Picophytoplankton abundance and community structure in the Philippine Sea, western Pacific

Abstract

Flow cytometric determinations of the abundance distribution and community structure of picophytoplankton (i.e., Prochlorococcus spp., orange fluorescence Synechococcus spp. and picoeukaryotes) were used for samples taken from the Philippine Sea in the western tropical Pacific Ocean from September to October of 2004. A fluorescence probe was employed to detect Chlorophyll a (Chl a). Abundances of Prochlorococcus spp., orange fluorescence Synechococcus spp. and picoeukaryotes ranged from 0.1 to 58×103 cells ml−1, 0.38 to 17×102 cells ml−1 and 0.42 to 26×102 cells ml−1, respectively. Synechococcus spp. and picoeukaryotes co-occurred in relatively shallow water with the maximum abundance observed at 50 to 70 m depth, while Prochlorococcus spp. only occurred in the 70 to 200 m layer. Prochlorococcus spp. was the dominant picophytoplankton population in terms of abundance and biomass. The cell size and carbon biomass content were estimated for the three picophytoplankton groups. In addition, among the three groups of picophytoplankton, the relative contribution of red fluorescence to the total red fluorescence varied with depth. The fluorescence and light scatter properties of individual cells indicated that in the upper 100 m layer, picoeukaryotes were a major contributor to total red fluorescence, while at the depth below 100 m, Prochlorococcus spp. and Synechococcus spp. made an important contribution to the total red fluorescence.