Complex effects of winter warming on the physicochemical characteristics of a deep lake

Recent winter warming over Central Europe associated with a positive phase of the North Atlantic Oscillation (NAO) strongly influenced the thermal and water column stability properties of deep Lake Constance (z(mean) = 101 m). Volumetrically weighted average water temperatures have increased since the 1960s by an average of 0.017degreesC yr(-1), and its interannual variability was strongly related to the variability in winter air temperature and the NAO winter index (NAO(w)). The influence of NAO(w) on water temperature was more persistent than its influence on air temperature. The seasonal persistence of the NAO(w) signal increased with water depth. Deep-water temperatures were related to the NAO(w) from one spring mixing period to the next. This caused a time lag of 1 yr in the response of deep-water winter temperatures to the NAO(w). Reduced winter cooling during high-NAO(w) years resulted in the persistence of small temperature gradients that possibly resisted complete mixing. This, in turn, resulted in less upward mixing of nutrients (total phosphorus and total silica), which accumulated in the hypolimnion during the previous stratification period. A second effect of incomplete mixing was the lack of the replenishment of deep-water oxygen during high-NAO(w) years. Hence, besides its strong impact on the thermal regime, climate variability influenced both the causes (nutrient supply for phytoplankton growth) and symptoms (the degree of hypolimnetic oxygen deficiency) of trophic changes in Lake Constance.