Diagnosing Causes: Background
Diagnostic protocols confirm a cause based on the presence of a particular symptom or suite of symptoms. Some diagnostic protocols have been used and tested sufficiently to be considered authoritative, and some have been formalized into a set of rules or a key (e.g., Meyer and Barclay 1990).
In medicine, diagnostic protocols identify a disease by examining its signs and symptoms. As in medical practice, diagnostic information in the SI process comes from the exposed organisms and includes symptomology (i.e., signs of the action of the causal agent on the organisms), and sometimes measures of internal exposure (e.g., isolation of pathogens or analysis of chemicals in organisms).
Diagnosis is a good method for SI when organisms are available for examination; the candidate causes have been made into a protocol; and there is a high degree of specificity in the cause, effect, or both. As an example, protocols for the investigation of fish kills are particularly well established (e.g., Meyer and Barclay 1990) and consist of collection of site data concerning candidate causes (e.g., oxygen, pH, temperature, contaminant levels, and presence of toxic algae), site data concerning effects (e.g., taxa killed, duration of event, behavior of live fish), and necropsy results (e.g., lesions, pathogens, tissue contamination, or clinical signs such as blue stomach which indicates molybdenum toxicity). Meyer and Barclay (1990) provide a dichotomous key for determining the causes of fish kills. Since an SI investigation is more likely to examine current biological community compositions that might reflect past chronic exposures rather than the effects of acute lethality, the methods for fish kill investigations often are not directly applicable.
Diagnostic tools are well developed for pathogens and to a slightly lesser extent for chemicals (e.g., certain bill deformities are diagnostic of exposure to dioxin-like compounds) (Gilbertson et al. 1991). Diagnostics are also well developed for a few other agents such as low dissolved oxygen (low blood oxygen, gasping at the surface, etc.). For many other stressors and for most invertebrate aquatic organisms, reliable diagnostics are seldom available. Many common types of biological impairments are produced by many different causes. For instance, an impairment that is frequently described is a decrease in the abundance of Ephemeroptera (mayflies). Unfortunately, there are too many possible causes for this effect to be proven by diagnostic methods. Nevertheless, there is hope that future research may reveal symptomatic characteristics from more than one taxon and at different levels of organization, such as a combination of genetic, biochemical, physical deformities, sensitive taxa or species or other characteristics that will provide a diagnostic set of symptoms. Expert judgment has been used to assign tolerance values to taxonomic groups for nutrients and this concept has been extended to other stressor types (Hilsenhoff 1987, Huggins and Moffet 1988). The use of these tolerance values in multimetric indices along with some recent statistical analyses indicate that the structure of fish and invertebrate communities may prove valuable for diagnosis (Yoder and Rankin 1995, Norton et al. 2000).
You also can use diagnostic protocols to refute a cause, when symptoms that are always associated with a candidate cause are not observed. However, we expect that it will be rare to have sufficient confidence to eliminate a cause from further consideration using symptoms alone.