Effects of a pyrethroid insecticide in an aquatic community

Background/Question/Methods

Agricultural pollutants that enter lakes, ponds and streams through runoff and aerial spray heavily impact aquatic organisms. Peak application of these pesticides often coincides with vulnerable early life stages of some species in aquatic habitats, including amphibians. The unshelled embryos of amphibians and semi-permeable skin of amphibian larvae, may make them particularly sensitive to pesticides. While the effects of pesticides are regularly studied by toxicologists in laboratory-based, single species studies, the effects of pesticides on communities are less well known. We used outdoor mesocosms to simulate a natural environment that examined the effects of a synthetic pyrethroid insecticide, cypermethrin, on an amphibian species, zooplankton and algae. Cypermethrin is a neuro-toxic synthetic, pyrethroid insecticide used to control lepidopterous pests of cotton, fruit and many vegetables. After experimental manipulations, we compared the sensitivity of the community to varying levels (0, 1 or 5 ug/L) of cypermethrin. To measure how the pond community was affected by cypermethrin treatments, zooplankton, phytoplankton, and periphyton abundances were sampled in each mesocosm.  

Results/Conclusions

Cypermethrin affected the aquatic community, even at the lowest dose. Amphibian growth and size were inhibited with exposure in high treatments. Amphibian survival was not significantly effected by cypermethrin, through we observed a trend towards reduced survival with higher treatments. Conversely, cypermethrin decreased the abundance of zooplankton and increased the abundance of both periphyton and phytoplankton compared to controls in all treatments. The increase in periphyton and phytoplankton is likely due to the reduction of grazing pressure from zooplankton and amphibians in the cypermethrin-exposed treatments. Like other synthetic pyrethroid insecticides, cypermethrin is extremely hydrophobic; it readily adheres to soil and suspended matter in natural water bodies. Consequently, it is often assumed that the bioavailability of cypermethrin is reduced to the point of posing no or negligible threats to aquatic life. However our experiment shows direct deleterious effects of cypermethrin on both invertebrate and vertebrate aquatic species.