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 otherwise 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.
Highlights
Sea has been affected by this rapid oceanic and atmospheric warming; both of which have had consequences on its ice shelves, such as Wilkins Ice Shelf (Graham et al, 2011)
We present the results separately for winter sea ice zone (WSIZ) and permanent open ocean zone (POOZ)
We evaluated the relationship between the c(O2) and θ and S (Fig. 4b and d)
Summary
Sea has been affected by this rapid oceanic and atmospheric warming; both of which have had consequences on its ice shelves, such as Wilkins Ice Shelf (Graham et al, 2011). K. Castro-Morales et al.: Biological production in the Bellingshausen Sea and community composition (Arrigo et al, 2008). Previous estimates of marine biological production in the Bellingshausen Sea were based on discrete measurements of 14C assimilation, chlorophyll a concentrations, seasonal inorganic nutrient deficits and phytoplankton abundance (i.e., Boyd et al, 1995; Garibotti et al, 2003; Serebrennikova and Fanning, 2004; Turner and Owens, 1995; Vernet et al, 2008). We present estimates of mixed-layer average net community oxygen production (N) and gross oxygen production (G) in the Bellingshausen Sea during the period of February to April 2007, based on continuous ship measurements of O2/Ar ratios and discrete samples for oxygen triple isotopes. The study area of Huang et al (2012) was located in the Palmer Long-Term Ecological Research (LTER) region, north of our area of study, and comprised a coarser grid of hydrographic stations than the present study, mainly outside the sea ice zone
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