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 substrate preference experiments to test the hypotheses that amphipods prefer (i) high-complexity substrates over low-complexity substrates and (or) (ii) substrates with high mussel feces and pseudofeces deposition over substrates with low deposition. We measured amphipod preference for bare rock, live mussels, and dead mussels in spring (May 1996) and summer (July and August 1995, June and August 1996). Habitat complexity affected amphipod habitat preference, 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 substrates. Amphipods never preferred low-complexity substrates. We also evaluated effects of zebra mussel presence on fish-amphipod interactions in laboratory feeding trials. We tested the hypothesis that mussel presence decreases bluegill (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 increase in amphipods upon zebra mussel invasion is due to increased habitat complexity, possibly by reducing predation risk. However, the effects of zebra mussel on fish-amphipod interactions depended on predator species.