Bridging the gap between hypothesis-based test results and point-estimates in freshwater chronic toxicity tests: a...

There has been a history of debate within the ecotoxicological community on the use of point estimates (e.g., effect concentration for 20% of test organisms; EC20) versus effect levels determined through hypothesis-based testing, such as the no observed effect concentration (NOEC) and lowest observed effect concentration (LOEC). This study provides analyses of the percent effect that is expected to occur at the NOEC, LOEC, and maximum acceptable toxicant concentration (MATC) based on available chronic data but does not debate the merits of point estimates versus hypothesis-based test results. We also developed adjustment factors that can be applied to NOECs, LOECs, MATCs, EC20, and EC10 values to equate them to EC5 values, which generally represent an effect level that is within the variability of control responses (e.g., NOEC/Adjustment Factor = Approximate EC5). Our analyses showed median percent effect occurring at the NOEC (8.5%), LOEC (46.5%), and MATC (23.5%) was not strongly influenced by chemical or taxon type (invertebrate vs. vertebrate). The median NOEC, LOEC, and MATC to EC5 adjustment factors were 1.2, 2.5, and 1.8, respectively. The median EC20 to EC5 adjustment factor was 1.7, and the median EC10 to EC5 adjustment factor was 1.3. Adjustment factors were not strongly influenced by chemical or taxon type, suggesting they can be applied across chemicals and taxa. Our results provide context to the use of hypothesis-based testing results. The adjustment factors developed could be considered in efforts to streamline screening-level ecological risk assessments and individual-level endangered species evaluations by providing an approach that could be used to adjust commonly reported toxicity test results (i.e., NOEC, LOEC, MATC, EC20, EC10) into approximate EC5 values.