Biological production in the Bellingshausen Sea from oxygen-to-argon ratios and oxygen triple isotopes

Abstract. We present estimates of mixed-layer net community oxygen production (N) and gross oxygen production (G) of the Bellingshausen Sea in March and April 2007. N was derived from oxygen-to-argon (O₂/Ar) ratios; G was derived using the dual-delta method from triple oxygen isotope measurements. In addition, O₂ profiles were collected at 253 CTD stations. N is often approximated by the biological oxygen air–sea exchange flux (F_bio based on the O₂/Ar supersaturation, assuming that significant horizontal or vertical fluxes are absent. Here we show that the effect of vertical fluxes alone can account for F_bio values < 0 in large parts of the Bellingshausen Sea towards the end of the productive season, which could otherwise be mistaken to represent net heterotrophy. Thus, improved estimates of mixed-layer N can be derived from the sum of F_bio, F_e (entrainment from the upper thermocline during mixed-layer deepening) and F_v (diapycnal eddy diffusion across the base of the mixed layer). In the winter sea ice zone (WSIZ), the corresponding correction results in a small change of F_bio = (30 ± 17) mmol m⁻² d⁻¹ to N = (34 ± 17) mmol m⁻² d⁻¹. However, in the permanent open ocean zone (POOZ), the original F_bio value of (−17 ± 10) mmol m⁻² d⁻¹ gives a corrected value for N of (−2 ± 18) mmol m⁻² d⁻¹. We hypothesize that in the WSIZ, enhanced water column stability due to the release of freshwater and nutrients from sea ice melt may account for the higher N value. These results stress the importance of accounting for physical biases when estimating mixed-layer marine productivity from in situ O₂/Ar ratios.