Long-term variations in oxygen in sub-tropical coastal waters: Influence of sewage effluent

Abstract

The increase in population in the Pearl River delta region has increased domestic sewage and industrial discharges, which influences biogeochemical (e.g. carbon dioxide release) and environmental (e.g. oxygen depletion) conditions. Dissolved oxygen and dissolved inorganic carbon concentrations along with salinity, temperature, pH, and chlorophyll a at the surface were collected during seven cruises at eight stations in different months in 2005–2006 to document the seasonal influence of anthropogenic inputs on dissolved oxygen in different areas of Hong Kong waters. Long-term data during 1997–2006 were used to assess air-sea exchange of dissolved oxygen. Near the Pearl River estuary, dissolved oxygen was undersaturated, while partial pressure carbon dioxide was oversaturated, which indicated that Pearl River estuary influenced waters were heterotrophic and represented net sources of carbon dioxide to the atmosphere. In Victoria Harbour, where there was greater sewage effluent influence, however, the degree of dissolved oxygen undersaturation was even more than the Pearl River estuary influenced waters throughout the year. In contrast, the eastern waters, where there was less influence of anthropogenic inputs, showed seasonal variability: undersaturation of dissolved oxygen (∼90%) in the dry season shifting to slight oversaturation (∼105%) in the wet season. The monthly average air-sea influx of oxygen decreased by ∼50 to 200 mmol m−2 d−1 in the wet season relative to that in the dry season in Victoria Harbour, which was coupled with an increase in chlorophyll a from the dry to wet season. These findings are important in understanding why the eutrophication impact from nutrient enrichments in Hong Kong waters is not as severe as one would expect, and also how sewage effluent influences biogeochemical processes of dissolved oxygen and carbon in coastal waters.