Temporal-spatial Variation Of Phytoplankton And Zooplankton Community And Its Relationship With The Particulate Organic Carbon Export In The Northern South China Sea Basin

The marine biological pump is an essential process in carbon fixation and carbon sequestration in the marine ecosystem,and the plankton plays a key role in marine biological pump.Therefore,it is important to study the temporal-spatial variations of plankton community structure and its relationship with particulate organic carbon(POC)export.This study focuses on the northern South China Sea(NSCS),conducted researches at the NSCS basin and Luzon strait from January to July 2018 and from June to July 2019 respectively.To elucidate the temporal spatial variations of plankton communities in the NSCS and also evaluate the impact from Kuroshio Intrusion(KI)and seasonality,this study uses High Performance Liquid Chromatography(HPLCCHEMTAX)and Flow Cytometry(FCM)to analyze the phytoplankton biomass and its community structure in full size category,and the zooplankton samples were towed vertically in discrete layers or in single layer,combined with the ZooScan analysis to determine the abundance and community composition of mesozooplankton.Based on floating sediment traps and Underwater Vision Profiler(UVP5)to analyze the particle distribution pattern and particle composition in the NSCS,to investigate the variation of POC export flux in the euphoric zone and twilight zone,explore the influence of lateral advection on POC export.The results summarized as follows:The Prochlorococcus,Synechococcus,prasinophytes and haptophytes₈ are dominant groups both in winter and summer at NSCS basin.TChl-a concentration exhibited clear temporal-spatial variation,with the value higher in winter than in the summer,higher in coastal area than in the basin.In vertical profile,the seasonal variation of Prochlorococcus is not clear,and detected high Chl-a concentration even in DCM layer(DCML);the surface layer of Chl-a concentration in Synechococcus is higher in winter than in the summer,but in summer the DCML was deeper;the pattern for prasinophytes in surface layer is higher in winter than in the summer;the vertivcal distribution of haptophytes₈ was in higher concentration at subsurface layer,which is consistent with the pattern of picoeukaryotes in higher cell abundance at subsurface layer based on FCM.The mesozooplankton abundance in the upper 200 m was higher in summer than in the winter,higher in Luzon Strait area than in the basin area,the averaged mesozooplankton abundance in winter was 115.4±75.9 ind.m-3,with 480.7±255.5 ind.m-3 in the summer,the abundance was significantly different between winter and summer(t-test,p<0.05).Adopting the two end-member isopycnal mixing model,the clear response of plankton community to the KI was identified.In the winter and summer in 2018,the TChl-a biomass was clearly increased during KI,the peak value appears in the area with Rk-100ave value around 50%?60%.From the perspective of plankton community structure,in winter,tracking the increase of Rk-100ave value,the major groups of phytoplankton in which its portion was increased including prasinophytes,haptophytes₈ and haptophytes₆,and the portion of Prochlorococcus and Synechococcus was decreased,but in summer the response of phytoplankton to KI in terms of community structure was not clear as in winter,among stations with Rk100avc>60%and stations with Rk-100ave<60%,the phytoplankton biomass and community structure were significantly different(p<0.01,p<0.05).For zooplankton,the contribution of Copepoda to total zooplankton abundance was increased following the increase of Rk-100ave,ranged from 79.5%?90.6%,but in summer the response of zooplankton community composition to KI was not clear.The observation in summer 2019 revealed that phytoplankton community structrure clearly displayed a pattern of diatom dominance in coast,Synechococcus dominance in surface,Prochlorococcus dominance in open ocean;the zooplankton abundance in basin area showed a subsurface maximum,and was decreased with increasing depth in the slope area,the dominance of Copepoda was varied with changing depths,the Foraminifera exhibits opposite distribution pattern at surface and subsurface layer in the basin and slope area.Based on floating sediment traps,the 50 m,100 m and 200 m POC export flux at Sta.SEATS was 46.5 mg C m-2 d-1?53.5 mg C m-2 d-1 and 35.1 mg C m-2 d-1 repectively,and was 66 mg C m-2 d-1?76 mg C m-2 d-1 and 49.8 mg C m-2 d1 at Sta.M4,it is both comparable and consistent with the POC export flux value which derived from the UVP5,and we observed a high POC export flux even at the depth of 450?750 m,possibly impacted by the lateral advection.In the basin area,the particle was contributed most by detritus,and the linear fitting relationship was also better between particle concentration and Chl-a concentration.