Extreme events and ice cover determine dissolved oxygen in a mountain lake

Climate change often leads to a decrease in hypolimnetic dissolved oxygen (DO). However, annual mean DO (% saturation) has increased from near anoxia to >20% in the bottom layer (35–39 m) of oligotrophic Lake Tovel (Italy), a deep mountain lake. We analysed long‐term patterns of DO (1937–2019) using correlation and trend analysis and identification of extreme events to link dissolved oxygen to drivers and indices of mixing. While spring mixing remained temporally limited, delayed ice‐in (5.1 days/decade) and the positive relationship between ice‐in and DO the following year indicated autumn mixing as the main driver for hypolimnetic DO increase. Using DO and conductivity (1995–2019), we identified 14 deep mixing events with hypolimnetic DO > 40% indicating that extreme meteorological events also replenished hypolimnetic DO. These events were only partially captured by density‐based indices of mixing (Schmidt stability, relative thermal resistance, Lake Number, and Wedderburn Number) and were related to snowmelt, flooding, and cold spells during spring and autumn, with a carryover effect sometimes lasting >1 year. Recently, annual mean DO in the upper layer decreased beyond temperature‐dependent solubility. Statistical tests did not comprehensively confirm this decrease but it was possibly linked to atmospheric stilling. We suggest that the observed shift from meromixis to dimixis was driven by climate warming (i.e., increasing air temperature 0.6°C/decade) that delayed ice‐in and increased autumn mixing in Lake Tovel. Our work emphasises the vulnerability of mountain lakes and their different response to climate change with respect to more studied lowland lakes.