Ecological Risk Assessment (ERA) is charged with evaluating the potential effect of a stressor on the environment, however the data used in the ERA framework is collected at the individual level. Specifically, most data used in ERA when considering the ecological risk of a contaminant come from standardized toxicity tests, in which Daphnia are exposed to potential toxicants alone or in small groups to allow for detailed tracking of effects on survival, growth, and reproduction. Since environmental concerns relate to how contaminants affect daphnid populations, not individual Daphnia, there is a great need to understand the link between individual- and population-level effects of toxicants. Further, these standardized toxicity tests require that the Daphnia are fed unrealistically high algal food levels. Except during algal blooms, natural Daphnia populations experience food levels as low as 1% of those used in toxicity tests. Therefore, it is important to analyze the effects of a toxicant on Daphnia fed environmentally relevant concentrations of algal food at the individual level to quantify the interactions between ecological stressors (food and toxicant stress) at multiple levels of biological organization. We conducted a suite of experiments investigating the effect of silver nanoparticles (AgNPs) on individuals and populations of Daphnia. We measured the direct effect of AgNPs on daphnid survival, growth and reproduction using food levels similar to those found in natural populations. We followed the dynamics of populations experiencing the same food and AgNP environments for 60 days.
Results/Conclusions
The most interesting result was that a concentration that was lethal to individual Daphnia such that few individuals reached adulthood and even fewer reproduced had no significant effect on daphnid populations. To understand the differences observed between AgNPs exposed to individuals or daphnid populations, we simulated daphnid populations with population models parameterized using the individual-level data. We found that a structured population model including feedback between the daphnid population and its algal food explains the observed pattern of population persistence. This study emphasizes the importance of feedbacks when extrapolating individual-level effects to whole populations experiencing multiple stressors.