A years' study of the drifting organisms in a brown-water stream of Alberta, Canada

Abstract

Ten 24-h drift samples were taken from a brown-water stream of Alberta, Canada over a 1-year period with drift nets having a mesh size of 320 μ. Cladocerans, cyclopoids, and ostracods, collectively called entomostracans, made up a large part of the drift by numbers and contributed substantially to the total biomass of the drift. Drift densities of entomostracans tended to increase as the ice-free season progressed, but drift densities of immature insects remained relatively constant throughout the ice-free season. Total daily drift of both the entomostracans and non-entomostracan fractions tended to decrease as the ice-free season progressed, being dependent on water volume. Drift densities, total daily drift, and number of taxa in the drift were very low in winter. Most of the species exhibited nighttime behavioral drift. At the sampling site, the entomostracans and immature aquatic insects were found to be essentially evenly distributed throughout the water column. For part of the study period, drift densities of taxa caught in the 320-μ net were compared with drift densities of the same taxa caught in a 720-μ net. The 720-μ net caught a much smaller fraction of the aquatic insects than did the 320-μ net, and almost all the entomostracans passed through the 720-μ net, When compared with other regional drift studies, the large fraction of entomostracans in the brown-water stream seems to be a unique feature; there is evidence that most of the drifting entomostracans originate in the marshy area drained by the main stream.