The role of waterway connections and downstream drift of veliger larvae in the expanding invasion of inland lakes by zebra mussels in Minnesota, USA

Zebra mussel spread to inland waters in Europe and North America is often attributed to overland transport by trailered boats and water-related equipment. Much less attention is paid to spread between lakes connected by streams that may serve as conduits for larval dispersal. Previous studies have produced few data on the magnitude and distance of downstream drift of veliger larvae. Here we present an evaluation of the contribution of streamflow connections to zebra mussel spread, and new data on the numbers of larvae transported from infested lake stream outlets. We studied lake/stream connections in Minnesota (MN) and found that lakes connected to upstream infested lakes are 27 times more often infested than unconnected lakes. In 4 lake/stream systems and at increasing distances downstream of infested source lakes, we estimated recruitment and larval “flux” (numbers per unit time), and found recruitment to be localized, in most cases to within 1 km from source lakes. A short distance from lake outlets, larval flux reached 10⁹ larvae per day or more at peak times in the 2 systems with the highest supply. In all 4 systems, flux dropped steeply and exponentially with distance. Nevertheless, flux reached 0 (observed) through the entire season, and/or was below a low-risk threshold (estimated) at the downstream inlet (as far as 64 km downstream) in only 1 system. Spread risk by downstream drift is so high that prevention and treatment efforts should be focused on upstream lakes in pristine watersheds. At the same time, localized recruitment confirms that zebra mussel populations are unlikely to persist far from source waters, limiting impact to unionids and other native species in small streams.