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

摘要

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 waterin 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 (DIG), bacterial production (BP) and bacterial respiration (BR). Surface DOsaturation was usually -80%, and surface partial pressure of carbon dioxide (pCO_2) averaged -570 uatm 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 theatmosphere, except during periods of phytoplankton blooms when pCO_2 became very low (<200 uatm). 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.