Where does the river end? Drivers of spatiotemporal variability in CO 2 concentration and flux in the inflow area of a large boreal lake

River inflow affects the spatiotemporal variability of carbon dioxide (CO 2 ) in the water column of lakes and may locally influence CO 2 gas exchange with the atmosphere. However, spatiotemporal CO 2 variability at river inflow sites is often unknown leaving estimates of lake‐wide CO 2 emission uncertain. Here, we investigated the CO 2 concentration and flux variability along a river‐impacted bay and remote sampling locations of Lake Onego. During 3 years, we resolved spatial CO 2 gradients between river inflow and central lake and recorded the temporal course of CO 2 in the bay from the ice‐covered period to early summer. We found that the river had a major influence on the spatial CO 2 variability during ice‐covered periods and contributed ~ 35% to the total amount of CO 2 in the bay. The bay was a source of CO 2 to the atmosphere at ice‐melt each year emitting 2–15 times the amount as an equally sized area in the central lake. However, there was large interannual variability in the spring CO 2 emission from the bay related to differences in discharge and climate that affected the hydrodynamic development of the lake during spring. In early summer, the spatial CO 2 variability was unrelated to the river signal but correlated negatively with dissolved oxygen concentrations instead indicating a stronger biological control on CO 2 . Our study reveals a large variability of CO 2 and its drivers at river inflow sites at the seasonal and at the interannual time scale. Understanding these dynamics is essential for predicting lake‐wide CO 2 fluxes more accurately under a warming climate.