EFFECTS OF HABITAT STRUCTURE ON HABITAT USE BY GAMMARUS-PULEX IN ARTIFICIAL STREAMS

1. The influence of coarse substratum and flow, coarse substratum and food, and predation risk and Bow on habitat use by Gammarus pulex: was studied in three experiments conducted in artificial stream channels. Each stream channel consisted of a riffle and pool habitat. 2. Locati on of coarse substrata and food was manipulated by placing cobbles (co arse substratum) and leaf packs (food) in different habitats. Predatio n risk was varied by running experiments in the presence and absence o f sculpins (Cottus gobio), and flow was varied by pumping water with o ne or two pumps. 3. In all experiments Gammarus were most abundant in pools but placement of cobbles in riffles increased use of the latter. An even greater percentage of Gammarus used rifles if leaf packs were also placed there. Decreased discharge and the presence of sculpins ( Cottus gobio) also caused Gammarus to increase use of riffles. These d ata indicate that Gammarus is able to evaluate differences in habitat quality and respond accordingly.