Toxicity of silver nanoparticles (AgNPs) has been studied in various culture media. However, these media notably differ from the natural aquatic system, thus the conclusions may be inapplicable for real environment condition. The toxicity and its underlying mechanism of AgNPs in surface waters warrant more investigations. This study investigated the acute toxicity, chronic toxicity, bioaccumulation, and alga-daphnia food chain transfer of citrate-coated AgNPs (C-AgNPs) and Ag+ (from AgNO3) to D. magna in a culture medium (M4) and a surface water sample. Results show that the acute toxicity in the surface water was significantly lower than that in the M4 medium and the toxicity of Ag+ was greatly higher than that of C-AgNPs. The 48h median effect concentration (EC50) of C-AgNPs to D. magna in the M4 medium and the surface water was 110±9.3μg/L and 270±26μg/L, respectively, while that of Ag+ was 1.8±0.7μg/L and 8.0±0.6μg/L, respectively. The released Ag+ contributed to but not dominated the acute toxicity of C-AgNPs. At the EC50 of C-AgNPs, the contribution of released Ag+ was 35.7% and 28.0% to the apparent nanotoxicity in the M4 medium and the surface water sample, respectively. The chronic toxicity of C-AgNPs and Ag+ was also lower in the surface water sample than in the M4 medium as indicated by the significantly higher survival of daphnia in the surface water during the 21d exposure. The daphnia took up less but depurated more Ag in the surface water than in the M4 medium, which could account for the lower toxicity in the surface water. Biological magnification of Ag through the alga-daphnia food chain was not observed. These findings will be helpful for assessing the environmental risk of AgNPs and understanding the mechanism of nanotoxcity.