SEMINAR: Understanding Eutrophication Impacts in Hong Kong Waters: Physical and Chemical Factors Reduce Impacts

The Pearl River Estuary is a sub-tropical estuary and the second largest in China based on discharge volume from the Pearl River. Processes in the estuary vary spatially and temporally (wet vs dry season). In the wet season, the SW monsoon windspush the Pearl River estuarine plume into Hong Kong waters and hence eutrophication impacts are a combination of nutrient loads from the Pearl River plus local sewage discharge. Eutrophication is not as severe as one would expect from these very high nutrient loads. This estuary shows a remarkable capacity to cope with excessive nutrients. Physical processes such as river discharge, tidal flushing, turbulent dispersion, wind-induced mixing, estuarine circulation and a shallow water column play important roles in controlling the production and accumulation of algal blooms and the potential occurrence ofhypoxia. Superimposed on the physical processes of the estuary are the chemical and biological processes involved in the production of algal blooms. For example, the 100N:1P ratio of estuarine waters indicates that phosphorus potentially limits the amount of algal biomass (and potential biological oxygen demand) in summer. While extended periods of hypoxia are rare in Hong Kong waters, episodic events have been reported to occur during late summer due to factors such as low wind, high rainfall and river discharge which result in strong density stratification that significantly dampens vertical mixing processes. Nutrient loads are likely to change over the next several decades and monitoring programs are essential to detect the response of the ecosystem.