We used artificial substrates (rocks < 1500 cm2 surface area) in shallow water (2 m) to assess the development of epilithic macroinvertebrate communities in the presence of zebra mussels. At a turbulent site (Wheatley, Lake Erie), previously colonized (with a non-zebra mussel community) and uncolonized rocks left for 1 year both had lower densities of total invertebrates than previously colonized rocks recovered after only 1 day. As zebra mussels colonized the rocks, Gammarus sp. (amphipods) increased in density, while Chironomini and Tanypodinae (midges), Polycentropus sp. (caddisflies), and Physella sp. and Pleurocera sp. (snails) declined. At a protected site (Stoney Point, Lake St. Clair), previously colonized rocks initially (2 months) had higher densities of many taxa, including zebra mussels, than uncolonized rocks. This difference disappeared after 1 year, as zebra mussels increased on all rocks. Gammarus sp. maintained its numbers, while Tricladida (flatworms) increased and Oecetis sp. (caddisflies), Physella sp., Pleurocera sp., and Tanypodinae declined. Although a similar "zebra mussel – amphipod" community developed on rocks at both sites, we hypothesize that at the turbulent site, zebra mussels and amphipods have a shared tolerance of unstable habitats, and zebra mussels facilitate amphipod colonization of rocks by increasing microhabitat stability and food supply. At the protected site, zebra mussels outcompete other surface dwellers like snails for space, and facilitate the colonization of scavenger–omnivores like amphipods and flatworms.
Subtidal littering: Indirect effects on soft substratum macrofauna?