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Kyoko Hagino, Hisatake Okada:
Distribution of the coccolithophore assemblages in the equatorial and subequatorial Pacific Ocean
(Talk)

Floral compositions of coccolithophore assemblages were investigated in the equatorial and subequatorial Pacific Ocean by analysing 196 surface and 206 subsurface samples. A cluster analysis of coccolithophore assemblages revealed seven assemblages: E. huxleyi abundant assemblage (A) and (B), G. oceanica abundant assemblage (A) and (B), U. irregularis-dominated assemblage (A) and (B), and Florisphaera abundant assemblage.

The E. huxleyi abundant assemblage (A) is characterised by a common occurrence of E. huxleyi and U. tenuis, occurring in areas close to continents. E. huxleyi abundant assemblage (B), which was characterised by common occurrences of E. huxleyi, G. crassipons, G. oceanica and R. parvula, was restricted to the eutrophic upwelling centre.

The G. oceanica abundant assemblage (A) contains common E. huxleyi, G. oceanica and C. leptoporus, and occurred in the upper photic zone of the eutrophic central equatorial Pacific Ocean. Vertical distribution of G. oceanica abundant assemblage (B), which is characterised by the abundant occurrence of G. oceanica and O. antillarum, was differentiated between the dynamic waters and the stratified waters. In the dynamic eutrophic waters, the G. oceanica abundant assemblage (B) was adapted to the upper photic zone. In the stratified water, however, this assemblage occurred in the moderately eutrophic middle photic zone.

Both the U. irregularis-dominated assemblages (A) and (B) were dominated by U. irregularis, and they dwelled in the oligotrophic upper photic zone waters. U. irregularis-dominated assemblage (A) is differentiated from the (B) assemblage in the common presence of U. sibogae var. sibogae. In the stratified waters, the U. irregularis-dominated assemblage (A) occurred below the U. irregularis-dominated (B) assemblage.

The Florisphaera abundant assemblage, which is characterised by the common occurrence of F. profunda and G. flabellatus, was restricted to the lower photic zone.

Horizontal and vertical distributions of assemblages were controlled by temperature and nutrient concentration. In the stratified oligotrophic stations, assemblages that require high nutrient-concentrations were constrained in the moderately eutrophic middle photic zone. Floral stratification is more complex in the stratified waters rather than in the dynamic waters.

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