THE ROLE OF THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA) IN STRUCTURING MACROINVERTEBRATE COMMUNITIES ON HARD SUBSTRATA

We examined the importance of the Eurasian zebra mussel (Dreissena pol ymorpha) in structuring macroinvertebrate communities on hard substrat a in the Great Lakes -St. Lawrence River system. An experiment using a rtificial substrata (i.e., cement bricks with either a layer of living zebra mussels, a layer of intact empty shells that mimicked living mu ssels, or with no added layer) showed that macroinvertebrate abundance is enhanced in the presence of zebra mussels and that macroinvertebra te responses to physical versus biological attributes of mussel beds ( e.g., spatial habitat created by clumped shells; biodeposition) vary a mong taxa. Moreover, densities of zebra mussels and associated epifaun a have increased severalfold at various sites in the Great Lakes -St. Lawrence River system within the past decade; changes in community com position were similar to those observed in our artificial substrate ex periment. Our results suggest that dense zebra mussel colonization alt ers macroinvertebrate communities on hard substrata by enhancing condi tions for deposit-feeding organisms, small gastropods, and small preda tory invertebrates, and by displacing large gastropods and certain lar ge filterers. In the St. Lawrence River, these effects were associated with zebra mussel densities of 1500-4000 individuals/m(2), which are likely to be supported by most waterbodies in North America.