The ecological significance of mesozooplankton feeding in marine planktonic food web

This dissertation addresses the role of mesozooplankton feeding in influencing the trophic interactions in marine planktonic food web. Several related hypotheses were tested in the framework of five individual chapters. Research started with two field investigations studying the response of mesozooplankton biomass and composition to estuarine nutrient loading in Hong Kong coastal waters and the spatial and temporal grazing importance in northern South China Sea (SCS). Trophic cascades induced by mesozooplankton resulted from the release of microzooplankton grazing was demonstrated in laboratory feeding experiments involving simplified triangle food chains and the strengths of trophic cascades were examined for a range of prey concentrations and compared for different predator species with different feeding characteristics. Finally, I went back to field works to test the hypothesis that mesozooplankton carnivory causes a decrease of clearance rate on phytoplankton community. To address this hypothesis, feeding experiments were conducted at two contrasting sites of Hong Kong coastal waters over a two year period to measure mesozooplankton clearance rate and the identity of mesozooplankton grazers in feeding experiments was determined to estimate a net carnivory degree of each assemblage. Field investigation in Hong Kong water indicated that mesozooplankton biomass in the estuarine eutrophic station was not elevated by estuarine nutrient loading compared to the coastal mesotrophic station. Instead, estuarine eutrophic conditions significantly modified the composition of mesozooplankton by increasing the proportion of omnivorous copepods in community and by decreasing some particular groups that are primarily herbivorous such as marine cladocerans, bivalve larvae and gastropod larvae. Field investigation carried out in northern SCS indicated that mesozooplankton in shelf water generally have higher biomass and grazing impacts on phytoplankton than in oceanic water becau