Abstract
AbstractArctic primary production is sensitive to reductions in sea ice cover, and will likely increase into the future. Whether this increased primary production (PP) will translate into increased export of particulate organic carbon (POC) is currently unclear. Here we report on the POC export efficiency during summer 2012 in the Atlantic sector of the Arctic Ocean. We coupled 234‐thorium based estimates of the export flux of POC to onboard incubation‐based estimates of PP. Export efficiency (defined as the fraction of PP that is exported below 100 m depth: ThE‐ratio) showed large variability (0.09 ± 0.19–1.3 ± 0.3). The highest ThE‐ratio (1.3 ± 0.3) was recorded in a mono‐specific bloom of Phaeocystis pouchetii located in the ice edge. Blooming diatom dominated areas also had high ThE‐ratios (0.1 ± 0.1–0.5 ± 0.2), while mixed and/or prebloom communities showed lower ThE‐ratios (0.10 ± 0.03–0.19 ± 0.05). Furthermore, using oxygen saturation, bacterial abundance, bacterial production, and zooplankton oxygen demand, we also investigated spatial variability in the degree to which this sinking material may be remineralized in the upper mesopelagic (<300 m). Our results suggest that blooming diatoms and P. pouchetii can export a significant fraction of their biomass below the surface layer (100 m) in the open Arctic Ocean. Also, we show evidence that the material sinking from a P. pouchetii bloom may be remineralized (>100 m) at a similar rate as the material sinking from diatom blooms in the upper mesopelagic, contrary to previous findings.
Highlights
The Southern Ocean south of 44∘S is responsible for approximately 25–30% of the global ocean uptake of anthropogenic carbon [Fletcher et al, 2006; Lenton et al, 2013], but accurately quantifying this sink and understanding the processes behind it remains challenging
dissolved inorganic carbon (DIC) was measured by coulometry [Johnson et al, 1985] following standard operating procedure (SOP) 2 of Dickson et al [2007], and total alkalinity (TA) was measured by potentiometric titration [Mintrop et al, 2000] following SOP 3b of Dickson et al [2007]
There is a gradual increase from around April to maximum DIC concentrations of ∼2200 μmol kg−1 in September. This increase during autumn and winter is caused by net heterotrophy and mixing with relatively old, carbon-rich Circumpolar Deep Water (CDW) as the mixed layer deepens
Summary
The Southern Ocean south of 44∘S is responsible for approximately 25–30% of the global ocean uptake of anthropogenic carbon [Fletcher et al, 2006; Lenton et al, 2013], but accurately quantifying this sink and understanding the processes behind it remains challenging. Seasonal sea ice cover increases the net annual CO2 uptake, but its effect on gas exchange remains poorly constrained. The net ocean-atmosphere CO2 flux of the high-latitude, seasonally ice-covered Southern Ocean is especially difficult to quantify due to a scarcity in observational data, during the ice-covered winter months [Bakker et al, 2014].
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