Phytoplankton abundance, community structure and nutrients in cultural areas of Daya Bay, South China Sea

Abstract

In order to provide a better understanding of phytoplankton structure and their relationship with environmental parameters in typical maricultural areas in Southern China, observations were carried out to estimate the phytoplankton structure and nutrients in eight stations in annual survey from July 1997 to June 1998 and three springs from 1998 to 2000 in Daya Bay, South China Sea. Phytoplankton and ambient chemical, physical, meteorologic data were examined. A total of 198 species of phytoplankton were identified. Diatom was the most diversified group in which 98 species in 39 genera were observed, while dinoflagellate was the second one with 83 species in 14 genera. The annual mean chlorophyll a concentration and cell density was 4.7 mg/m 3 and 425 cells/ml, respectively. The abundance of total phytoplankton ranged from 10 cells/ml to 6698 cells/ml. Water temperatures were mostly over 20 °C with the annual average of 25.0 °C. Salinities remained constant except for the rainy typhoon season from July to September. DSi was sufficient in Daya Bay, and never limited for the growth of diatoms. Diatoms were preponderant in all seasons, while dinoflagellates were prevalent only in spring. Dominant diatoms included Asterionella japonica, Chaetoceros spp., Pseudo-nitzschia spp., Skeletonama costatum and Thalassiosira subtilis, which predominated alternately or co-dominated together. An unidentified dinoflagellate, Gymnidinium sp., was the dominant dinoflagellate, causing two blooms in May 1998. Results from statistical analyses revealed that DIN and DSi had high loadings in population dynamics of diatoms, and DIP was important for dinoflagellates. Meanwhile, peak abundances of diatoms were coinstantaneous with the low values of DIN and DSi concentrations, while high abundances of dinoflagellates with low levels of DIP. The variety in nutrient requirement and utilization by diatoms and dinoflagellates resulted in frequent occurrence of spring blooms caused by them. From the results of this survey, it could be suggested that appropriate water temperature, salinity, sufficient DSi, as well as quick recovery of nutrients, played important roles in the high abundance of phytoplankton and frequent outbreak of blooms in Daya Bay.