Seasonal Variations Of Phytoplankton And Its Far-Field Responses To Typhoon In The South China Sea Basin With BGC-Argo Floats

The South China Sea(SCS)is the largest marginal sea in the northwestern Pacific Ocean,and its basin is a typical oligotrophic region.Many studies have analyzed the seasonal variation of phytoplankton and their responses to typhoon events based on the ship-based or satellite observations.However,these observations are with low temporal or vertical resolution.In addition,most studies considered the chlorophyll a concentration(Chla)as the indicator of phytoplankton biomass,ignoring the contribution of phytoplankton physiological adjustment to Chla changes.Based on the measurements of two BGC-Argo floats deployed in the SCS basin,this study aimed to analyze the seasonal variation of phytoplankton from the vertical perspective,and explore the impact of photoacclimation effect on the Chla changes,and explore the farfield impacts of typhoon on the upper phytoplankton dynamics.Moreover,this study also analyzed the bio-optical relationships between particles and optical backscattering in the SCS basin based on the in-situ observations.Particulate backscattering coefficient(bbp)provides effective proxies for particulate organic carbon(POC)and phytoplankton carbon(Cphy);however,their biooptical relationships in the oligotrophic basin are rarely reported.In this work,based on the in-situ synchronous optical and biogeochemical measurements in SCS basin,we refined the regional relationships between POC(and Cphy)and bbp,and investigated the impacts of phytoplankton community compositions and size classes on the bbp variability.The results showed that:1)POC and Cphy exhibited good linear relationships with bbp;2)the relationship between Cphy and POC could also be fitted in a linear function with a positive POC intercept,and the POC was dominated by the non-algal particles,and the contribution of phytoplankton to total POC would increase with the phytoplankton biomass;and 3)the POC-specific bbp(b*bp)did not vary with the phytoplankton biomass,but was positively correlated with the fraction of the phytoplankton groups Haptophytes(Type 8)and Diatoms to total Chla,while negatively correlated with the fraction of pico-phytoplankton to Chla(fpico).These findings suggest that in oligotrophic waters,the variability of b’bp was mainly controlled by the variability in the relative contribution of large phytoplankton with complex structures,which was contrary with the prediction of classic Mie theory.Seasonal variations of the float-observed phytoplankton were analyzed.1)In the northern SCS basin,the depth-integrated phytoplankton in the whole water column was the highest in winter,and thus the winter bloom occurred,but they varied weakly in the other seasons.In the central SCS basin,the seasonal variation of the total phytoplankton biomass was insignificant.The total phytoplankton increase depended on the fact that large amount of nutrients can be pumped into the upper layers.2)In the northern SCS basin,the increase of surface Chla during winter bloom period was contributed by the combined effects of phytoplankton vertical redistribution(physical process),more nutrient injection(biological alternation),and phytoplankton photoacclimation(physiology adjustment).In the central SCS basin,the increase of the surface Chla was mainly controlled by phytoplankton photoacclimation adjustment.3)In the stratified waters,the seasonal variation of Chla vertical distribution resulted from a light-driven low-frequency process,but was more impacted by the physical adjustment during highfrequency events(e.g.,eddies).Super typhoon Rammasun(2014)traveled across the South China Sea.Its far-field impacts on phytoplankton dynamics in the upper ocean were documented by a BGCArgo float located 200 km to the left of its track.Both surface Chla and bbp were observed to increase substantially within two distinct stages.The initial increase occurred during the passage of the typhoon,and the subsequent increase happened 5 days after the typhoon.In contrast,depth-integrated Chla and bbp in the upper 150 m underwent negligible changes throughout the entire period.The key lies in the fact that surface phytoplankton increases in the far-field region were mainly resulted from the physically driven vertical redistribution of particles,rather than from biological alternations.The first increase was attributed to the typhoon-induced strong turbulence which deepened the surface mixed layer,and thus entrained subsurface particles to the surface;the second was due to the post-typhoon adiabatic quasi-geostrophic adjustment of the upper ocean that gradually raised the isopycnals(and thus subsurface particles).These results challenge the prevailing wisdom on typhoon impacts,and thus shed new lights on the nature of the upper ocean responses to typhoons from the vertical perspective.