The biological toxicity of aquatic ecosystems should be considered when assessing the ef-fects of toxicity on the water environment.The aim of this study was to identify the main pollutants in the Baiyangdian(BYD)and the factors that contribute to biological toxicity.We determined various physical and chemical indicators in the surface water of the BYD,in-cluding nutrients and heavy metals,and the biological toxicity.We also explored the sources of the main pollutants and how the pollutants contributed to toxicity in the lake,using corre-lation analysis and an index of the biological toxicity.The results showed that total nitrogen(TN),ammoniacal nitrogen(NH4+-N),chromium(Cr),and zinc(Zn)were the main pollutants in the BYD surface water.The average concentration of Cr was 2.3 times greater than the Class V threshold,and the concentrations at about 65％of the sampling points,mainly those in the southern part of the BYD,exceeded the threshold standard.The average concentra-tion of Zn was 1.25 times higher than the Class V threshold,with the concentrations of about 35％of the samples greater than the threshold concentration.The integrated toxic-ity of the surface water to luminescent bacteria ranged from 0.51％to 58％,and averaged 24.07％,which was within the range of moderate toxicity.The inhibition rates were high near Diantou(59％)and Duan(51.6％).The pollutant levels in the BYD tend to be related to the population density,with pollution mainly caused by sewage and domestic garbage,with little influence from local industries.Cr and TN were strongly correlated,but the biological toxicity was not correlated with any of the individual environmental indicators,which sug-gests that the toxicity in the surface water of the BYD reflected the combined effects of the environmental factors,rather than a single factor.The information from this study,about the main pollutants and the relationships between the physical and chemical properties of the surface water in the BYD,can be used to support plans for restoring the BYD.