Environmental Toxicology EVALUATION AND COMPARISON OF THE RELATIONSHIP BETWEEN NOEC AND EC10 OR EC20 VALUES IN CHRONIC DAPHNIA TOXICITY TESTING

yMiddle Tennessee State University, Murfreesboro, Tennessee, USAzEnvironmental Stewardship and Sustainability, Mason Business Center, The Procter & Gamble Company, Cincinnati, Ohio, USA(Submitted 21 January 2015; Returned for Revision 24 February 2015; Accepted 19 May 2015)Abstract: Hypothesis-based no-effect-concentration (NOEC) and regression-based x% effect concentration (EC ) values are commonstatisticalapproachesusedtosummarizeecotoxicologicaleffects.ControversyovertheNOECmodelhaspromptedamovementtowarddiscontinuationoftheNOECinfavorofECx,butthebestx%effectsurrogateforNOEChasnotyetbeendetermined.Historically,10%and 20% effect concentrations (EC10 and EC20) have been treated as NOEC analogs. Given these measurements’ importance toecotoxicology,furtherunderstandingoftherelationshipsbetweenNOECandEC10orEC20iscrucial.Inthepresentstudy,ametadatasetof daphnid chronic toxicity tests was compiled to analyze the strength and significance of NOEC:EC10 and NOEC:EC20relationships.Theimpactofendpoint(e.g.,mortality,reproduction)andtestconditionparameters(e.g.,pH,temperature)onNOEC:EC10andNOEC:EC20wasevaluated.Mortalityendpointsweremostsensitive51%ofthetime,withgrowthandreproductiveendpointsconstitutingtheremainder,underscoringthevalueofusingmultipleendpointstoevaluatetoxiceffectsratherthanrelyingonreproductionastheapriorimostsensitiveendpoint.Whentestconditionparameterswerelessrestricted(e.g.,pH,hardness),theNOEC:EC20associationwasmorerobust,suggestingthatvariabilityintroducedbytestimplementationincreasedvariabilityinECxcalculation.Theanalysisrevealedthat,overall, EC10 was a more suitable analog than EC20 for NOEC. Recommendations include refinement and reporting of the testparameters pH and hardness to minimize variability in ECx calculation. Environ Toxicol Chem 2015;9999:1–7. # 2015 SETACKeywords: Ecotoxicology Statistics No-observed-effect concentration (NOEC) Effective concentration (EC)Risk assessmentINTRODUCTION