Nanoparticles have had a large impact and driven a growing commercial industry of nano-enabled products. Nanoparticles’ small size causes large changes in physicochemical properties compared to their “bulk” states, and this has been utilized for a wide variety of applications, ranging from biomedical to electronic, to cosmetic use. However, these novel properties also carry with them unknown effects on our environment. Numerous laboratory studies have found deleterious effects of nanoparticles on aquatic organisms, but we are missing key knowledge on how these toxic effects amplify in ecological systems. The aim of my dissertation is to investigate the effects of nanoparticles on freshwater systems through a series of experiments on phyto- and zooplankton and the development of quantitative models to explain these empirical results and extrapolate effects to other systems. My work has identified the importance of ecological feedbacks in nanotoxicology, specifically two novel feedbacks: algae produce dissolved organic carbon (DOC) that mitigates the toxicity of nanomaterials (silver and iron nanoparticles) to the cells themselves and a concentration of silver nanoparticles (AgNPs) that is toxic to individual Daphnia has no effect on populations of zooplankton due to population-level feedbacks.

The development of quantitative models of both of these feedbacks has enabled the estimation of the strength of both nanomaterial toxicity and of the mitigating feedback. Through the development of models of algal growth, DOC production, and nanoparticle toxicity, I estimated the strength of both the inactivation of toxicity by DOC as well as the toxic strength of the various contributors to nanotoxicity (nano versus ionic, different transformations of nanoparticles). Estimating the relative contributions of different forms or products of nanoparticles to their overall toxic effect can be useful in ecological risk assessment, as it could identify toxic factors that are already regulated (such as ionic silver) along with those that are currently unregulated (nanosilver) and allow for direct comparison of their toxicity. The development of models of daphnid growth and reproduction, parameterized with individual-level data of AgNP exposure of Daphnia at multiple food rations, allowed me to identify the feedback that seemingly disrupts extrapolation between levels of biological organization. A concentration of AgNPs that is toxic to individual Daphnia has no effect on small populations of Daphnia due to population-level feedbacks in which the zooplankton population equilibrates at a lower consumer and higher resource biomass. This increase in the amount of food per individual allows the zooplankters to survive AgNP exposure. Overall, my dissertation work highlights the importance of ecological and environmental complexity when estimating the impacts of nanoparticles on freshwater systems.