Comparative acute toxicity values for Ceriodaphnia dubia, Scapholeberis sp. and Pimephales promelas exposed to copper were used to calculate water effect ratios (e.g., site water LC50 value/reference water LC50 value), which reflect the difference in the biological availability and/or toxicity of copper between water from the Naugatuck River, Connecticut and Lake Superior reference water. These ratios were used to modify EPA ambient aquatic life criteria for copper to site- and station-specific criteria, using the indicator procedure of the EPA guidelines for deriving site-specific water quality criteria. It was concluded that the national and site-specific criteria derived for copper would be protective of the river's aquatic life because a relatively health aquatic community existed where these criteria were exceeded slightly. Generally, C. dubia survival and young production data from receiving water tests and copper addition tests, conducted just prior to the acute toxicity tests, were also indicative of reduced copper biological availability and/or toxicity in the Naugatuck River at downstream stations. (Copyright (c) Pergamon Journals Ltd. 1986.)