| Siliceous organisms uptake dissolved silicate(DSi)and form siliceous frustules converting DSi to biogenic silica(BSi).Due to its slow dissolution rate,high preservation efficiency and it can facilitate carbon fixation,BSi is an effective biomarker in reconstructing past environmental changes and exploring the carbon cycle.Mesoscale cyclonic eddies,ubiquitous oceanic feature pump new nutrients from the depth to the euphotic zone,which stimulate the growth of larger phytoplankton such as diatoms,and enhance primary production,which is often followed by enhanced export of particulate organic matter.However,BSi responds to the different lifecycle of the cyclonic eddy and the coupling relationship between Si and C are still many uncertainties.This research conducts high-resolution sampling of suspended BSi particulates in two cyclonic eddies,CE2 and CE4;observed in oligotrophic waters of the Western Pacific Ocean.The concentration and spatio-temporal distributional characteristics of BSi has been studied to explore controlling mechanisms.Comparing BSi/234Th ratio and BSi export flux with different sampling techniques to enable a reliable estimation of BSi export flux.Finally,a conceptual model was constructed to describe the response of the BSi with respect to mesoscale cyclonic eddies in Western Pacific Ocean.The cruise "KK1902-SILICON" conducted 4 transect surveys from the 3rd to 6th week of cyclonic eddy CE2 and 1 transect survey on CE4 from March to April 2019 in western pacific.The average concentration of BSi was 5.39±3.75 nmol/L(0.49～24.82 nmol/L).Compared with the outside and warm eddy stations,the concentration of BSi in the cold eddy increased at the DCM(Depth of chlorophyll a maximum)(90～150 m),and was about 1-3 times that in the outside and warm eddy.The surface BSi concentration demonstrates no significant difference.However,the value at the DCM decreased from 14.40 nmol/L in the 3rd week to 8.43 nmol/L in the 4th week,then reached to lowest 5.72 nmol/L in the 5th week,with an increase of 13.76 nmol/L in the 6th week.Spatially,the BSi concentration in the core is approximately 2～3 times that in the edge of DCM.In addition,it is found that the fronts of the cold eddy may promote siliceous organisms flourish.The nutrients basis of biological growth is mainly dominated by physical upwelling,strong upwelling leads to the higher BSi concentration in the core,but has no significant differences between the core and edge when upwelling is weaker.Nutrient concentrations above the DCM are lower than the minimum threshold for growth,showing typical oligotrophic characteristics,and Si:N:P was(3 8±23):(4±3):1(CE2);(25±1):8:1(CE4),which indicated the diatom growth is limited by nitrogen or phosphorus.The correlation between BSi and TChl a standing stock is weak,indicating that the small-sized organisms were the majority of the phytoplankton biomass rather than BSi.This research also confirmed that the difference in BSi/234Th ratio and BSi export flux through different sampling procedures(bottle,in situ pump and sediment trap),were due to the decoupling between BSi and 234Th in particle size.BSi export flux ranged from-0.011 ± 0.001～0.074±0.008 mmol/m2/d.The decoupling of Si and C exists in production and export processes.During production,BSi and POC are decoupled with respect to particle size,and BSi mainly consisted of large particle size(>53 μm),whilst POC mainly consisted of small particle size(1～10 μm).During export,the Si/C ratio within the euphotic layer is lower than 200 m,which indicates that the dissolution degree of BSi is weaker than POC when exported from the euphotic zone.An adequate correlation between BSi and POC flux(R2=0.6398,p<0.001)was observed,which implies that BSi plays an important role and contributed towards POC export from the euphotic zone.A conceptual model was constructed to describe the response of the BSi:In the 3rd week of CE2,BSi concentration of the core is higher than that of the edge at DCM,and the BSi was carried by the current from the core to the edge,then dissolved and export,which lead to the export flux of the core is lower than that of the edge at the export layer.In the 4th week,BSi concentration and export flux of the core is higher than that of the edge at DCM and the export layer.During 5～6th week,BSi concentration and export flux were low,and had no significant differences between the core and edge.To sum up,the concentrations of BSi in the DCM enhanced,within the cyclonic eddies of CE2 and CE4 in the Western Pacific.Its spatio-temporal distribution,dynamics and export flux are controlled by various factors such as life-stage,physical conditions and availability of nutrients.The differences in BSi/234Th ratio with different sampling methods mainly related to the decoupling of BSi and 234Th with respect to particle size.In order to estimate the BSi export flux accurately in future,it is necessary to select an appropriate methodology for estimation. |
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