Characteristics of Phytoplankton community in Suzhou river in summer and autumn and their affecting factors

In order to explore characteristics and evolution laws of the phytoplankton community in Suzhou River in the summers and autumns of the four years after implementation of the Suzhou River Rehabilitation Project, and status quos of the phytoplankton communities and water pollution in various sections of the river, a survey was conducted, collecting samples six times from the five sampling sites set along Suzhou River during the period from Jun. to Nov. of 2012. A total of 259 species of phytoplankton belonging to 95 genera of 8 phyla were identified and recorded during the survey. On the whole the phytoplankton community was composed mostly of Chlorophyta and Bacillariophyta, with Cyclotella bodanica, Cryptomons erosa and Scenedesmus quadricanda being the predominant species and the indicator species of organic pollution of waterbodies. Density of the phytoplankton community varied in the range from 0.46×10⁴ to 1.91×10⁴L^-1, with the highest observed under the Wuning Road Bridge in June and the lowest at Zhaotun in August. The Shannon-Wiener index and Pielou index of the phytoplankton community in Suzhou River was 3.22±0.38 and 0.75±0.09,respectively. In light of the three parameters, the water of Suzhou River was moderately polluted. Compared with the corresponding period of 2005, apparent changes had taken place in dominancy of species in Suzhou River, with severe pollution indicator species (Chlorella vulgaris and Oscillatoria tenuis) replaced by moderate pollution indicator species (Cyclotella bodanica, Cryptomons erosa and Scenedesmus quadricanda). The river was no longer pitch black and odorous, but the water in the lower reaches was still obviously worse in quality than that in the upper reaches. The concentration of TN, TP and N/P ratio in the down-streams reached 4.61 mg · L^-1, 0.44 mg · L^-1 and 16.48, respectively, during the survey. Correlation analysis of the Shannon-Wiener index with various environmental factors shows that temperature and N/P ratio were the main factors affecting diversity index of the phytoplankton community of the whole river, and the correlations of diversity index with TN and N/P ratio were rather significant in the downstreams. Redundancy analysis (RDA) of the density of the dominant species of phytoplankton with environmental factors shows that besides temperature, N: P ratio was the most important factor influencing density of the phytoplankton community in Suzhou River.