Differential effects of malathion and nitrate exposure on American Toad and Wood Frog tadpoles

Organisms living in aquatic ecosystems are increasingly likely to be exposed to multiple pollutants at the same time due to the simultaneous use of several pesticides and fertilizers. We examined the single and interactive effects of environmentally realistic concentrations of nitrate and malathion on two species of tadpoles common in agricultural regions of the United States-the American Toad (Bufo americanus) and the Wood Frog (Rana sylvatica)-using a fully factorial mesocosm experiment that crossed four concentrations of malathion ranging from 0 to 1000 μg/l and two concentrations of nitrate (0 or 8 mg/l). In both B. americanus and R. sylvatica, malathion delayed metamorphosis compared to controls, even at the lowest concentration of malathion. Malathion did not affect survivorship in either species. B. americanus metamorphs were smaller in malathion treatments, whereas R. sylvatica were larger in malathion treatments. Nitrate did not affect survivorship or metamorph size in either species, but did accelerate time to metamorphosis in R. sylvatica. The interaction between nitrate and malathion had no effects in B. americanus and had no effect on R. sylvatica survivorship or metamorph size. However, in the 250 μg/l and 500 μg/l malathion treatments, nitrate reduced the negative effect of malathion on time to metamorphosis in R. sylvatica such that there was little if any delay in metamorphosis compared to the controls in these treatment combinations. This observation suggests that the presence of nitrate might ameliorate the effects of malathion on R. sylvatica. Our results suggest that malathion could have significant effects on anuran populations and communities and that nitrate might potentially mediate such effects in some species.