An empirical comparison of effective concentration estimators for evaluating aquatic toxicity test responses

Aquatic toxicity tests are statistically evaluated by either hypothesis tes ting procedures to derive a no-observed-effect concentration or by invertin g regression models to calculate the concentration associated with a specif ic reduction from the control response. These latter methods can be describ ed as potency estimation methods. Standard U.S. Environmental Protection Ag ency (U.S. EPA) potency estimation methods are based on two different techn iques. For continuous or count response data, a nominally nonparametric met hod that assumes monotonic decreasing responses and piecewise linear patter ns between successive concentration groups is used. For quantal responses, a probit regression model with a linear dose term is fit. These techniques were compared with a recently developed parametric regression-based estimat or, the relative inhibition estimator, Rip. This method is based on fitting generalized linear models, followed by estimation of the concentration ass ociated with a particular decrement relative to control responses. These es timators, with levels of inhibition (p) of 25 and 50%, were applied to a se ries of chronic toxicity tests in a U.S; EPA region 9 database of reference toxicity tests. Biological responses evaluated in these toxicity tests inc luded the number of young produced in three broods by the water flea (Cerio daphnia dubia) and germination success and tube length data from the giant kelp (Macracystis pyrifera). The greatest discrepancy between the RIp and s tandard U.S. EPA estimators was observed for C. dubia. The concentration-re sponse pattern for this biological endpoint exhibited nonmonotonicity more frequently than for any of the other endpoint. Future work should consider optimal experimental designs to estimate these quantities, methods for cons tructing confidence intervals, and simulation studies to explore the behavi or of these estimators under known conditions.