The influence of late summer typhoons and high river discharge on water quality in Hong Kong waters

A typhoon produces a rapid mixing and flushing event and it can be added to the list of other factors such as shallow water depth, spring tidal mixing, the Pearl River discharge, summer upwelling that make Hong Kong waters relatively resistant to eutrophication impacts. Two typhoons passed over Hong Kong waters and provided an opportunity to document the changes in water quality in late summer 2003. Before the typhoon (Aug 19–20) and during a neap tide, a large algal bloom (>10 μg Chl-a L−1) occurred in the stratified southern waters influenced by the Pearl River estuarine waters with high NO3. However, PO4 and SiO4 were drawn down to near limiting concentrations by the large bloom. After the typhoons, Chl-a decreased to 2 μg L−1 due to vertical mixing and advection. The heavy rainfall and increased river discharge quickly re-set the water column to the usual strong summer stratification in only a few days. As a result, high nutrients in the river discharge stimulated another large algal bloom a few days after the next neap tide when tidal mixing was reduced. In the southern waters, the deeper station showed stronger stratification and lower bottom dissolved oxygen (DO) than the shallower station suggesting that the low DO in the bottom water may have come from offshore transport.