RELATIVE EFFICIENCIES OF WET AND DRY EXTRACTION TECHNIQUES FOR SAMPLING AQUATIC MACROINVERTEBRATES IN A SUB-ARCTIC PEATLAND

Two behavioural extraction techniques, wet and dry extraction, were ev aluated as to their relative efficiencies in removing invertebrates fr om samples of wet peat collected from a fen near Schefferville, Quebec . Dry extraction involved drying a substrate sample from above, forcin g any macroinvertebrates contained therein to move downward out of the substrate matrix. In wet extraction, peat samples were suspended in a water bath where a vertical temperature-dissolved oxygen gradient had been established; invertebrates responded to the gradient by moving d ownward out of the sample and into the water bath. Wet extraction yiel ded approximately three times more invertebrates per unit volume of su bstrate than dry extraction, but the two extraction methods differed c onsiderably in their ability to extract different invertebrate taxa. D ry extraction was more effective than wet for obtaining larvae of Taba nidae, Tipulidae, Empididae, and Dolichopodidae, but larval Chironomid ae, sphaeriid clams, and oligochaete worms were collected more efficie ntly by wet extraction. Other invertebrate taxa were collected with ap proximately equal efficiency by both methods.