A seasonal change in the distribution of a stream-dwelling stonefly nymph reflects oxygen supply and water flow

We examined the effects of oxygen availability, which has been viewed as a minor factor in streams, on the seasonal and spatial microhabitat distribution of a stonefly. Surveys were conducted in winter and summer in a mountain stream by collecting stones from the streambed and determining the presence or absence of the insect. At each stone sampling, we also measured physical conditions. The probability of the stonefly presence increased significantly with current velocity in summer, but not in winter. Because current influences oxygen renewal rates, our results suggest that the distribution of the insect could be restricted by oxygen.