The effect of methoxychlor on carbon assimilation by freshwater phytoplankton

The effect of the insecticide methoxychlor (2,2-bis(p-methoxyphenyl)-1,1,1-trichloroethane) on photosynthetic carbon assimilation by natural phytoplankton populations was determined by incubation of plankton samples in situ at a 1.0-metre depth with [¹⁴C]bicarbonate for 4-h periods with or without 100 parts per billion (ppb) (100 μg/l) pure or technical grade methoxychlor. The different algal populations occurring over a 5-week period in May and June in Lake Opinicon, Leeds County, Ontario, were found, in 19 experiments, to be seldom affected by the insecticide. In the few cases where carbon assimilation in methoxychlor-treated samples was found to be significantly lower than that of the controls, the inhibition ranged in extent from 9.7% to 67.4% and these effects could not be correlated unequivocally with cell number or with the presence of any particular algal species.