Acute and chronic toxicity of ammonia, nitrite, and nitrate to the endangered Topeka shiner (Notropis topeka) and fathead minnows (Pimephales promelas)

Toxicity tests with ammonia, nitrite, and nitrate were conducted on the endangered Topeka shiner (Notropis topeka) to determine if current U.S. Environmental Protection Agency (U.S. EPA) water quality criteria are protective of this species. Results from acute lethal and chronic growth tests are reported for both Topeka shiners and fathead minnows (Pimephales promelas). Embryolarval tests were conducted with only fathead minnows because Topeka shiner embryos were not available. Predicted outcomes for Topeka shiner embryo-larval toxicity endpoints were calculated by comparing relationships between growth tests and embryo-larval tests for fathead minnows and extrapolating those relationships to Topeka shiners. Results show that the U.S. EPA's criterion for total ammonia-nitrogen (TAN), 1.24 mg/L when early life stages are present, would be protective, given that our most sensitive result was a predicted maximum acceptable toxicant concentration (MATC) for Topeka shiners at 5.63 mg/L TAN, calculated from the fathead minnow chronic embryo-larval test. The U.S. EPA's criterion for nitrite (5 mg/L) would not be protective, given that our most sensitive result was a predicted MATC for Topeka shiners of 3.97 mg/L NO₂-N, calculated from the fathead minnow chronic embryo-larval test. However, nitrite is generally transient, and unpublished field data show levels far lower than the criterion. Finally, the U.S. EPA's recommendation of a maximum of 90 mg/L NO₃-N for the protection of warmwater fishes would protect Topeka shiners but not fathead minnows. For Topeka shiners, the MATC from the 30-d juvenile growth test was 360 mg/L NO₃-N, but for fathead minnows, the MATC was 84 mg/L. More field sampling may be needed to determine if levels comply with criteria, especially in Topeka shiner critical habitat.