Aquatic toxicology is the study of the adverse effects of toxins and their activities on aquatic ecosystems. Aquatic toxicologists assess the condition of aquatic systems, monitor trends in conditions over time, diagnose the cause of damaged systems, guide efforts to correct damage, and predict the consequences of proposed human actions so the ecological consequences of those actions can be considered before damage occurs. Aquatic toxicologists study adverse effects at different spatial, temporal, and organizational scales. Because aquatic systems contain thousands of species, each of these species can respond to toxicants in many ways, and interactions between these species can be affected.

Consequently, a virtually unlimited number of responses could be produced by chemicals. Scientists study effects as specific as a physiological response of an important fish species or as inclusive as the biological diversity of a large river basin. Generally, attention is first focused on responses...