The effect of surface films on the air–sea gas exchange in the Baltic Sea

Gas exchange experiments were performed with water from a coastal station in the southwestern Baltic Sea in order to investigate the effect of organic films on the gas transfer velocity. Samples were taken on a weekly basis during January to July in 2007 and 2008. Using a 1.5 l glass vessel, the evasion of oxygen from the sample into an O2-free headspace was recorded with an O2 optode. The transfer velocities were calculated and related to the surface film coverage that was characterized by the reduction of the surface tension with respect to a surfactant-free water sample (Δσ). For the determination of Δσ, surface tension measurements of organic-free water samples were performed and an equation was derived that describes σ as a function of temperature and salinity.The normalized transfer velocity (Schmidt number = 660), k660, decreased by 55%–65% at low film densities (Δσ < 0.3 mN m− 1) and remained almost constant at this level despite increasing film coverage (Δσ up to 10 mN m− 1). These findings are qualitatively consistent with other studies which, however, report a less drastic film-induced decrease of k660. The Δσ were low in January (0.1–0.2 mN m− 1) but increased during the development of the spring bloom. This implies a seasonality of the k660 which may have a strong effect on gas exchange calculations. It was shown that the annual net CO2 uptake of the Baltic Sea decreases by a factor of two when a seasonal k660 amplitude of ± 20% is included into the flux calculations.

Research highlights
► New equation describing surface tension as a function of temperature and salinity.
► A drastic reduction of k660 occurs already at low surface film coverage.
► Ignoring the seasonality of film effects may bias annual CO2 balances.