Carbon Dioxide and Methane Dynamics in a Small Boreal Lake During Winter and Spring Melt Events

In seasonally ice‐covered lakes, carbon dioxide (CO 2 ) and methane (CH 4 ) emission at ice‐off can account for a significant fraction of the annual budget. Yet knowledge of the mechanisms controlling below lake‐ice carbon (C) dynamics and subsequent CO 2 and CH 4 emissions at ice‐off is limited. To understand the control of below ice C dynamics, and C emissions in spring, we measured spatial variation in CO 2 , CH 4 , and dissolved inorganic and organic carbon from ice‐on to ice‐off, in a small boreal lake during a winter with sporadic melting events. Winter melt events were associated with decreased surface water DOC in the forest‐dominated basin and increased surface water CH 4 in the mire‐dominated basin. At the whole‐lake scale, CH 4 accumulated below ice throughout the winter, whereas CO 2 accumulation was greatest in early winter. Mass‐balance estimates suggest that, in addition to the CO 2 and CH 4 accumulated during winter, external inputs of CO 2 and CH 4 and internal processing during ice‐melt could represent significant sources of C gas emissions during ice‐off. Moreover, internal processing of CO 2 and CH 4 worked in opposition, with production of CO 2 and oxidation of CH 4 dominating at ice‐off. These findings have important implications for how small boreal lakes will respond to warmer winters in the future; increased winter melt events will likely increase external inputs below ice and thus alter the extent and timing of CO 2 and CH 4 emissions to the atmosphere at ice‐off.