Intraspecific diversity is critical to population-level risk assessments

Environmental risk assessment (ERA) is a critical tool for protecting life and its effectiveness is predicated on predicting how natural populations respond to contaminants. Yet, routine toxicity testing typically examines only one genotype from surrogate species, which may render risk assessments inaccurate as populations are most often composed of genetically distinct individuals. To determine the importance of intraspecific variation in the translation of toxicity testing to populations, we quantified the magnitude of phenotypic variation within 20 Daphnia magna clones derived from one lake. We repeated these assays across two exposure levels of microcystins, a cosmopolitan and lethal aquatic contaminant produced by harmful algal blooms. We found considerable intraspecific genetic variation in survival, growth, and reproduction, which was amplified by microcystins exposure. Using simulations, we demonstrate that the common practice of employing a single genotype to calculate toxicity tolerance failed to produce an estimate within the 95% confidence interval over half of the time. Finally, we conducted whole genome sequencing of all 20 clones to test whether differences in toxicological responses were associated with overall genomic divergence or divergence at candidate loci based on prior gene expression work. We find no overall correlations, suggesting that clonal variation, but not variation at candidate genes, is an important predictor of population-level responses to toxic insults. These results illuminate the importance of incorporating overall intraspecific genetic variation, without focusing specifically on variation in candidate genes, into ERAs to reliably predict how natural populations will respond to contaminants.