Complex effects of winter warming on the physicochemical characteristics of a deep lake, pp1432-1438

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 (zmean 5 101 m). Volumetrically weighted average water temperatures have increased since the 1960s by an average of 0.0178C yr21, and its interannual variability was strongly related to the variability in winter air temperature and the NAO winter index (NAOW). The influence of NAOW on water temperature was more persistent than its influence on air temperature. The seasonal persistence of the NAOW signal increased with water depth. Deep-water temperatures were related to the NAOW 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 NAOW. Reduced winter cooling during high-NAOW 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 deepwater oxygen during high-NAOW 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. During the past few decades, many large and deep Central European lakes have undergone strong eutrophication and subsequent oligotrophication (Sas 1989) as well as changes in temperature (Weyhenmeyer et al. 1999; Livingstone and Dokulil 2001; Livingstone 2003) associated with a positive phase of the North Atlantic Oscillation (NAO) (Hurrell 1995). The NAO is a large-scale fluctuation in the atmospheric pressure difference between the subpolar Icelandic low and the Azores high. Climatic conditions in Europe are influenced by the NAO predominantly during winter, when it is strongest and has the clearest teleconnections (Barnston and Livezey 1987). Hence, interannual variability in the NAO is usually expressed in terms of the winter NAO index (NAOW), which is the December through March average of standardized sea level pressure differences between Lisbon, Portugal, and Stykkisholmur/Reykjavik, Iceland (Hurrell 1995). The increased pressure difference during a positive phase of the NAO corresponds to more and stronger winter storms crossing the Atlantic Ocean, which results in high winter temperatures in western and northern Europe. A neg1 Corresponding author ([email protected]).