The length of winter influences the dissolved oxygen dynamics in a large ice‐covered dimictic lake

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In dimictic lakes, under‐ice dissolved oxygen (DO) is shaped by stratification and mixing, oxygen consumption, and late‐winter algal production, which can increase upper‐water DO as light availability rises. Despite growing interest in these dynamics, late‐winter DO remains poorly documented due to inherent dangers of sampling on thin ice. To address this, we analyzed 6 yr of high‐frequency temperature and DO data from Lake Simcoe (2015–2017 and 2022–2024), a large dimictic lake in Ontario, Canada. Our observations indicate that under‐ice DO at the lake bottom decreased in early winter due to hypolimnetic oxygen demand. However, the DO increased in the upper‐water column, leading to near‐surface supersaturation in the winters with longer snow‐free ice cover. The net result is that after ice‐off, the depth‐averaged DO levels were usually higher than at the start of winter, and thus set the initial DO for spring, with volumetrically averaged DO ranging from 12.6 to 14.5 mg L −1 over the 6 yr. After the onset of summer stratification, hypolimnetic oxygen demand reduced bottom DO in late summer to 4.0–5.1 mg L −1 . As the hypolimnetic summer oxygen demands were similar for 6 yr, the years with higher initial spring DO levels and a shorter stratification period had higher DO at the end of summer stratification. These findings highlight the important but understudied role of late winter DO dynamics in shaping seasonal oxygen budgets and underscore the need to monitor these processes due to the rapid shifts in ice quality and duration in many ice‐covered dimictic lakes.