Intraspecific genetic variation is critical to robust toxicological predictions in Daphnia

Environmental risk assessment is a critical tool for protecting biodiversity and its effectiveness is predicated on predicting how natural populations respond to environmental stressors. Yet, routine toxicity testing typically examines only one genotype, which may render risk assessments inaccurate at the population scale. To determine the importance of intraspecific variation in the translation of toxicity testing to populations, we quantified the magnitude of genetic variation within 20 Daphnia magna clones derived from one lake using whole genome sequencing and phenotypic assays. We repeated these assays across two exposure levels of Microcystis aeruginosa, a cosmopolitan freshwater contaminant. We found considerable intraspecific genetic variation in survival, growth, and reproduction. Exposure to Microcystis amplified the degree of intraspecific variation. Simulations demonstrate assays employing one clone failed to produce an estimate within the 95% confidence interval over half of the time. These results illuminate the importance of incorporating intraspecific genetic variation, but not necessarily genome sequences, into toxicity testing to reliably predict how natural populations will respond to environmental stressors.