Abstract

Rapid biological assays may be attractive for monitoring toxicity in effluents and trade wastes where mixtures of toxins may have complex additive or antagonistic effects but they are of very little use in assessing the effect of toxins on aquatic macrophytes in flowing water because the importance of the pattern of concentration and period of exposure is often neglected. In particular, there is an alarming scarcity of data for gauging the effect of toxins on submerged aquatic macrophytes. Their response to many photosynthetic inhibitors is complicated at low times of exposure by their resilience; and at low concentrations by the protective and competitive effects of epiphytes. The availance (concentration time product, or integral of concentration with respect to time) is a useful and convenient measure of toxic effect, but only for pulses of moderate concentration and duration. It is of very limited use for assessing the effects of toxins on macrophytes in river systems, where the pattern of exposure is often extreme. The pattern is determined by processes of dispersion and dilution, together with rates of dissipation and inactivation of the toxin. Therefore all these factors should be examined in any toxicity-testing program. In addition, since extremely low concentrations of some inhibitors are deleterious for macrophytes, background contamination from diffuse sources must also be considered.