This large‐scale quasi‐synoptic study gives a comprehensive picture of sea‐air CO₂ fluxes during the melt season in the central and outer Laptev Sea (LS) and East Siberian Sea (ESS). During a 7 week cruise we compiled a continuous record of both surface water and air CO₂ concentrations, in total 76,892 measurements. Overall, the central and outer parts of the ESAS constituted a sink for CO₂, and we estimate a median uptake of 9.4 g C m⁻² yr⁻¹ or 6.6 Tg C yr⁻¹. Our results suggest that while the ESS and shelf break waters adjacent to the LS and ESS are net autotrophic systems, the LS is a net heterotrophic system. CO₂ sea‐air fluxes for the LS were 4.7 g C m⁻² yr⁻¹, and for the ESS we estimate an uptake of 7.2 g C m⁻² yr⁻¹. Isotopic composition of dissolved inorganic carbon (δ¹³C_DIC and δ¹³C_CO2) in the water column indicates that the LS is depleted in δ¹³C_DIC compared to the Arctic Ocean (ArcO) and ESS with an offset of 0.5‰ which can be explained by mixing of δ¹³C_DIC‐depleted riverine waters and 4.0 Tg yr⁻¹ respiration of OC_ter; only a minor part (0.72 Tg yr⁻¹) of this respired OC_ter is exchanged with the atmosphere. Property‐mixing diagrams of total organic carbon and isotope ratio (δ¹³C_SPE‐DOC) versus dissolved organic carbon (DOC) concentration diagram indicate conservative and nonconservative mixing in the LS and ESS, respectively. We suggest land‐derived particulate organic carbon from coastal erosion as an additional significant source for the depleted δ¹³C_DIC.