Ecological Characteristics of Phytoplankton and Causes for Frequent Occurrence of Dinoflagellate Red Tide in the Nanji Islands Sea Area

Based on the data observed in four seasons during May 2004 to February 2005 and collected du-ring the red tide occurrence period from 2001 to 2005 in the Nanji Islands sea area,the seasonal variations in species composition,dominant species and cell abundance of phytoplankton in the sea area,their relations to the environmental factors and the occurrence conditions of dinoflagellate red tide in spring are analyzed.It is shown from the analysis results that 105 species of phytoplankton in the sea area were identified,including 73 species of diatom and 28 species of dinoflagellate,and 16 dominant species include Skeletonema costatum,Ditylum brightwellii and Noctiluca scintillans;the cell abundance in autumn is the highest,being 2.8×108 ind./m3.The species composition and cell abundance data observed during May 2004 to February 2005 have changed considerably compared with those observed in 1990.The seasonal variations in species composition are mainly affected by the Jiangsu-Zhejiang coastal current and Taiwan Warm Current.Many species of red tide plankton and cysts of dinoflagellate provide the species sources for the frequent occurrence of red tide,and the suitable hydrometeorological conditions,specific living habits of some dinoflagllates and high N/P ratio may be the main causes for the occurrence of dinoflagellate red tide in the sea area,and the eutrophication has less relation to the frequent occurrence of red tide in the sea area in spring.