Impact of Methoxychlor on Selected Nontarget Organisms in a Riffle of the Souris River, Manitoba

Abstract

The insecticide methoxychlor was applied at 300 μg∙L−1 for 15 min to a riffie on the Souris River, located about 18 km downstream from Souris, Manitoba. Physical, chemical, and biological variables were measured and aquatic insect community structure was monitored using drift, emergence trap, and artificial substrate samplers. All taxa monitored, irrespective of functional feeding group, drifted catastrophically for 4–24 h immediately following methoxychlor addition. Different species demonstrated varying abilities to recoionize artificial substrates following treatment. Species having a high propensity to drift naturally, recolonized most rapidly. Taxa that required the longest period to recoionize following methoxychlor treatment were generally univoltine, had a low propensity to drift, and a limited ability to disperse as adults. Impact of methoxychlor was influenced by the prevalent life-cycle stage of some species at the time of treatment. Catostomus commersoni fry and juvenile Orconectes virilis were more sensitive to methoxychlor than previous research on mature individuals has indicated. Invertebrate drift appeared to be more sensitive to pesticide treatment than benthic invertebrates on artificial substrates. Species richness and total numbers of drift were significantly reduced for at least 33 d following treatment, whereas richness and numbers on artificial substrates were significantly lower for only 4 and 8 d, respectively.