Umeå universitets logga

The CO₂ flux (FCO₂) from lakes to the atmosphere is a large component of the global carbon cycle and depends on the air–water CO₂ concentration gradient (ΔCO₂) and the gas transfer velocity (k). Both ΔCO₂ and k can vary on multiple timescales and understanding their contributions to FCO₂ is important for explaining variability in fluxes and developing optimal sampling designs. We measured FCO₂ and ΔCO₂ and derived k for one full ice-free period in 18 lakes using floating chambers and estimated the contributions of ΔCO₂ and k to FCO₂ variability. Generally, k contributed more than ΔCO₂ to short-term (1–9 d) FCO₂ variability. With increased temporal period, the contribution of k to FCO₂ variability decreased, and in some lakes resulted in ΔCO₂ contributing more than k to FCO₂ variability over the full ice-free period. Increased contribution of ΔCO₂ to FCO₂ 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 FCO₂ variability in shallow waters. Accordingly, knowing the variability of both k and ΔCO₂ over time and space is needed for accurate modeling of FCO₂ from these variables. We conclude that priority in FCO₂ assessments should be given to direct measurements of FCO₂ at multiple sites when possible, or otherwise from spatially distributed measurements of ΔCO₂ combined with k-models that incorporate spatial variability of lake thermal structure and meteorology.