Zooplankton abundance, biovolume and size structure in the Yellow Sea and East China Sea in summer of 2017

Zooplankton abundance, biovolume, and size structure were assessed in the Yellow Sea (YS) and East China Sea (ECS) in summer of 2017 using ZooScan imaging analysis. Generally, zooplankton abundance and biovolume showed similar spatial patterns. In the YS, zooplankton abundance and biovolume were higher in the northern and southern regions. In the ECS, lower levels of zooplankton abundance and biovolume were observed in the offshore regions. Based on the biovolume of the categorized size classes of 26 identified taxonomic groups, the zooplankton community was classified into five groups, and each group was coupled with distinctive oceanographic features. Under the impacts of the Yellow Sea Cold Water Mass (YSCWM), the Yellow Sea offshore group (YSO) was characterized by the lowest bottom temperature and the most abundant large size Calanoids (2-3 mm size class). In the Yellow Sea inshore group (YSI), Hydrozoans showed the largest biovolume and dominated in the 3-4 mm and >5 mm size classes. The East China Sea offshore group (ECSO), which was mainly affected by the Kuroshio Branch Current, featured high temperature and salinity, and the lowest bottom dissolved oxygen. The lowest values of zooplankton abundance and biovolume in the ECSO group might be attributed to the low bottom dissolved oxygen. The East China Sea inshore group (ECSI), which was mainly influenced by the Zhejiang-Fujian Coastal Current (ZFCC) and Changjiang Diluted Water (CDW), was characterized by high Chlorophyll a (Chl a) and the largest biovolume of Siphonophores. The Changjiang River Estuary offshore group (CREO) showed the most abundant Cyclopoids, which might be associated with the less turbid water mass in this region.