Drift of invertebrates in an intermittent stream draining marshy terrain of west-central Alberta

Seven 24-h drift samples were taken with a plankton net (pore size: 76 μ) over a 1-year period from an intermittent stream that drains marshy, muskeg-type terrain of west-central Alberta, Canada. The drift was mainly composed of planktonic and benthic animals originating in the marsh. The only abundant lotic taxon in the drift was simuliid larvae. Rotifers and cyclopoid nauplii were numerically the most important taxa. Drift densities for the fauna as a whole tended to decrease as the ice-free season progressed, but there was no consistent correlation between drift densities and flow. However total daily drift across a point varied directly with flow. All the abundant taxa drifted more during the day than at night, and nematodes, harpacticoids, simuliid larvae, chironomid larvae, chydorids, and rotifers were found in significantly (P < 0.05) greater numbers in the daytime drift. Drift rates of taxa caught in the plankton net were compared with drift rates of the same taxa caught in a 320-μ drift net. Rotifers, entomostracans (especially the immature stages), and even small simuliid and chironomid larvae would have been seriously underestimated using only the 320-μ net. The marshy areas via drift through the intermittent tributaries contribute a very large number of small organisms to the main stream. Draining the wetlands might have a pronounced detrimental effect on the main stream's ecosystem.