Zebra mussel dispersal in lake-stream systems: Source-sink dynamics?

We investigated the ability of zebra mussels (Dreissena polymorpha) to colonize small streams (<30 m wide), which have been considered to have low susceptibility to invasion. We examined European literature concerning riverine mussel populations and sampled lake and stream sites in the St. Joseph River basin (Indiana-Michigan, USA) for mussels. The presence of colonized upstream lakes (rather than stream size) was the critical watershed feature determining zebra mussel invasion of streams because such lakes served as a source of veligers that drifted into outflowing streams. For all sites, there was a significant positive association (p < 0.001) between zebra mussel presence in lakes and in their out flowing streams. Two streams in the St. Joseph River basin (average widths: 20 m and 7 m) had mussel densities that declined exponentially from >1000/m² at the lake outlet to about 10/m² within 1 km downstream of the colonized lake, although isolated mussels were found up to 12 km downstream. This pattern persisted for 3 y (1993-1995) with no substantial change in mussel distribution or abundance. Stream populations appear not to be self-sustaining, but rather they rely on an upstream source of larvae. Our findings suggest that zebra mussel distributions in flowing water ecosystems are best described by a source-sink model, wherein streams ('sinks') are the recipient for larval mussels produced in lakes ('sources').