LAUSR

Global warming affects the thermal stratification of freshwater lakes, and thus can lead to thermocline deepening. To follow the direct and indirect effects of thermocline shifts on processes at the sediment–water interface, the thermoclines in six of twelve lake enclosures were deepened by pumping warm surface water below the thermocline. Hypolimnetic temperatures increased and oxygen concentrations decreased due to the treatment. Path modeling suggests a correlation of the treatment to epilimnetic chlorophytes and hypolimnetic diatoms, while the dominant cryptophytes and cyanobacteria remained unaffected by the treatment. Sedimentation rates, ranging between 0.1 and 1.9 g m−2 day−1 per enclosure, correlated strongly with the selected phytoplankton groups, but were not affected by the treatment. Concentrations of elements (Ba, Mn, S, Sr), nutrients (NH4+, PO43−, Si), and greenhouse gases (CO2, CH4, N2O) in the upper centimeter of the sediment were not affected by the hypolimnetic changes. Methane and carbon dioxide emission from the enclosures to the atmosphere were negligible. Bacterial and viral abundance in sediments were not affected by the treatment. Results suggest that the benthic ecosystem remains unaffected by slight changes in the pelagial. Further, we conclude that short-term warming does not increase greenhouse gas emissions from temperate, deep, oligo-mesotrophic lakes.