Transitional circulation patterns from full ice cover to ice-off in a seasonally ice-covered lake

There is consensus that under-ice circulation presents multiple phases through the winter, and that different mechanisms dominate each period. In this work, measurements of temperature, water velocity, conductivity, and dissolved oxygen from Lake Massawippi, Quebec, Canada, obtained during the ice-covered season in 2019, were used to characterize the time scales of different winter regimes and transitions among dominating circulation mechanisms. Lake circulation during this period began with a single-cell convection induced by sediment flux pulses in early winter. The single-cell convection decayed into a brief quiescent period. Radiatively driven convection then formed a convectively mixed layer in late winter. The defined mixed layer and temperature structure provided the necessary conditions for the formation of a potential rotational feature, which briefly formed immediately prior to ice break-up. Ice break-up led to complex hydrodynamics that persisted for nearly 28 days following full ice-off. Dissolved oxygen was directly correlated with the varying circulation features throughout the field campaign. This work provides a quantitative measure to delineate the transitions between under-ice regimes and provides novel insights into the subsequent circulation during and after ice break-up.