ABSTRACTIce-covered periods might significantly contribute to lake emissions at ice-melt, yet a comprehensive understanding of under-ice carbon dioxide (CO2) dynamics is still lacking. This study investigated the processes driving spatiotemporal patterns of under-ice CO2in large Lake Onego. In March 2015 and 2016, under-ice CO2, dissolved inorganic carbon (DIC), and dissolved organic carbon (DOC) distributions were measured along a river to an open-lake transect. CO2decreased from 120/129 μmol L−1in the river to 51/98 μmol L−1in the bay, and 34/36 μmol L−1in the open lake, while DOC decreased from 1.18/1.55 mmol L−1in the river to 0.67/1.04 mmol L−1in the bay in 2015 and 2016, respectively. These decreases in concentrations with increasing distance from the river mouth indicate that river discharge modulates spatial patterns of under-ice CO2. The variability between the 2 years was mainly driven by river discharge and ice transparency affecting the extent of under-ice convection. Higher discharge during winter 2016 resulted in higher CO2concentrations in the bay. By contrast, intensive under-ice convection led to lower, more homogeneously distributed CO2in 2015. In conclusion, the river-to-bay transition zone is characterized by strong CO2variability and is therefore an important zone to consider when assessing the CO2budget of large lakes.