Water-level fluctuations in the Great Lakes are of concern for a number of reasons, including the potential effects they may have on coastal developments, shipping, recreation, hydropower, and natural resources. Most previous attempts to address the effects of fluctuations on wetland resources used aerial photograph interpretation as the major assessment tool. As a result, conclusions were sometimes drawn suggesting that high water levels destroy wetland vegetation. Sometimes this destruction was recognized as part of a cyclic process, other times it was not. High and low water levels are forms of natural disturbance in wetland systems. Disturbance alters competition and succession processes and, if in the proper form, may result in greater plant species diversity and improved fish and wildlife habitat.

The vegetation of shallow water areas in the Great Lakes is the one biotic resource most directly affected by natural or regulated changes in water level. Individual plant species and communities of species have affinities and physiological adaptations for certain water depth ranges, and their life forms may show adaptations for different water-level environments. Changes in water level add a dynamic aspect to the species/depth relationship. Water-level dynamics result in shifting mosaics of aquatic vegetation types. In general, high water levels kill trees, shrubs, and other emergent vegetation, and low water levels following these highs result in seed germination and growth of a multitude of species. Some species are particularly well-suited to recolonizing the drawdown zone, and several emergents may coexist there because of their diverse responses to natural disturbance.

In the first year following a reduction in water levels, the distribution of new seedlings is due to the distribution of seeds in the sediments. In ensuing years, the distribution of adults is due to seedling survival through competitive interactions. If one species is favored in early colonization, its density may be great enough that it can maintain dominance of an area (site preemption). In most cases, early colonizing species or communities are later lost through competitive displacement, but the opportunity to go through a life cycle allowed them to replenish the seed bank.

The magnitude of water-level fluctuations is of obvious importance to wetland vegetation because it directly results in different water-depth environments. The frequency, timing, and duration of water-level fluctuations is also important for several reasons. Water-level changes on a seasonal time scale are likely to have different effects than fluctuations on a scale of a decade or longer, and infrequent, unpredictable fluctuations will result in greater diversity than annual fluctuations. Drawdowns occurring in spring will produce different results than drawdowns in summer. Drawdowns in winter can have other effects, such as ice-induced sediment erosion. Stable water levels with little fluctuation during the growing season will likely result in a stable shoreline plant community, while unstable summer water levels (wide fluctuations) will likely result in variability in the vegetation. The duration of flooding thus becomes a controlling factor.

The different plant communities that develop in a lakeshore wetland shift from one location to another in response to changes in water depth. The water-depth history largely determines the species composition of a particular site at a given point in time, with the plant communities generally corresponding to zones that are A) almost never covered with water, B) occasionally covered with water, C) covered/uncovered on a short-term basis, D) often covered with water, and E) almost always covered with water. Future water-level changes couple with seed bank composition to determine future species composition.

Wetlands provide valuable habitat for fish and wildlife. Many invertebrates are closely associated with macrophyte beds; waterfowl, aquatic mammals, and small fish are attracted to these areas because they provide food and shelter. When water levels change, habitats and organism interactions change also. Flooding of emergent plant communities allows access for spawning fish, reduces mink predation on muskrats, and increases hemi-marsh habitat (half vegetated, half open water) preferred by waterfowl. Flooded, detrital plant materials are also colonized by invertebrates that are fed on by waterfowl. Low water levels can jeopardize fish spawning and reduce waterfowl nesting area; yet, they provide the opportunity for regeneration of the plant communities that are the foundation of the habitat. Water-level fluctuations promote the interaction of aquatic and terrestrial systems and result in higher quality habitat and increased productivity. When the fluctuations in water levels are removed through stabilization, shifting of vegetation types decreases, more stable plant communities develop, species diversity decreases, and habitat value decreases.

In 1988, during Phase 1 of the International Joint Commission Water-Levels Reference Study, data were collected at wetland sites in lakes Superior, Michigan, Huron, and St. Clair to demonstrate the manner in which water-level changes between the historic high of 1986 and the ensuing 0.5 m drop in 1988 had affected plant communities. Sampling was conducted according to topographic gradients. Specific altitudes were selected for each lake that represented different water-depth histories (i.e., different number of years since last flooded or last dewatered). Following the topographic contour for each of these altitudes, transects were established roughly parallel to the shoreline, and plant communities were sampled quantitatively.

In 1991, during Phase 2 of the IJC study, 17 sites on Lake Ontario and 18 sites on Lake Superior were selected for sampling. They were initially stratified into three general morphological categories: 1) embayments protected from direct wave action by sand spits or other barriers, 2) coastal areas open to direct wave action, and 3) drowned river or stream mouths. Wetlands affected by disturbance factors (e.g., marinas and shoreline protection structures) were eliminated from consideration as study sites because they would not accurately reflect responses to water-level change. The study sites were then selected as a random sample drawn from each strata and were considered representative of the wetlands in each lake. Transects were again established that followed topographic contours that differed in water-depth history, and plant communities were sampled quantitatively.

