(Table 1) Biogeographic distribution of living coccolithophores in the Pacific sector of the Southern Ocean

Résumé

This paper adds to a series of studies addressing the distribution of living coccolithophores in the Southern Ocean (SO). We investigated plankton samples collected during RV Polarstern cruise ANT-XXVI/2 (from 27th November 2009 to 27th January 2010) along a broad E-W transect in the Pacific sector of the SO during austral summer. One hundred and fifty samples fromtwenty-nine stationswere collected fromthe upper 150mof the water column. Both coccoliths and coccospheres per samplewere counted separately using a scanning electronmicroscope (SEM). The highest abundances of 640 · 103 coccospheres/l were reached close to the Subtropical Front (STF) and increases in the numbers of coccospheres and coccoliths were found both at the Subantarctic Front (SAF) and the Polar Front (PF). However, the numbers decrease southward until almost a monospecific assemblage and sporadic record of Emiliania huxleyi (types B/C and C) south of the PF. Thirty-three coccolithophore species, including sixteen species found as isolated coccoliths, were identified of which E. huxleyi is clearly the most dominant coccolithophore taxon in the studied samples. Two main coccolithophore assemblages were established coincident with areas bounded by the oceanographic fronts: the Polar Front Zone (PFZ) and Subantarctic Zone (SAZ). In the upper photic zone of the SAZ, Acanthoica quattrospina, Calcidiscus leptoporus, Coccolithus pelagicus (sensu lato) HOL, E. huxleyi type A, Ophiaster spp. and Syracosphaera spp. among others were found. The PFZ was characterized by a reduced number of species, i.e., Calciopappus caudatus, E. huxleyi types B, B/C and C, as well as Pappomonas spp. and Papposphaera spp. The sea surface temperature measured in situ was the most prominent factor influencing coccolithophore diversity, distribution and assemblage compositions in the Pacific sector of the SO during austral summer. Coccolithophore biogeography in the study area showed marked differences with the northern high latitudes; the reduced presence of the cold water species Coccolithus pelagicus, abundant in the (sub) Arctic region, and the dominance of E. huxleyi type B/C and C in the SO contrasts with the dominance of E. huxleyi types A and B in the North Atlantic. Findings such as these cover existing gaps in an unexplored area of the SO as well as supporting previous research performed in neighboring areas. The current coccolithophore numbers and assemblage distribution in relation to the frontal dynamics of the SO provide valuable information for potential future paleoceanographic reconstructions.