Effects of ivermectin-spiked cattle dung on a water-sediment system with the aquatic invertebrates Daphnia magna and Chironomus riparius.

A two-species test using a water-sediment test system was performed to investigate chronic effects of the parasiticide ivermectin on Daphnia magna and Chironomus riparius. To simulate exposure by direct excretion of cattle into surface waters, ivermectin was applied via spiked cattle dung. The parasiticide was applied once, at concentrations ranging from 11 to 1314 microg kg(-1) dung dry weight. The highest concentration corresponds to the maximum concentration in dung 3 days after topical application to cattle. Test vessels were stocked with chironomid larvae and daphnids of defined, mixed age. Replicates were sampled 10, 24, 38 and 51 days post application. Survival, growth and emergence of chironomids, and abundance and biomass of daphnids were evaluated. In case of extinction of the D. magna population in all replicates of a concentration level, daphnids were re-introduced into the remaining vessels of this concentration to simulate immigration. In addition, a second batch of chironomid larvae was introduced into the vessels on day 27 post application. At 1314 microg ivermectin kg(-1) dung dry weight, survival, larval growth and emergence of the initially stocked chironomids were strongly affected. A significant effect on emergence was also observed for the second batch of chironomids. The two highest test concentrations led to 100% mortality of the initially stocked daphnids. At 1314 microg kg(-1) dung dry weight, no daphnids survived following re-introduction on days 11, 28 and 42. At 263 microg kg(-1) dung dry weight, the daphnids that were re-introduced on day 11 survived and reproduced, but abundance and biomass were reduced. The results of the present study indicate that following single application, toxic ivermectin concentrations persisted for an extended period. Possible effects on aquatic invertebrates, which may be caused by direct excretion of ivermectin-containing dung into surface water, deserve further attention.