Spatiotemporal Distribution Of Phytoplankton And Its Ecological Effects On Dimethyl Sulfide Production In The Eastern Shelf Of China

The eastern shelf sea area of China is one of the most important marginal seas in the world,which plays an important role in global climate change and biogeochemical cycle.Marine phytoplankton community and dimethyl sulfide(DMS)play an important role in marine biogenic sulfur cycle.The dynamic change of marine phytoplankton community is closely related to dimethylsulfoniopropionate(DMSP),DMS and air sea flux.In recent years,it has been found that the concentration of DMSP in the interstitial water of marine sediments is even orders of magnitude higher than that in the overlying water,and phytoplankton in the water-sediment interface occupies an important position in the transformation,but it is unclear whether the phytoplankton in interface conversion process of dimethyl sulfide,the contribution of the research of phytoplankton cells to the sediment surface settlement formation resting cells and the resuspension growth can reveal the phytoplankton in the watersediment interface between the role of sulfur cycle.On the other hand,DMSP can be accumulated in zooplankton and other aquatic animals,which is an important DMSP repository in the ocean.The animals will cause a large number of phytoplankton cell rupture in the process of feeding,and make release of high concentration of DMSP into the water body which is further transformed into DMS by DMSP lyase.At the same time,these animals will transfer DMSP from phytoplankton to feces and settle on the seafloor.Therefore,it is of great significance to study the effect of zooplankton feeding on the production and circulation of DMS in the ocean.Based on the above background,this study focused on the main line of "community structure and spatiotemporal distribution of phytoplankton-resting and germination of phytoplankton(sediments algae)-feeding of zooplanktondistribution and yield of dimethyl sulfide",to answer the following three key scientific questions:(1)How do changes of phytoplankton community structure and distribution affect marine DMS concertrations in the invstiogated sea area?(2)What is the contribution of the germination of diatoms in sediments to DMS content in the ocean?Changes of DMS or DMSP content during the formation and germination of diatom resting cells?(3)The effect of zooplankton feeding on phytoplankton on the release of DMS(P)?In this study,phytoplankton in the Yellow Sea and the East China Sea were investigated.The spatial and temporal distribution of phytoplankton community structure in the Yellow Sea and the East China Sea was investigated in spring 2017,summer 2018 and winter 2019.Combined with the survey data of hydrology,nutrients,DMSP and DMS,the response and relationship of dimethyl sulfide to phytoplankton community structure and dominant species changes in different seasons were analyzed and discussed.Combined with the summer voyage,the deposition of some stations in the survey area was found The concentration of DMSP in interstitial water was significantly higher than that in overlying water.In winter,the sediments from the areas with significant upwelling along the coast of Fujian and Zhejiang and the areas with cold water mass in the South Yellow Sea were collected to explore the contribution of the recovery of algal growth in dormancy period to dimethyl sulfide in seawater;the model algal species-Thalassiosira pseudonana,and the dominant species of sediments culture-Skeletonema costatum,were selected to explore the mechanism of DMSP production.The contribution of resting cells to DMSP and DMS in the process of forming and germinating was studied.Prorocentrum donghaiense(dominant species in spring voyage)and Platymonas subcordiformis(common unicellular microalga as aquaculture food in inshore)were isolated and cultured.The relationship between the changes of main physiological indexes of phytoplankton and the release of DMS by algal cells during zooplankton feeding pressure was studied by feeding experiment with the copepod Pseudodiaptomus annandalei.The main results are as follows:1.The average concentration of Chl-a in the surface layer of the eastern shelf is 1.162 μg/L in spring,1.135 μg/L in summer and 0.420 μg/L in winter.The concentration of Chl-a in spring was higher than that in summer and winter,and the distribution of high value area was influenced by season.The cross section of the Yellow Sea cold water mass(35° N)is higher in the nearshore and lower in the center of the South Yellow Sea in three seasons,but the range of the nearshore high value area in summer is smaller than that in spring.The difference between the Changjiang Estuary