THE PRELIMINARY STUDY ON PHYTOPLANKTON COMMUNITY STRUCTURE IN THE CENTRAL BOHAI SEA AND THE BOHAI STRAIT AND ITS ADJACENT AREA

Phytoplankton species composition and species succession were analyzed in 1998-1999 during 2 nestle investigation cruises in the Central Bohai Sea, the Bohai Strait and adjacent waters, and two monthly monitoring stations at Penglai and Changdao for 15 months. The results show that diatoms and dinoflagellates are the major components of phytoplankton community in the survey region, with cynobacteria, green algae and siliconflagellates being also common found in some stations. The ecotype of these phytoplankters is temperate and neritic species; some warm oceanic species can be imposed from the Yellow Sea and distributed near the Bohai Strait. The size fractional features of phytoplankton community in the survey area show that nanophytoplankton was a major component and the picophytoplankton was also important (i.e., it accounted for 21.3% and 28.4% biomass separately in autumn 1998 and spring 1999). The horizontal distribution of phytoplankton in the survey area has a close relationship with physical chemical conditions in ambient waters. Three phytoplankton provinces can be discerned at the Huanghe Estuary, north of Bohai Bay and the Bohai Strait. Short term (days) development of phytoplankton community in the central Bohai Sea depends on the species changes of the dominant species but the key species. Annual variations of the phytoplankton community in the Bohai Strait show a typical double peak cycle in the north hemisphere. The higher cell abundance peak is in April and the lower peak in September. In spring, the phytoplankton community is mainly made of small cell diatoms, and in autumn the phytoplankton community is characterized by big cell diatoms coexisting with dinoflagellates. The species succession was the major process of seasonal changes of phytoplankton community development pathway in the Bohai Sea. Long term observations from 1958-1999 in the Bohai Sea show that the replacement of diatom by dinoflagellates is the main feature of phytoplankton community changes in recent years, which is indicated by N/P ratio increment and Si/N ratio decrement. Red tides are inescapable in the Bohai Sea due to the anthropogenic nutrient input and drying up of the Huanghe River.