Thermobaric stratification in very deep Norwegian freshwater lakes

Thermobaric stratificationwas investigated in very deep lakes of elongated shape. We considered the spring time configuration when large freshwater lakes may have their deepest recirculation. In five very deep freshwater lakes in Norway, profiles weremeasured in spring 2006 withmultiparameter probes. Temperature profiles confirmed thermobaric stratification in all of these lakes, i.e. the temperature profiles crossed the 4 °C line and thus were affected by pressure effects on the temperature of maximum density. Electrical conductancewas low in all of the investigated lakes, and vertical gradients were too small to play a role in the density stratification. In all lakes, deep water was well supplied with oxygen. Three out of five lakes showed a temperature gradient close to Eklund's stability theorem. However, the remaining two lakes clearly opposed the theorem. Bottom temperaturesweremore closely related to lake length than their maximumdepths. Oxygen saturation profiles documented recent intensive gas exchange with the atmosphere. They agree well with the possible formation of deep water from density driven flows at horizontal temperature gradients close to 4 °C.