Influence of monsoons and oceanographic processes on red tides in Hong Kong waters

Abstract

Hong Kong waters in the northern part of the South China Sea are subjected to seasonal oceanographic processes due to monsoonal winds and Pearl River outflow. Several 100 red tides have occurred in Hong Kong waters during 1983 to 1998, and show a clear spatial and temporal distribution. Most (74%) occurred in semi-enclosed NE bays away from the Pearl River estuary, and fewer occurred in western estuarine waters. Most red tides (70%) occurred between December and May, and fewer in summer. However, nutrients are high in the Pearl River estuary, whereas nutrients are generally low in the NE bays (Mirs Bay and Port Shelter) and cannot support the high biomass of red tides. This suggests that there are concentrating mechanisms promoting the formation of red tides in NE waters, either by vertical migration or horizontal aggregation under suitable wind conditions (NE winds and moderate speeds around 6 m s-1). The prevailing NE monsoon winds in winter and spring result in downwelling and a longer residence time of waters in the NE bays, creating conditions similar to those of a batch culture, and allowing local inputs of nutrients and vertical migration of phytoplankton in shallow waters to play a dominant role in favor of local red tides. This may be one of the reasons why more red tides occur in winter and spring in semi-enclosed waters. In summer, the SW monsoon winds result in upwelling along the coast, and high river discharge and rainfall cause an increased estuarine circulation in the Pearl River estuary and rapid outflow of the surface water from these semi-enclosed waters. As a result, residence time of these waters decreases, and they may be analogous to semi-continuous or continuous cultures. This may explain why there are fewer red tides in summer. The species occurring most frequently are (in descending order) Noctiluca scintillans, Gonyaulax polygramma, Skeletonema costatum, Mesodinium rubrum, Prorocentrum minimum and Ceratium furca. In general, dinoflagellate red tides occur mostly in April when Si is low, whereas diatom red tides occur in June when Si increases due to the freshwater discharge. Deep oceanic water on the continental shelf is drawn into the NE bays during the upwelling; it is poor in nutrients, and does not favor nutrient acquisition in deep water by vertical migrating dinoflagellates.