Environmental factors controlling zooplankton accumulation in the near-surface layer in the western North Pacific marginal seas

Abstract

Vertical distribution of zooplankton abundance is essential for understanding the linking primary production to higher-trophic marine organisms. In this study, we investigated the zooplankton abundance at the near-surface layer (0–20 m depth; ZA 20 ) and those at 0–200 m depth (ZA 200 ) from 240 stations in the Sea of Japan (SOJ) and the East China Sea–Kuroshio (ECSK) area during summer to evaluate the environmental parameters controlling zooplankton accumulation in the near-surface layer. The total numerical ZA 20 was 22 ± 15% (mean ± standard deviation) and 15 ± 11% of ZA 200 in the SOJ and ECSK, respectively. Temperature and chlorophyll a concentration at 20 m depth, as well as the sampling time, had a significant impact on the degree of surface accumulation (ZA 20 : ZA 200 ratio) in a generalized additive model (GAM); the ZA 20 : ZA 200 ratio increased in cold and eutrophic conditions and during the nighttime. The difference between SOJ and ECSK was insignificant, while the zooplankton communities were significantly different between the areas. Therefore, the vertical distribution of zooplankton is seamlessly explained by environmental parameters when setting the zooplankton as one ecological functional group. Members belonging to six orders, Calanoida, Harpacticoida, Cyclopoida, Ctenopoda ( Penilia avirostris ), Onychopoda (family Podonidae ), and Copelata (appendicularian), were dominant based on microscopic observations. When the ZA 20 : ZA 200 ratio was estimated at the order level, the responses to temperature differed between copepods and non-copepods groups: the copepods stay in the subsurface water when the surface water is warm. • We compared the zooplankton abundance in the ≤20 m and ≤ 200 m depth. • Zooplankton accumulated the ≤20 m depth in cold and eutrophic water and at night. • Almost a quarter of zooplankton was in the ≤20 m depth. • Copepods stayed in the subsurface water when the surface water is warm.