No abstract

In 2001, the Hong Kong government implemented the Harbor Area Treatment Scheme (HATS) under which 70% of the sewage that had been formerly discharged into Victoria Harbor is now collected and sent to Stonecutters Island Sewage Works where it receives chemically enhanced primary treatment (CEPT), and is then discharged into waters west of the Harbor. The relocation of the sewage discharge will possibly change the nutrient dynamics and phytoplankton biomass in this area. Therefore, there is a need to examine the factors that regulate phytoplankton growth in Hong Kong waters in order to understand future impacts. Based on a historic nutrient data set (1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001), a comparison of ambient nutrient ratios with the Redfield ratio (N:P:Si=16:1:16) showed clear spatial variations in the factors that regulate phytoplankton biomass along a west (estuary) to east (coastal/oceanic) transect through Hong Kong waters. Algal biomass was constrained by a combination of low light conditions, a rapid change in salinity, and strong turbulent mixing in western waters throughout the year. Potential stoichiometric Si limitation (up to 94% of the cases in winter) occurred in Victoria Harbor due to the contribution of sewage effluent with high N and P enrichment all year, except for summer when the frequency of stoichiometric Si limitation (48%) was the same as P, owing to the influence of the high Si in the Pearl River discharge. In the eastern waters, potential N limitation and N and P co-limitation occurred in autumn and winter respectively, because of the dominance of coastal/oceanic water with low nutrients and low N:P ratios. In contrast, potential Si limitation occurred in spring and a switch to potential N, P and Si limitation occurred in eastern waters in summer. In southern waters, there was a shift from P limitation (80%) in summer due to the influence of the N-rich Pearl River discharge, to N limitation (68%) in autumn, and to N and P co-limitation in winter due HHS Public Access Author Manuscript Author ManuscriptAuthor ManuscriptAuthor Manuscript to the dominance of N-poor oceanic water from the oligotrophic South China Sea. Our results show clear temporal and spatial variations in the nutrient stoichiometry which indicates potential regulation of phytoplankton biomass in HK waters due to the combination of the seasonal exchange of the Pearl River discharge and oceanic water, sewage effluent inputs, and strong hydrodynamic mixing from SW monsoon winds in summer and the NE monsoon winds in winter.

Hong Kong waters are influenced by the Pearl River discharge in the west, coastal/ oceanic waters in the east, and year-round domestic sewage effluent in the Victoria Harbour area. Seven cruises were conducted at 12 stations across the Hong Kong water in the dry and wet seasons to examine how the Pearl River outflow and sewage effluent discharge influenced the distributions of dissolved oxygen (DO), dissolved inorganic carbon (DIC), bacterial production (BP) and bacterial respiration (BR). Surface DO saturation was usually ~80%, and surface partial pressure of carbon dioxide (pCO 2 ) averaged ~570 µatm at all 12 stations. The undersaturated DO and supersaturated pCO 2 indicated that Hong Kong waters were heterotrophic and a net source of CO 2 to the atmosphere, except during periods of phytoplankton blooms when pCO 2 became very low (< 200 µatm). The Pearl River discharge in the wet season decreased DO and increased pCO 2 and BP in the estuarine-influenced waters in comparison with the coastal/oceanic waters. Sewage effluent exerted a strong influence on carbon dynamics and CO 2 efflux as surface pCO 2 was significantly correlated with NH 4 , an indicator of sewage effluent. BR in Hong Kong waters was ~100 to 400 mmol C m -2 d -1 in July and November 2005. The BR fraction accounted for 50 to 80% of total dark community respiration (DCR) in coastal/oceanic waters with less eutrophic inputs, but increased to > 90% of DCR in more eutrophic waters near the sewage discharge site, likely due to the decrease in phytoplankton biomass. Contribution of BR to the CO 2 efflux was ~3 to 10 mmol C m -2 d -1, which was ~50% (varied from 16 to 130%) of the total CO 2 efflux. This study has important implications for severely anthropogenically impacted coastal areas, as they may be an important source of atmospheric CO 2 due to active BR.