Seawater-atmosphere O-2 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 relationship s between turbulence and oxygen transfer velocity, and thus correct continu ously measured day-night changes in dissolved oxygen as estimates of plankt onic primary production and respiration. After saturating 15-1 sterile seaw ater microcosms with an oxygen-poor gas mix (4.9% O-2, 95.1% N-2), the micr ocosms were left to equilibrate with the atmosphere under different turbule nce conditions. The rate of increase in dissolved O-2 was measured at 15-mi n intervals with polarographic-pulsed electrodes and the corresponding valu es of the oxygen transfer velocity (the K-O2 constant for the different tur bulence conditions) were determined. After pooling these and literature dat a obtained in similar experimental conditions, the relation between epsilon (turbulent kinetic energy dissipation rates) and K-O2 was determined. Theo retical K-O2 values were also calculated using semi-empirical models in whi ch 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 underestimat e respiration rates, even resulting in nocturnal O-2 increase. The magnitud e of the differences between experimentally derived and theoretically calcu lated 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 provid es statistically reliable estimations of primary production and respiration . (C) 2001 Elsevier Science B.V. All rights reserved.