Seasonal changes in the zooplankton community and population structure in the northern Bering Sea from June to September, 2017

Zooplankton community structure in the northern Bering Sea may change significantly over relatively short periods due to the inflow of different water masses and the seasonal release of meroplankton, although details of these changes are still unclear. We studied the zooplankton community in the northern Bering Sea from June to September of 2017 and examined seasonal changes in the community structure and stage structure of the dominant species. Zooplankton abundance ranged from 41,000 to 928,000 ind. m(-2), with the greatest abundances near 174 degrees W during July. Copepods were the dominant taxa, comprising 10-98% of zooplankton abundance, with benthic larvae such as bivalves dominant at some stations during July and August. Cluster analysis of abundances divided the station/zooplankton communities into seven groups. West of 172 degrees W, clear seasonal changes were not observed, because the Bering Chukchi Winter Water persisted in the deep layer and sampling was only conducted in this region in July and August. In contrast, the community structures east of 172 degrees W differed every month due to water masses changes, meroplankton release, and copepod production associated with the phytoplankton bloom. Despite the changes of water mass, development for the dominant large copepods (Calanus glacialis/marshallae, Eucalanus bungii and Metridia pacifica) was revealed from their population stage structures. Seasonal shifts in species within Neocalanus and appendicularians were driven by water mass exchanges. This study demonstrates that zooplankton community in the northern Bering Sea varies substantially on a monthly time scale. Therefore, to evaluate the impact of climate change on zooplankton, it is important to consider both the seasonal period and the dominant water masses present.