Vegetation survey data from both Phase 1and Phase 2 studies were analyzed using summary statistics and detrended correspondence analysis (DECORANA) ordination procedures. Correlations between specific elevations and accompanying plant communities were assessed across all wetlands sampled in each lake to determine the range of elevations in which the most diverse plant communities occur and to identify any specific habitat requirements of individual plant species.

A further objective in Phase 2 was to evaluate proposed new water-level regulation plans for lakes Ontario and Superior and plans to regulate all of the Great Lakes. Model environmental scenarios that provide the necessary range of water-level fluctuations to sustain wetlands were also developed for each of the Great Lakes.

The results of Phase 1 studies confirmed that fluctuations of water levels in the Great Lakes are necessary to perpetuate cycling of successional processes in wetlands. High lake levels periodically eliminate competitively dominant emergent plants. When levels recede, less competitive species are able to grow from seed, complete at least one life cycle, and replenish the seed bank before being replaced through competitive interactions. Habitat diversity in wetlands is thus maintained.

Phase 1 studies also showed that each wetland responds differently to water-level fluctuations as a result of differences in basin morphology and positioning of plant communities within a basin. Therefore, site-specific data from a few locations cannot be used to generalize across a whole lake or the entire Great Lakes system. Instead, data are necessary from a number of sites in a lake to describe typical basin morphology and identify critical water levels to which wetland plant communities respond. Therefore, numerous sites on lakes Ontario and Superior were studied in Phase 2.

In both lakes Ontario and Superior, wetland plant communities differed at different elevations. The plant communities developed as a result of the water-level history of each elevation that was sampled; these elevations were selected to define different water-level histories. In general, plant communities at elevations that had not been flooded for many years were dominated by shrubs, grasses, and old-field plants. If flooding was more recent, small shrubs that became established after flooding were present, as were grasses, sedges, and forbs. These plant communities at elevations that are flooded periodically each ten to twenty years and dewatered for successive years between floods had the greatest diversity of wetland vegetation. The plant communities contained the most wetland taxa and the most diversity of plant types. Dominants included grasses, forbs, sedges, rushes, short emergent plants, and submersed aquatic vegetation. At elevations that are rarely or never dewatered, submersed and floating plants were dominant, with emergent plants also occurring at some sites. In Lake Ontario, floating mats of cattail were present in many locations, and purple loosestrife was widespread.

The regulation plans for Lake Ontario offered few choices that would protect over 3400 ha (over 8400 ac) of wetlands on the U.S. side alone. The current regulation plan and all proposed plans extended the moderation of fluctuations that has existed since the mid-1970s. The lack of high lake levels has allowed floating cattail mats to form, purple loosestrife and other exotics to thrive, and shrub and old field communities to take over higher elevations. If no actions are taken to change the manner in which Lake Ontario levels are regulated, the species richness of the wetlands will likely decrease as competitive dominants eliminate more and more species and are themselves unchecked by environmental conditions. The lack of multi-year fluctuations in these regulation plans makes them unacceptable from the standpoint of wetlands protection. The proposed environmental scenario or some modification of it that provides periodic high lake levels followed by low lake levels was the only scenario that will promote the cyclic, regenerative processes that maintain wetland diversity. If diverse habitats are to be maintained in wetlands of Lake Ontario, a new regulation scenario must be developed, and a long-term scientific evaluation must be initiated. Such studies are necessary to develop a better understanding of wetland response to water-level changes when they occur, rather than attempting to gather information after the fact. These studies would also provide a fine-tuning mechanism to guide regulation decisions in the future.

The current regulation plan and proposed plans for Lake Superior did not have the dire environmental consequences posed by those for Lake Ontario. However, the lowest summertime highs were not frequent enough to allow cyclic, regenerative wetland processes to occur over a large enough range of elevations. The result was a restriction of the area occupied by the most diverse plant communities. The amplitude of the peak summertime highs also restricted development of these plant communities. The proposed environmental scenario that provided an increased range of elevation between the highest and lowest summertime highs could increase the diversity of plant communities and faunal habitats in over 4200 ha (over 10,500 ac) of wetlands on the U.S. side alone. Achieving reduced summertime peaks in paired years is most critical.

The proposed 3- and 5-lake regulation plans that included lakes Erie, St. Clair, Michigan, and Huron showed no multi-year variability across the 90-year period of record. The likely result would be substantially reduced wetland diversity, development of stable plant communities and floating cattail mats, and invasion of purple loosestrife. In each lake, the proposed environmental scenario provided an increased range of elevation between the highest and lowest summertime highs, established natural cyclic patterns of highs and lows, and would increase the diversity of wetland plant communities and faunal habitats.

This study demonstrated the necessity of cyclic water-level fluctuations in the Great Lakes to periodically stress competitively dominant plants both on the shore and in the water. However, short-term studies that attempt to assess long-term processes on one or two years of data collection cannot provide complete insight into the interactions between water-level changes and wetland plant communities. The single most important fact that the Water-Levels Reference Study has made clear is that a long-term evaluation of the effects of lake-level changes on many features of the Great Lakes community is necessary. This evaluation should include long-term studies of individual representative wetlands and should also include sedimentological studies on all lakes to provide lake-level histories similar to that which has been produced for lake Michigan-Huron. Partially in response to these and other study results, the IJC began a new reference study for Lake Ontario in 2000 to evaluate options for new regulation plans (Link). A new reference study for Lake Superior is currently under evaluation.

