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Zebra Mussels in Southwestern Lake Michigan


by
Tammy Keniry
Illinois Natural History Survey
J. Ellen Marsden
Illinois Natural History Survey
The zebra mussel (Dreissena polymorpha) is a European species that was accidentally introduced into North America. It has had a tremendous impact on freshwater ecosystems of the United States and Canada. Since the zebra mussel was first discovered in Lake St. Clair in 1988, it has spread to each of the Great Lakes and to the major river systems of central and eastern United States. Communities along the affected lakes and rivers rely on these waters for drinking, industrial water supplies, transportation, commercial fishing and shelling, and recreation. Rapidly expanding populations of zebra mussels could ultimately affect many of these activities, in addition to changing the structure of the ecosystem.
By firmly attaching to hard surfaces, zebra mussels have clogged water-intake pipes and fouled hard-shelled animals such as clams and snails. In addition, zebra mussels have reduced plankton populations as colonies of mussels filter large volumes of water for food (e.g., Holland 1993), potentially depleting food resources of larval and planktivorous fishes such as smelt, chub, and alewife (Alosa pseudoharengus). Transfer of suspended material to the lake bottom in mussel waste products also leads to increased water clarity (Reeders et al. 1992) and increased growth of aquatic plants, a phenomenon already observed in some of the shallower harbors of Chicago. Although clear water is often considered aesthetically pleasing, this clarity indicates that drastic changes have occurred at the base of the food web and that energy flow through the ecosystem has been altered.
Zebra mussel (Dreissena polymorpha) on fragile papershell mussel (Leptodea fragilis). Courtesy D.W. Schlorsser, NBS
The first live zebra mussel was discovered in Lake Michigan near Chicago in 1989. We documented the subsequent establishment of the zebra mussel in southern Lake Michigan by monitoring larval and adult zebra mussels in 1991-93. Monitoring was conducted primarily along the Illinois and Indiana shorelines; limited sampling occurred along the southern Wisconsin shoreline. We also quantified the initial effects of the invasion on water clarity and native fauna.

Zebra Mussel Densities
Larval zebra mussels were present at all sampling locations during 1991-93; however, the number of sampling locations decreased from 8 to 3 over the 3 years. Peak numbers were collected each year at Burns Harbor, Indiana, where the highest average density was 37,044 veligers/m3 (1,049/ft3) in 1991; 74,493/m3 (2,109/ft3) in 1992; and 42,099/m3 (1,192/ft3) in 1993.
Attached zebra mussels were found in quite low numbers (less than 150/m2 or 14/ft2) in 1991 at one Wisconsin and four Illinois locations sampled by divers. The maximum density in 1991 (up to 2,389/m2 or 222/ft2) was recorded on concrete blocks in the intake channel of an Indiana power plant inaccessible to divers. By 1992, sampling at 2 Wisconsin and 4 Illinois sites revealed that the population had exploded, with a minimum average density of 57,115/m2 (5,306/ft2) near Glencoe, Illinois. The maximum average density in 1992 was 267,885/m2 (24,885/ft2) at a Waukegan site that 1 year previously had only 25 mussels/m2 (2/ft2) (Marsden et al. 1993). Densities at two Illinois locations remained high in 1993, with average densities of 224,428/m2 (20,858/ft2) at Waukegan and 52,428/m2 (4,870/ft2) at Lake Forest.
High reproductive success during 1991 was clearly responsible for the huge increase in the number of attached mussels during 1992. It is interesting that although 1992 levels of reproduction were generally twice as high as in 1991, the population increase did not continue in 1993 at the two locations sampled.
Water Clarity
Water visibility (using a secchi disk) increased from a maximum depth of 4 m (13 ft) in 1990, to 6 m (20 ft) in 1991, to 10 m (33 ft) in 1992. Water remained clear in 1993, with a maximum depth at disappearance of 9.5 m (31 ft). At the site for which data are most consistently available (Waukegan), minimum water visibility measurements during 1991-93 were higher than any measured values during 1990. This trend should be interpreted with caution given the natural variability in water clarity values. The data suggest, however, that the water clarity of southern Lake Michigan may be increasing due to colonization of the lake by massive numbers of zebra mussels. This trend has been documented in other recently colonized lakes, such as Lake Erie (Leach 1992).
Impacts on Snails
Most native snails we collected were colonized by one or more zebra mussels. Stagnicola was the most common genus collected in nonquantitative samples. In 1991, 72% of these snails had attached zebra mussels, with an average of 1.6 mussels per snail. By 1992, 99% of Stagnicola were fouled, with the average number increasing to 3.7 zebra mussels per individual snail. Elimia snails dominated the quantitative samples from rocky areas. In 1992, 99% of 94 Elimia were fouled with mussels; in 1993 divers failed to find any live Elimia at the Waukegan reef.

