| According to the increasingly serious water pollution of urban rivers, this thesis mainly studied the characteristics of primary water quality factors and the plankton community structure of different water quality grade of urban rivers by carrying out researches on comprehensive assessment of water quality and the structure and diversity of plankton community based on different water quality grade of urban rivers. And then it thoroughly explored the response pattern between the structure, density, diversity of main plankton community and different water quality grade of urban river. The research results showed that:The research results showed that the pollution of Xinjiaopu River was fairly light (Grade V, X1.X2=5.7), Gongye River was worse(worse than Grade V, but not malodorous black, X1.X2=6.6), Jiushanwai River came next(worse than Grade V and seasonally malodorous black, X1.X2=7.4), Shanxia River was the severity water pollution river of the four (worse than Grade V and annually malodorous black, X1.X2=9.1). Considering the time, water quality of the four rivers were the worst during July and August, better in September and October, and the water quality in June and November was the best. The monthly water quality variations of these four rivers were in coincidence with the analysis results.As for phytoplankton, Chlorophyta was found to be the most species in the species composition of phytoplankton of the researched rivers throughout the test period, Cyanophyta came next, Bacillariophyta came the third. The monthly phytoplankton density of Gongye River, Xinjiaopu River, Jiushanwai River and Shanxia River were respectively 2.48×106 ind./L,7.98×106 ind./L,8.91×105 ind./L and 3.11×105 ind./L. There was Water Bloom Pollution in Gongye River and Xinjiaopu River. The results showed that comprehensive water quality identification index had a negative correlation with phytoplankton density (r=-0.837). The phytoplankton density decreased as the water pollution became more serious. There's a significantly negative correlation between Shannon-Wiener index and water quality grade(r=-0.998,p<0.01), a negative correlation between Pielou evenness index and water quality grade(r=-0.835), and a significantly negative correlation between Margalef index and water quality grade(r=-0.994,p<0.05), which indicated that as the water pollution became more serious, the phytoplankton diversity decreased.As for protozoa,48 Ciliata species and 16 Sarcodina species were found in the researched rivers. The proportion of Ciliata and the water quality grade were found to be in very negative correlation(r=-0.983,p<0.05). As the water pollution became more serious, the proportion of Ciliata decreased correspondingly. There was a significantly positive correlation between the proportion of Sarcodina and the water quality grade(r=0.983, p<0.05), the proportion of Sarcodina increased with the aggravation of pollution. The analysis showed that the more close the water quality grade was, the more similar protozoa density and diversity were, and the similarity of Sarcodina was higher than that of Ciliata. The results showed that protozoa density of Xinjiaopu River was the greatest in the four rivers(57643 ind./L), Gongye River came next(22383 ind./L), Jiushanwai River the third(14901 ind./L), the protozoa density of Shanxia River was the smallest(8479 ind./L). Which indicated that there was a negative correlation between protozoa density and water quality identification index (r=-0.850), protozoa density would decline with more serious water pollution. Further more, Shannon-Wiener index, Pielou evenness index and Margalef index were observed to be in negative correlation with water quality identification index(r=-0.832, r=-0.554, r=-0.930). Species diversity of protozoa was reduced with the aggravation of water pollution.As for rotifer, the number of Rotifer's sorts in Xinjiaopu River was found to be the biggest(40 kinds) in the four selected rivers of different water quality grade, and that of Gongye River came next(38 kinds), that of Jiushanwai River was the third(32 kinds), Shanxia River last(24 kinds). The correlative analysis showed that Rotifer's class number of the four river was negatively correlated with the comprehensive water quality identification index(r=-0.989, p<0.05), more water pollution would lead to a decrease in the number of Rotifer species. The correlative analysis of Rotifer's Czekanowisiki similarity coefficient in the four rivers indicated that the more close the water quality grade was, the more similar Rotifer density and diversity were. Rotifer density of Xinjiaopu River was the grcatest(5145 ind./L), then came Gongye River(3839 ind./L), followed by Jiushanwai River(2593 ind./L), and that of Shanxia River was the smallest(1754 ind./L). It showed a significantly negative correlation between Rotifer density and comprehensive water quality identification index(r=-0.961, p<0.05) by relatively analysis, that is, Rotifer would gradually diminished in quatity with more water pollution. Further more, Shannon-Wiener index, Pielou evenness index and Margalef index of Rotifer were observed to be in negative correlation with water quality identification index(r=-0.801, r=-0.515, r=-0.879). Species diversity of Rotifer were reduced with the aggravation of water pollution.In the test,191 kinds of plankton were identified, within which,87 kinds were phytoplankton,64 kinds were protozoa,40 kinds were rotifer. A significantly negative correlation was observed between species number of plankton and the water quality grade by correlation analysis(r=-0.988, p<0.05). The species number would decline with more serious water pollution. Studying the Czekanowisiki similarity coefficient of every two of phytoplankton, protozoa and rotifer revealed that the more close the water quality grade was, the higher Czekanowisiki similarity coefficient got. The community structure of phytoplankton, protozoa and rotifer were negatively correlated with water quality grade (respectively are r=-0.906, p<0.05; r=-0.861, p<0.05; r=-0.912, p<0.05). According to extend, the order of correlation degree was rotifer, phytoplankton, and protozoa. As the water pollution became more serious, density of phytoplankton would significantly decrease, which promoted the density proportion of protozoa and rotifer. Correlation analysis between monthly mean density of various organism groups and the comprehensive water quality identification index of corresponding rivers indicated that the density of phytoplankton, protozoa, and rotifer were all in negative correlation with water quality grade (respectively were r=-0.837; r=-0.850;r=-0.961,p<0.05). Density of plankton would decrease with the aggravation of water pollution. Correlation analysis between plankton density and the mainly water quality factors shows that there existed a significant positive correlation between plankton density and DO(r=0.985, p<0.05), a negative correlation between plankton density and NH3-N(r=-7.46), a negative correlation between plankton density and TP(r=-0.605). As DO mass concentration increases, concentration of N and P would decrease, as a result, water equality would gradually improved, making a better environment for the plankton. thus, the plankton density would increase.
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