6.0 Aquatic Macroinvertebrate Distribution and Abundance in Long Point Bay

6.4 Influence of zebra mussels on native aquatic macroinvertebrate abundance

The ability of zebra mussels to alter water quality and clarity could directly influence the species composition and biomass of native invertebrates species over space and time. Zebra mussels could also influence invertebrates indirectly, as changes in water quality would influence plant species composition, which is a major determining factor in invertebrate species composition. Zebra mussels tend to attach themselves to native Unionids and this has apparently caused Unionid numbers to decline in certain areas. Zebra mussel use of freshwater molluscs as a substrate declined by 50% from 1991 to 1995 in the Inner Bay, suggesting that Unionid populations may have declined during that time (Table 6.4). This theory is supported by studies showing that zebra mussels caused a decline in native Unionid densities and a loss in the biodiversity of the mussel community in Lake St. Clair (Gillis and Mackie 1994). The density of living Unionids declined from 1.75 per mē in 1990 to 0.06 in 1991, while the number of living species declined from 11 to 4. Gillis and Mackie (1994) did not record any living native Unionids in the southwestern region of Lake St. Clair in 1992. They also found that colonized Unionids had 50% less lipid reserves than uncolonized individuals (Hebert et al. 1991). Zebra mussels tend to form dense colonies on native Unionids, particularly in areas with soft sediments such as Long Point (Herbert et al. 1991). The number of zebra mussels found attached to a single North American Unionid has been reported to be as high 10,000 (Hebert et al. 1991). These extremely high densities may interfere with native Unionid's functioning by competing directly with them for resources or interfering with their filtering activities through deposition of feces and pseudofeces (Gillis and Mackie 1994). As suggested by Gillis and Mackie (1994), the ultimate result of the rapidly expanding zebra mussel population, and the resultant decline in native Unionids has been that the mussel community has shifted from a slow-growing one with a minor influence on ecosystem functioning, to a single taxa that dominates the benthic community, and strongly affects the dynamics of the entire aquatic ecosystem.