Comparative responses of aquatic ecosystems to toxic chemical stress
I reviewed a variety of studies on the effects of toxic chemical stress on aquatic ecosystems to search for common patterns of response or sensitivity. Information was obtained from whole-ecosystem experiments, mesocosm experiments, and well-conducted studies of toxic discharges (such as accidental oil spills) into natural ecosystems. Whole-ecosystem experiments have provided the most powerful tool for determining the ecological effects of toxic substances in lakes, but there have been no experiments at this scale in marine ecosystems. A variety of whole-ecosystem and mesocosm experiments in both freshwaters and seawater have found that single-species tests conducted in the laboratory can grossly underestimate the potential for environmental damage from toxic substances. My review suggests a similarity in response across different types of aquatic ecosystems to a variety of toxic substances. Toxic chemical stress can affect both the structure and function of aquatic ecosystems. However, structural changes are generally more predictable and may occur earlier or at lower levels of stress than do changes in ecosystem processes. I found several examples of structural changes occurring with no noticeable change in rates of ecosystem processes. However, I found no examples of ecosystem processes being affected without an accompanying change in structure. When ecosystem processes are affected by toxic chemical stress, rates can either increase or decrease. The most predictable structural change appears to be the loss of sensitive species. Changes in the structure of aquatic ecosystems can be of importance even when not accompanied by changes in ecosystem processes such as primary production. For instance, fish populations can be lost as a result of changes in food web structure. Ecosystems dominated by opportunistic species appear more resistant to stress than those dominated by more specialized organisms. Ecosystems with greater diversity are probably less resistant to stress. The relative sensitivity of an ecosystem (or a mesocosm designed to mimic a natural ecosystem) needs to be considered when extrapolating results on the effects of toxic chemical stress, as well as in designing and interpreting experiments or monitoring programs in these systems. Identification of general patterns in the sensitivity and resistance of ecosystems to toxic chemical stress can aid in environmental management by helping to identify which ecosystems require special protection or particularly close monitoring. I hypothesize that ecosystems with more open element cycles may be more resistant to toxic chemical stress than are more closed ecosystems. If true, then eutrophication has some effects in common with toxic chemical stress. As systems are stressed by eutrophication or by toxic substances, they become more resistant to further change. However, many desirable characteristics of the original ecosystems may already have been lost. If the hypothesis proves true, it should be useful in designing experiments to test the ecological effects of toxic substances. It may also prove extremely useful in siting decisions and other aspects of environmental management.