The effect of the lampricide, 3-trifluoromethyl-4-nitrophenol (TFM), on the benthic macroinvertebrates within the hyporheic region of Dam Creek, Ontario was investigated . Organisms were regularly found to a depth of 70 cm in the substrate throughout the study period . Of the eight major taxa found at both the treated and untreated sites only Tubificoidea exhibited a decrease in abundance attributable to TFM .One day after treatment, TFM concentrations were greatest at a depth of 55 cm, the greatest depth to which water samples were taken . The movement of TFM into the hyporheic region during the present study may be due to the large convective forces created by the rapid decrease in surface water temperature . These convective forces are usually greatest in the late fall and winter when TFM is not applied . The greater part of TFM stream treatments are carried out during the summer when surface water temperatures remain fairly constant . It is suggested that the hyporheic region may act as a refuge zone for benthic macroinvertebrates against the lampricide during most of the TFM application season .
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.
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