| Marine phytoplankton has been playing an important role in ocean ecosystem, so studies on phytoplankton in ecology has become one of the most dynamic frontiers in marine ecology research.In this paper, from 2003 to 2005 eight times of field trips monitoring phytoplankton in Bohai Bay was carried out and the results were analyzed accordingly, 80 species of phytoplankton were identified during these field trips, which contained 39 genus, 28 families and 6 phylum. These species of phytoplankton were sorted as follows according to different phylum: 15 families of Bacillariophyta had 23 genus and 51 species; 8 families of Pyrrophyta had 25 species and 12 genus; Prozotoa, Chrysophyta, Cyanophyta and Chromophyta all individually just had one family, one genus and one species. The results showed that Bacillariophyta were the preponderant species in the surface waterbody in Bohai Bay. The varying trend of average phytoplankton species showed obvious seasonal characteristics during three years: the least in spring, more in summer, the most in autumn; in low-flow season, the cells abundance of phytoplankton ranged from 2052 cells/L to 53963 cells/L, the mean value is 17985 cells/L, in high-flow season the cells abundance of phytoplankton ranged from 1424 cells/L to 316200 cells/L, the mean value is 46180 cells/L, in mean-flow season the cells abundance of phytoplankton ranged from 3116 cells/L to 1203464 cells/L, the mean value is 125830 cells/L.The seasonal variation of phytoplankton abundance was obvious during these three years, it was shown as follows: The phytoplankton abundance not only gradually decreased along mean-flow, high-flow and low-flow season but also were slightly high in inshore area compared with offshore area and were much higher in southern part than that of northern part in near 3 years. The phytoplankton abundance got the highest value in June of 2004, which was related to a large-scale red tide in Bohai Bay. From 2003 to 2005 the variation tendency of phytoplankton preponderant algae in Bohai Bay showed that: the coastal alga increased, the abroad alga reduced, the harmful algae and toxic algae was distributed broadly. By means of cluster analysis of phytoplanktonic community, the results were concluded that data gathered in the same season or from neighboring stations had higher community similarity.In this paper 18 kinds of diversity indices were studied in order to appraise the validity of eutrophication by setting up three ideal gradient phytoplanktonic community sequences and they were verified to be effective using Shannon's Index(base 2 or e), Simpson's Reciprocal Index, Berger-Parker's Index, Simpson's Encounter Index and Pielou's Evenness Index. The nutrition level of Bohai Bay water quality was appraised in near three years combining the effect of Shannon's Index(base 2), Pielou's Evenness Index and Naughton's Dominance Index, and the result indicated that: in general the water quality of Bohai Bay was in the state of mesotrophication and eutrophication, and it was most serious in spring; less serious in summer; and best in autumn, furthermore the extents of eutrophication inshore was more serious than that of offshore.Compared with the assessment results from chemistry index model and diversity indices one, it was verified to be effective in appraising the nutritional level of sea areas using the diversity indices model. Seen from the analyzed data, it is indicated that the method of the phytoplankton abundance alone has its disadvantages in appraising the nutritional level of the water quality, and the method of diversity index will be much more effective especially from the biology view.
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