Net and gross oxygen production from O-2/Ar, O-17/O-16 and O-18/O-16 ratios

In aquatic systems, variations in the O-2/Ar ratio and in the O-17 anomaly ((17)Delta, calculated from O-17/O-16 and O-18/O-16) are affected by both physical and biological processes. Laboratory observations suggest that (17)Delta is independent of respiration rate, in contrast to the O-2/Ar ratio. This makes it possible to determine gross O-2 production from measurements of O-17/O-16 and O-18/O-16 of dissolved O-2, while the O-2/Ar ratio can be applied to estimate net O-2 production. We demonstrate how biological and physical forcing modifies the O-2/Ar ratio and (17)Delta during the day and at night in a coral reef near Eilat. This field study confirmed our hypothesis that (17)Delta of dissolved O-2 ((17)Delta(dis)) is affected by photosynthesis, ocean dynamics and air-water gas exchange, but not by respiration. We also show, for the first time, a detailed record of (17)Delta responses to biological fluxes and subtropical ocean dynamics on a seasonal scale. The measurements were conducted in the Atlantic Ocean near Bermuda during monthly cruises of the Bermuda Atlantic Time-series Study (BATS) from March 2000 to January 2001. With the formation of the seasonal thermocline (17)Delta(dis) significantly increased below the oceanic mixed layer, reflecting the seasonal integration of photosynthesis in the thermocline. Hence, (17)Delta(dis) accumulation may be used to calculate the overall rate of photosynthesis below the mixed layer. We used the O-2/Ar ratio and (17)Delta(dis) to estimate net and gross O-2 production (NOP and GOP) and directly compared them with production values derived from C-14 incubations. In general, GOP based on (17)Delta(dis) was several times greater than production determined via C-14 fixation [P(C-14)], which in turn was somewhat greater than NOP based on the O-2/Ar ratio. The NOP/GOP ratio was 0.08 to 0.21. This implies that the observed high GOP/P(C-14) ratios result mainly from rapid cycling of O-2 between photosynthesis and O-2 uptake mechanisms.