Contribution of gas concentration and transfer velocity to CO2 flux variability in northern lakes

Abstract The CO 2 flux () from lakes to the atmosphere is a large component of the global carbon cycle and depends on the air–water CO 2 concentration gradient (ΔCO 2 ) and the gas transfer velocity ( k ). Both ΔCO 2 and k can vary on multiple timescales and understanding their contributions to is important for explaining variability in fluxes and developing optimal sampling designs. We measured and ΔCO 2 and derived k for one full ice‐free period in 18 lakes using floating chambers and estimated the contributions of ΔCO 2 and k to variability. Generally, k contributed more than ΔCO 2 to short‐term (1–9 d) variability. With increased temporal period, the contribution of k to variability decreased, and in some lakes resulted in ΔCO 2 contributing more than k to variability over the full ice‐free period. Increased contribution of ΔCO 2 to variability over time occurred across all lakes but was most apparent in large‐volume southern‐boreal lakes and in deeper (> 2 m) parts of lakes, whereas k was linked to variability in shallow waters. Accordingly, knowing the variability of both k and ΔCO 2 over time and space is needed for accurate modeling of from these variables. We conclude that priority in assessments should be given to direct measurements of at multiple sites when possible, or otherwise from spatially distributed measurements of ΔCO 2 combined with k ‐models that incorporate spatial variability of lake thermal structure and meteorology.