||In this thesis, several fundamental aspects of phytoplankton growth, microzooplankton grazing, and microbial food web structure were addressed in Hong Kong coastal waters and the oligotrophic South China Sea (SCS). An oligotrichous ciliate isolated from Port Shelter and fed with two nanophytoplankton at different concentrations exhibited higher clearance rates and lower growth rates at lower prey concentrations, which apparently validated the dilution technique. Monthly dilution experiments conducted at two stations with contrasting trophic conditions in Hong Kong coastal waters showed that higher Chl a, microzooplankton biomass, phytoplankton growth and microzooplankton grazing rates were found at the estuarine station, NM3, than the eastern station, PM7. The extent of Chl a increase (3.8 times) was higher than the increase of phytoplankton growth (1.5 times) and microzooplankton grazing rates (1.3 times). Our results confirm that even in eutrophic coastal environments, microzooplankton grazing remained a dominant loss pathway for phytoplankton. In the western SCS, phytoplankton growth rates were highly variable, being much higher in upwelling regions, while Chl a concentration was relatively constant. Microzooplankton grazing closely coupled with phytoplankton growth, consuming on average 88% of primary production. We found positive correlations among biomass, growth, and grazing for phytoplankton, probably due to the addition of larger size classes with higher growth rates, into the system. Size-fractionation experiments were conducted to define potential trophic levels within the microbial food web in SCS. Removing grazers by filtration would relieve the grazing pressure on lower trophic levels which finally influenced the net growth rates of picoplankton. Based on the growth patterns of picoplankton, a general pattern of five potential trophic levels (<2 μm, 2-5 μm, 5-10 μm, 10-60 μm, 60-200 μm) was revealed combining all the experiments, confirming the existence of multiple trophic levels within the microbial food web of SCS.