Fachbereich 5 – Geowissenschaften

: Coccolithophores are globally distributed microscopic marine algae that exert a major influence on the global carbon cycle through calcification and primary productivity. There is recent interest in coccolithophore polar communities, however field observations regarding their biogeographic distribution are scarce for the Southern Ocean. This study documents the latitudinal, as well as in depth, variability in the coccolithophore assemblage composition and the coccolith mass variation of the ecologically dominant 20 Emiliania huxleyi across the Drake Passage. Ninety-six water samples were taken between 10 and 150 m water depth from 18 stations during POLARSTERN Expedition PS97 (February-April, 2016). A minimum of 200 coccospheres per sample were classified identified in scanning electron microscope and coccolith mass was estimated with light microscopy. We find that coccolithophore abundance, and diversity and maximum depth habitat decrease southwards marking different oceanographic fronts as ecological boundaries. We characterize three zones: (1) the Chilean margin, where E. huxleyi type A (normal and 25 overcalcified) and type R are present; (2) the Subantarctic Zone (SAZ), where E. huxleyi reaches maximum values of 212.5*10 3 cells/L and types B/C, C, O are dominant. (3) The Polar Front Zone (PFZ), where E huxleyi types B/C and C dominate. We link the decreasing trend in E. huxleyi coccolith mass to the poleward latitudinal succession from type A to type B group. Remarkably, we find that coccolith mass is strongly anticorrelated to total alkalinity, total CO 2 , bicarbonate ion and pH. We speculate that low temperatures are a greater limiting factor than carbonate chemistry in the Southern Ocean. However, further in 30 situ oceanographic data is needed to verify the proposed relationships. We hypothesize that assemblage composition and calcification modes of E. huxleyi in the Drake Passage will be strongly influenced by the ongoing climate change. Sample preparation Ninety-six water samples were taken at 18 sStations located in the southern Chilean continental margin and across the western end of the Drake Passage (Fig. 1) from February to April 2016 during Expedition PS97 (Lamy, 2016). Seawater samples were obtained at different depths using a rosette sampler with 24 × 12 L Niskin bottles (Ocean Test Equipment Inc.) attached to a CTD Seabird SBE911 plus device (Lamy, 2016). The bottles were fired by an SBE32 carousel. For the study of coccolithophore assemblages, 4 25 to 7 samples per station, between 10 and 150 m water depth, were chosen. Two litres of water were filtered onto 0.45 µm pore size Polycarbonte Track-Etch Membrane, air-dried and stored over silica gel. A small part of the filter was cut out, fixed on an aluminum Scanning Electron Microscope (SEM) stub and sputtered with gold/palladium. A specific area of the center of the filter was analysed with Zeiss DSM 940A SEM at the University of Bremen, to determine quantitative cell counts for all morphotypes, species and total coccolithophore abundance at magnifications of 1000x, 2000x and 5000x when required. A minimum of 200 30 whole coccospheres per sample were counted and classified following Young et al. (2003), the revised classification of Jordan et al. (2004)(2004) and the electronic guide to the biodiversity and taxonomy of coccolithophores Nannotax 3 (ina.tmsoc.org/Nannotax3/index.html) by Young et al. (2019). The existing relationships between the physico-chemical parameters and the coccolithophore components in the Drake Passage suggest that assemblage composition and calcification modes of E. huxleyi will be strongly affected by the ongoing climate change.