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Sewage treatment plant (STP) effluent has been used as ecological supplementary water in dry and polluted urban rivers. However, more consideration should be given to the impact of STP effluent on water quality and the environment. In this paper, a typical urban tributary river of Dianchi was selected to study the impact of STP effluent on water quality and plankton communities. Using a paired-samplet-test, factor analysis and canonical correspondence analysis, changes in water quality, plankton communities and their relationships were analyzed. The results showed that STP effluent decreased the concentrations of suspended solid, chemical oxygen demand, five-day biological oxygen demand (BOD5) and total phosphorus. However, it significantly increased the concentration of nitrate, total nitrogen and nitrate nitrogen (NO3-N). A distinct shift in the phytoplankton community from Chlorophyta (green algae) to Bacillariophyta (diatoms) occurred after the influx of STP effluent and further analysis indicated that this shift was primarily caused by the increase in NO3-N and facilitated by the seasonal rise in water temperature. Zooplankton communities were less sensitive to changes in water pollution condition than phytoplankton communities and the zooplankton community structure of the Chuanfang River was mainly influenced by water temperature. [ABSTRACT FROM AUTHOR]