Introduction
During the last twenty years, there has been increasing evidence of the effects of chemical contaminants on the environment in the Great Lakes region. Most visible have been the reproductive problems found in fish-eating mammals, particularly mink, and populations of birds, such as herring gulls, eagles and cormorants. These effects appear to be linked to PCBs and other organochlorine compounds which entered the food chain at very low levels but progressively bioaccumulated and biomagnified until reaching potentially toxic levels in creatures near the top of the food chain, such as birds and mammals.
While the evidence of effects has been increasing, the amounts of PCBs and related compounds in the Great Lakes, as reflected in levels measured in fish, have been decreasing. Thus, the situation is complex and a comprehensive assessment of the current risks and prediction of future risks will require increased interdisciplinary efforts and the collection of much more data.
Measuring ecological risk
The assessment of ecological risk from chemicals is much more difficult that the estimation of risks to humans. First, human risk assessment deals with only one kind of living creature, humans, while environmental risk assessment must consider the whole range of organisms in nature from the very small to the very large; those which live in water as well as those living in the air or on the ground; and those which have very long life spans as well as those that live only a short period of time. These organisms vary not only in size and life span but also in sensitivity to particular chemicals and how these chemicals are broken down after being absorbed into the organism.
Second, human risk assessment is focused on the individual: assessors try to determine the levels of chemicals at which any one person will be affected. The goal of ecological risk assessment is not as clearly defined. While some scientists think the focus should be on the individual organism, others have suggested that the focus should be on the survival of the population of animals rather than any single organism. Still others have another view: that the larger ecosystem -- such as a river or lake system -- is the unit that should be protected.
As a result of this lack of agreement, and in contrast to the situation for human risk assessment, there is no standard procedure for assessing overall ecological risk. There are intense efforts currently underway to remedy this situation, but it will undoubtedly take several more years before such procedures are established. In addition, further time will be needed before these procedures are applied to all contaminants and sites of interest. Thus, a commonly accepted comprehensive appraisal of risks to the non-human biota -- living organisms -- in the environment from chemical contaminants will not be available for some time.
However, this does not mean that little or nothing is presently known. A large number of studies have been performed indicating that specific chemicals cause adverse effects in the environment. Some of these are simple studies of effects that occur rapidly after animals in the wild or in laboratory environments are exposed to contaminants. In these studies, death is the usual effect studied and there is a large literature base on the levels at which particular chemicals cause death to specific types of animals, both aquatic and terrestrial.
In addition, there are many studies that have investigated other adverse effects, especially changes in reproductive capacity. Besides the research on PCBs and terrestrial organisms, other investigations have shown a relationship between reproduction and tributyltin, a pesticide often added to marine paints. Other work relating fish tumors to contaminants is not as well advanced and many tumors have been shown to be caused by naturally occurring viruses.
A number of these effects, especially death and decreased reproductive capacity, have impacts on populations as well as individuals and also have the potential to affect ecosystem health. Thus, whatever the final framework for comprehensive environmental risk assessment, there are many currently available studies that can be directly used decreasing the need for additional research.
While these studies provide a great deal of important information, there are a variety of uncertainties associated with the results. When the studies are done in the field one significant source of uncertainty is determining the degree of exposure of the animal to various agents, both chemical and physical, as it moves from place to place and as the environment around it changes. This can be especially problematical when dealing with animals that migrate and so face radically different environments at different times of the year.
When attempts are made to control the environment by performing studies in the laboratory, there are uncertainties in how well the artificial situation corresponds to the real one and thus how applicable the results are to the situation of interest. These result from the impossibility of duplicating the complexity of the natural environment in any laboratory situation. The uncertainties increase as the level of organization increases -- from the individual to the ecosystem -- so that assessing ecosystem effects in the laboratory is associated with a great degree of uncertainty.
Summary
To best manage the environment, it is necessary to assess the extent of current and future risks to the biota of this environment and to characterize the causative agents. Since chemical contamination appears to be the greatest threat, determining the amounts of each chemical that cause adverse environmental effects is the first priority. Achieving this goal, however, is a difficult task.
One significant problem is that there isn't a consensus on how to measure adverse ecological effects, especially with respect to the level of organization of most importance: the individual organism, the population, or the ecosystem. A second problem is that the diversity of environments and organisms suggests that a great deal of research is needed, much more than is presently being performed. Third, there are inherent uncertainties in performing ecological risk assessments.
However, in spite of these difficulties, it has been possible to identify some significant problem chemicals and to roughly estimate the levels at which these compounds are of concern. Increased understanding by citizens is critical to the development and implementation of policies that are responsive to the concerns of the people inhabiting the ecosystem.
Source: Summarized from "Assessing the Risk to Humans of Toxic Chemicals in the Great Lakes Ecosystem" Instiute for Environmental Technology, Michigan State University
