DYNAMICS ON CELL DENSITIES OF DIATOM, DINOFLAGELLATE AND RELATIONSHIP WITH ENVIRONMENTAL FACTORS IN AOTOU AREA, DAYA BAY, SOUTH CHINA SEA

Daya Bay of the South China Sea, where a major nuclear power plant resides, is a large local inlet and an important aquaculture area in Guangdong Province. This area has been seriously affected by human activities in the last decade. In order understand better the phytoplankton structure and their relationship to environmental factors in aquaculture areas, a study was carried out on phytoplankton structure and bloom dynamics from July 1997 to June 1998 in Aotou area of Daya Bay. Water samples, ambient chemical, physical and meteorological data were examined. 198 species of phytoplankton were identified during one-year study period. Diatom was the most diversified group of which 98 species in 39 genera were observed, while dinoflagellate ranked second with 83 species in 14 genera. More than 50 species of harmful algal bloom (HAB) causative phytoplankton were recognized in this survey, such as genus Alexandrium, Dinophysis acuminate, Gonyaulax, Gymnodinium, Karenia, Akashiwo, Scrippsiella, Chattonella, Heterosigma and Phaeocystis. Nutrient contents were not very high, but when supplemented by mariculture and domestic discharge, they increased quickly, resulting in high primary production. The maximum and annual average phytoplankton cell densities were 6689.8 and 424.7 cells/ml, respectively. Blooms occurred frequently in the year, four occurred in spring and one in each of the other three seasons. Most blooms occurred in offshore areas first, before spreading and assembling two or three days later, by way of currents and winds, in inshore areas. Diatoms were donminant in all seasons making up an average of 94.9% of total phytoplankton. Dinoflagellates, however, were prevalent only in spring. Dominant diatoms included Asterionella japonica, Chaetoceros affinis, Pseudo-nitzschia spp., Skeletonama costatum and Thalassiosira subtilis, which predominated alternately or co-dominated together. Nutrient concentrations were high prior to spring blooms of diatom, and large amounts of nutrients were assimilated as cell densities increased. Nutrient content especially that of DIN reached to the minimum points at the maximum scale of diatom blooms. Growth of dinoflagellates then accelerated as nutrient content rose and diatom blooms declined. High DIP promoted the blooms of dinoflagellate. The DIP was almost fully used during bloom period. Furthermore, DIP exhaustion dissipated dinoglagellate blooms. Nutrient elements needed by diatoms and dinoflagellates are different. This was probably the reason for frequent occurrence of spring blooms caused alternately by these two different phytoplankton groups. Results suggest that favourable water temperature, salinity, meteorological conditions and sufficient DSi, rapid influx of nutrients are major causes of high cell concentrations of phytoplankton and frequent outbreaks of bloom in Aotou area of Daya Bay. Moreover, accumulation of algal cells from the outer areas to the inner bay by physical factors as wind and current, play an important role in the occurrence of blooms.