A literature review of freshwater model ecosystem studies with herbicides was performed to assess the NOEC[sub]ecosystem for individual compounds, to compare these threshold levels with water quality standards, and to evaluate the ecological consequences of exceeding these standards. Studies were judged appropriate for this purpose if the test systems simulated a realistic freshwater community, if the experimental design was generally sound (ANOVA or regression design, exposure concentrations described), and if published not earlier than 1980. Almost half of the collected papers did not meet these selection criteria. Effects were classified according to their magnitude and duration. The most sensitive endpoints for photosynthesis inhibitors, the most widely studied group of herbicides, were responses related to community metabolism and the structure of phytoplankton, periphyton and macrophytes. These endpoints showed a clear dose-response relationship. The criteria as set by the Uniform Principles appeared to provide sufficient protection for aquatic ecosystems against herbicides. Possible exceptions are the herbicides with an auxin-simulating mode of action, because aquatic macrophytes appeared to be more sensitive to these substances than algae.Functional responses of communities in phytoplankton-dominated ecosystems sometimes recovered rapidly through shifts in algae species composition and adaptation. Indirect effects on the zooplankton in such systems generally occurred at higher concentrations than primary effects. Adequate studies in macrophyte-dominated systems were rare, but in several experiments a pronounced long-term decline of macrophytes was observed at chronic concentrations only slightly above the NOEC[sub]ecosystem. This may result in considerable indirect effects on the macrophyte-associated fauna. The most important modifying factors with respect to types of effect and recovery rates following the application of herbicides to freshwater ecosystems are also discussed.
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