Effects of an anticyclonic eddy on the distribution and community structure of zooplankton in the South China Sea northern slope

Mesoscale anticyclonic eddies have been frequently observed in the South China Sea (SCS) northern slope. Here we present the first multidisciplinary cruise conducted in an anticyclonic eddy to characterize zooplankton community structure in the northern SCS during March 2017. In-situ net sampling combined with a ZooScan integrated system was used to determine the zooplankton biomass, abundances, and biovolumes in four size classes (0.2-0.5 mm, 0.5-1 mm, 1-2 mm, and 2-5 mm). According to the temperature and salinity, the stations were divided into three groups, corresponding with the inside, edge, and outside of the anticyclonic eddy, respectively. Elevated zooplankton biomass, abundances, and biovolumes were clearly observed at the edge and outside of the anticyclonic eddy compared to the inner eddy area. This was associated with the significantly increased proportions of zooplankton in the 1-2 mm size class. Copepoda were the major contributors to the zooplankton, with 68.7% of the total abundance and 51.5% of the total biovolume. The analysis of similarity revealed that the zooplankton size composition and taxonomic composition were significantly different between the inner and outer areas of the anticyclonic eddy. In addition, we also found obvious diel variations for the zooplankton, particularly in the inner anticyclonic eddy area. Shrimp showed significantly diel variations (0-200 m) in this region. The anticyclonic eddy substantially influenced the zooplankton distribution and community structure in the SCS northern slope during March 2017.