BENTHIC MACROINVERTEBRATE COMMUNITIES OF SOUTHWESTERN LAKE-ONTARIO FOLLOWING INVASION OF DREISSENA

Changes in benthic macroinvertebrate communities inhabiting natural co bble and artificial reef substrates in southwestern Lake Ontario were quantified following invasion of zebra mussels (Dreissena polymorpha) and ''quagga'' mussels (Dreissena sp.). Post-Dreissena invasion data ( 1991-1992) were compared with pre-invasion data (1983) from the same s ites. In 1991-1992, Dreissena comprised 79% and 93% of macroinvertebra tes collected at cobble and artificial reef sites, respectively, repla cing the amphipod Gammarus fasciatus as the numerically dominant taxon at both sites. Total abundance of non-Dreissena macroinvertebrates wa s significantly greater at both sites in 1991-1992 than in 1983. Taxa showing the greatest increases in abundance at the cobble site include d the annelids Manayunkia speciosa, Spirosperma ferox, and unidentifie d tubificids; the gastropods Helisoma anceps, Physa heterostropha, Sta gnicola catascopium, Valvata tricarinata, Goniobasis livescens, and Am nicola limosa; the amphipod Gammarus fasciatus; and the decapod Orcone ctes propinquis. At the artificial reef site, significant population i ncreases of Physa heterostropha, Valvata tricarinata, Goniobasis lives cens, Amnicola limosa, Gammarus fasciatus and the trichopteran Polycen tropus were observed. No taxon was less abundant in 1991-1992 than 198 3. Comparisons of macroinvertebrate community similarity in 1983 and 1 991-1992 by Morisita's Index, excluding Dreissena, indicated that prev iously established taxa did not change substantially between 1983 and 1991-1992 at either site. Although many factors may have contributed t o the changes observed, our results support theories that Dreissena is facilitating energy transfer to the benthos through pseudofecal/fecal deposition, and that mussel colonies are providing additional habitat for other invertebrate taxa.