Effects of several variables on whole effluent toxicity test performance and interpretation

Protocol changes and options contained within U.S. Environmental Protection Agency whole effluent toxicity tests represent variables that have the pot ential to affect bioassay performance and interpretation of results. Variab les evaluated in this study include: the change in allowable age in the Pim ephales promelas acute bioassay from up to 90 d to a maximum of 14 d, age-s pecific acute responses of P. promelas among the allowable ages of 1 to 14 d, change in the chronic growth endpoint definition from final mass to biom ass, differences between hemacytometer and fluorometer measurements in the Selenastrum capricornutum protocol, and options for statistical interpretat ion of species sensitivity in multiple test/species screening bioassays. Cl ear age-related sensitivity and precision differences were observed in acut e responses of P. promelas. Results obtained using the younger age classes were typically more variable in studies of both 1- to 14-d-old and 14- to 9 0-d-old P. promelas. In the experiments on 1- to 14-d-old organisms, larvae at 1 d of age were significantly less sensitive. In the tests on 14- to 90 -d-old organisms, the 14-d-old organisms were significantly less sensitive. The change in endpoint definition in the P. promelas chronic bioassay resu lted in an apparent increase in toxic response in the inhibition concentrat ion (ICp) value for each bioassay, evaluated by the biomass method, with no general improvement in statistical intertest precision estimates and no pr edictable impact on the no-observed-effect concentration endpoint. Fluorome tric scoring in the Selenastrum bioassay was significantly more precise and better capable of estimating counts than hemacytometer measurements. Discr epancies associated with commonly used statistical endpoints used to determ ine the most sensitive species were identified, and potential solutions wer e proposed.