Abstract: By using the survey method for freshwater plankton, the species composition and abundance variation of protozoa in Dianshan Lake in Shanghai was investigated from 2004 to 2006, and the relationships between protozoan density, Margalef diversity index, and lake water eutrophication were studied. A total of 48 species belonging to 33 genera of 7 orders were identified, including 10 Sarcodina species and 38 Ciliata species. The dominant species were Tintinnidium fluviatile Stein, T. entzii Chiang, Strombidium viride Stein, Didinium balbiamii nanum Kahl, and Vorticella companula Ehrenberg, and the sum of the dominant species densities accounted for 69.8% of the total protozoan density. The annual average abundance of protozoa in 2004, 2005, and 2006 was 3365, 5298, and 2549 ind·L^-1, respectively, and the seasonal fluctuation in the annual average density of protozoa in the three years was shown as spring (7685 ind·L^-1) > winter (3241 ind·L^-1) > autumn (2153 ind·L^-1) > summer (1680 ind·L^-1). Two sampling sites (St2 and St5) near the villages had the higher densities of protozoa, and the horizontal distribution of protozoan density was consistent with that of modified Carlson trophic state index. The Margalef index ranged from 1.15 to 1.99, with an average of 1.45, which was closely related with water quality.

Key words: Heterosigma akashiwo, Alexandrium tamarense, Skeletonema costatum, UV-B radiation enhancement, DNA damage