Seasonal Variations in Structure and Distribution of Zooplankton Community in Vostok Bay (Peter the Great Bay, Northwestern Sea of Japan)

The composition and spatial distribution of the zooplankton abundance and biomass in Vostok Bay during the period from July to November 2023 were analyzed. A total of 37 zooplankton taxa including neritic (71%) and oceanic (29%) species were identified. Copepods (Acartia hudsonica, Oithona brevicornis, O. similis, Paracalanus parvus, and Pseudocalanus newmani) and cladocerans (Penilia avirostris and Podon leuckartii) contributed more than 70% to the total zooplankton abundance and biomass. In the early summer, with southeasterly winds and freshening of surface water, the cold-water copepods A. hudsonica, P. newmani, and O. similis and the cladoceran Podon leuckartii dominated the total abundance and biomass; brackish-water copepods of the genera Centropages, Tortanus, and Sinocalanus were more abundant in the early summer than in the other seasons. In the late summer, with maximum warming of water in the bay, the warm-water copepods P. parvus, O. brevicornis and the cladoceran P. avirostris dominated the total abundance, while the copepod P. parvus, the cladoceran P. avirostris, and chaetognaths dominated the total biomass. In the fall, as the water cooled down to 8–11°C, with northerly winds and the change of the summer monsoon to the winter one, the copepods P. newmani, O. brevicornis, O. similis and copepod nauplii dominated the total abundance, while the copepods P. newmani, O. brevicornis,and chaetognaths dominated the total biomass. Three types of zooplankton assemblages (estuarine, mixed, and marine) spatially coincided with the shallow, transitional, and deep parts of the bay. In the early summer, the total abundance and biomass increased in the direction from the deep to the transitional parts; in the late summer and fall, the values of these parameters increased, vice versa, from the shallow toward the deep parts. The 25-year dynamics of the total zooplankton abundance and biomass during the summer and fall seasons showed a negative trend with a pronounced shift to lower values. A decrease was recorded in the contribution of large-sized and tropical/subtropical species such as copepods of the genera Calanus, Eucalanus, Metridia, Labidocera, Sapphirina, Scolecithricella, chaetognaths, gammarids, hyperiids, isopods, and salpids to the total zooplankton abundance.