Studies on species composition and abundance distribution of phytoplankton in the South China Sea

The species composition and cell abundance of phytoplankton in a large spatial scale of the South China Sea(5°～25°N,105°～120°E) were reported in the summer and winter of 1998 and in the northern South China Sea in the summer of 1999. A total of 119 taxa belonging to 54 genera of phytoplankton in the study waters were identified. The average cell abundance of phytoplankton is 0.833(SD±3.224) ×10~3 cells/dm~3 (n=61) in the summer of 1998, among which, diatoms was accounting for 75.8%, dinoflagellates are accounting for 10.80% and cyanobacteria is accounting for 13.5% of total cell abundance of phytoplankton.The high abundance of phyto-plankton occurred (5×10~3 cells/dm~3 ) in the northeast of Hainan, among which, diatoms were the predominant. The dominant species is Thalassionema nitzschioides. The cell abundance of phytoplankton (mean 8.46×10~3cells/dm~3) was higher in winter than in summer, among them, diatoms are accounted for 95.8%, dinoflagellates are accounted for 1.9% and cyanobacteria is accounted for 2.3% of total cell abundance of phytoplankton. The high abundance of phytoplankton occurred in the southwest and northwest of Nansha Islands, and in sea area of the northeast of Hainan, and off the Dongsha Islands. The cell abundance of phytoplankton is greater than 10~3 cells/dm~3 in the above sea areas, among them, diatoms are predominant. The dominant species is Thalassiothrix frauenfeldii, Pseudonitzschia pungens, Pseudonitzschia delicatissima, Chaetoceros spp., Bacteriastrum spp. etc. The dominant groups are warm water group and eurytopic group. The species composition and cell abundance of phytoplankton are influenced by water temperature, salinity, nutrient, water masses, circulation and upwelling, etc. The cell abundance of phytoplankton and nutrition concentration are high in upwelling zone. The cell abundance of phytoplankton and contents of nutrition are low in central area of anticyclonic gyre. Phytoplankton abundance shows a distinct correlationship with the nutrient concentration,of which the nitrate-N and phosphate-P are positive while water temperature and salinity are negative in the South China Sea.