Biological production in the NE Pacific and its influence on air‐sea CO2 flux: Evidence from dissolved oxygen isotopes and O2/Ar

Abstract

We determine rates of gross photosynthetic O2 production (GOP) and net community O2 production (NCP) using the triple oxygen isotope and O2/Ar approach on two spring and two late summer meridional transects of the NE Pacific. Observed GOP and NCP in the subtropical (89 ± 9 and 8.3 ± 1.3 mmol O2 m−2 d−1, respectively) and subarctic (193 ± 16 and 16.3 ± 3.8 mmol O2 m−2 d−1) were in agreement with rates previously determined at time series stations in each region, validating the regional representativeness of these sites. At the transition zone chlorophyll front (TZCF), which migrates seasonally from 32°N in spring to 40°N in summer, GOP and NCPwere elevated by 2–4× compared to adjacent areas. Coincident with the TZCF, increases in surface nitrate concentration and extensive changes in phytoplankton community composition were observed. HPLC pigment data indicated substantial increases in a prymnesiophyte (e.g., coccolithophore) biomarker at the TZCF on a spring and summer cruise, and a diatom biomarker on the spring cruise. Increases in remotely sensed surface particulate inorganic carbon concentration were also observed at the TZCF on all four cruises, indicating that coccolithophore production may contribute to increased productivity at the TZCF. Meridional trends in observed air‐sea CO2 flux on each cruise resembled those of the biologically induced CO2 flux (NCP), but with an overprinting of the response of air‐sea CO2 exchange to summer warming. A simple carbon budget based on regional CO2 flux climatology demonstrates the importance of NCPfor net annual air‐sea CO2uptake, although slow air‐sea equilibration and seasonal solubility effects obscure this term.