Water column changes under ice during different winters in a mid-latitude Mediterranean high mountain lake

Winter limnology has been developed mainly in northern-temperate or subarctic lakes, while mid-latitude Mediterranean high mountain lakes have been less studied during the ice-covered season. We present a five-year study in one of these lower latitude high mountain lakes, conducted with multiprobes located at two different depths recording high-frequency data. We have studied highly contrasted winters regarding temperature and snowfall. The results of a principal component analysis and a subsequent hierarchical clustering show that bottom oxygen depletion under ice is strongly influenced by winter cover thickness and transparency. A thick and opaque winter cover (dark mode) leads to an intense oxygen depletion until anoxia, while a thin and clear winter cover (clear mode) allows higher dissolved oxygen saturation due to oxygen production by photosynthesis. Under unusual conditions of cold and dry climate, only a weak decrease in bottom dissolved oxygen was observed. The extent of dissolved oxygen depletion and its relationship with meteorological parameters and climate patterns are further investigated with generalized additive models. Winter cover thickness and phenology are driven by the joint effect of the North Atlantic Oscillation and East Atlantic climatic patterns. Ice phenology could also influence the following ice-free period given the correlation of lake heat content with the ice-off date and the ice cover length. We propose some features jointly occurring in Lake Cimera as a model for mid-latitude Mediterranean high mountain lakes, including (yet not exclusive) the occurrence of very thick winter covers, the prevalence of dark mode, the relatively high and similar through the years oxygen depletion rates, and high water temperatures during the ice-free periods. It is unclear how the ongoing climate change will affect the prevalence of dark over clear modes due to the expected antagonistic effects of warmer temperatures and lower snowfalls.