DRIFTING MACROPHYTES AS A MECHANISM FOR ZEBRA MUSSEL (DREISSENA-POLYMORPHA) INVASION OF LAKE-OUTLET STREAMS

Veligers spawned from lake populations are thought to be the major mea ns for spread of zebra mussels (Dreissena polymorpha) into outflowing rivers. We hypothesized, however, that adult zebra mussels attached to lake macrophytes could enter outflowing streams by 'rafting' on uproo ted macrophytes originating in lakes. Drifting macrophytes were collec ted on three occasions (8-h, mid-channel, seine samples) from Christia na Creek (the outflow of invaded Christiana Lake, southwestern Michiga n). On average, 2620 adult mussels entered the stream per day attached to plants, mostly on wild celery (Vallisneria americana). To measure experimentally the transport of macrophytes, 50 V. americana plants we re tagged with flagging tape, released and recollected (N = 5 releases ). Average transport distance of macrophytes was 333 m, and the presen ce of attached mussels did not affect transport distance. Woody debris was the most important retention item, entrapping 85% of the released macrophytes. Drift of macrophytes from upstream lakes provides a mech anism by which adult zebra mussels can invade outlet streams, but rete ntion appears to limit downstream transport to relatively short distan ces. Regardless, high potential survival of adult mussels compared to veligers suggests that adult drift on macrophytes is an important comp onent of riverine invasion.