Abstract

 Paracelsus' Dictum, which addressing human toxicology, forms the basis for both modern medicine and ecotoxicology. The Dictum can be paraphrased as "the dose makes the poison". What this means in terms of ecotoxicology, is that for toxicity to occur, an organism must be exposed to a contaminant and at the correct concentration. Effectively, this Dictum also separates pollutants (toxicity occurs) from contaminants (toxicity does not occur). However, as presently constituted, the Dictum neither includes nor considers two critical modifying factors in the sedimentary environment: bioavailability; and realistic exposure scenarios. Too many scientists are explicitly or implicitly, applying the Dictum directly, without modification which has resulted in incorrect interpretations regarding, for instance: PAH phototoxicity; bioavailability of PAH and metals/metalloids in sediments; the relative importance of pore water, overlying water and dietary exposure routes; bioavailability in dynamic environments such as estuaries; biological tolerance mechanisms (avoidance, acclimation, non-genetic adaptation, genetic adaptation, metabolism); and appropriate toxicity test organism sensitivities. Each of these issues is discussed, as are secondary (or indirect) toxicity resulting from sediment instability, and genetic adaptation to particular contaminants resulting in susceptibility to other stressors. A modification of Paracelsus' Dictum is proposed, as part of a weight of evidence framework, for determining the present and future quality of aquatic sedimentary environments.