Latitudinal and temporal distributions of diatom populations in the pelagic waters of the Subantarctic and Polar Frontal zones of the Southern Ocean and their role in the biological pump

Abstract

Abstract. The Subantarctic and Polar Frontal zones (SAZ and PFZ) represent a large portion of the total area of the Southern Ocean and serve as a strong sink for atmospheric CO2. These regions are central to hypotheses linking particle fluxes and climate change, yet multi-year records of modern flux and the organisms that control it are, for obvious reasons, rare. In this study, we examine two sediment trap records of the flux of diatoms and bulk components collected by two bottom-tethered sediment traps deployed at mesopelagic depths (~ 1 km) in the SAZ (2-year record; July 1999–October 2001) and in the PFZ (6-year record; September 1997–February 1998, July 1999–August 2000, November 2002–October 2004 and December 2005–October 2007) along the 140° E meridian. These traps provide a direct measure of transfer below winter mixed layer depths, i.e. at depths where effective sequestration from the atmosphere occurs, in contrast to study of processes in the surface ocean. Total mass fluxes were about twofold higher in the PFZ (24 ± 13 g m−2 yr−1) than in the SAZ (14 ± 2 g m−2 yr−1). Bulk chemical composition of the particle fluxes mirrored the composition of the distinct plankton communities of the surface layer, being dominated by carbonate in the SAZ and by biogenic silica in the PFZ. Particulate organic carbon (POC) export was similar for the annual average at both sites (1.0 ± 0.1 and 0.8 ± 0.4 g m−2 yr−1 for the PFZ and SAZ, respectively), indicating that the particles in the SAZ were relatively POC rich. Seasonality in the particle export was more pronounced in the PFZ. Peak fluxes occurred during summer in the PFZ and during spring in the SAZ. The strong summer pulses in the PFZ are responsible for a large fraction of the variability in carbon sequestration from the atmosphere in this region. The latitudinal variation of the total diatom flux was found to be in line with the biogenic silica export with an annual flux of 31 ± 5.5 × 108 valves m−2 yr−1 at the PFZ compared to 0.5 ± 0.4 × 108 m−2 yr−1 at the SAZ. Fragilariopsis kerguelensis dominated the annual diatom export at both sites (43 % at the SAZ and 59 % in the PFZ). POC fluxes displayed a strong positive correlation with the relative contribution of a group of weakly silicified and bloom-forming species in the PFZ. Several lines of evidence suggests that the development of these species during the growth season facilitates the formation of aggregates and carbon export. Our results confirm previous work suggesting that F. kerguelensis plays a major role in the decoupling of the carbon and silicon cycles in the high-nutrient low-chlorophyll waters of the Southern Ocean.

Coccolithophore populations and their contribution to carbonate export during an annual cycle in the Australian sector of the Antarctic zone

“…The lower coccolithophore production in the AZ-S is also reflected in the lower carbonate export at this site, i.e. 6 g m −2 yr −1 versus 10-13 g m −2 yr −1 at the 47 • S site (Rigual-Hernández et al, 2015b;Wilks et al, 2017). The non-proportional latitudinal change in coccolith and carbonate fluxes (i.e.…”

The fate of diatom valves in the Subantarctic and Polar Frontal Zones of the Southern Ocean: Sediment trap versus surface sediment assemblages

“…This is despite the fact that the two A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 37 zonal systems host dramatically different phytoplankton communities. The poor silica particle content registered by the sediment traps at the 47°S site (Table 4) suggest a secondary role for diatoms in the control of carbon export in this region (Closset et al 2015;Rigual-Hernández et al 2015b). Thus, our data indicate that a system dominated by non-siliceous phytoplankton can export the same amount of POC as a diatomdominated ecosystem such as the PFZ.…”

“…The splitting procedure and chemical analyses of the < 1 mm fraction were carried out following the methodology described by Bray et al (2000), Trull et al (2001a) and Rigual-Hernández et al (2015b). A total of 105 samples were processed for chemical analysis.…”

“…The composition and seasonal variability of the diatom sinking assemblages captured by the shallower traps at the 54°S and 47°S sites are described in detail in Rigual-Hernández et al (2015b). The same species occurring at the shallower traps (Rigual-Hernández et al, 2015b) were indentified in the deeper traps (Fig.…”

“…The same species occurring at the shallower traps (Rigual-Hernández et al, 2015b) were indentified in the deeper traps (Fig. 4).…”