Relative contribution of surface water concentrations (pCO2aq) and gas transfer velocity (k) to CO2 flux variability in boreal lakes

Lakes emit CO₂ to the atmosphere at magnitudes significant for the global carbon cycle, in the form of diffusive CO₂ flux (F_CO2). As direct F_CO2 measurements are time-consuming, F_CO2 is often estimated from the air-water CO₂ concentration gradient (Δp_CO2) and the gas transfer velocity (k), representing the two components considered to regulate F_CO2. However, extrapolating measurements of ΔpCO₂ and k to whole-year estimates require understanding of their variability in time and across different types of lakes, which is often insufficient. As a result, simple linear interpolations are typically used in extrapolations which risk producing bias as spatiotemporal variability is not included. Further insight to the variability of Δp_CO2 and k may contribute to more representative extrapolations and provide guidance for focusing sampling campaigns on capturing times of high variability. We used floating flux chambers and surface water samples to measure F_CO2 and Δp_CO2, respectively, both within-weeks and over seasons during the open water period at 12 locations in each of 15 boreal lakes across a latitudinal gradient in Sweden. We combined these measurements to derive spatially resolved values of k in order to identify: i) the contributions of Δp_CO2 and k to F_CO2 variability over time; and ii) if differences in the contributions of Δp_CO2 and k to F_CO2 variability can be related to lake characteristics. The results presented are relevant for improved modelling of lake CO₂ emissions.