AbstractThe phytoplankton community in the southern waters of Hainan Island was characterized based on four cruise surveys carried out in 1998 and 1999. A total of 290 species (including 23 varieties and 5 forms) were identified by microscope, and species composition, richness and community structure were analyzed using statistical and clustering methods. The results showed that phytoplankton species were abundant in the study area, and were mainly composed of diatoms and dinoflagellates. The species composition showed a significant spatio-temporal variation. In general, the dominant species were characterized by warm water and halophilous or euryhalinous affinities. In the cool seasons, Navicula subminuscula, a eurythermal species, was the major dominant species, while warm water species in-creased rapidly with increasing air temperature. Phytoplankton richness showed no remarkable seasonal variations, and diatoms were the most abundant group throughout the year. However, the richness of dinoflagellate and blue-green algae increased significantly in warm seasons. The center of phytoplank-ton richness occurred in the western waters around the mouth of the Beibu Bay in April and September, and then shifted to the middle-eastern waters in December and January. A clustering diagram indicated that the phytoplankton community had no obvious block distribution. The diversity index of the phyto-plankton community was relatively low in December, at which time the community was unstable in middle-southern waters. In conclusion, this phytoplankton community comprised a unique tropical open sea biota, showing significant seasonal variation, and influenced by hydrologic features.