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Abstract

Small dimictic Lake Plusssee in North Germany experienced major changes of the temperature and oxygen regime from 1969 to 2006, which reflected regional and large-scale changes in meteorological forcing. We used long-term weekly measurements of the lake's epilimnetic, metalimnetic, and hypolimnetic temperatures and dissolved oxygen concentrations and meteorological forcing variables to estimate long-term trends for each week. Except for large warming trends of air temperatures in the last week of April (0.14 degrees C yr(-1)) and the first week of May (0.11 degrees C yr(-1)), the extent of epilimnetic warming trends during the same time period (similar to 0.06-0.08 degrees C yr(-1)) was also caused by a cumulative effect resulting from an earlier onset of stratification. Negative hypolimnetic temperature trends at the same time (similar to 0.01 degrees C yr(-1)) also resulted from an earlier onset of stratification and increased water-column stability in spring. In summer and autumn, large increases in epilimnetic temperature and water-column stability were also affected by a shift of the timing of the yearly maxima of epilimnetic temperatures and water-column stability, which resulted in a later breakdown of stratification. This annual pattern of long-term trends of epilimnetic and hypolimnetic temperatures agrees with models driven by the expected increase of air temperatures due to anthropogenic greenhouse gases. Due to a prolonged stratification period, oxygen concentrations during the last weeks of the stratified period were lower, although oxygen concentrations in the water column increased within the first weeks of stratification.