Seasonal And Interannual Variability Of Coccolithophore Downward Fluxes And Potential Environmental Drivers In The Northern South China Sea

Coccolithophore is the most abundant calcified marine phytoplankton which take apart a key component in both biological pump and carbonate counter pump in global oceans.The calcified scales(coccoliths)of coccolithophore contribute to regulate global carbon cycle through calcification and act as mineral ballast to accelerate biogenic export to deep layer.Coccolithophore subsidize 5-40%of world ocean primary production and over 50%of carbonate pump,perhaps 80%in bloom conditions.Therefore,seasonal and interannual changes of coccolithophore assemblage composition and their export production responses to oceanic environment processes are significant in marine biogeochemical studies.Since,coccolithophores are also sensitive indicators of changes in physical-chemical properties of the surface water masses,it preserved in sediments as good proxy for long-term climatic signals,hence it is used in paleo-oceanography.Although,scattered studies of coccolithophore downward flux have been conducted on open global oceans,a systematic unremitting study on coccolithophore export behaviour in marginal seas with response to climatic changes was not done.The South China Sea(SCS)is the largest semi enclosed,oligotrophic marginal sea which connects the West Pacific through the Luzon Strait to the northeast and the Indian ocean through the Kalimantan Strait to the south.The SCS has>5000 m deep central basin and is always subjected to influence of two main weather regimes,Indian ocean and East Asian monsoon systems,hence sensitive to climate change.Therefore,coccolithophore studies in the SCS is crucial in paleo-oceanographic and paleo-climatic elucidations,but limited modern studies are documented.In order to fill the gap,temporal(seasonal and interannual)and vertical variation of coccolith fluxes in sinking particles in relation to potential environmental drivers were investigated in time-series sediment traps deployed in the northern SCS.The sediment trap mooring(Mark 7G-21)was deployed in the northern SCS(SCS-N,18.5°N,116°E with 3736 m bottom depth)and sediment samples collection was done for three years which covered weak El Nino(2009-2010),weak La Nina(2011-201 2)and strong El Nino period(2015-2016).The SCS-N mooring was mounted three sediment traps at 1000 m,2100 m and 3200 m depths during the weak El Nino and La Nina years while only two traps were installed at 1000 m and 3200 m depths during the strong El Nino year.Each sediment trap was collected 20 samples with 16-day sampling intervals for the weak El Nino and weak La Nina years while 17-day sampling interval was used for the strong El Nino year.Sample cups pre-treated with 3.3%mercuric chloride(HgCl2)and 66.6%sodium chloride(NaCl)prior to deployment were used so as to minimize diffusion and decomposition of the trapped materials.After recovery,the wet samples were sieved(1mm)to remove zooplankton and<1 mm fraction were split into aliquots with a high-precision rotary splitter,then filtered over pre-weighed Nucleopore filters(0.45 ?m)and dried at 40? for 12 hours.The dry weights of these fractions were used to calculate the total particle fluxes CaCO3,opal and particulate organic carbon(POC)analyses.Dried samples were used to prepare of smear slides and copper stubs for light microscopy(LM)and scanned electron microscopy(SEM)analysis.The numbers of coccoliths and coccospheres were counted at a magnification of 1000 in 20 observation fields or at least over 400 coccoliths per sample were counted in less abundance condition.Coccoliths were converted to coccospheres according to the number of coccoliths per coccosphere for each species,to calculate the relative abundance of extant coccolithophore species.Environmental factors(chlorophyll a,sea surface temperature,sea surface height anomaly,mixed layer depth and sea surface wind vector)around the SCS-N were retrieved by remotely sensed observations and correlated with coccolith flux patterns.According to the findings,downward fluxes of coccoliths/coccosphere were varied annually and seasonally due to different seasonal and non-seasonal physical oceanic processes while average fluxes and peak time were different in each year.The highest annual average flux of coccoliths(49.72 × 109 coccoliths·m-2 ·d-1)was recorded during the strong El Nino year and it was 3 and 1.3 times higher than during the weak El Nino and weak La Nina years respectively.The highest peak fluxes were noticed in summer(38.8 × 109 coccoliths·m-2·d-1),winter(119.0 × 109 coccoliths·M-2·d-1)and spring(104.8 × 109 coccoliths·m-2·d-1)during the weak El Nino,weak La Nina and strong El Nino years respectively.The average fluxes of those three seasons contributes 43.1%,49.4%and 32.4%to the total annual coccolith fluxes of the corresponding years.Low fluxes of coccoliths in the winter of 2009-2010(26.26%)and 2015-2016(24.00%)were possibly related to diminish water circulation and upwelling influenced by weak El Nino in 2009 and strong El Nino in 2015.Apart from the influence of El Nino-Southern Oscillation(ENSO),mesoscale eddy occurrence,lateral advection and resuspension of particles could influence the coccoliths fluxes variation.Both extinct and extant species of coccoliths in the surface sediments,were added to the pelagic layer during winter time due to strong winter mixing and circulation.The average daily flux of extinct or reworked forms was much higher during the La Nina year(19.15 × 108 coccoliths·m-2·d-1)compared to other two time-series years.The source of the fossil coccoliths(extinct)most likely were the reworked Pleistocene sands which cover the outer shelf and upper slope to the west and south of the Dongsha Islands between 20 and 600 m water depth.These sediments contain limestone fragments with foraminiferal assemblages of Miocene to Pliocene age,and,to the north and south of the islands.Miocene strata are exposed on the sea floor.Mesoscale eddies,both cyclonic and anticyclonic,were probably the main agent for resuspending and transporting the coccoliths as they propagated westwards along-slope from the Dongsha area to the mooring site.The high amount of reworked forms could be carried by enhanced current intensity and resuspension brought by La Nina condition during 2011.In contrast to the coccolith fluxes,annual and daily fluxes of total particles,CaCO3,and opal were nearly equal during both weak El Nino and La Nina years.But those fluxes considerably decreased during the strong El Nino year except that POC was slightly higher than during the weak El Nino year.The possible reason was the high rate of decomposition of particles due to poor mixing related to weak current intensity brought by weak East Asian Monsoon during strong El Nino.The vertical variation of coccoliths/coccosphere sinking fluxes showed that higher fluxes were in middle layer traps compared to shallow and deep-water depths during both weak El Nino and weak La Nina years.The high flux in middle layer during La Nina was impacted by enhanced resuspension and lateral advection during that period.A total of 61 coccolithophore taxa were identified at 1000 m depth during all three time-series years including 29 species of non-living/reworked forms while the highest species richness of coccoliths were found in strong El Nino year(51 taxa).The annual coccolithophore assemblage in the northern SCS was dominated by three key species;Florisphaera profunda,Gephyrocapsa oceanica,and Emiliania huxleyi,which cumulatively accounted for 90.9%,87.0%and 90.4%of the total annual fluxes in weak El Nino,La Nina and strong El Nino years,respectively.Among these,F.profunda was prominent except in the winter and spring of 2009/2010 and spring of 201 5/2016 when G.oceanica and E.huxleyi were the major species of those seasonal assemblages respectively.Comparatively,strong wind mixing and deepened mixed layer depth(ML.D)with high nutrients could suppress F.profunda,leading to dominance of G oceanica in the winter of 2009/2010,but lateral advection of F.profunda from adjacent waters might be stronger in the winter of 2011/2012,which caused F.profunda to dominate the winter assemblage during the La Nina year.Species composition of other coccolithophores also varied seasonaly and interannually,and were impacted by non-seasonal oceanic processes.Lateral advection of extant taxa may have also taken place but the extent to which this may have masked primary signals from in-situ coccolithophore production remains open.Therefore,current results together with environmental drivers have to be considered deeply in future paleo-climatic elucidations and quantification of carbon fluxes in biogeocheimal cycles.