Perspectives in winter limnology: closing the annual cycle of freezing lakes

Winter has traditionally been considered as an ecologically insignificant season and, together with technical difficulties, this has led winter limnology to lag behind summer limnology. Recently, rapidly expanding interest in climate warming has increased water research in winter. It has also become clear that neither winter conditions of lakes nor under-ice communities are as static as often supposed. Although interannual differences in water temperature are small, close to the maximum density temperature, they may have profound effect on under-ice hydrodynamics. Thus, stochastic variations in weather, particularly those preceding the time of freezing and ice melting, may have important consequences for hydrodynamics which then affect the distributions and conditions of microorganisms and probably further to higher trophic levels. Even fish distributions can be dictated by under-ice conditions and their activities as well as behavior can sometimes approach those in summer. Life in freshwater ice is one of the least studied aspects of winter limnology and recent studies suggest that a thorough evaluation is needed. Altogether there are strengthening signs that winter should be considered as an integral part in the functioning and dynamics of lakes affecting quantitative and qualitative characteristics of aquatic communities in summer. There are great prospects that more thorough understanding of the prevailing limnological conditions in winter will improve our understanding of lake ecosystems in their entirety, and there is no doubt that such an approach requires multidisciplinary and long- term studies at different spatial scales.