Bacterial production and respiration in subtropical Hong Kong waters: influence of the Pearl River discharge and sewage effluent

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 similar to 80 %, and surface partial pressure of carbon dioxide (pCO(2)) averaged similar to 570 mu atm at all 12 stations, The undersaturated DO and supersaturated pCO(2) indicated that Hong Kong waters were heterotrophic and a net source of CO2 to the atmosphere, except during periods of phytoplankton blooms when pCO(2) became very low (< 200 mu atm). 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 CO2 efflux as surface pCO(2) was significantly correlated with NH4, an indicator of sewage effluent. BR in Hong Kong waters was similar to 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 CO2 efflux was similar to 3 to 10 mmol C m(-2) d(-1), which was similar to 50% (varied from 16 to 130 %) of the total CO2 efflux. This study has important implications for severely anthropogenically impacted coastal areas, as they may be an important source of atmospheric CO2 due to active BR.