/ Laboratory stream microcosms have been used to study transport, fate, and effects of toxic substances in stream ecosystems. Several general concerns exist in utilizing laboratory streams in this way. We summarize some of the most important and difficult of these problems and endeavor to provide theoretical understanding, evaluation, and empirical approaches necessary for making laboratory stream ecosystem studies more useful in solving problems of toxic substance behavior in natural stream ecosystems. Welldesigned laboratory streams and other microcosms are complex dynamic systems that can contribute to our understanding of the behavior of toxic substances. But such systems are far too complex and dynamic to be employed as bioassay, monitoring, or predictive tools, as individual organisms have been.Our culture is faced with the unprecedented task of evaluation and control of toxicants entering the environment as a result of human activities. The problem is of such technical, social, and natural complexity that one may well wonder by what means and how adequately it can be managed. While questions concerning the transport, fates, and effects of toxic substances in the environment are by no means only scientific in nature, scientific evaluation and understanding of the overall problem and its particulars are necessary.There is a serious need for general approaches to the study of transport, fates, and effects of toxic substances in natural ecosystems (Levin and others 1984, Herricks andSchaeffer 1985). Recently, there has been increased interest in the use of laboratory microcosms of various designs to evaluate the behavior and effects of toxic substances in ecosystems (Draggen 1976, Witt and Gillett 1979, Clark and others 1980, Coats 1980, Giesy 1980, Sanders 1982, Shriner and Gregory 1984, Lynch and others 1985, Ribeyre 1985, Warren and others 1987. Although much microcosm and laboratory stream work has been done, several general problems remain. Shriner and Gregory (1984) believe it is necessary to know how processes in artificial microcosms differ from those in natural situations. Levin and others (1984) state that the criteria important in assessing the applicability of microcosms to natural systems are accuracy, replicability, reproducibility, KEY WORDS: Laboratory streams; Toxic substances; Classification; Theory; Ecosystems *Author to whom correspondence should be addressed.and simplicity. Herricks and Schaeffer (1985) regard microcosms as biomonitoring test systems of great complexity producing qualitative data differing in variability, sensitivity, and validity. Swartzman and Rose (1984) propose a strategy for studying toxic effects in aquatic ecosystems that combines the use of microcosms with single species bioassays, mesocosm studies, and simulation models, yet they remain uncertain about applying microcosm results to natural systems. King (1980) addresses some concerns in applying the results of simple microcosm experiments to complex natural systems. Perhaps the most important constrain...
