BACTERIAL INTERFERENCE IN WHOLE-EFFLUENT TOXICITY TESTS

Routine National Pollutant Discharge Elimination System (NPDES) coolin g-water toxicity tests demonstrated an acute effect on Ceriodaphnia du bia and fathead minnows (Pimephales promelas). Test results were chara cterized by irregular mortality dose-response curves and a brown filam entous mass on the opercular area of affected fish. Some fish and Ceri odaphnia also became entrapped in filamentous material present on the bottom of the test vessels. Microscopic examination of test fish revea led severe fouling of the oral and branchial cavities with a mixed pop ulation of microbes, the major component being a filamentous sheathed bacteria. Treatment of water samples with a 0.45-mu m membrane filter and/or ultraviolet light sterilizer were effective in eliminating the bacteria and test organism mortalities. Regulatory test procedures wer e modified to allow filtration of the water prior to testing to elimin ate this naturally occurring biological interference in toxicity tests .