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 found in 10-m(2) quadrats sampled along transects: Elliptio complanata, Pyganodon grandis, and Lampsilis radiata. Overall, the mean density of unionids declined 5- to 8-fold from 1993 to 1997, coinciding with a rapid increase in D. polymorpha densities on unionids. By 1997, i.e., 4 years after fouling began, P. grandis and L. radiata had been extirpated from those sites, with only E. complanata remaining. By 2000, i.e., 7 years after fouling began, all three unionid taxa had been essentially extirpated from the 30-km section of the river, with only one live individual (E. complanata) collected during 10 extensive diving surveys (the total estimated area of riverbed surveyed was 4000 m(2)). This study indicates that heavy fouling by D. polymorpha causes the extirpation of unionids in small impounded river systems (less than or equal to100 km).