The distribution of oceanic coccolithophorids in the Pacific

Abstract

Horizontal and vertical distributions of oceanic coccolithophorids were investigated along five traverses in the North and Central Pacific. The population was characterized by: 1.(1) a high standing crop in high latitude,2.(2) an abrupt decrease in the temperature and subtropical areas except in the Kuroshio Water, and3.(3) a significant increase in the equatorial and Kuroshio ares. When most abundant, there were as many as 1·7 × 105 individuals per liter of water in the −5 m level at 50°N 155°W, but at some deeper localities in the photic layer there were no cells at all, particularly in the south Philippine Sea.Approximately 90 species of the Coccolithophoridae were identified. Six coccolithophorid zones were established in the surface water along the 155°W meridian based on the distribution pattern of characteristics species: Subarctic, Transitional, Central North, Equatorial North, Equatorial South and Central South.The Subarctic Zone corresponded to the Pacific Subarctic Current, where the specimens were almost exclusively a subarctic variety of Emiliania huxleyi. Emiliania huxleyi, a cold variety and Rhabdosphaera clavigera were dominant in the Transitional Zone, which coincided with the North Pacific Current. The Central zones were dominated by Umbellosphaera irregularis. The North Equatorial Current and Equatorial Countercurrent comprise the Central North Zone where the largest variety of species was observed. The Equatorial zones includes the South Equatorial Current and were characterized by an abundance of three placolith-type species: Gephyrocapsa oceanica, Cyclococcolithina leptoporus and Cyclococcolithina fragilis. The Central South Zone coincides with the northern portion of the South Pacific circulation.Three vertical coccolithophorid zones were designated in the 200 m water column of the Transitional and Central zones, while the Subarctic and Equatorial zones had one and two vertical zones respectively. Umbellosphaera irregularis and Rhabdosphaera clavigera preferred shallow water while Umbellosphaera tenuis and Cyclococcolithina fragilis were mainly observed in the middle euphotic waters. Florisphaera profunda and Thorosphaera flabellata occurred only in lower euphotic waters.Water temperature and light intensity appeared to be most influential factors in the distribution of coccolithophorid species, but currents also delimited the distribution of some species.