Seawater-atmosphere O2 exchange rates in open-top laboratory microcosms: application for continuous estimates of planktonic primary production and respiration

Abstract

Seawater-atmosphere O2 exchange rates were experimentally measured in open-top laboratory microcosms. The objective was to establish the relationships between turbulence and oxygen transfer velocity, and thus correct continuously measured day–night changes in dissolved oxygen as estimates of planktonic primary production and respiration. After saturating 15-l sterile seawatermicrocosms with an oxygen-poor gas mix (4.9% O2, 95.1% N2), the microcosms were left to equilibrate with the atmosphere under different turbulence conditions. The rate of increase in dissolved O2 was measured at 15-min intervals with polarographic-pulsed electrodes and the corresponding values of the oxygen transfer velocity (the KO2 constant for the different turbulence conditions) were determined. After pooling these and literature data obtained in similar experimental conditions, the relation between ϵ (turbulent kinetic energy dissipation rates) and KO2 was determined. Theoretical KO2 values were also calculated using semi-empirical models in which oxygen transfer velocity (KO2) is related to wind velocity. Theoretical, wind related KO2 values were significantly higher than the experimental ones, and as a consequence overestimate primary production and underestimate respiration rates, even resulting in nocturnal O2 increase. The magnitude of the differences between experimentally derived and theoretically calculated oxygen transfer velocity, precludes the use of wind-derived equations to calculate KO2 in meso- and microcosms experiments not affected by wind, while the equation obtained relating experimental ϵ and KO2 provides statistically reliable estimations of primary production and respiration