Fate and biological effects of methyl parathion in outdoor ponds and laboratory aquaria. II. Effects.

Methyl parathion (MEP) applied to three outdoor ponds at a nominal concentration of 100 micrograms liter-1 was toxic to some species of aquatic insects and crustaceans but not to fish. The spectrum of toxicity was similar to predictions based on a literature survey of data obtained from laboratory tests. Various secondary effects occurred that could not be predicted from laboratory toxicity tests. An increase in populations of Diaptomus in treated ponds was probably caused by mortality of predators and competitors. A bloom of filamentous algae which then collapsed, leading to severe depletion of dissolved oxygen and fish deaths, may have been triggered by mortality of herbivorous mayflies and daphnids. The growth of juvenile rainbow trout in treated ponds was significantly less than in untreated ponds. On the other hand their growth in laboratory aquaria was not affected when rainbow trout were exposed to higher concentrations of MEP than occurred in the outdoor ponds. It was concluded that growth of rainbow trout in the ponds was probably affected by mortality among aquatic insects and crustaceans on which they feed.