Recent developments in the field of ecotoxicological effects assessment of pollutants on aquatic systems make use of methods founded on statistical principles and requirements (Kooijman 1987; Van Straalen & Denneman 1989). Van Straalen and Kooijman methods estimate a hazardous concentration for p percent of species in a community (HCp) at a chosen protection level. Evaluations on the application of the methodology to different toxic compounds and test species using both chronic and acute toxicity data have been undertaken (Van Leeuwen 1990).Centro de Investigaciones del Medioambiente