Bacterial interference in whole‐effluent toxicity tests

Routine National Pollutant Discharge Elimination System (NPDES) cooling-water toxicity tests demonstrated an acute effect on Ceriodaphnia dubia and fathead minnows (Pimephales promelas). Test results were characterized by irregular mortality dose-response curves and a brown filamentous mass on the opercular area of affected fish. Some fish and Ceriodaphnia also became entrapped in filamentous material present on the bottom of the test vessels. Microscopic examination of test fish revealed severe fouling of the oral and branchial cavities with a mixed population of microbes, the major component being a filamentous sheathed bacteria. Treatment of water samples with a 0.45-μm membrane filter and/or ultraviolet light sterilizer were effective in eliminating the bacteria and test organism mortalities. Regulatory test procedures were modified to allow filtration of the water prior to testing to eliminate this naturally occurring biological interference in toxicity tests.