Vertical variations in zooplankton size spectra down to 3,000 m depth and significant effects of the sizes of Calanoida and Ergasilida across the subarctic, transitional, and subtropical regions of the western North Pacific

Zooplankton size spectra are crucial for evaluating marine ecosystem structure, with copepods being the predominant taxa within the marine zooplankton communities. Among copepods, Calanoida and Ergasilida are the two numerically dominant taxa. Despite their importance, limited information exists regarding the effects of these two taxa on the overall zooplankton size spectra. This study aimed to comprehensively evaluate the regional and vertical changes in zooplankton size spectra, normalised biomass size spectrum (NBSS), and size diversity across subarctic, transitional, and subtropical regions of the western North Pacific Ocean. Additionally, the study aimed to investigate the effects of size variations in two dominant copepod taxa, Calanoida and Ergasilida, on the overall zooplankton size spectra. To achieve this, vertically stratified zooplankton samples were collected from 12 layers, ranging from the sea surface to a depth of 3000 m, at five stations across the subtropical to subarctic western North Pacific. The samples were analysed using ZooScan to assess the overall zooplankton size spectra and the effects of Calanoida and Ergasilida on it. Across all stations, the NBSS slopes became moderate, and the size diversity increased with increasing depth, particularly evident for the oxygen minimum layer (OML) at approximately 1000 m depth. These patterns reflect a high proportion of large-sized zooplankton in the deeper layers and a lower predation pressure from micronektonic fish around the OML. Calanoida and Ergasilida accounted for 43.3% and 24.6% of the mean zooplankton abundance, respectively. Among the two taxa, Calanoida exhibited significant changes in body size depending on the station and depth, being larger in the subarctic region and deeper layers. Ergasilida showed minimal changes in body size relative to location and depth. The body size of Calanoida significantly influenced the overall zooplankton size spectra. Therefore, the dominance of large-sized Calanoida induced a moderate NBSS slope and high size diversity in the overall zooplankton size spectra. The results of this study indicate that the size of Calanoida, the dominant taxon in the zooplankton community, primarily governs the size spectra of the overall zooplankton community.