VERTICAL DISTRIBUTION OF DINOFLAGELLATE RESTING CYSTS IN RECENT SEDIMENTS FROM DAYA BAY,THE SOUTH CHINA SEA

Six sediment cores from length of 8 to 18cm were collected from DapengAo area,Daya Bay,the South China Sea,by TFO core sampler in Aug. 2001 to investigate the vertical distribution of dinoflagellate resting cysts. 48 different cyst morphotypes representing 20 genera and 6 groups were identified from 35 sediment samples in this study. There were 20 autotrophic species and 28 heterotrophic ones. Cyst species richness in each sample varied from 12 to 29,while the values of Shannon-Weaver’s Diversity Index(H′)were between 0.61 and 4.13. There were an obvious increase of species richness and H’values in the depth of 2—4cm. Cyst concentrations varied from 154 to 2.38×104 cysts/g D Wt,and were between 1000 and 2000 cysts/g D Wt in most samples,while the highest concentration was obtained at the depth of 2—4cm in fish raised area of St.1.Scrippsiella trochoidea was the most common and abundant cyst type. Meanwhile,the abrupt increase of cysts of S. trochoidea in the depth of 2—4cm layer of sediment reflected the bloom of this species in nearby sea areas in 2000. The results from cyst assemblages showed some trend of changes in water quality in this area to some extent,and indicated the typical type of pollution caused by cultural eutrophication,which began in the late 1990s. Cysts of Alexandrium mainly those of A. catenella and A. tamarense complex occurred frequently and abundantly in this area,with the highest concentration and relative frequency of 503cysts/g D Wt and 12.0%respectively. Thus,high concentrations of Alexandrium cysts provided rich“seed bed”for Alexandrium blooms and an important resource of PSP toxin especially in winter.