The effects of Masoten, an organophosphate parasiticide, on phytoplankton and zooplankton in earthen ponds were studied. In 2 separate trials, 3 ponds (0.04 hal received a single application of Masoten at a rate of 0.25 mg/l (active ingredient) and 3 ponds served as untreated controls. Net plankton samples were collected at pretreatment, and 5, 24 and 48 hI's following treatment. Toxic effects were based on quantitative.qualitative plankton analyses. Phytoplankton and rotifers were unaffected by treatment. The copepod Diaptomus sp. and nauplii were also unaffected. However, variability in response of copepods and nauplii to Masoten can be anticipated. Cladocerans were the most sensitive to Masoten; losses are to be expected when the compound is employed for control purposes. Information regarding residues and decomposition of Masoten is presented.
