Abstract
-Veligers spawned from lake populations are thought to be the major means 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 uprooted macrophytes originating in lakes. Drifting macrophytes were collected on three occasions (8-h, mid-channel, seine samples) from Christiana Creek (the outflow of invaded Christiana Lake, southwestern Michigan). 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 were tagged with flagging tape, released and recollected (N = 5 releases). Average transport distance of macrophytes was 333 m, and the presence 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 mechanism by which adult zebra mussels can invade outlet streams, but retention appears to limit downstream transport to relatively short distances. Regardless, high potential survival of adult mussels compared to veligers suggests that adult drift on macrophytes is an important component of riverine invasion.
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