| Summary: | Analyses by Optical Plankton Counter (OPC) were made on mesozooplankton samples (n = 1,396) collected by NORPAC net from 0-150 m along eight north-south transects (located 35-60°N, 144°E-145°W) covering whole area of North Pacific during May to August of 1981-2009. Based on OPC data, spatial (north-south and east-west) changes in mesozooplankton abundance, biovolume and Normalized Biovolume Size Spectra (NBSS) were evaluated. Throughout the transects, north-south variations in zooplankton communities were similar : i.e. abundance was greater in the subarctic (SA) region, biovolume was greater in the transitional (TR) region and slope of NBSS was moderate in TR. High biovolume and moderate NBSS slope in TR were caused by the dominance of late copepodid stages (C5) of Neocalanus copepods. While zooplankton fauna was similar through SA and TR (subarctic fauna), high temperature in TR could be accelerate copepod growth, and induce dominance of Neocalanus C5 there. Concerning east-west differences, zooplankton abundance and biovolume tended to be greater in the western transects. High zooplankton abundance and biovolume in the western regions would be caused by the east-west differences in primary productivity, which reported to be reflection of the east-west gradients in iron concentration.
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