Conclusions
In the Great Lakes and associated river systems, populations of native clams are threatened because of the colonization of their shells with massive numbers of zebra mussels (Mackie 1991). Our data indicate that snails are also being used as substrate for mussel attachment in Lake Michigan. As grazers, snails are an important part of the bottom community. They are also a source of food for fishes such as yellow perch (Perca flavescens), sunfish, and whitefish (Scott and Crossman 1973). Given the limited knowledge of the role of snails in Lake Michigan and other large lakes, it is not possible to fully anticipate the effects of reduced or decimated snail populations.
The rapid increase in zebra mussel densities we observed in the open waters of the lake was reflected in their colonization of municipal and industrial water-intake pipes. In 1991 and 1992 facilities drawing raw water from Lake Michigan began treatment programs to reduce infestation of intake pipes. The cost of retrofitting plants in Chicago and northern Illinois shoreline communities had totaled $1,778,000 by 1992 (Nelson 1992). This value does not include chemical costs, or increased personnel costs as workers dealt with mussel-related problems. In addition to economic costs of retrofitting and chemical treatments, Lake Michigan has an increased ecological risk of accidental chemical spills or leakages.
Zebra mussels also affect the aesthetic and recreational value of the lake. Boat owners are concerned about zebra mussels fouling boat hulls and engine cooling systems, and windrows of broken shells have begun to appear along Lake Michigan beaches.
The economic impact of zebra mussels is not limited to industrial and recreational interests, however. Native clams from the Illinois River are shipped to Japan for use in the cultured pearl industry; in 1991 the value of this resource was $1.4 million annually. The infestation of clams by zebra mussels has increased dramatically, resulting in significant clam mortality. Commercial shelling on the Illinois River was recently banned, following a drop in harvest from over 454,000 kg (1 million lb) in 1991 to 67,646 kg (149,000 lb) in 1993 (Don Duffert, Illinois Department of Conservation, personal communication).
Zebra mussels are a permanent addition to the Lake Michigan ecosystem and connected waters. Chemical and mechanical controls for zebra mussels are only useful in localized areas such as intake pipes and other artificial structures, but not in the open waters of the lake. Ultimately, zebra mussel populations will exceed the capacity of the environment to support them, after which their numbers will likely decline. Native predators such as freshwater drum (Aplodinotus grunniens), diving ducks, and crayfish may also keep mussel populations in check in some areas. The adverse effects of zebra mussels on human activities and native aquatic species will never be totally eliminated, but eventually they may become a more tolerable nuisance.
For further information:
Tammy Keniry
Illinois Natural History Survey
Lake Michigan Biological Station
400 17th St.
Zion, IL 60099
References
Holland, R.E. 1993. Changes in the planktonic diatoms and water transparency in Hatchery Bay, Bass Island area, western Lake Erie since the establishment of the zebra mussel. Journal of Great Lakes Res. 19:617-624.

Leach, J.H. 1992. Impacts of the zebra mussel (Dreissena polymorpha) on water quality and fish spawning reefs in western Lake Erie. Pages 381-395 in T.F. Nalepa and D.W. Schloesser, eds. Zebra mussels: biology, impacts, and control. Lewis Publishers, Boca Raton, FL.

Mackie, G.L. 1991. Biology of the exotic zebra mussel, Dreissena polymorpha, in relation to native bivalves and its potential impact on Lake St. Clair. Hydrobiologia 219:251-268.

Marsden, J.E. 1992. Standard protocol for monitoring and sampling zebra mussels. Illinois Natural History Survey Biological Notes 138. 40 pp.

 Marsden, J.E., N. Trudeau, and T. Keniry. 1993. Zebra mussel study on Lake Michigan. Final report to the Illinois Department of Conservation. Illinois Natural History Survey Tech. Rep. 93/14. 51 pp.

Nelson, S. 1992. A pound of cure for a ton of mussels. Aquaticus: Journal of the Shedd Aquarium 23:28-29.

Reeders, H.H., A. bij de Vaate, and R. Noordhuis. 1992. Potential of the zebra mussel (Dreissena polymorpha) for water quality management. Pages 439-451 in T.F. Nalepa and D.W. Schloesser, eds. Zebra mussels: biology, impacts, and control. Lewis Publishers, Boca Raton, FL.

Scott, W.B., and E.J. Crossman. 1973. Freshwater fishes of Canada. Bull. 184. Fisheries Research Board of Canada, Ottawa. 966 pp.


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