Ecological Modelling, 22 (1983/1984) 123--134 123 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands SIMULATING THE BIOLOGICAL EFFECTS OF TOXICANTS IN AQUATIC MICROCOSM SYSTEMS GORDON SWARTZMANand KENNETHA. ROSE School of Fisheries HR-20, University of Washington, Seattle, WA 98195 (U.S.A.) ABSTRACT Swartzman, G. and Rose, K.A., 1984. Simulating the biological effects of toxicants in aquatic microcosm systems. Ecol. Modelling, 22: 123-134. In this paper we discuss a strategy for developing an improved prediction of the biological effects of toxicant additions to aquatic ecosystems. This strategy involves combining single species bioassay results with microcosm and mesocosm studies using simulation models to predict the response of natural ecosystems. Work presently being conducted by the authors on intercomparing process equations from different models in the literature with respect to behavior of biota in microcosms is introduced. The usefulness of microcosm experiments for model development, calibration and validation is illustrated. Finally, the next phase of our research which involves combining the microcosm model with single species bioassay results is discussed. INTRODUCTION Assessing the effect of toxicant additions to aquatic ecosystems offers a new challenge to the ecological community to add to the already existing problems of studying the effects of harvesting, removal of biota, and eutrophication. The programs in modeling the biological effects of toxicants are i) the difficulty in quantifying exposure of organisms to the initial dose and in comparing this with bioassay results due to the decay over time of many toxicants via biological and physical processes, 2) the speciation of metals and biodegradation of organic toxicants and the lack of knowledge of the toxic effects of many of the metal species and intermediate products, 3) the chemical interaction of toxicants within an aquatic system which are often sensitive to pH, buffering capacity, temperature, nutrients, etc. In the parlance of process oriented simulation models, modeling the effects of toxicant addition requires a new set of mechanisms for the mortality and process inhibition induced by the toxicant. This is a difficult task when we con- sider that the processes comprising ordinary ecosystem models have been studied for a relatively long time, while toxicants in ecosystems have for the most part only a short history of study or none at all. Many toxicant effects, especially sublethal or long term effects, have not yet been well described. With this challenge before us it seems that we require an integrated strategy for assessing the effect of toxicant loading to an aquatic system. We propose here a multistage experimental approach involving laboratory bioassays, microcosms (artificial) species assemblages in a completely controlled laboratory environment)