Effects of the organophosphorus insecticide fenitrothion on growth in fivefreshwater species of phytoplankton

The acute toxicity of the insecticide fenitrothion was measured using four freshwater algae (Chlorella saccharophila, Chlorella vulgaris, Scenedesmus acutus, and Scenedesmus subspicatus) and one cyanobacteria (Pseudanabaena g aleata). Insecticide concentrations eliciting 50% growth reduction over 96 hr (EC50) ranged from 0.84 to 11.9 mg/L. Fenitrothion was more toxic than o ther pesticides studied with the same algal species such as chlorsulfuron, molinate, and pyridaphenthion. The transformation of effective concentratio ns of fenitrothion and other pesticides obtained from toxicity measurements into percent of the saturation level in water is used as a first evaluatio n of potential hazard to aquatic systems. The insecticides fenitrothion and pyridaphenthion were less hazardous than the herbicides atrazine, benthioc arb, cinosulfuron, chlorsulfuron, methyl-bensulfuron, and molinate. The two species of Chlorella and the cyanobacterium Pseudanabaena were more tolera nt to fenitrothion than the two species Of Scenedesmus. (C) 2001 by John Wi ley & Sons, Inc.