The impact of methoxychlor treatment of the saskatchewan river system on artificial substrate populations of aquatic insects

The impact of methoxychlor exposure on aquatic insects inhabiting artificial substrates was monitored at three downstream sites relative to an upstream untreated site of the North Saskatchewan River. Treatment impact was studied for selected species of Simuliidae (Diptera), Perlodidae (Plecoptera), Hydropsychidae (Trichoptera), Baetidae and Heptageniidae (Ephemeroptera). At sites subjected to methoxychlor exposure which were 21, 38, and 107 km from injection, population changes varied depending on species and distance from the injection point. Although populations of some species were not significantly affected by treatment at any downstream site (P > 0.05), others were significantly reduced at one or more of the sites (P < 0.05- P < 0.01). Nymphs of Stenonema terminatum (Walsh) and Baetis tricaudatus Dodds (Ephemeroptera) apparently recolonized after dislodgement due to methoxychlor exposure. Species are categorized on the basis of their responses to methoxychlor treatment. Factors which probably caused different treatment impacts among species are discussed.