The seamount effect on phytoplankton in the tropical western Pacific.

The tropical western Pacific, one of the most oligotrophic seas on Earth, has a large number of seamounts. In this sea, we hypothesize that although the presence of shallow seamounts can enhance phytoplankton biomass, they do not alter the phytoplankton community composition. The concentration and distribution of size-fractionated chlorophyll a (Chl a) on the C4 Seamount were measured, and the phytoplankton community was analyzed based on photosynthetic pigments. The results showed that the deep chlorophyll maximum (DCM) was located between 100 and 150 m and that picophytoplankton such as prochlorophytes and cyanobacteria (mainly Synechococcus) were the dominant groups, contributing approximately 85% of the Chl a. The integrated Chl a concentration above the seamount was significantly higher than that off the seamount, and most communities were enhanced near the peak. The physical, chemical, and biological coupling processes of the C4 Seamount support the classic hypothesis of the seamount effect. However, the relative contributions of different phytoplankton groups to Chl a were not significantly different between stations above and off the seamount, and the analysis of similarities among results also indicated that the community composition on the C4 Seamount did not differ significantly from that off the seamount. In addition, size-fractionated Chl a data from two nearby shallow seamounts also supported our hypothesis. More studies on the ecology of seamounts in the tropical western Pacific are required to further confirm and refine these results.