Preliminary Study Of Microbial Community Structure In The East China Sea Shelf Area And The Frontal Region Of The Northern Part Of The South China Sea

Relationships between distributions of pico- and nano-sized photosynthetic organisms, bacteria and viruses, and physical and chemical factors or species competition were studied in this dissertation based on observations during the cruises conducted in the East China Sea in fall 2003 (including ship-board incubation experiments), in the front regions of the northern part of the South China Sea in winter 2004, and in the Changjiang Estuary (Yangtze Estuary) and adjacent coastal region in fall 2004, to further our understanding towards the significance of these organisms within marine biogeochemical cycles.In Chapter one, general characters of pelagic and benthic pico- and nano-sized photosynthetic organisms, bacteria and viruses, and the compositon of microbial food web were introduced, and the objective of this dissertation were put forward as well.Methods of enumeration of pelagic and benthic pico- and nano-sized photosynthetic organisms, bacteria and viruses by flow cytometry were discussed in Chapter two. Besides, evaluations of the effects of three different fixatives (including paraformaldehyde, glutaraldehyde and the mixture of the two) and time of storage on the determination of picoplankton were compared. On-board determination of picoplankton is preferred, if possible. Otherwise, it is better to use paraformaldehyde or the mixture of paraformaldehyde and glutaraldehyde as fixatives, store the samples in liquid nitrogen, and do the analyses as soon as possible when back to laboratory.The results of field studies (including ship-board incubation experiments) were shown in Chapeter three:The average abundances of Synechococcus, Prochlorococcus, picoeukaryotes and heterotrophic bacteria were 105, 105, 104 and 106 cells ml~-1, respectively, according to on-board observations in the East China Sea in fall 2003. Distributions of different picoplankton groups were dominated by different water masses. Synechococcus, picoeukaryotes and heterotrophic bacteria were abundant at all the stations and presented higher concentration in the inner shelf, where influences from the Changjiang effluent plumes and the coastal upwelling were evident, while Prochlorococcus was absent from the near-shore stations of relative low salinity and high turbidity, and became the dominant picophytoplankton population in offshore waters, especially in the Kuroshio-controlled waters, where its abundance was comparable to that for heterotrophic bacteria. Besides, all picoplankton groupsshowed a reduction in cell number with depth, and a positive correlation with water temperature were observed, which reflected the importance of light and temperature on picoplankton growth. A negative relationship with salinity was found for heterotrophic bacteria along two sections across the East China Sea Shelf as well.Data from ship-board incubation experiments carried out in 2003 showed that Synechococcus and picoeukaryotes were more euryhaline and more adaptable to mutive environments (e.g. mixing of water masses) than Prochlorococcus. The sensitivity of pico- and nanophytoplankton to variations in nutrient levels were probably one reason for the improvement of their adaptability to the environments. In regions where brakish and saline waters actively mix, picoeukaryotes can be most competitive in biomass among the pico- and nano-phytoplankton groups when nutrients are rich, and replaced by nanophytoplankton under conditions of comparatively low nutrient levels.Distributions of different picoplankton groups were studied at a coastal front region in the northern part of the South China Sea in winter 2004. Prochlorococcus was confined to relatively high temperature and high salinity waters. Heterotrophic bacteia were the greatest contributors to the total picoplankton biomass and kept stable in biomass and surface/bottom biomass ratio on both sides of the front. Increases of the total picophytoplankton biomass and surface/bottom biomass ratio mainly due to the contribution of Synechococcus and. Prochlorococcus on the offshore side were remarkable. Temperature...