Regional differences in rates and patterns of North American inland lake invasions by zebra mussels (Dreissena polymorpha)

Zebra mussels (Dreissena polymorpha) have spread rapidly in North America b y dispersal within connected bodies of waters. This study provides the firs t systematic evaluation of rates of zebra mussel dispersal to inland lakes separated from source populations by functional dispersal barriers. Plankto n samples were examined for this exotic species from 140 lakes during a 3-y ear period (1995-1997). Infestations were detected in 19% of lakes surveyed : seven of 28 Indiana lakes (25%), 15 of 49 Michigan lakes (31%), but only five of 63 Wisconsin-Illinois lakes (8%). Annual rates of infestation varie d from 0 to 12%.year(-1) among the three regions. Wisconsin-Illinois lake i nfestations were only detected in 1995 and 1996, whereas new Indiana and Mi chigan infestations were detected in all three years. Lakes with surface ar eas less than 100 ha had lower infestation rates than larger lakes. Inciden tal sightings of inland lake colonization within the study region qualitati vely supported observed regional differences in rates and spatial patterns of colonization. These results demonstrate that the spread of zebra mussels into inland lakes is not occurring as rapidly as through connected waterwa ys, and rates of inland lake colonization vary according to regional condit ions and lake size.