O2 and 18O: CO-Tracers of Oceanic Productivity

Author

John Afton Kiddon, University of Rhode Island

Date of Award

1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Department

Oceanography

First Advisor

Michael L. Bender

Abstract

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 ε_resp for ten representative marine organisms. The average ε_resp 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 ε_resp 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 p^B(I) curves, constructed using gross o₂ production rates, indicated that magnitudes of p^B 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's photoperiod, and a second component proportional to the consumption rate of carbon fixed prior to the day's 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.

Recommended Citation

Kiddon, John Afton, "O2 and 18O: CO-Tracers of Oceanic Productivity" (1993). Open Access Dissertations. Paper 1313.
https://digitalcommons.uri.edu/oa_diss/1313