Mechanisms underlying amphipod responses to zebra mussel (Dreissena polymorpha) invasion and implications for fish-amphipod interactions

We examined mechanisms underlying increased amphipod abundance after zebra mussels (Dreissena polymorpha) invaded Lake Erie. We conducted field substr ate preference experiments to test the hypotheses that amphipods prefer (i) high-complexity substrates over low-complexity substrates and (or) (ii) su bstrates with high mussel feces and pseudofeces deposition over substrates with low deposition. We measured amphipod preference for bare rock, live mu ssels, and dead mussels in spring (May 1996) and summer (July and August 19 95, June and August 1996). Habitat complexity affected amphipod habitat pre ference, and preference varied seasonally. In spring, amphipod density was highest on dead mussels, but the response was highly variable. In midsummer (June and July), amphipods showed no substrate preference. In late summer (August), amphipods consistently preferred high-complexity mussel substrate s. Amphipods never preferred low-complexity substrates. We also evaluated e ffects of zebra mussel presence on fish-amphipod interactions in laboratory feeding trials. We tested the hypothesis that mussel presence decreases bl uegill (Lepomis macrochirus) and yellow perch (Perca flavescens) predation on amphipods. Predation by bluegill but not yellow perch was significantly lowered by mussel presence. Our results support the hypothesis that the inc rease in amphipods upon zebra mussel invasion is due to increased habitat c omplexity, possibly by reducing predation risk. However, the effects of zeb ra mussel on fish-amphipod interactions depended on predator species.