Biogeographic distribution of extant Coccolithophores in the Indian sector of the Southern Ocean

Abstract

Water samples from nine vertical profiles down to 110m water depth and 19 samples from the sea-surface were studied for coccolithophore abundance and distribution across oceanic frontal regions of the Indian sector of the Southern Ocean. Sampling was performed along a north-south transect (between 39�S and 65.49�S, ~57.3�E) during the 4th Indian Southern Ocean Expedition (between 31st January and 18th February 2010). Coccospheres and coccoliths were counted separately using a Scanning Electron Microscope (SEM). A total of 39 taxa (including morphotypes, types and subspecies) were recorded as intact coccospheres with abundances reaching up to 750�103 coccospheres/l. In addition, 85 taxa (including varieties, morphotypes) were counted as coccoliths reaching up to 900�105coccoliths/l. Emiliania huxleyi was recognized as the most abundant species, accounting for >86% of the total coccolithophore assemblage at each station. Elevated coccolithophore diversity was observed in the subtropical zone whereas high coccolithophore abundance was observed in the Subantarctic zone. A monospecific Emiliania huxleyi assemblage was recorded within and south of the Polar frontal zone. Three assemblages were recognized based on coccolithophore abundance and diversity. The assemblage of the Agulhas Retroflection frontal zone and Subtropical zone is highly diverse (39 taxa) and can be linked to relatively warm, high saline and oligotrophic waters. The Subantarctic zone assemblage is characterized by a reduced number (thirteen) of coccolithophore taxa, whereas the Polar Frontal zone comprises a monospecific E. huxleyi assemblage (preferentially morphotypes C and B/C). Multivariate statistics indicated that regions with elevated temperature and low nutrient concentration show high coccolithophore diversity whereas regions with high nutrient concentrations and low temperature show a strongly reduced coccolithophore diversity with abundant monospecific E. huxleyi (morphotypes B/C and C) assemblages.