and the open sea is small in winter and spring.The size fraction of small phytoplankton was dominant,in spring,summer and winter,the proportion of Ch1-a concentration in particles>20 μm was 77.69%,62.96%and 56.84%respectively.2.In spring,275 species of phytoplankton belonging to 6 phyla and 90 genera were identified in the surface layer,including 208 species of 62 genera of Bacillariophyta and 56 species of 20 genera of Pyrrophyta.In summer,256 species of phytoplankton belonging to 7 phyla and 95 genera were identified,including 168 species of 55 genera of Bacillariophyta and 63 species of 21 genera of Pyrrophyta.In winter,252 species of phytoplankton belonging to 6 phyla and 98 genera were identified,including 192 species of 56 genera of Bacillariophyta and 42 species of 29 genera of Pyrrophyta.The average cell density of phytoplankton was 21.46×104 cells/L in spring,6.17×104 cells/L in summer and 1.20×104 cells/L in winter.The average density was spring>summer>winter.The Bacillariophyta/Pyrrophyta ratio(B/P ratio)was 41.13 in spring,5.96 in summer and 25.45 in winter.3.There are obvious seasonal differences in dominant species.In spring,S.dohrnii(8.05×104 cells/L),C vanheurckii(1.94x104 cells/L)and P.donghaiense(0.68×104 cells/L)were dominant species;in summer,Scrippsiella trochoidea(1.84×104 cells/L)was dominant species;in winter,P.sulcate(0.19×104 cells/L),C.acuta(0.12×104 cells/L),N.longissima(0.04×104 cells/L)were dominant species.Through correlation analysis,it was found that phytoplankton cell density was significantly correlated with dissolved oxygen and pH in spring,significantly positively correlated with nitrate,nitrite and silicate concentrations in summer,and highly correlated with dissolved oxygen in winter.Phytoplankton in different seasons is affected by different environmental factors.4.Phytoplankton plays an important role in the production of DMSP and DMS in the eastern China continental shelf,and is the main source of DMS in the ocean.The dominant diatom species were positively correlated with cell density and DMS and DMSP concentrations(P<0.05).Although some studies have shown that the yield of per cell DMS and DMSP of diatoms is not high,the contribution of diatom to DMS can not be ignored when the density of diatoms in the ocean is high or when a diatom bloom occurred,for example,the concentration of DMSPp at St.E3(B/P=550.86,diatom cell densities is 417.96×104 cells/L)was 3.83 times that at St.E5(B/P=0.10,diatom cell densities is 0.05 ×104 cells/L),and the concentration of DMS was 1.24 times that at St.E5.The results showed that diatoms played a greater role in the production of DMS in the investigated sea area.5.To study DMSP content variation in marine phytoplankton resting stage cells during the process of "sedimentation-resuspension growth",the experiment on"resting stage cells formation-germination-vegetative cell growth" of the typical diatom species were performed.The results showed that the sediments played important role in marine phytoplankton resting cells "seeds",especially the bottom water is seawater important contributor of dimethyl sulfide.This could be showed in the following two aspects:(1)The resting cells of phytoplankton have high vitalityand contribute to the development of phytoplankton community after germination.During germination and growth,they promote the generation of DMSP and the release of DMS in the ocean,forming the sulfur release process at frome the sediment to seawater interface.(2)Dark and low temperature promoted the production of DMSP and the release of DMS from diatom vegetative cells.It was inferred that the deposition of diatom at the late stage of the outbreak of red tide would promote the production of DMSP and the release of DMS,and formed a sulfur deposition process from seawater interface to sediments.Therefore,diatom resting stage cells are the"sulfur pumps" that cannot be ignored in the ocean "sediment-seawater interface"circulation.6.In the feeding experiment with the copepod P.annandalei on two microalgae P.donghaiense and P.subcordiformis,the maximum concentrations of dissolve DMSP(DMSPd)were 1.29 and 1.10 times of control;and DMS were 2.53 and 1.98 times of control,respectively.When feeding on P.donghaiense of P.annandalei(300 inds.and 150 inds.was added respectively),the maximum concentrations of DMSPd and DMS were 2.08 times and 8.38 times of the control,respectively.Feeding on P.donghaiense and P.subcordiformis,P.annandalei promoted the release of DMSP and DMS,which increased with time in the early and middle stages of culture,significantly reduced the concentration of DMSPp in the culture system(P<0.05),and significantly promoted the release of protein and polysaccharide(P<0.05).