m and surface areas from 1 to 150 ha. The focus is on the period of early winter from the beginning of the ice-covered period to approximately 30 days after ice-on. During early winter, temperatures varied between 1 and 3.7°C and depended on summer temperatures and meteorological conditions preceding ice-on. Sediment heat fluxes of several W/m2 drove gravity currents with velocities in the order of several mm/s.  Oxygen depletion was higher in early winter compared to late winter periods but lowered in early winter periods with penetrative convection occurring.

In summary, the results show a high interannual variability and variability between lakes in early winter temperatures, gravity currents and oxygen depletion rates all of which depending on meteorological conditions and lake morphometry.