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 (<i>N</i>) and gross oxygen production (<i>G</i>) of the Bellingshausen Sea in March and April 2007. <i>N</i> was derived from oxygen-to-argon (O<sub>2</sub> / Ar) ratios; <i>G</i> was derived using the dual-delta method from triple oxygen isotope measurements. In addition, O<sub>2</sub> profiles were collected at 253 CTD stations. <i>N</i> is often approximated by the biological oxygen air-sea exchange flux (<i>F</i><sub>bio</sub>) based on the O<sub>2</sub> / Ar supersaturation, assuming that significant horizontal or vertical fluxes are absent. Here, we show that the effect of vertical fluxes alone can account for <i>F</i><sub>bio</sub> 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 <i>N</i> can be derived from the sum of <i>F</i><sub>bio</sub>, <i>F</i><sub>e</sub> (entrainment from the upper thermocline during mixed-layer deepening) and <i>F</i><sub>v</sub> (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 <i>F</i><sub>bio</sub>= (30 ± 17) mmol m<sup>−2</sup> d<sup>−1</sup> to <i>N</i>= (34 ± 17) mmol m<sup>−2</sup> d<sup>−1</sup>. However, in the permanent open ocean zone (POOZ), the original <i>F</i><sub>bio</sub> value of (−17 ± 10) mmol m<sup>−2</sup> d<sup>−1</sup> gives a corrected value for <i>N</i> of (−2 ± 18) mmol m<sup>−2</sup> d<sup>−1</sup>. 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 O<sub>2</sub> / Ar ratios