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

The zebra mussel, Dreissena polymorpha, is invading inland lakes and their outflowing streams in North America and Europe, yet little is known about m ussel population dynamics in streams that provide natural conduits for disp ersal. We compared veliger abundance, mussel recruitment, and growth of adu lt mussels in two connected lakes and their common outlet stream in southwe stern 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 10(4) settlers m(-2) month( -1)) than in the stream (142.0 +/- 16.7 settlers m(-2) month(-1)). Recruitm ent in the stream declined exponentially with distance downstream (y = 679 e(-0.006d). R-2 = 0.75) from the lake outlet and no recruitment was detecte d farther than e 1.8 km downstream. Mean adult growth was lowest in Eagle L ake (3.2 +/- 0.1), but growth in Christiana Lake (8.6 +/- 0.3 mm) was simil ar to growth in the stream (9.5 +/- 0.3 mm) during the summer. Veliger abun dance in the stream did not decline with distance downstream in 1994 or 199 5, 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 vel igers through the stream was about 800 million veligers/d in 1996. We concl ude that small outflowing streams can provide a corridor for veliger disper sal into downstream ecosystems, but appear themselves to support only small populations of zebra mussels.