Net and gross oxygen production from O2/Ar, 17O/16O and 18O/16O ratios

Abstract

In aquatic systems, variations in the O 2 /Ar ratio and in the 17 O anomaly ( 17 , calculated from 17 O/ 16 O and 18 O/ 16 O) are affected by both physical and biological processes. Laboratory observations suggest that 17 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 17 O/ 16 O and 18 O/ 16 O 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 during the day and at night in a coral reef near Eilat. This field study confirmed our hypothesis that 17 of dissolved O 2 ( 17 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 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 dis significantly increased below the oceanic mixed layer, reflecting the seasonal integration of photosynthesis in the thermocline. Hence, 17 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 dis to estimate net and gross O 2 production (NOP and GOP) and directly compared them with production values derived from 14 C incubations. In general, GOP based on 17 dis was several times greater than production determined via 14 C fixation [P( 14 C)], 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( 14 C) ratios result mainly from rapid cycling of O 2 between photosynthesis and O 2 uptake mechanisms.