Geographical and seasonal variations of copepod communities from Ise-Mikawa Bay to open-ocean waters affected by the warm Kuroshio Current

Abstract

Copepods play a crucial role in coastal ecosystems and support fisheries production. This study revealed distinct spatial and seasonal variations in copepod communities in and around semi-enclosed Ise-Mikawa Bay, Japan. A warm-water community (Group I) dominated the outer bay throughout the year, whereas the dominant communities in the inner bay underwent strong seasonal transitions. From winter to early spring, a cold-water-period community (Group IIA) prevailed. Group IIA was followed by a spring shelf-slope water community (Group IIB). In summer, a mixed community of cold- and warm-water species (Group IIC) appeared. The variations of the duration of dominance by these inner-bay communities between Ise Bay and Mikawa Bay reflected the stronger estuarine characteristics of Mikawa Bay. In autumn, copepod communities in the inner bay shifted to warm-water communities (Group I); an inner-bay community (IC) prevailed in September, and a coastal-to-open-ocean community (IB) prevailed in November. These spatiotemporal shifts were shaped by intrusion of open-ocean water, freshwater input, and seasonal changes of environmental factors such as temperature, salinity, and dissolved oxygen concentrations, particularly in the bay. The distribution of the genus Acartia, which frequently dominates in shallow, estuarine environments and has evolved a life cycle involving temperature-cued resting eggs, responded to species-specific, fine-scale conditions and provided a valuable indicator of seasonal and spatial environmental changes, freshwater discharge, open-ocean water exchange, and potential climate change impacts in the bay. This study highlighted complex hydrographic-biological interactions, the sensitivity of planktonic communities to physical and chemical environmental changes, and the spatial heterogeneity in semi-enclosed coastal ecosystems.