Three-Dimensional Distribution of Winter Stonefly Nymphs, Allocapnia pygmaea, within the Substrate of a Southern Ontario River

Changes in the three-dimensional distribution patterns of stonefly nymphs, Allocapnia pygmaea, beneath the streambed in the Speed River, southern Ontario, were monitored throughout their 1-yr life cycle using 270 colonization chambers. These were filled with organism-free, sieved stream gravel and buried in vertical groups of three, at three depth intervals, in three trenches positioned across a riffle. Nymphs were present throughout the year. Seasonal changes in the distribution pattern of nymphs indicated that they were able to move beneath the streambed in both the horizontal and vertical planes. Nymphs were most abundant at depth during the summer diapause, but moved up to the surface once diapause had been broken in the autumn. There was no evidence of any bankwards migration of nymphs prior to emergence. We have therefore confirmed in detail previous suggestions that stream insects move freely into and out of the hyporheic, using it as a refuge from adverse conditions on the streambed. Stream ecologists should therefore be aware of the possibilities of movement to and from the hyporheic when working with benthic invertebrates.