Production, respiration, and the isotope geochemistry of O₂ in the upper water column

The isotopic composition (or 18 O/ 16 O ratio) of O 2 produced by photosynthesis and the isotopic composition of O 2 consumed during respiration are different from each other as well as from the isotopic composition of ambient dissolved O 2 . In a closed body of water in which photosynthesis and respiration are co‐occurring, the 18 O/ 16 O of dissolved O 2 will be a function of both the rate of respiration and the gross rate of photosynthesis. Therefore gross primary production can be calculated if one determines both net O 2 production and the 18 O/ 16 O of dissolved O 2 . As a test of the feasibility of this approach, we present data on the 18 O/ 16 O ratio of dissolved O 2 , along with ancillary parameters, in the top 200 m of waters in the North Pacific gyre in August and September 1985. The results are shown to reflect isotope fractionation during photosynthesis and respiration. We outline the additional information needed before natural 18 O/ 16 O variations can be used to constrain quantitatively the gross production.