Plankton abundance, biovolume, and normalized biovolume size spectra in the northern slope of the South China Sea in autumn 2014 and summer 2015

Plankton abundance, biovolume and distribution in the northern slope of the South China Sea in summer 2015 and autumn 2014 were assessed by using flow cytometry and microscopy (FlowCAM) and ZooScan methods. Copepoda was the most dominant zooplankton in both seasons. Rhizosolenia dominated the phytoplankton community in autumn. In summer, the dominant phytoplankton were Chaetoceros and Thalassionema. In autumn, the abundances of phytoplankton and zooplankton ranged from 0.66 x 10(3)-7.83 x 10(3) ind. m(-3) and 0.34 x 10(3) 0.88 x 10(3) ind. m(-3), respectively, whereas the biovolumes of phytoplankton and zooplankton ranged from 41.67 to 204.66 mm(3)m(-3) and 56.19-157.24 mm(3)m(-3), respectively. A wide range of normalized biovolume size spectra (NBSS), consisting of phytoplankton, mesozooplankton and macrozooplankton, were built, and the resulting NBSS slopes were flatter than the single zooplankton NBSS slope. The NBSS slopes were steeper in summer (mean +/- SE = - 0.93 +/- 0.04) than in autumn (mean +/- SE = - 0.69 +/- 0.05). Based on the biovolume (0.00024mm(3)-131.07mm(3)) of plankton in 19 size classes and the taxonomic groups in each size class, Bray-Curtis cluster analysis divided the plankton into five groups. The distributions of the five plankton groups coincided with changes in currents during autumn and summer. Groups A and B were distributed in a region with an anticyclonic and a cyclonic eddy, respectively. Group A was affected by SCSW (South China Sea Water) and KW (Kuroshio Water) and had a NBSS slope of nearly -1. Group B was influenced by SCSW, KW and SHW (Shelf Water), and the NBSS slope of group B showed a higher ecological transfer efficiency than that of group A. Group C and group D were influenced by westward currents. The sea surface temperature (SST) (r = -0.52, P < 0.01), chlorophyll a concentration at the depth of the chlorophyll maximum (SMChl-a) (r= -0.61, P < 0.01), and average chlorophyll a concentration (SAChl-a) (r = -0.59, P < 0.01) were negatively correlated with the NBSS slope.