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

Seawater-atmosphere O-2 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 15-1 sterile seawater microcosms with an oxygen-poor gas mix (4.9% O-2, 95.1% N-2), the microcosms were left to equilibrate with the atmosphere under different turbulence conditions. The rate of increase in dissolved O-2 was measured at 15-min intervals with polarographic-pulsed electrodes and the corresponding values of the oxygen transfer velocity (the K-O2 constant for the different turbulence conditions) were determined. After pooling these and literature data obtained in similar experimental conditions, the relation between epsilon (turbulent kinetic energy dissipation rates) and K-O2 was determined. Theoretical K-O2 values were also calculated using semi-empirical models in which oxygen transfer velocity (K-O2) is related to wind velocity. Theoretical, wind related K-O2 values were significantly higher than the experimental ones, and as a consequence overestimate primary production and underestimate respiration rates, even resulting in nocturnal O-2 increase. The magnitude of the differences between experimentally derived and theoretically calculated oxygen transfer velocity, precludes the use of wind-derived equations to calculate K-O2 in meso- and microcosms experiments not affected by wind, while the equation obtained relating experimental epsilon and K-O2 provides statistically reliable estimations of primary production and respiration. (C) 2001 Elsevier Science B.V. All rights reserved.