Zooplankton Taxonomic Structure And Its Influencing Factors In The South China Sea Northern Slope

Zooplankton plays key component of marine planktonic food webs,material circulation,and energy flow.The biomass and taxonomic structure of zooplankton affect largely the efficiency of carbon transportation.Mesoscale eddies have been frequently observed in the northern South China Sea(SCS)and affect the marine ecosystems.Here we focused on zooplankton and conducted three cruises in the SCS northern slope during spring,summer and autumn.Zooplankton biomass,size composition and taxonomic structure were analyzed with a ZooScan integrated system and discussed their relationships with the hydrological,chemical and chlorophyll a parameters.In spring,an anticyclonic eddy affected the SCS northern slope.The sampling stations were divided into inside,at the edge and outside the anticyclonic eddy according to the satellite images,in-situ temperature and salinity.The zooplankton wet weight,dry weight and carbon weight were 43.32±20.44 mg m^-3,3.39±2.14 mg m^-3 and 1.36±0.83 mg C m^-3,respectively.The zooplankton biomass were significantly lower in the inside of anticyclonic eddy than in the edge or in the outside of anticyclonic eddy(p<0.05).The zooplankton abundance and biovolume were 573.2±284.8 ind.m^-3 and 92.2±38.9 mm³ m^-3,respectively.The zooplankton abundance and biovolume were significantly lower in the inside of anticyclonic eddy than in the outside of anticyclonic eddy(p<0.05).Zooplankton in the 0.2-0.5 mm size class dominated the zooplankton abundance with a mean proportion of 64.9%.The proportion of zooplankton abundance decreases with the increasing size.However,larger zooplankton contributed higher biovolume percentage compared to small organisms.The biovolume proportion in the 2-5 mm and 1-2 mm size classes were39.5%and 33.3%,respectively.The size compositions were significantly different between the inside of anticyclonic eddy and the outside of anticyclonic eddy(p<0.05).Copepoda,Noctiluca and Tunicata dominated the zooplankton abundance,with mean proportion of 69.1%,7.0%and 3.4%,respectively.Copepoda,Chaetognatha and Medusa dominated the zooplankton biovolume,with mean proportion of 52.8%,17.1%and 8.3%.The taxonomic compositions were significantly different between the inside of anticyclonic eddy and the outside of anticyclonic eddy(p<0.05).The zooplankton taxonomic compositions showed different patterns between the size classes.Copepoda made a major contribution in small size classes,but with a lower contribution in large size classes.In contrast,Chaetognatha and Medusa made higher contributions in large size classes.The results indicated that the zooplankton in smaller size classes was obviously affected by the anticyclonic eddy.We also found obvious diel variations of zooplankton in the larger size classes.In addition,diel variations of Copepoda,Amphipoda and Ostracoda were observed,but only Shrimp was significantly increased during night(p<0.05).In summer,a mesoscale dipole eddies affected the SCS northern slope.The sampling stations were divided into the anticyclonic eddy area,frontal area,cyclonic eddy area and shelf area according to the satellite images,in-situ data.The zooplankton wet weight and dry weight in SCS northern slope were 63.55±31.92 mg m^-3 and 9.13±4.75 mg m^-3,respectively.The zooplankton abundance and biovolume were 1328.6±842.2 ind.m^-3 and 156.3±100.6 mm³ m^-3,respectively.Lower zooplankton abundance and biovolume was observed in anticyclonic eddy than in shelf area,frontal area and cyclonic eddy.Significant difference of zooplankton abundance and biovolume was found between the anticyclonic eddy and the cyclonic eddy(p<0.05).The proportion of zooplankton abundance in the 0.2-0.5 mm size class was 72.4%and the proportion in shelf area and frontal area were higher than in anticyclonic eddy(p<0.05).The biovolume proportion in the 1-2 mm and 2-5 mm size classes were 35.9%and 31.3%,respectively.Copepoda,Egg and Noctiluca were the most abundant taxa,with mean proportions of 67.0%,4.5%and 3.8%,respectively.Copepoda,Chaetognatha and Medusa dominated the zooplankton biovolume,with mean proportions of 50.3%,14.5%and 9.6%,respectively.The ANOSIM analysis showed that the zooplankton taxonomic compositions was significantly different between the shelf area and the anticyclonic eddy(p<0.05).The zooplankton taxonomic compositions showed different patterns between the size classes.Copepoda made a major contribution in small size classes and the proportion decreases with the increasing size.In contrast,Chaetognatha,Medusa and Shrimp made higher contributions in large size classes.The vertical distribution of zooplankton abundance and biovolume showed that higher values concentrated on 0-100 m layer.Furthermore,the proportion of zooplankton in smaller size class was higher in upper layer,while the proportion of the zooplankton in larger size class was higher in deeper layer.In autumn,the zooplankton wet weight,dry weight and carbon weight in SCS northern slope were 72.97±41.63 mg m^-3,10.41±5.07 mg m^-3 and 3.49±1.72 mg C m^-3,respectively.Higher zooplankton biomass was located in the northwestern regions.The zooplankton abundance and biovolume were 662.5±259.9 ind.m^-3 and112.7±35.8 mm³ m^-3,respectively.Higher zooplankton biovolume was located in the southeastern regions.The proportion of zooplankton abundance in 0.3-0.5 mm and0.5-1 mm size classes were higher,with mean proportion of 57.1%and 35.8%,respectively.The proportion of zooplankton biovolume in 1-2 mm and 2-5 mm size classes were higher,with mean proportion of 33.2%and 32.1%,respectively.Copepoda,Foraminifera and Chaetognatha were the most abundant taxa,with mean proportions of 65.3%,8.0%and 4.5%,respectively.Copepoda,Chaetognatha and Shrimp dominated the zooplankton biovolume,with mean proportions of 37.6%,16.4%and 8.5%,respectively.Copepoda made a much higher proportion in small size classes and the proportion decreases with the increasing size.In contrast,Chaetognatha,Medusa and Shrimp made higher contributions in large size classes.The vertical distribution of zooplankton abundance and biovolume showed that higher values concentrated on 0-100 m layer.In addition,the proportions of zooplankton abundance and biovolume in smaller size class were higher in upper layer,while the proportion of the zooplankton in larger size class was higher in deeper layer.The results also showed that the zooplankton abundance and biovolume were higher at night than at day in the upper layer,while the opposite diel variation was in the deeper layer.Furthermore,we also found obvious diel variations of zooplankton in the larger size classes.Diel variations of Copepoda,Chaetognatha and Ostracoda were observed,but only Shrimp biovolume was significantly increased at night(p<0.05).In general,the seasonal variations of zooplankton wet weight and dry weight in the SCS northern slope were autumn>summer>spring,with averaged values of58.65±33.16 mg m^-3 and 7.35±5.06 mg m^-3,respectively.The seasonal variations of zooplankton abundance and biovolume were summer>autumn>spring,with averaged values of 872.2±645.0 ind.m^-3 and 121.1±71.9 mm³ m^-3,respectively.The seasonal difference of zooplankton biomass and abundance was mainly due to the different physical progresses,which resulted the difference of size composition and taxonomic structure of zooplankton.The lowest zooplankton level in spring could be attributed to the anticyclonic eddy with oligotrophic conditions.In summer,the amount of fresh water of Pearl River was transported to our sampling stations due to mesoscale dipole eddies,which associated with higher proportion zooplankton in smaller size classes and the higher proportion of gelatinous zooplankton.However,the zooplankton in autumn was just the opposite of summer.