O2 and 18O: CO-Tracers of Oceanic Productivity

Metabolic processes naturally and subtly alter the isotopic composition (δ18O) as well as the concentration of the dissolved O2 in the sea. Three investigations employing o2 and δ18O as co-tracers of metabolic activity are presented. In a laboratory investigation, we measured the respiratory isotope effects εresp for ten representative marine organisms. The average εresp of the dominant marine respirers (the bacteria, microalgae and zooplankton) was 20 ± 3 ‰. This value supports the hypothesis that the photosynthesis-respiration cycle is responsible for the 23.5 ‰ enrichment in the δ18O ratio of atmospheric O2 relative to seawater (the Dole effect). The large value and large variability in the average εresp limits the usefulness of a proposed method using the δ18O of naturally fractionated dissolved o2 in seawater as a tracer of primary production in the oligotrophic ocean. In a second investigation, we measured gross and net o2 production rates, determined respectively by H2 18O assimilation and Winkler titration, during the 1989 JGOFS North Atlantic (Spring) Bloom Experiment at 47°N, 20°W. Production versus irradiance pB(I) curves, constructed using gross o2 production rates, indicated that magnitudes of pB m, α, and B were high during the bloom. Integrated gross o2 production was linearly related to integrated incident irradiance. 24-hour O2 respiration rates consisted of one component proportional to the amount of carbon fixed in the day\u27s photoperiod, and a second component proportional to the consumption rate of carbon fixed prior to the day\u27s photoperiod. POC turnover times in the euphotic region ranged from two days to two weeks. In a third investigation, we examined natural changes in the δ18O of dissolved O2 in the mixed layer during the spring bloom event, and in the mixed layer and upper seasonal thermocline in the Sargasso Sea during the summer production season. In all settings, the δ18O remained constant despite large changes in O2 concentration. Modeling exercises incorporating metabolic processes (photosynthesis and respiration) and physical processes (air-sea exchange, bubble injection, and eddy-diffusion) satisfactorily replicated the observed δ18O vs O2 concentration trends in the mixed layer regions, but not in the Sargasso Sea upper-thermocline