Difference in O2 and CO2 gas transfer velocities in Funka Bay

Abstract

At a station in Funka Bay (92 m deep), regular observations were made almost once a month for 16 months; and the water column concentrations of dissolved oxygen (DO), phosphate, total carbonate and the δ13C of CO2, as well as some auxiliary components, were measured. Funka Bay has a characteristic water exchange which occurs twice a year and is otherwise limited. This enabled us to apply a closed-system model for the water, except during the flushing periods, to obtain gas transfer velocities from the changes in concentrations. The TC budget corrected for biological production and decomposition showed an increase in water column content, due to atmospheric CO2 absorbed into the bay water at the rate of 7 mol m−2/year or more. This result was in agreement with the observation that the fugacity, f(CO2), of surface seawater was always lower than the atmospheric f(CO2), by about 200 ppm, especially in April. By applying the closed-system model, we obtained a mean gas transfer velocity for O2 of 3.3±1.6 m/day in summer and 7.4±4.3 m/day in winter. The gas transfer velocity of CO2 from carbonate was larger than that of CO2 converted from O2. The mean ratio of the gas transfer velocity of CO2 obtained from carbonate to that of CO2 from O2 was 1.76±0.34. The transfer velocity of CO2 estimated from the δ13C budget was even larger; its ratio to that of CO2 from O2 was 2.8±1.3, although the velocity was obtained for only one period in winter. This order of increase in the gas transfer velocities coincides with the increase in the times necessary for gas exchange equilibration at the surface. This suggests the importance of bubbles or air masses being taken into deeper water.