Spatial dynamics of coccolithophore assemblages in the equatorial Western-Central Pacific Ocean

The spatial distribution of living coccolithophores was studied in the West ern-Central Equatorial Pacific Ocean during November-December, 1990 and Sep tember-October, 1992. The highest local concentration of coccolithophores o ccurred at the thermocline in well-stratified waters, but at sea-surface le vel in dynamic waters. In total, 111 coccolithophore taxa were recognized, some of which exhibited hydrographically controlled variation in their abso lute abundance. Gephyrocapsa oceanica and Oolithotus antillarum were abunda nt in the upwelling front. Most of the lower photic dwellers were abundant in the tropical to subtropical stations regardless of the water stratificat ion. The coccolithophore flora of well-stratified waters could be distingui shed from the upwelling front flora by the higher abundance of Umbellosphae ra irregularis and lower abundance of G. oceanica. The temperature mixed-wa ter flora was characterized by a high abundance of Emiliania huxleyi. The v ertical distribution of all coccolithophore taxa, except three placolith-be aring species, Gephyrocapsa ericsonii, G. oceanica and E. huxleyi, was cont rolled by upper photic-zone temperature and water stratification. The upper or lower vertical distribution limits of many coccolithophore taxa coincid ed with the top of the thermocline. The most common 27 taxa were grouped in to four ecological groups, Upper Photic-zone Group (UPG), Middle Photic-zon e Group (MPG), Lower Photic-zone Group (LPG) and Omnipresent Group (OPG), o n the basis of their vertical distribution. By analyzing the hydrographic c ontrol on the vertical distribution of these four ecological groups, four e cological assemblages were recognized: High Temperature; Warm Oligotrophic; Warm Eutrophic; and Temperate Mixed-water Assemblages. In equatorial water s, the total coccolithophore assemblage across the photic-zone was controll ed by the population in the upper photic-zone. The UPG monopolized the uppe r photic-zone flora in the High Temperature Assemblage. In the Warn Oligotr ophic-water Assemblage, common OPG accompanied abundant UPG in the upper ph otic-zone. The upper photic-zone of the Warm Eutrophic Assemblage consisted of UPG, MPG and OPG. Emiliania huxleyi and Gephyrocapsa oceanica, the major component of OPG, di splayed intra-specific morphological variations. G. oceanica Type 1 was res tricted to the upper photic-zone of well-stratified oligotrophic waters. Co nversely, in these waters E. huxleyi Type C and G. oceanica Type 2 only occ urred below the thermocline. These two taxa also coexist with G. oceanica T ype 3 and E. huxleyi Type A in the upper photic zone of dynamic waters. (C) 2000 Elsevier Science B.V. All rights reserved.