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

Abstract

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.

Influence of seasonal monsoons on net community production and CO&lt;sub&gt;2&lt;/sub&gt; in subtropical Hong Kong coastal waters

“…Harrison et al (2008) concluded that there is no massive hypoxia in the lower Pearl River estuary due to a short residence time of the seawater. Hence, the eutrophication impact in Hong Kong waters was not as severe as expected for such a eutrophic area in the wet season (Ho et al, 2008;Yuan et al, 2010a), as DO remained >150 µM with solubility ranging from 210 to 240 µM (Fig. 4a).…”

“…Data on salinity, temperature, primary production, DO, DIC and pCO 2 at stations 1 to 8 were presented in Ho et al (2008 and and Yuan et al (2010a). In this study, these data along with wind, respiration and gaseous air-sea fluxes were grouped into three main regions (Pearl River estuary, Victoria Harbor and eastern waters) and two seasonal patterns.…”

“…Dissolved organic carbon (DOC) also increased to 200 µM in Victoria Harbor in both the dry and wet seasons (Yuan et al, unpublished data, 2010). Hence, the increase in inorganic nutrients and organic carbon not only increased primary production in summer (Ho et al, 2010), but also produced an increase in bacterial production (BP) and BR (Yuan et al, 2010a).…”

“…Zhai et al (2005a) have reported that aerobic respiration is the most important process in maintaining a high pCO 2 level (>3000 µatm) in the upstream section of the Pearl River estuary. Being downstream of the Pearl River estuary, Hong Kong coastal waters have also been reported to be generally a CO 2 source due to the influence of the Pearl River discharge and sewage effluent (Yuan et al, 2010a). However, little is known about the seasonal variations in O 2 and CO 2 in response to monsoonal winds and the freshwater discharge of the Pearl River estuary.…”

High bacterial carbon demand and low growth efficiency at a tropical hypereutrophic estuary: importance of dissolved organic matter remineralization

“…The high and continuous input of allochthonous nutrients and organic material from the surrounding lands and the domestic sewage leads to high phytoplankton and bacterial growth (BARRERA-ALBA et al, 2008;GAULKE et al, 2010;VALENTIN, 2008b;MORTAZAVI et al;SCHUMANN et al, 2003;YUAN et al, 2010). However, most of estuarine BGE in situ estimates were performed in temperate zones, with few exceptions on the tropics (GUENTHER et al, 2008a;LEE et al, 2009;RAM et al, 2003;RAM et al, 2007).…”