Pulse vs. press disturbance of wetland mesocosms by an emerging contaminant, silver nanoparticles
It is well known that the frequency and magnitude of disturbance drive responses in aquatic ecosystems. With chemical contaminants prone to sequestration in sediments, a low concentration press-exposure may have greater long-term impacts than a high concentration pulse-exposure. One such class of contaminants are engineered nanoparticles, emerging contaminants with interesting physicochemical properties due to their small diameters <100 nm. In wetland systems these particles are prone to aggregation and sedimentation, which can diminish the importance of their unique properties.
In this experiment, we ask how the fate and impacts of silver nanoparticles (AgNPs) differ under pulse and press exposure scenarios. We focused on silver nanoparticles as they are both biocidal and widely used in commercial and consumer applications. AgNPs were added to triplicate wetland mesocosms under either pulse (one-time addition, 450 mg Ag, resulting in 0.75 mg Ag/L) or press (weekly addition, 8.65 mg Ag, resulting in 0.014 mg/L initially) treatments. Here we report watercolumn and submersed plant silver concentrations, dissolved organic carbon (DOC), and dissolved greenhouse gas concentrations during the first 120 days of this experiment. These measurements allow us to examine both how pulse and press AgNP perturbations differ in terms of how they impact ecosystems and how ecosystems impact the AgNPs.
The pulse-disturbance with AgNPs stimulated the release of dissolved organic carbon (DOC) from macrophytes, which showed a small but significant increase from 24.6±1.4 to 29±0.5 mg C/L. This in turn caused a cascade of effects (decreased dissolved oxygen, increased dissolved carbon dioxide and methane). In the press-treatment there were no measured impacts on DOC, carbon dioxide, or methane concentrations as compared to controls. The stability of the AgNPs and rate of removal from the watercolumn was fundamentally different under pulse versus press exposures, with greater stability evident by the lower removal rate in the pulse exposure due to acquired macrophyte-derived DOM coatings protecting the particles against aggregation and sedimentation. By day 24 the watercolumn concentration of the press treatment surpassed that of the pulse treatment, despite the enhanced stability of particles in the pulse treatment and that 17-fold more silver had been added compared to the press by that date. The higher watercolumn concentrations in the press treatment—which matched with higher concentrations in macrophytes—suggest that the press exposure has a higher probability of having long term chronic impacts on organisms and ecosystem processes, in contrast to the strong short term impacts of the pulse exposure.