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

Changes in benthic macroinvertebrate communities inhabiting natural cobble 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 sites. In 1991-1992, Dreissena comprised 79% and 93% of macroinvertebrates collected at cobble and artificial reef sites, respectively, replacing the amphipod Gammarus fasciatus as the numerically dominant taxon at both sites. Total abundance of non-Dreissena macroinvertebrates was significantly greater at both sites in 1991-1992 than in 1983. Taxa showing the greatest increases in abundance at the cobble site included the annelids Manayunkia speciosa, Spirosperma ferox, and unidentified tubificids; the gastropods Helisoma anceps, Physa heterostropha, Stagnicola catascopium, Valvata tricarinata, Goniobasis livescens, and Amnicola limosa; the amphipod Gammarus fasciatus; and the decapod Orconectes propinquis. At the artificial reef site, significant population increases of Physa heterostropha, Valvata tricarinata, Goniobasis livescens, Amnicola limosa, Gammarus fasciatus and the trichopteran Polycentropus were observed. No taxon was less abundant in 1991-1992 than 1983. Comparisons of macroinvertebrate community similarity in 1983 and 1991-1992 by Morisita's Index, excluding Dreissena, indicated that previously established taxa did not change substantially between 1983 and 1991-1992 at either site. Although many factors may have contributed to 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.