An integrated approach to aquatic ecosystem health: top-down, bottom-up or middle-out?

Due to the ecological complexity of many stressed ecosystems, large field studies have the disadvantage of having to deal with diverse sets of stressors involving both chemical impacts and habitat changes. Toxicologically-oriented studies have traditionally been mechanistically focused, attempting to develop an understanding of the interactions between chemical availability and physiological responses of aquatic organisms. This has led to the development of epidemiological techniques, reductionist approaches, ‘cause-effect’ studies and ‘biomarkers’. On the other hand, ecological studies have traditionally been descriptive in nature, attempting to develop an understanding of the interaction of aquatic communities with their habitat. This philosophy has encouraged holistic approaches and integrative measurements. The two approaches have not evolved from a common philosophical basis and, despite the attempted marriage of these philosophies into the discipline of ecotoxicology, studies are often forced into a compromise between the two approaches. This paper summarizes some of the differences between the two approaches, and describes a conceptual framework. This framework may aid managers in developing an improved understanding of field surveys, allowing for improved study designs and more appropriate interpretation of data. Examples from recent studies will be used to show how the development of a tiered approach to impact assessment and management may aid in the evolution of the field studies to address the complex issues facing the management of natural ecosystems, such as the Great Lakes.