East-west comparison of the zooplankton community in the subarctic Pacific during summers of 2003-2006

The subarctic Pacific is known to have east-west gradients in the oceanic environment and phytoplankton community. The western subarctic Pacific is characterized by low temperature and high chlorophyll-a while the eastern region by high temperature and low chlorophyll-a. Although there is little information on the differences in the zooplankton community between the eastern and western subarctic Pacific, the gradients in the oceanographic environment and phytoplankton community may markedly affect the zooplankton community in this region. The aim of this study is to clarify east-west differences in the subarctic Pacific zooplankton community. Zooplankton were sampled at stations along the 165°E line (western subarctic Pacific from 41°30'N to 49°30'N) and 165°W line (eastern subarctic Pacific from 39°N to 53°30'N) using 0.335 and 0.100 mm mesh size Twin NORPAC net during the summers of 2003-2006. East-west differences in the zooplankton community were characterized as: 1) greater total zooplankton abundance in the west-and 2) larger body size of calanoid copepods within same copepodid stage in the west. Differences in east-west zooplankton abundances are attributed to differences in the magnitude of primary production (high in the west) and the size of primary producers (large in the west). Larger body sizes of calanoid copepods in the west are attributed to the lower temperature. Thus, differences in zooplankton abundance and body size are concluded to be due to east-west gradients in the oceanographic environment and phytoplankton community.