| Phytoplankton species composition and ecological distribution of genus Chaetoceros were analyzed seasonally from 2006-2007 in Daya Bay, Zhanjiang (Zhanjiang Harbor and Leizhou Bay) and Shantou (Shantou Harbor and Zhelin Bay) watersheds, respectively with combining light microscopic and scanning electronic microscopic methods.The results indicated 37 species of Chaetoceros were identified totally, including C. affinis, C. atlanticus var.atlanticus, C. atlanticus var.neapolitana, C. atlanticus var.skeleton, C. borealis, C. brevis, C. coarctatus, C. compressus, C. costatus, C. curvisetus, C. danicus, C. debilis, C. decipiens f.decipiens, C. decipiens f.singularis, C. denticulatus, C. denticulatus f.denticulatus, C. diadema, C. didymus, C. didymus var. anglisa, C. distans, C. diversus, C. indicus, C. knipowitschii, C. laevis, C. lauderi, C. lorenzianus, C. messanensis, C. pelagicus, C. peruvianus, C. pseudocurvisetus, C. pseudodichaeta, C. rostratus, C. siamense, C. socialis, C. teres, C. tortissimus, C. castracanei. Two unidentified species, Chaetoceros spl and Chaetoceros sp2, were also observed. Diagnostic descriptions and illustrations are provided for all 39 species of Chaetoceros from studied area. Some new recorded ultrastructural features of Chaetoceros were observed by SEM, such as intercalary setae of C. affinis and C. messanensis, and the setae of C. costatus, C. diadema and C. pseudocurvisetus with spiral rows of spines. Intercalary setae of C. coarctatus have 4 rows of spines and the terminal setae of C. coarctatus have 10 rows of spines. It's worth noting that they are differ from previous studies. Morphological description and taxonomic analysis of these new ultrastructural features was carried out. The ecotypes of Chaetoceros in the coastal water of Guangdong are dominated by warm water and temperature inshore species, with a few of oceanic species.Species of Chaetoceros in Daya Bay, Zhanjiang and Shantou sea area were observed with numbers of 33,26 and 26, respectively. Seasonally variation of species number of Chaetoceros was found in these study areas. The species number of Chaetoceros in Zhanjiang sea area was highest in winter and lowest in summer and autumn, comparably highest species richness were found in Daya Bay and Shantou sea area in spring. The variations patterns of phytoplankton abundance decreased seasonally.The annual average of cell abundance in these three sea areas were 49.89×103cells/L, 602.31×103cells/L and 1032.11×103cells/L, respectively. The cell abundance and distribution of Chaetoceros showed also a distinct variation pattern seasonally. Cell abundance of Chaetoceros in Zhanjiang sea area was higher in summer and lower in spring with a range of 7.14×10-2300.23×103cells/L. C. pseudocurvisetus was the dominant species in Zhanjiang sea area in summer with cell abundance of 2289.870xl03cells/L, contributioning 87.05% to the total phytoplankton. Cell abundance of Chaetoceros in Daya Bay ranged from 2.10 to 99.93×103cells/L, was higher in spring and lower in winter. And C curvisetus was the dominant species in Daya Bay in spring and its cell abundance was 87.128×103cells/L, with its percentage contribution to total phytoplankton reaching 50.27%. Cell abundance of Chaetoceros in Shantou sea area ranged from 0.23～3799.50×103cells/L, was higher in spring and lower in winter. C. curvisetus and C. debilis were the dominant species in Shantou sea area in spring and their cell abundance were 1917.220×103cells/L and 1294.411×103cells/L, with their percentage contribution to total phytoplankton reaching 43.13% and 29.12% respectivly. Horizontal distribution of cell abundance of dominant species in studied area varied seasonally and differed significantly among the three study areas.
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