Effects of water masses on the zooplankton community structure in the northern East China Sea during the East Asian Summer Monsoon in 2020

Abstract

The prolonged rainy season in 2020 was classified as the second-largest rainfall event since 1961. In this study, we conducted a survey to assess fluctuations in the zooplankton community structure of the northern East China Sea (nECS) after the rainy season in August 2020. We examined 24 stations and observed several environmental factors, including water temperature, salinity, chlorophyll-a concentration, suspended particulate matter (SPM), and phytoplankton density. Zooplankton samples were collected from 12 stations using MOCNESS to target the surface mixed layer (SML), pycnocline layer (PNC), and lower layer (LL). In regions with low water temperatures, the Yellow Sea and northern East China Sea shelf mixed water community in each layer exhibited a decrease in the relative abundance of Calanus sinicus, whereas the incidence of Paracalanus parvus s. l. showed an increasing trend. Similarly, in areas with low salinity, the CDW and northern East China Sea shelf mixed water community in SML and PNC displayed comparable distributions, with a tendency for the relative abundance of Acartia pacifica and Paracalanus parvus s. l. to show a replacement pattern. Moreover, the Tsushima Warm Current (TWC) community in each layer, characterized by high water temperature and salinity, showed a high relative abundance of Oncaea spp. and a diverse species distribution, mainly in the eastern part of the study area. Our study identified multiple vertically distributed communities, providing detailed information on water mass structures through cluster analysis. This approach enhanced previous studies by offering novel insights into the vertical structure of water masses and their association with zooplankton communities in the nECS.