The spread of zebra mussel (Dreissena polymorpha) from the lower Susquehanna River into the upper Chesapeake Bay, Maryland, USA.

The invasion of North American waters by Dreissena polymorpha (zebra mussel) and its subsequent ecological impacts has been well documented. Their spread continues to new states and watersheds despite a wealth of research and outreach campaigns, albeit at a slowed pace. We describe the spread and proliferation of zebra mussel into one of the more recently invaded states, Maryland, USA. Veliger counts and fixed hard surface monitoring suggest that a population is established within the lower Susquehanna River to the head of the Chesapeake Bay. A relatively narrow tidal-freshwater habitat envelope with suitable salinity may serve to restrict zebra mussel dispersal further downstream. Dispersal to other freshwater habitats in Maryland seems likely as the current area of infestation is highly used by commercial and recreational boaters, although regulations are in place that could help slow or prevent spread. Successful invasion will likely be a function of water chemistry, which may be limiting in some parts of the state.