Recruitment and growth of zebra mussels (Dreissena polymorpha) in a coupled lake-stream system

The zebra mussel, Dreissena polymorph a, is invading inland lakes and their outflowing streams in North America and Europe, yet little is known about mussel population dynamics in streams that provide natural conduits for dispersal. We compared veliger abundance, mussel recruitment, and growth of adult mussels in two connected lakes and their common outlet stream in southwestern Michigan, USA. Mean veliger abundance in the lakes was 31-92 % higher and more variable over time than in the stream. Recruitment also was higher and more variable in the lakes (10.2 ± 9.9 x 104 settlers m-2 month-12 than in the stream (142.0 ± 16.7 settlers m-2 month-1). Recruitment in the stream declined exponentially with distance downstream (y = 679 e- 0.006d; R2 = 0.75) from the lake outlet and no recruitment was detected farther than 1.8 km downstream. Mean adult growth was lowest in Eagle Lake (3.2 ± 0.1), but growth in Christiana Lake (8.6 ± 0.3 mm) was similar to growth in the stream (9.5 ± 0.3 mm) during the summer. Veliger abundance in the stream did not decline with distance downstream in 1994 or 1995, but declined significantly from 8.8 veligers/L at the lake outlet to 2.1 veligers/L near the mouth (ca. 18 km downstream) in 1996. Mean flux of veligers through the stream was about 800 million veligers/d in 1996. We conclude that small outflowing streams can provide a corridor for veliger dispersal into downstream ecosystems, but appear themselves to support only small populations of zebra mussels.