Structure and size distribution of plankton communities down to the greater depths in the western North Pacific Ocean

As part of the research program “WEST-COSMIC (Western Pacific Environment Study on CO2 Ocean Sequestration for Mitigation of Climate Change)”, vertical distribution patterns of community structure and size spectra of plankton organisms were studied at three sites: Station SA at 44°N, 155°E (down to 4000 m depth); Station I at 39°N, 147°E (2000 m depth); and Station ST at 25°N, 147°E (4800 m depth) in the western North Pacific Ocean. The plankton organisms were divided into four major groups (bacteria, phytoplankton, protozooplankton, and mesozooplankton) and their sizes and biomass were quantified. Total plankton biomass in the water column ranged from 8180 (night) to 8630 (day) mg C m−2 at Station ST to 29,800 (day) to 32,800 (night) mg C m−2 at Station SA. The water column-integrated major group compositions (biomass) were different between stations: mesozooplankton were the most dominant group (47–52%) at the two northern stations, while they constituted 9–14% at the southern station. An appreciable contribution of dormant copepods to higher mesozooplankton biomass was noted at the northern station, but there were few copepods at the intermediate station and nil at the southern station. The water column-integrated size distribution patterns of plankton communities were characterized by three marked peaks [pico-, micro- (20 μm), and meso- (2000 μm) size] at Station SA, the same three peaks, but with less marked in the micro- and meso-sizes at Station I, and only one peak (pico-size) at Station ST. Biomass each plankton group decreased with increasing depth, and their declining patterns below 100 m depth were well described by a negative power function, with different slopes between groups and also between stations. Within stations, the slope was the greatest for mesozooplankton, followed by phytoplankton and bacteria or protozooplankton. From correlation analyses between the biomass of the four major groups of plankton organisms, a close relation was observed between bacteria and protozooplankton. This bacteria–protozooplankton link, combined with the results of the depth-related changes in the abundance of each major group and size spectrum, is discussed in the light of regional and bathymetric differences in the structure and functioning of plankton community contributing to the ‘biological pump’ in the western North Pacific Ocean.