Seasonal changes in zooplankton swimmer community collected by sediment trap moored in the western North Pacific Ocean

For high-latitude oceanic region, life cycle of zooplankton is difficult to evaluate by ordinary ship-board observation. To overcome this problem, zooplankton monitoring on swimmer samples collected by sediment trap may be a powerful tool. In this study, we studied seasonal changes in zooplankton community based on the swimmer samples (>1 mm) collected by a sediment trap moored at 200 m depth at St. K2 (47°N, 160°E) in the western subarctic Pacific with one- to two-week intervals during July 2013 to May 2014. Zooplankton abundance and biomass showed clear seasonal pattern, and were higher during July-August. Cluster analysis (Bray-Curtis methods) separated samples into three groups. Occurrence of each group had clear seasonal pattern: i.e. group A characterized with high abundance with dominance of copepods Eucalanus bungii and Neocalanus plumchrus occurred during July to September, followed by group B with few abundance dominated by chaetognaths during October to December, then group C dominated by Neocalanus cristatus and Paraeuchaeta elongata during January to March. For dominant copepods, seasonal changes in population structure, lipid content and gonad developmental stage were observed. Thus, most males of E. bungii were C4 and C5 until February, while the composition of adults (C6M) suddenly increased and reached 80% at end of March. These drastic changes in copepod population structure are considered as a reflection of their arousal from diapause at that depth. Carnivorous P. elongata showed high abundance during March to July, and both egg-sac-carrying and spermatophore-attached adult females (C6F) were occurred during that period. These facts suggest that active reproduction of P. elongata was at that season. Results of this study suggest that seasonal monitoring on zooplankton swimmer collected by sediment trap is a powerful tool to evaluate life cycle of the oceanic zooplankton species.