Silver nanoparticles(AgNPs)are used in various commercial products because of their antimicrobial properties.However,there is a high probability that AgNPs are released into the environment and AgNP concentrations in surface waters are predicted to be in the ng L-1 to low ? g L-1 range.Toxicity to aquatic organisms has been demonstrated at ?g L-1 levels,which raises concerns about the ecological risks of exposure to AgNPs.This study aimed to evaluate the persistence and transformations of 50 nm PVP AgNPs in natural lake water using various analytical techniques.Studies were conducted in mesocosms deployed in a lake in the Experimental Lakes Area(ELA),ON,Canada and mesocosms were spiked with a one-time addition of AgNPs at a nominal concentration of 45 ?g L-1.Samples were collected for analysis by centrifugal ultrafiltration,CPE,spICP-MS,and AF4-ICP-MS to monitor changes in AgNP agglomeration and dissolution over time.Total Ag levels in the two mesocosms declined relatively slowly,with a t1/2 of~20 days.There was no evidence of extensive agglomeration of AgNPs.The stability of AgNPs may have been due to the low ionic strength and moderate concentrations of dissolved organic carbon(DOC)in the lake water.CPE,spICP-MS,and AF4-ICP-MS analysis all indicated that there was dissolution of AgNPs over time,but the concentrations of dissolved silver were generally below detection limits.Modelling of Ag+ speciation predicted that all free ion released from AgNPs would complex with DOC,which is expected to reduce the toxicity of Ag+ in natural waters.The results from this study emphasize that multiple analytical techniques are required in order to fully characterize AgNP transformations in the aquatic environment and to overcome the analytical limitations of each method.