Dosing, Not the Dose: Comparing Chronic and Pulsed Silver Nanoparticle Exposures.

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The environmental impacts of manufactured nanoparticles are often studied using high-concentration pulse-additions of freshly synthesized nanoparticles, while predicted releases are characterized by chronic low-concentration additions of weathered particles. To test the effects in wetlands of addition rate and nanoparticle speciation on water column silverconcentrations,ecosystem impacts, andsilveraccumulation by biota,we conducted a year-long mesocosm experiment. We compared a pulse addition of Ag0-NPs to chronic weekly additions of either Ag0-NPs or sulfidized silver nanoparticles. The initially high water column silver concentrations in the pulse treatment declined such that after 4 weeks it was lower on average than in the two chronic treatments. While the pulse caused a marked increase in dissolved methane in the first week of the experiment, the chronic treatments had smaller increases in methane concentration that were more prolonged between weeks 28-45. Much like water column silver, most organisms in chronic treatments had comparable silver concentrations to the pulse treatment after only 4 weeks, and all but one organism had similar or higher concentrations than the pulse treatment after one year. Pulse exposures thus both overestimate the intensity of short-term exposures and effects and underestimate the more realistic long-term exposure, ecosystem effects, and accumulation seen in chronic exposures.