| Phytoplankton is a key component in both global biogeochemical cycles and climate changes.The brief cell cycle combine to the high dynamics made phytoplankton cells very responsive to their environment and their variations.It's important to recognize the community structre and dynamics of phytoplankton.In this dissertation,phytoplankton community structure and dynamics was studied in different marine environments such as bay,continental slope and open sea based on the new phytoplankton flow cytometry(CytoSub)technique.The major aim of this strudy was to under standand the community structure of whole size phytoplankton,the dynamics of phytoplankton growth rates,and the relationships between environments and phytoplankton communities.This study layed the foundation for Cyto Sub used in different marnie environmets and fields.In the study of phytoplankton community structure,the spatial distribution of phytoplankton composition,abundance and biomass was studied in the Jiaozhou Bay,South China Sea northern slope and north-western Pacific.The affecting of environment limiting factors and key physical processes on phytoplankton distributions was discussed.The main conclusions were as follows:Phytoplankton community of Jiaozhou Bay was investigated in April 2014 using CytoSub.The data treatment conducted on the basis of pulse shape analysis,resolved 6 clusters,which were named Synechococcus,picoeukaryotes,cryptophytes,nanoeukaryotes,microeukaryotes and chain-forming diatoms.chain-forming diatoms were dominant in Jiaozhou Bay.The average abundance and biomass of chain-forming diatoms were 1187 cells/mL,46.31 ?g C/L inside of Jiaozhou Bay.However,the average abundance and biomass of chain-forming diatoms were 116 cells/m L,4.54 ?g C/L outside of Jiaozhou Bay.Correlation analysis showed chain-forming diatoms were positively correlated with temperature and nutrients.Compared with other phytoplankton size fraction analysis,the CytoSub provide accurate evaluation on phytoplankton community dominated by the chain-forming alages.Phytoplankton community of South China Sea northern slope was investigated in June 2016 using CytoSub.The data treatment conducted on the basis of pulse shape analysis,resolved 6 clusters,which were named Prochlorococcus,Synechococcus,picoeukaryotes,nanoeukaryotes 1,nanoeukaryotes 2 and microeukaryotes.In the vertical distribution,the abundance of Synechococcus were dominant during the 0-25 m depth,the abundance of Prochlorococcus were dominant during the 40-200 m depth,the biomass of Synechococcus and nanoeukaryotes 2 were dominant dominant 0-25 m depth,and the biomass of nano-and micro-eukaryotes were dominant in 40-200 m depth.This work also showed that the phytoplankton were most abundant in the continental shelf,and less abundant in the slope and open water.The maximum abundance of Prochlorococcus,nanoeukaryotes 1 and microeukaryotes were in 50 m depth layer,while the maximum anbundance of Synechococcus was in surface layer.The maximum abundance eddy was found in 114.5-115.5°E,19-20°N between 0 m and 50 m layers.However,the lowest abundance edyy was found in 114.5-115.5°E,19-20°N between 100 m and 200 m layers.The relationship between phytoplankton community structure and environment factors was analyzed.The results showed that temperature,salinity,nutrients and eddies influenced the phytoplankton community structure.The spatial distribution of phytoplankton was investigated in Auguse and November of 2014 in two different regions of the Pacific western boundary current,namely the Warm Pool near the equator and the subtropical Kuroshio south area.Traditional approaches(size-fractionated chlorophyll-a(Chl-a)and microscopic analyses)combined with single-cell analysis(using a flow cytometer)were used to analyse the whole range of phytoplankton.Flow cytometry analysis resolved five clusters,two belonging to the pico-size fraction and three belonging to the nano-size fraction.Microscopy analysis revealed that the genera Coscinodiscus,Rhizosolenia,Chaetoceros and Ceratium were numerically dominant in the region studied.The lowest values of Chl-a,phytoplankton abundance and carbon biomass were found in the Kuroshio south area.Both Chl-a concentration data and flow cytometry analysis revealed that picophytoplankton were the predominant contributors to phytoplankton in the Pacific western boundary currents.Along the three transects during the summer cruise,Synechococcus and nanocyanobacteria-like organisms numerically dominated in surface waters with higher temperature.In contrast,eukaryotes were primarily distributed in subsurface waters with higher nutrients,especially in the eddy upwelling region mainly associated with the North Equatorial Coudnter Current.The vertical distribution of phytoplankton over the epipelagic layer reinforces the important role of currents in the north-western Pacific during summer.In the study of phytoplankton community dynamics,the phytoplankton community structure,temporal and spatial distribution of phytoplankton and the dynamics of phytoplankton growth rates were observed using high frequency CytoSub/Sence in the Jiaozhou Bay and Bay of Marseille.The relationships between environments conditions and phytoplankton abundance were also studied in order to search for the possible controlling factors on phytoplankton growth.The main conclusions were as follows:Phytoplankton community of Jiaozhou Bay was investigated in October 2015 using CytoSub.The data treatment conducted on the basis of pulse shape analysis,resolved 8 clusters,which were named Synechococcus,picoeukaryotes,cryptophytes,nanoeukaryotes 1,nanoeukaryotes 2,nanoeukaryotes 3,microeukaryotes and chain-forming diatoms.The estimated in situ cluster growth rates of Jiaozhou Bay was low in autumn.Correlation analysis showed the abundance and in situ cluster growth rates were positively correlated with temperature.The highest picophytoplankton abundance were found during the night,however,the highest nanoeukaryotes and microeukaryotes were found during the night.Redunced Picoeukaryotes,cryptophytes and chain-forming diatoms desities were found in Jiaozhou Bay,with a stratified vertical distribution during the day and tendency to a more even distribution from the end of night time to sunrise.On the other hand,the highest phytoplankton densities were observed in Jiaozhou Bay,being concentrated mainly in superficial layers(0-4 m)during a 24-hour cycle.Light was the main hydrological factor which controlled the diel variations of phytoplankton in Jiaozhou Bay.Phytoplankton community of Bay of Marseille was investigated in November 2015 using CytoSence.The data treatment conducted on the basis of pulse shape analysis,resolved 10 clusters,which were named Synechococcus,picoeukaryotes1,picoeukaryotes2,picoeukaryotes3,nanoeukaryotes 1,nanoeukaryotes 2,nanoeukaryotes 2,microeukaryotes and LFNano.One pioneering work unveiled clusters of autotrophic picoeukaryotes whose abundance variability was explained by the occurrence of diel vertical migration.To test this hypothesis,we designed an elaborated experimental set up at the same site,involving one enclosure isolating a water column with its superficial sediment,and a multisampler coupled to an updated automated flow cytometer.Among the 7 resolved clusters,3 were composed of autotrophic picoeukaryotes undergoing diel vertical migration.One occupied the water column the first half of the day and disappeared supposedly in the sediment while the other two moved upward to occupy the water column the rest of the day.To our knowledge,this is the first time that autotrophic picoeukaryotes are shown to achieve diel vertical migration.Survival strategy was considered to explain this phenomenon.One nanophytoplankton cluster was found to be composed of mixotroph cells.Further studies are needed to identify the involved species,to better characterise the migratory mechanism,both at the cell level and at the spatial scale and to evaluate its ecological impact within the microbial trophic network. |
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