Influence of the Pearl River estuary and vertical mixing in Victoria Harbor on water quality in relation to eutrophication impacts in Hong Kong waters

Abstract

This study presents water quality parameters such as nutrients, phytoplankton biomass and dissolved oxygen based on 11 years of water quality data in Victoria Harbor and examined how the Pearl River estuary discharge in summer and year round sewage discharge influenced these parameters. Nutrients in Victoria Harbor were strongly influenced by both the Pearl River and sewage effluent, as indicated by the high NO3 inputs from the Pearl River in summer and higher NH4 and PO4 in Victoria Harbor than both its sides. N:P ratios were low in the dry season, but increased to >16:1 in the wet season, suggesting that P is potentially the most limiting nutrient in this area during the critical period in the summer. Although there were generally high nutrients, the phytoplankton biomass was not as high as one would expect in Victoria Harbor. In fact, there were high concentrations of chl near the bottom well below the photic zone. Salinity near the bottom was lower in Victoria Harbor than at the two entrances to Victoria Harbor, suggesting strong vertical mixing within Victoria Harbor. Therefore, strong vertical mixing and horizontal advection appear to play an important role in significantly reducing eutrophication impacts in Victoria Harbor. Consequently, dissolved oxygen near the bottom was low in summer, but only occasionally dipped to 2 mg/L despite the high organic loading from sewage effluent.