||The study of bacterial production and respiration is a relatively new field of investigation in Hong Kong waters and the South China Sea. The objectives of this thesis were to determine: 1) the temporal and spatial variations of bacteria and their role in oxygen consumption and carbon release; and 2) whether P is the most limiting nutrient for bacterial respiration in the coastal waters of Hong Kong. Surface bacterial production was 10 to 50 μg C L-1 d-1 in the eastern waters, and was significantly higher (20 to 150 μg C L-1 d-1) near the Stonecutters Island sewage outfall discharge site (p <0.05). Integrated bacterial respiration was ~1.2 to 4.5 g C m-2 d-1 in June and November 2005, which accounted for >90% of the dark community respiration (DCR) near the sewage discharge site, and 40 to 75% in the shelf waters. Bottom DO was usually >3 mg L-1 (i.e. ~75 % saturated) in Victoria Harbor. DO was often undersaturated and pCO2 supersaturated, indicating that Hong Kong waters were heterotrophic and a net source of CO2 to the atmosphere. Surface pCO2 averaged ~570 μatm at all 12 stations during the study period, except during the episodic periods of phytoplankton blooms when CO2 was <200 μatm. Surface pCO2 was significantly correlated with NH4 (p <0.05), an indicator of sewage effluent, and therefore, sewage effluent appeared to exert a strong influence on carbon dynamics and CO2 efflux. Only ~10% of the ambient DOC (~200 μM) near the sewage discharge site was consumed by bacteria, and the remainder was most likely transported to the coastal waters. The DIN:PO4 ratio could often be up to ~50:1 in the wet season, and hence PO4 might be the first nutrient to be depleted and became limiting for bacterial growth, especially during phytoplankton blooms in the wet season. In addition, bacteria require a higher proportion of P relative to N (N:P = ~5:1), and hence they are more susceptible to P limitation. The significant correlation between apparent oxygen utilization (AOU) and phosphorus also indicated that the lack of large areas of hypoxia in the area may be linked to phosphorus limitation. Therefore, a PO4 addition experiment was conducted to determine the responses in bacterial respiration, production and growth efficiency to PO4 limitation. Natural seawater was obtained from southern Hong Kong waters in July 2006. PO4 addition treatment increased both bacterial respiration (BR) and phytoplankton respiration (PR) by ~20% when phytoplankton assimilated most of the PO4. However, the >0.2 μM PO4 addition treatment did not further increase BR. Bacterial growth efficiency decreased with higher phytoplankton abundance, but increased due to PO4 addition. Therefore, despite the high nutrient loading from the Pearl River estuary and sewage effluent, there is still potential limitation of inorganic phosphorus for both bacterial and phytoplankton respiration, which may partially decrease eutrophication impacts in Hong Kong waters. In addition to the coastal waters of Hong Kong, the spatial variation in bacterial abundance and production were also investigated in the northern South China Sea during the summer of 2004 and 2005. The South China Sea (SCS) is an oligotrophic ocean with low nutrients, and low phytoplankton biomass and productivity in surface waters. Surface bacterial abundance was relatively high (~8 to 12 x 105 cells ml-1) in the Pearl River estuarine waters near Hong Kong due to high nutrient and organic matter concentrations. In the northwest SCS, high surface bacterial abundance (~12 x 105 cells ml-1) was observed near Hainan Island probably due to upwelling. In contrast, the invasion of the Pacific Ocean through Luzon Strait led to low surface bacterial abundance (~5 x 105 cells ml-1). Bacterial biomass and production were clearly related to Chl a in the shelf and open ocean, which indicated a bottom-up control by DOC produced by primary producers. Further evidence for bottom-up control was provided by bioassay nutrient addition experiments, which showed that a P addition enhanced bacterial production in coastal waters, and a DOC addition in combination with a NO3 or PO4 addition, gave the largest enhancement of bacterial growth in all bioassay experiments, indicating that inorganic nutrients exerted some control of labile DOC consumption in this oligotrophic ocean. UVR inhibited bacterial production (BP), phytoplankton production (PP), and enhanced viral decay rates (VDR) by 20, 14 and 36% respectively. UVR inhibition on BP was significantly higher than PP in most samples, which suggests that UVR inhibition on heterotrophic activities was higher than autotrophic activities in the northern South China Sea on sunny days.