THE ABUNDANCE CYCLE AND INFLUENCE FACTORS ON RED TIDE PHENOMENA OF NOCTILUCA-SCINTILLANS (DINOPHYCEAE) IN DAPENG BAY, THE SOUTH-CHINA SEA

?he seasonal occurrence of Noctiluca scintillans and the causative mec hanisms of red tide phenomena were investigated in the northwestern pa rt of Dapeng Bay, the South China Sea, from March 1990 to June 1992. I t occurred in the plankton usually from January through May/June with peak abundance from March to early May. At least eight red tides of th is organism, when the surface water was covered with the pink patches and slicks, were observed during the investigation period, and all of them took place between March and early May. Noctiluca occurred at tem peratures from 15.8 to 28.6 degrees C, and the population density was the highest at temperatures between 19 and 25 degrees C. It disappeare d from the plankton in summer and fall when the temperature warmed to lethal level of similar to 30 degrees C and then the population probab ly was transported to outside of the bay. During the peak period of ab undance between March and May, the log transformed average density of Noctiluca at each station showed a significantly positive relationship with average water temperature, indicating that within its optimum te mperature a higher temperature promotes the growth of Noctiluca popula tion. Sharp declines of the population were associated with abrupt dec reases in salinity due to heavy rain. Hence, rich food supply is basic ally necessary for Noctiluca to reproduce massively, but the suitable temperature, stable muggy weather without heavy rain are considered to be the important factors for the Noctiluca blooms. The surface conver gence, due to the geographical and tidal features, may also constitute a direct driving force leading to its frequent occurrences in the stu dy area. The Noctiluca density showed a negative relationship with chl orophyll a concentration, indicating that this species gives a predati on pressure on the phytoplankton.