Impacts Overview
Federal, State, and Municipal Facilitys
Recreation
Other Organisms

Impacts

Since their introduction, zebra mussels have impacted man-made structures, as well as the structure and function of natural communities they have invaded. Zebra mussel interactions with federal, state, and municipal facilities have been primarily negative to date. Indeed, most of the initial interest in zebra mussels was a direct result of the economic cost to municipal facilities with large zebra mussel infestations. The impacts of mussel interactions with natural communities are more detailed, and currently under intensive study.

This section provides a brief overview of known zebra mussel impacts to both man-made structures and natural communities. Zebra mussel impacts on a system relate directly to zebra mussel biology. Major problems for either man-made or natural systems result from mussel attachment to a structure or organism with byssal threads, the ability to filter large quantities of water, and high fecundity under suitable environmental conditions.

Impacts on Federal, State, and Municipal Facilities

Zebra mussels in infested areas impact facilities such as locks, dams, and hydroelectric plants. Mussels have blocked pipes , screens , condenser units, and many other small openings which water must pass through. No structures in the water are immune . They do this by forming druses (clumps of mussels) or simply attaching to available hard substrates with byssal threads. Once a single mussel is in place, others may settle on or around it and the colonization increases until a pipe or opening is completely blocked . Even if mussels are killed by a control technique, the shells and any decaying animal material remain behind causing additional problems. Thus killing the mussels is only part of the solution to opening a blockage. A complete solution requires the removal of the mussels.

Dreissena has formed such dense encrustations that entire facilities have been forced to shut down for mechanical cleaning, as demonstrated by these pictures of the interior of the Detroit Edison Hydroelectric Plant undergoing such cleaning . In the first picture, note the dense mats of zebra mussels on all exposed surfaces. These shut downs cost power companies thousands of dollars in terms of both maintenance costs and lost revenue.

Corps of Engineers lock facilities are vulnerable to heavy encrustations. In addition to the problems posed by dense mussel encrustations, there is evidence that zebra mussel byssal threads may accelerate corrosion around joints and rivets of metal structures of power plants . Note the dense zebra mussel encrustation and corrosion around the visible rivets . This is after only one year of zebra mussel settlement. A more detailed discussion of the Black Rock Lock facility may be found in Risk Assessment, Detection/Monitoring, and Management Options.

Impacts on Recreation

Smaller man-made structures are also vulnerable to zebra mussel infestation. Zebra mussels have caused increased maintenance costs for waterfront property owners, recreational boaters, and fisherman. Docks, sea walls, boats, boat lifts, and ladders all provide hard substrate for mussels to settle on in inland lakes and rivers, as well as the Great Lakes. Owners must scrape these substrates annually to remove mussels that could otherwise impede function and damage structural integrity. This boat lift and ladder were removed from Lake Wawasee, Indiana in October, 1992, less than a year after mussels were first reported in this inland lake . Note the zebra mussels attached to the lift in the close-up . These mussels settled out and attached to this lift in one season (June-October).

Impacts on Other Organisms

Naturally occurring substrates are also vulnerable to zebra mussel attachment and even more prone to serious long term damage. Native clams, such as the unionid mussels, provide hard substrates for zebra mussel settlement in lakes and rivers. Dense encrustations such as these will kill native clams since it is impossible for them to open their shell valves to filter feed, respire and burrow. The same unionid is shown before

and after zebra mussel removal . There are numerous examples of native mussels being impacted by zebra mussel encrustation . Unionid populations in Lake St. Clair have been reduced to extremely low levels. Unionid populations in inland lakes such as Lake Wawasee, Indiana, are rapidly being reduced by zebra mussels. It is estimated that the larger unionids in Lake Wawasee have more than 500 zebra mussels per unionid (Benbow et al. 1994). The impacts of unionids decline on local trophic structure are as yet unknown. However, many threatened and endangered unionid species are much more likely to decline in numbers if zebra mussels settle into their watersheds.

In addition a variety of organisms have been found with zebra mussels attached. These include macrophytes, gastropods, crayfish, trichopteran cases, as well as fish. Note the dense zebra mussel coating on the lower portion of Ceratophyllum, a submersed macrophyte . Numerous other macrophyte species in inland lakes have been observed with dense layers of zebra mussels (Myriophyllum sp.; ). The effects of such thick coatings of mussels on a plant's ability to photosynthesize and function are unknown at the present time. Other organisms are similiarly impacted .

More research on the zebra mussel's impact on local trophic structure is needed in order to understand the biological and ecological consequences of this invading species.

Impacts Bibliography

This section provides a bibliographic listing for all works cited in Impacts. A more complete listing of related zebra mussel information is provided in References.

Benbow, Mark E., Jason C. Doss, Juliana M. Harding, Kathleen A. Burky, Albert J. Burky and Carl M. Way. 1994. Water flow around Dreissena polymorpha encrusted Anodonta grandis in a lake system. Proceedings of Fourth International Zebra Mussel Conference, Madison, Wisconsin.

Garton, David W. and Wendell R. Haag. 1993. Seasonal reproductive cycles and settlement patterns of Dreissena polymorpha in western Lake Erie. p. 111-128 In: Zebra mussels: Biology, Impacts, and Control. (eds. T. F. Nalepa and D. W. Schloesser). Lewis Publishers. Boca Raton, Florida.

McMahon, Robert M. and John L. Tsou. 1990. Impact of European Zebra mussel infestation to the electric power industry. Proceedings of American Power Conference. 988-997.