Abstract
Substances such as Al3+, Cu2+, Cd2+, Zn2+, ammonia, atrazine, 3,4-dichloroaniline, linear dodecylbenzene sulfonate, pentachlorophenol and phenol were added to a microcosm （mixed flask culture）constituted of bacteria, Cyclidium, Lepadella, Philodina, Aeolosoma, Chlorella and Schizothrix, and the respiration rate and the production rate were determined. Good correlations were found between the observed maximum concentrations of the substances having no effects on the respiration rate and the production rate in microcosms（M_NOAECs）and the mean values of the no-observed-effect concentrations of the substances in experimental and natural ecosystems（NOECs）．An uncertainty factor for predicting the safe environmental concentrations in ecosystems（PNOECs）from the M_NOAECs was estimated to be 200 from the regression line（log [the mean of NOECs] / log [M_NOAEC]）and the log-normal distribution curves of the NOECs of each substance in experimental and natural ecosystems. The calculated PNOECs covered most of the reported NOECs in experimental and natural ecosystems. The microcosm could be used as a tool for screening tests of substances on generic ecosystem-level toxicity.
