Probability distributions of toxicant sensitivity for freshwater Rotifer species

Probability distributions of species sensitivities describe variation among species in response to toxicants. These distributions indicate what proportion of species assemblage is expected to suffer adverse effects upon toxicant exposure. Sensitivity distributions demonstrate how representative model species used in toxicity tests are of entire species assemblage. Despite theoretical importance in ecological risk assessment, few probability distributions were characterized for aquatic animals. We characterized sensitivity distributions for nine freshwater rotifer species to pentachlorophenol and cadmium. Endpoints were 24 h mortality and 30 min in vivo enzyme activity. The 95% confidence limits for mean PCP LC50s were 1.3-183 and 33-187 mg L-1 for cadmium. A log toxicant concentration plot vs rank species susceptibility was constructed for each toxicant. A Kolmogorov-Smirinov goodness of fit test indicated these distributions did not differ significantly from normality, with R(2)s ranging 0.75-0.89. A species' sensitivity rank commonly changed three or four ranks with toxicant or endpoint change. Across toxicants and endpoints, Asplanchna girodi was the least sensitive species with mean percentile rank 78 and Trichocerca pusilla was most sensitive with rank 25 percentile. The Rotifer species most commonly used in toxicity tests, Brachionus calyciflorus, ranked 72nd percentile, placing it among the least sensitive rotifer species. Tenth centile for rotifer exposure to PCP is 2.8 and 17.2 mu g L-1 for cadmium. These values are near both toxicants' lower 95% confidence limit for all nine rotifer species. Toxicity assessments based only on B. calyciflorus with PCP LC50 of 160 mu g L-1 and cadmium LC50 of 270 mu g L-1 were not protective of most sensitive rotifer species by more than an order of magnitude. Sensitivity distributions characterization is likely to define toxicant concentrations which are harmful for rotifer assemblages better than any single species toxicity test. (C) 1999 by John Wiley & Sons, Inc.