Dissolved oxygen in a wind-shielded mountain lake is determined by the interplay of ice cover and extreme events

Generally, hypolimnetic dissolved oxygen (DO) decreases with climate change. However, in oligotrophic Lake Tovel (Italy), a deep mountain lake, annual mean DO (% saturation) has increased from near anoxia to > 20 % in the bottom layer (35–39 m). Long‐term patterns of DO (1937–2019) were analysed to link DO to drivers and indices of mixing. While spring mixing remained temporally limited, later ice‐in (5.1 days decade per decade) and the positive relationship between ice‐in and DO the following year evidenced autumn mixing as the main driver for hypolimnetic DO increase. Hypolimnetic DO was replenished also by extreme meteorological events with 14 deep mixing events (i.e., hypolimnetic DO > 40%) only partially indicated by density dependent indices (Schmidt stability, relative thermal resistance, Lake Number, and Wedderburn Number). Lake Tovel's shift from meromixis to dimixis was driven by climate warming (i.e., increasing air temperature 0.6°C per decade) that delayed ice‐in and increased autumn mixing. This study underlines the vulnerability of mountain lakes and their different response to climate change with respect to more studied lowland lakes