Effects of an azinphos-methyl runoff event on macroinvertebrates in the Wilmot River, Prince Edward Island, Canada

High levels of azinphos-methyl (0.4-0.8 mu g/L) were detected in the Wilmot River, Prince Edward Island, Canada, following runoff from an agricultural field after a heavy rainfall on 19 July 2002. Benthic macroinvertebrate abundance and diversity were sharply lower in samples collected 1 d after the event compared with samples collected in the same manner in July or October 2001. The greatest effects were noticed on the aquatic insects, whose abundance declined from > 10000 individuals per 3-min kick sample in July 2001 to < 900 individuals per 3-min kick sample in July 2002. One family of Diptera, one family of Plecoptera, and three farmlies of Trichoptera disappeared entirely from the study reach after the runoff event, and several other families were severely depleted in number. This led to low taxonomic similarity values between the communities before and after the runoff event and a change relative to reference streams on PEI. Examination of biological metrics (including indices such as % EPT (Ephemeroptera, Plecoptera, or Trichoptera), % chironomids, % burrowers, etc.) confirmed that aquatic insects were more heavily targeted by the insecticide than non-insect invertebrates. This resulted in a shift in the community towards non-insect taxa that were better able to avoid or tolerate this type of pollution.