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

Abstract

The CO2 flux ((Formula presented.)) 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 (Formula presented.) is important for explaining variability in fluxes and developing optimal sampling designs. We measured (Formula presented.) 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 (Formula presented.) variability. Generally, k contributed more than ΔCO2 to short-term (1–9 d) (Formula presented.) variability. With increased temporal period, the contribution of k to (Formula presented.) variability decreased, and in some lakes resulted in ΔCO2 contributing more than k to (Formula presented.) variability over the full ice-free period. Increased contribution of ΔCO2 to (Formula presented.) 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 (Formula presented.) variability in shallow waters. Accordingly, knowing the variability of both k and ΔCO2 over time and space is needed for accurate modeling of (Formula presented.) from these variables. We conclude that priority in (Formula presented.) assessments should be given to direct measurements of (Formula presented.) 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.