The ecosystem size and shape dependence of gas transfer velocity versus wind speed relationships in lakes

Air-water diffusive gas flux is commonly determined using measurements of gas concentrations and an estimate of gas transfer velocity (k(600)) usually derived from wind speed. The great heterogeneity of aquatic systems raises questions about the appropriateness of using a single wind-based model to predict k(600) in all aquatic systems. Theoretical considerations suggest that wind speed to k(600) relationships should instead be system-specific. Using data collected from aquatic systems of different sizes, we show that k(600) is related to fetch and other measures of ecosystem size. Lake area together with wind speed provided the best predictive model of gas transfer velocity and explained 68% of the variability in individual k(600) measurements. For a moderate wind speed of 5 m.s(-1), predicted k(600) varied from 6 cm.h(-1) in a small 1 ha lake to over 13 cm.h(-1) in a 100 km(2) system. Wave height is also shown to be a promising integrative predictor variable. The modulating influence of system size on wind speed - gas transfer velocity relationships can have a large impact on upscaling exercises of gas exchange at the whole landscape level.