ABSTRACTThe US Environmental Protection Agency (USEPA) has historically used different methods to derive an aquatic level of concern (LoC) for atrazine, though all have generally relied on an expanding set of mesocosm and microcosm (“cosm”) studies for calibration. The database of results from ecological effects studies with atrazine in cosms now includes 108 data points from 39 studies and forms the basis for assessing atrazine's potential to impact aquatic plant communities. Inclusion of the appropriate cosm studies and accurate interpretation of each data point—delineated as binary scores of “effect” (effect score 1) or “no effect” (effect score 0) of a specific atrazine exposure profile on plant communities in a single study—is critical to USEPA's approach to determining the LoC. We reviewed the atrazine cosm studies in detail and carefully interpreted their results in terms of the binary effect scores. The cosm database includes a wide range of experimental systems and study designs, some of which are more relevant to natural plant communities than others. Moreover, the studies vary in the clarity and consistency of their results. We therefore evaluated each study against objective criteria for relevance and reliability to produce a weighting score that can be applied to the effect scores when calculating the LoC. This approach is useful because studies that are more relevant and reliable have greater influence on the LoC than studies with lower weighting scores. When the current iteration of USEPA's LoC approach, referred to as the plant assemblage toxicity index (PATI), was calibrated with the weighted cosm data set, the result was a 60‐day LoC of 21.2 μg/L.Integr Environ Assess Manag2018;14:489–497. © 2018 The Authors.Integrated Environmental Assessment and Managementpublished by Wiley Periodicals, Inc. on behalf of Society of Environmental Toxicology & Chemistry (SETAC)

Key PointsThe extensive body of microcosm and mesocosm (“cosm”) research with the herbicide atrazine has been a major source of data for establishing a regulatory level of concern (LoC)—the highest level of exposure considered by risk managers to be protective of aquatic plant communities.The approach described in this paper—evaluating cosm studies for relevance and reliability and weighting the LoC analysis with the information—is useful because studies that are more relevant and reliable have greater influence on the LoC than studies with lower weighting scores.The atrazine LoC derivation process used by the USEPA is highly influenced by interpretation of individual cosm studies, by the model used to predict cumulative atrazine effects, and by the results of the data evaluation.When the EPA's current LoC approach, referred to as the Plant Assemblage Toxicity Index, was calibrated with appropriate cosm data weighted according to the data evaluation scores, the result was a 60‐day time‐weighted average exposure of 21.2 µg/L.