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

[摘要] The Subantarctic and Polar Frontal zones (SAZ and PFZ) represent a largeportion of the total area of the Southern Ocean and serve as a strong sinkfor atmospheric CO₂. These regions are central to hypotheses linkingparticle fluxes and climate change, yet multi-year records of modern fluxand the organisms that control it are, for obvious reasons, rare. In thisstudy, we examine two sediment trap records of the flux of diatoms and bulkcomponents collected by two bottom-tethered sediment traps deployed atmesopelagic depths (~ 1 km) in the SAZ (2-year record; July1999–October 2001) and in the PFZ (6-year record; September 1997–February 1998, July 1999–August 2000, November 2002–October 2004 andDecember 2005–October 2007) along the 140° E meridian. Thesetraps provide a direct measure of transfer below winter mixed layer depths,i.e. at depths where effective sequestration from the atmosphere occurs, incontrast to study of processes in the surface ocean. Total mass fluxes wereabout twofold higher in the PFZ (24 ± 13 g m⁻² yr⁻¹) thanin the SAZ (14 ± 2 g m⁻² yr⁻¹). Bulk chemical composition ofthe particle fluxes mirrored the composition of the distinct planktoncommunities of the surface layer, being dominated by carbonate in the SAZand by biogenic silica in the PFZ. Particulate organic carbon (POC) export was similar for the annualaverage 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 theparticles in the SAZ were relatively POC rich. Seasonality in the particleexport was more pronounced in the PFZ. Peak fluxes occurred during summer inthe PFZ and during spring in the SAZ. The strong summer pulses in the PFZare responsible for a large fraction of the variability in carbonsequestration from the atmosphere in this region. The latitudinal variationof the total diatom flux was found to be in line with the biogenic silicaexport with an annual flux of 31 ± 5.5 × 10⁸ valves m⁻² yr⁻¹ at the PFZ compared to0.5 ± 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 strongpositive correlation with the relative contribution of a group ofweakly silicified and bloom-forming species in the PFZ. Several lines ofevidence suggests that the development of these species during the growthseason facilitates the formation of aggregates and carbon export. Ourresults confirm previous work suggesting that F. kerguelensis plays a major role in thedecoupling of the carbon and silicon cycles in the high-nutrientlow-chlorophyll waters of the Southern Ocean.

[学科分类] 地球化学与岩石