| The South China Sea is characterized by low surface nutrient concentrations and low phytoplankton biomass.Despite the oligotrophic environment,the northern continental slope area of the South China Sea possesses a highly intensive mesopelagic fish resource.Plankton provides food for mesopelagic fish.It is of interest how the plankton community structure and energy flux were.Body size is a well-established key plankton index.It can reflect physiological and ecological processes such as metabolic processes and predator-prey relationships.Size-based normalized biomass size spectra(NBSS)are attracting more and more attention as an important alternative means for the study of plankton community characteristics.The slopes of NBSS can reveal community structure and ecological conversion efficiency.The study of NBSS in China and the application of NBSS in continental slope area were little documented.Here NBSS was used to study plankton community structure and energy flow of northern continental slope area of the South China Sea.In this study,macro-,meso-and micro-plankton net and multi-net were used to sample plankton to construct plankton normalized biomass size spectra above 200 m depth and stratified water plankton normalized biomass size spectra.The seasonal and diel variability,horizontal and vertical variability of NBSS were revealed,the differences of zooplankton community energy flux among tropical and subtropical seas were compared,and the relationship between NBSS slopes and environmental factors was analyzed.The results are as followed:In summer,there were 39 phytoplankton groups in phytoplankton community.The dominant groups were Chaetoceros,Rhizosolenia,Synedra,Thalassionema,Navicula and Oscillatoria.The average phytoplankton abundance was 3.74×104cells/m3.The Shannon-Wiener index and Evenness index was 2.87and0.70,respectively.There were 16 zooplankton groups in zooplankton community,and the dominant groups were Copepoda,Tunicate,Gastropod and Amphipoda.The average zooplankton abundance was 0.19×104cells/m3.The Shannon-Wiener index and Evenness index was 1.44and0.40,respectively.In autumn,there were 11 phytoplankton groups and 11 zooplankton groups.The dominant groups in phytoplankton community were Chaetoceros,Rhizosolenia,Coscinodiscus,Thalassionema,Ceratium and Noctiluca.And dominant groups in zooplankton community were Copepoda,Gastropod,Chaetognatha,Cladocera,Egg,Euphausiacea and Foraminiferida.The average abundance of phytoplankton and zooplankton was 0.39×104cells/m3 and 0.05×104cells/m3,respectively.And the Shannon-Wiener index and Evenness index in phytoplankton and zooplankton communities were 2.15and0.83,1.89 and 0.57,respectively.High phytoplankton abundance was found near Pearl River Estuary stations,while there was no regularity of high zooplankton abundance.The plankton community average NBSS slopes above 200 m water were ?0.93(–1.09 ~ –0.61)in summer and –0.69(–0.76 ~ –0.50)in autumn,respectively.The NBSS slope range of stratified water zooplankton were ?1.03 ~ ?0.20 in summer and ?1.38 ~ ?0.38 in autumn.Compared with autumn,there were more plankton groups,higher biomass,steeper NBSS slopes,a lower proportion of large organisms,and a lower ecological conversion efficiency from small to large plankton in summer.The diel vertical migration of zooplankton was observed from diel NBSS slopes in summer and autumn.Zooplankton vertical migration could be found at about 0~600 m.The diel NBSS pattern of plankton community was similar in summer and autumn.In daytime,the NBSS slope was flatter,proportion of large organisms was higher.Slopes from inshore to offshore stations tended to be more stable and flatter.And there was a tendency for less negative slopes and higher proportion of larger plankton with increasing depth.Compared with other tropical and subtropical seas,there was higher proportion of large zooplankton,steeper NBSS slopes in the northern continental slope area of the South China Sea.The ecological conversion efficiency was at a middle level among tropical and subtropical seas.The NBSS slope in the northern continental slope area of the South China Sea in summer was similar to the NBSS slopes of coastal regions,such as Brazilian Continental Shelf Coastal stations and the neighboring waters of Japan,while the NBSS slope in autumn was similar to the NBSS slopes of Oceanic stations,Brazilian Continental Shelf Oceanic stations and Western Pacific.There was a significant negative correlation between NBSS slopes and temperature(r = –0.5065,P< 0.0001),and no correlation was found between NBSS slopes with salinity and chlorophyll a.NBSS slopes in high primary productivity area were flatter than those in low primary productivity area,and the ecological conversion efficiency was higher in high primary productivity area.This study showed that NBSS provides an effective approach to the study of plankton community structure and energy flow of the northern continental slope area of the South China Sea.Form the NBSS,the seasonal distribution pattern was revealed,the zooplankton diel vertical migration and the predation relationship were well described.This study provides more data to reveal the distribution and function of plankton community in northern slope of the South China Sea. |
