Trend analysis of weekly temperatures and oxygen concentrations during summer stratification in Lake Plußsee: A long‐term study

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°C yr −1 ) and the first week of May (0.11°C yr −1 ), the extent of epilimnetic warming trends during the same time period (∼ 0.06–0.08°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 (∼ 0.01°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.