The effects of the lampricide, TFM, on the abundance of macroinvertebrates in the benthos of Wilmot Creek, a hardwater tributary to Lake Ontario, was examined over 1 year. Drifting macroinvertebrates were also collected before, during and after TFM treatment. Significant decreases in benthic abundance were exhibited by Dolophilodes sp., Tubificoidea, Cricotopus sp. and Macrotendipes sp. throughout the 350 days following treatment. Only the decrease in abundance of Dolophilodes sp. and lbbificoidea could be attributed to TFM treatment. Increases in drift abundance observed during treatment were generaIly an accurate indicator of TFM-sensitive macroinvertebrates. The most sentive taxa (Dolophilodes sp., Dugesia sp. and Tubificoidea) responded immediately following the introduction of TFM. Branchiobdellida, Diamesa sp., Dicranota sp., Lumbricidae and Nemouridae exhibited increases in drift abundance 8-10 h after the introduction of TFM, however, were considered less sensitive than the former taxa because a decline in their abundance in the benthos was not detected.
The response of the benthic invertebrates found in this hardwater creek was similar to those observed during studies of softwater streams. Only the most severely affected taxa were not present in the benthos 350 days after treatment.