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Abstract The CO₂flux () from lakes to the atmosphere is a large component of the global carbon cycle and depends on the air–water CO₂concentration gradient (ΔCO₂) and the gas transfer velocity (k). Both ΔCO₂andkcan vary on multiple timescales and understanding their contributions to is important for explaining variability in fluxes and developing optimal sampling designs. We measured and ΔCO₂and derivedkfor one full ice‐free period in 18 lakes using floating chambers and estimated the contributions of ΔCO₂andkto variability. Generally,kcontributed more than ΔCO₂to short‐term (1–9 d) variability. With increased temporal period, the contribution ofkto variability decreased, and in some lakes resulted in ΔCO₂contributing more thankto variability over the full ice‐free period. Increased contribution of ΔCO₂to 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, whereaskwas linked to variability in shallow waters. Accordingly, knowing the variability of bothkand ΔCO₂over 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 ΔCO₂combined withk‐models that incorporate spatial variability of lake thermal structure and meteorology.