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

Abstract

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 (Fbio) 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 Fbio values < 0 in large parts of the Bellingshausen Sea towards the end of the productive season, which could be mistaken to represent net heterotrophy. Thus, improved estimates of mixed-layer N can be derived from the sum of Fbio, Fe (entrainment from the upper thermocline during mixed-layer deepening) and Fv (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 Fbio= (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 Fbio 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.