Relative efficiencies of wet and dry extraction techniques for sampling aquatic macroinvertebrates in a subarctic peatland

Two behavioural extraction techniques, wet and dry extraction, were evaluated as to their relative efficiencies in removing invertebrates from samples of wet peat collected from a fen near Schefferville, Quebec. Dry extraction involved drying a substrate sample from above, forcing 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 downward out of the sample and into the water bath. Wet extraction yielded approximately three times more invertebrates per unit volume of substrate than dry extraction, but the two extraction methods differed considerabiy in their ability to extract different invertebrate taxa. Dry extraction was more effective than wet for obtaining larvae of Tabanidae, Tipulidae, Empididae, and Dolichopodidae, but larval Chironomidae, sphaeriid clams, and oligochaete worms were collected more efficiently by wet extraction. Other invertebrate taxa were collected with approximately equal efficiency by both methods.