Climatic effects on vertical mixing and deep-water oxygen content in the subalpine lakes in Italy

Deep lakes south of the Alps (DSL: Maggiore, Lugano, Como, Iseo and Garda) are characterised by varying trophic states and dissolved oxygen (DO) concentrations. Some of these lakes experience anoxic conditions in deep waters. We hypothesised that the increase in temperature and water-column stability observed in these lakes during recent decades influenced the deep-water DO concentration. In particular, we tested whether the thermal regime of the lakes and the depth of mixing affect oxygen replenishment during winter–spring turnover. To this aim, we analysed long-term trends and seasonal variability of oxygen levels in the DSL during 1992–2016. We included in our analysis the effects of environmental variables, such as winter air temperature and atmospheric modes of variability. Our results showed a recent decrease in the deep-water oxygen content in lakes Maggiore, Como and Garda and an increase of the extent of anoxic conditions in lakes Lugano and Iseo. Our results suggest that, beside cultural eutrophication, rising environmental pressures, such as global warming, can influence the future trends of the oxygen levels and ecological states of deep lakes.