Zebra mussels (Dreissena polymorpha) and Unionids in Douglas Lake, Michigan.

Abstract

Zebra mussels were introduced into the Laurentian Great Lakes in 1986, and zebra mussel invasion has impacted freshwater ecosystems. Zebra mussels cause starvation of native unionid bivalve mollusks by out competing unionids for food resources. Invasion of zebra mussels to Douglas Lake, Michigan occurred in 2001. In this study, the effect of zebra mussels on unionids, in Douglas Lake is investigated. A survey of unionids was carried out in shallow and deep water sites. There is a significant difference in the ratio of live to dead unionids in shallow water sites (ratio=0.383) and deep water sites (ratio=0636) (ANOVA, p=0.04, df=23). A greater ratio of live to dead unionids is found in deep water sites below the thermocline in Douglas Lake. In addition, a greater abundance of zebra mussels are present in the shallow sites than in the deep sites. Zebra mussels may be impacting the unionids in shallow water, however there appears to be a refuge below the thermocline. Reports of unionid refuge in freshwater ecosystems have been reported. Considering the total mortality of unionids owing to zebra mussel infestation, identification and protection of refuge for unionids would be critical in preserving native unionid species in North America.