PHYTOPLANKTON IN THE NORTHERN SOUTH CHINA SEA IN SUMMER 2004

The investigation in the northern South China Sea (18°—25°N,110°—118°E) in summer 2004 on phytoplankton was carried out with samples taken from 18 grid stations and the results are reported in this paper. A total of 161 taxa belonging to 74 genera of 4 classes were identified by Utermhl method, in which diatoms were predominant and coexisted with dinoflagellates, silicoflagellates, and cyanobacteria. The average cell abundance of phytoplankton was (115046±438891) cells/L (n=91). The dominant species were Pseudo-nitzschia delicatissima, Chaetoceros curvisetus, Chaetoceros diadema, Prorocentrum dentatum and Asterionellopsis glacialis, etc. among them Pseudo-nitzschia delicatissima is most dominated. The average cell abundance of phytoplankton in surface water reduced rapidly from coastal zone to open sea. A high abundance of phytoplankton occurred in coastal seawaters of Guangdong Province and in the Zhujiang (Pearl River) Estuary. Compared with the historical phytoplankton data, the cell abundance of our survey doubled that of summer 1999, but horizontal pattern of the cell abundance was similar. Vertical profiles of cell abundance in survey area showed that, with the increase in depth, the cell abundance decreased rapidly. Subsurface maximum cell abundance found only at Section 2, cell abundance in the other stations was high in surface layers. Pearson Correlate Analysis of phytoplankton cell abundance with environmental factors showed that the phytoplankton abundance correlated positively to the ammonia, chlorophyll a and dissolved oxygen, and negatively correlated to the salinity. The Shannon-Wiener biological diversity index and Evenness of phytoplankton community in survey area were low.