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Under-ice dissolved oxygen (DO) metabolism and DO depletion are poorly understood, limiting our ability to predict how changing winter conditions will affect lake ecosystems. We analyzed under-ice DO dynamics based on high-frequency (HF) data at two depths (5 and 25 m) for three winters (January-March 2014, 2015, and 2016) in oligotrophic Lake Tovel (1178 m above sea level; maximum depth 39 m). Specifically, we assessed diel metabolic rates based on HF data of DO, temperature, and light for winter 2016 and seasonal DO depletion rates based on HF data of DO for all three winters. For 2016, calculations of metabolic rates were possible only for 34% and 3% of days at 5 and 25 m, respectively; these metabolic rates generally indicated net heterotrophy at both depths. Low success in modeling metabolic rates was attributed to low diel DO variability and anomalous diel DO patterns, probably linked to under-ice physical processes. Seasonal DO patterns for the three winters showed increasing, decreasing, or stable DO trends at 5 m while at 25 m patterns always showed decreasing DO trends but with different rates. Our multiyear study permitted us to hypothesize that the observed intraannual and interannual differences in DO depletion can be attributed to variable snow cover determining the penetration of radiation and thus photosynthesis. This study brings new insights to DO dynamics in ice-covered systems, highlights the challenges linked to under-ice lake metabolism, and advocates for a modeling approach that includes physical processes.

Plain Language Summary More than half of the world's lakes freeze in winter, but scientists still know little about what happens under ice. A major challenge in dealing with winter conditions is the difficulty of sampling when snow and ice often restrict site accessibility. The use of underwater sensors that continuously measure temperature, oxygen, and light has partly overcome this obstacle. However in most lakes, sensors are removed during winter because ice breakup can damage this expensive equipment; Lake Tovel is one of the few lakes where sensors are deployed all year round. Oxygen is needed by organisms for respiration, and how much oxygen is still available in the lake when ice melts will have important implications for the rest of the year. We analyzed 3 years of under-ice oxygen data and show that oxygen can increase at 5 m depth but always decreased at 25 m depth. Climate change predictions indicate less snow in the Alps and earlier ice breakup. We show how water movement and snow influence under-ice oxygen. Our study brings new insights to oxygen dynamics in ice-covered systems and highlights the challenges in understanding the biological and physical factors that influence oxygen in ice-covered lakes.