Aquatic toxicity of mixtures

The inherent toxicity of a substance to aquatic organisms is typically determined via single species laboratory tests. Results from these tests are used to determine a Predicted No Effect Concentration (PNEC) for ecosystems per substance. Since organisms in the environment are exposed simultaneously to a wide array of substances, it is important to understand the potential effects of mixtures to aquatic organisms. The effects of mixtures can be generally categorised as additive, greater than additive (synergism) and less than additive (antagonism). Effects that correspond to the addition of toxicities for each mixture component are considered additive. Via acute toxicity tests, mixtures of substances that are chemically related or have the same mode of action are generally found to be additive. However, some "groups" of substances when tested in relatively simple mixtures do not behave in a readily predictable manner (e.g. metals and some pesticides). Even so there are only a few examples in the literature of synergism where the effects are more than three times greater than those predicted from additivity of acute toxic effects. When large numbers of substances are present in mixtures at low concentrations relative to their individual acute toxicities, additivity of acute toxic effects is closely followed. This holds true even when the substances are not related chemically, or exhibit different modes of action when acting as acute toxicants alone. This phenomenon for organic substances has been called "baseline toxicity", or narcosis. The same theory applies to chronic toxicity tests. That is, if organic mixture components are at concentrations below the level exerting chronic toxicity, then additivity can be expected - thereby supporting the concept of baseline toxicity. Mixture components with the same mode of action can be expected also to act additively. However, it is not possible to make generalisations about the chronic toxicity of mixtures containing metals. Such mixtures can give responses across the entire range of interactions from antagonism to synergism. This may be due largely to different modes of action and differences in metal speciation in mixtures compared to single toxicant tests. While data from model ecosystems, field studies and effluent studies are generally limited and difficult to interpret from the standpoint of the toxic effects of mixtures of substances, the evidence tends to support the basic concept of additivity, particularly when the role bioavailability can play in reducing toxic effects under environmental conditions is taken into account. Predicted mixture effects based on body residues from organisms exposed in the field indicate that additivity of substances below their PNECs (baseline toxicity approach) is sufficiently conservative for protection of aquatic resources.