||This thesis research investigated the dynamics of nutrients and phytoplankton biomass and production in Hong Kong (HK) waters, with a focus on: 1) the effects of discharge from local sewage and the Pearl River Estuary (PRE), and as well as the effects of the implementation of sewage pollution abatement on the dynamics of nutrients and phytoplankton biomass; 2) the long-term changes in water quality and eutrophication impacts, and 3) the factors controlling phytoplankton biomass, production, and distribution of various algal groups in different eutrophic environments in HK waters. This thesis demonstrated that the marine ecosystem in HK waters has suffered from serious pollution problems over the last three decades, due to the seasonal input of nutrients from the PRE and year round discharge of local sewage. The summer wet season is the most productive period when the mean integrated chl a (Ichl a) and primary production (IPP) reached 120 mg m-2 and 9 g C m-2 d-1 respectively in HK waters. Phytoplankton production in VH, and southern and eastern waters (110-250 mg m-2 and 11-19 g C m-2 d-1, respectively) were higher than Stonecutters Island (SCI) and western waters (14-35 mg m-2 and 1.2-2.9 g C m-2 d-1). However, the mean bottom dissolved oxygen (DO) remained >3.5 mg L-1 in most regions in summer, indicating that the eutrophication impact in HK waters was not as severe as expected for such a eutrophic and productive area. The significant reduction in nutrients and the increase in bottom DO indicated that the water quality in Victoria Harbor (VH) has been improving after implementation of the Harbor Area Treatment Scheme (HATS) in 2001. However, Deep Bay exhibited severe eutrophication impacts such as high inorganic and organic nutrient concentrations, high E. coli counts and low bottom DO, which have exhibited a deteriorating trend over the last two decades. Western and southern waters are relatively resistant to the nutrient enrichment, and eutrophication impacts in these regions were not as severe as VH and Deep Bay. However, symptoms of eutrophication impacts such as significant long-term increases in E. coli counts, chl a and the occurrence of episodic hypoxic events were observed in western and southern waters. Sewage treatment with extensive removal of phosphate and chlorination should be applied to 'clean up' Deep Bay and to prevent severe eutrophication impacts in western and southern waters. The field and laboratory work showed that the phytoplankton biomass and production in HK waters are generally dominated (66 and 64%, respectively) by diatoms, probably due to: (1) high SiO4 loading from the PRE; (2) higher chl a-specific photosynthetic rates than small phytoplankton (<5 µm) in high light conditions, (3) vertical mixing which keeps the diatoms suspended in shallow water, and 4) higher growth rates and tolerance range under different environmental conditions, compared to dinoflagellates and raphidophytes.