Decline and regional extirpation of freshwater mussels (Unionidae) in a small river system invaded by Dreissena polymorpha: the Rideau River, 1993-2000

Data pertaining to the ecological impact of the exotic zebra mussel, Dreissena polymorpha, on benthic fauna in small river systems are scarce. We conducted a long-term study to assess the impacts of the D. polymorpha invasion in a small river system (100 km) in eastern Ontario during an 8-year period (1993–2000). A 30-km downstream section of the Rideau River was studied before and during rapid population growth of D. polymorpha in the area. During 1993–1995, D. polymorpha abundance on hard substrates increased by four to six orders of magnitude and remained high thereafter. A comparable temporal pattern of D. polymorpha abundance was observed on shells of live freshwater mussels (Unionidae). During peak fouling (1995–1997), mass ratios (mass of attached D. polymorpha / mass of live unionid host) ranged from 0.37 to 1.81. SCUBA-diving surveys (50-m transects) were conducted to examine the impact of D. polymorpha on native unionids in impounded river habitats. In 1993–1994, three unionid taxa were commonly...