The Great European Lake Onego provides important resources for Northern Russia. This lake is ice-covered during 5 months in winter. However, very few data are available for winter, as most studies covered the ice-free period. Actually, comprehension of seasonally ice-covered large lakes is limited compared to the knowledge available from small subpolar lakes or perennially ice-covered polar lakes. To tackle this issue, an international consortium of scientists has gathered around the project « life under ice » to investigate physical, chemical and biogeochemical changes during winter in Lake Onego. As part of this multidisciplinary project, our research focused on carbon dioxide (CO2) dynamics and CO2 sources during ice-covered period and during ice break-off. Our main hypothesis is that CO2 accumulates throughout the ice-covered period, mainly due to blocked gas exchange, the degradation of organic matter (OM) in the sediment, and direct inputs from the inflowing River Shuya. CO2 dynamics were measured in Petrozavodsk Bay using a mooring that was equipped with two CO2 sensors at 3 m and 25 m depth from March to May 2015. Vertical profiles of CO2, methane (CH4), dissolved organic and inorganic carbon were collected with water samples at different locations. The main results showed that Lake Onego was slightly over-saturated regarding CO2, while CH4 concentrations were extremely low. At 3 m depth, CO2 increased under the ice at a linear rate of 9.6 ppm/day. During the ice-covered period, CO2 accumulated four times more in the bottom boundary layer than in the mixed convection layer. This bottom boundary layer might indicate a quantifiable flux of CO2 from the sediment due to mineralization of settling organic matter (OM). OM mineralization in the sediment was assessed through sediment cores. According to CH4 sediment pore water profiles, about 70% of CH4 was oxidized at 7 cm below the sediment water interface. The total oxidation rate of methane in the sediment was estimated to -2.26 ± 0.22 mgC m-2d-1. In contrast, OM mineralisation rates in the water were shown to be extremely low in this oligotrophic system. This mineralization in the water originated mostly from autochthonous OM, while only a few percent stemmed from the abundant allochthonous OM. Lastly, River Shuya contributes significantly to the dissolved CO2 of Petrozavodsk Bay, as it represents 93% of the water inputs into the bay and riverine CO2 concentrations were 2.4 higher than in the bay water. In conclusion, this study contributes to better understand how CO2 accumulates under ice in large seasonally ice-covered lakes, and helps to predict how CO2 fluxes might be modified as a result of climate-driven changes.