Wilcox, D. A. 1989. Responses of selected Great Lakes wetlands to water-level fluctuations. Phase 1 Report to Working Committee 2, IJC Water-level Reference Study. International Joint Commission, Ottawa, ON, Canada and Washington, DC, USA.

Wilcox, D. A. 1990. Water-level fluctuations and Great Lakes wetlands. Great Lakes Wetlands 1(2):1-3.

Wilcox, D. A., J. E. Meeker, and J. Elias. 1992. Impacts of water-level regulation on wetlands of the Great Lakes. Phase 2 Report to Working Committee 2, IJC Water-Level Reference Study. International Joint Commission, Ottawa, ON, Canada and Washington, DC, USA.

Wilcox, D. A. 1993. Effects of water-level regulation on wetlands of the Great Lakes. Great Lakes Wetlands 4(1):1-2.

Wilcox, D. A., J. E. Meeker, and J. Elias. 1993. Appendix: Impacts of water-level regulation on wetlands of the Great Lakes—additional scenarios. Phase 2 Report to Working Committee 2, IJC Water-Level Reference Study. International Joint Commission, Ottawa, ON, Canada and Washington, DC, USA.

Wilcox, D. A. 1995. Wetland and aquatic macrophytes as indicators of anthropogenic hydrologic disturbance. Natural Areas Journal 15:240-248.

Wilcox, D. A. 1995. The role of wetlands as nearshore habitat in Lake Huron. p. 223-245. In M. Munawar, T. Edsall, and J. Leach (eds). The Lake Huron Ecosystem: Ecology, Fisheries and Management. Ecovision World Monograph Series, S.P.B. Academic Publishing, Amsterdam, The Netherlands.

Wilcox, D. A. and J. E. Meeker. 1995. Wetlands in regulated Great Lakes. p. 247-249. In E. T. LaRoe, G. S. Farris, C. E. Puckett, P. D. Doran, and M. J. Mac (eds.) Our Living Resources: a Report to the Nation on the Distribution, Abundance, and Health of U.S. Plants, Animals, and Ecosystems. U.S. DOI, National Biological Service, Washington, DC, USA.

Maynard, L. and D. A. Wilcox. 1997. Coastal Wetlands. State of the Lakes Ecosystem Conference Proceedings. Environment Canada, Burlington, ON, Canada and U.S. Environmental Protection Agency, Chicago, IL, USA.

Keough, J. R., T. A. Thompson, G. R. Guntensperten, and D. A. Wilcox. 1999. Hydrogeomorphic factors and ecosystem responses in coastal wetlands of the Great Lakes. Wetlands 19:821-834.

Wilcox, D. A. and T. H. Whillans. 1999. Techniques for restoration of coastal wetlands of the Great Lakes. Wetlands 19:835-857.

Meeker, J. E. and D. A. Wilcox. 1989. The importance of natural water-level fluctuations on aquatic plant communities: case studies on Lake Superior and Voyageurs National Park wetlands. International Association of Great Lakes Research, Madison, WI, USA.

Wilcox, D. A. 1989. The interaction of basin morphology and water-level change in determining Great Lakes wetland plant community composition. American Society of Limnology and Oceanography, Fairbanks, AK, USA.

Wilcox, D. A. 1989. The role of water-level fluctuations in maintaining Great Lakes coastal wetlands. Restoration and Preservation of Great Lakes Coastal Ecosystems, Buffalo, NY, USA.

Wilcox, D. A., S. J. Nichols, L. Kensler, and D. Schloesser. 1989. The effect of water-level fluctuations on plant zonation in a Saginaw Bay, Lake Huron wetland. International Association of Great Lakes Research, Madison, WI, USA.

Wilcox, D. A., D. Schloesser, and S. J. Nichols. 1989. The effects of water-level fluctuations on plant communities in a Great Lakes river-delta wetland. Society of Wetland Scientists, Orlando, FL, USA.

Wilcox, D. A. 1990. The influence of water-level change and basin morphology on plant community composition in Great Lakes wetlands. Association of Wetland Managers, Niagara Falls, NY, USA.

Madsen, B. J. and D. A. Wilcox. 1991. Long-term effects of lake-level fluctuations on a freshwater coastal marsh. Society of Wetland Scientists, Ann Arbor, MI, USA.

Meeker, J. E., D. A. Wilcox, and D. M. Waller. 1991. Macrophyte response to fluctuating water levels in a wild rice-dominated wetland on Lake Superior. Society of Wetland Scientists, Ann Arbor, MI, USA.

Meeker, J. E., D. A. Wilcox, and J. Elias. 1992. The importance of water-level fluctuations for plant species richness in Lake Superior wetlands. Society of Wetland Scientists, New Orleans, LA, USA.

Wilcox, D. A. 1992. Keeping wetlands in the process: a case history on Great Lakes water-level regulation. Society of Wetland Scientists, New Orleans, LA, USA.

Wilcox, D. A. 1992. Wetland and aquatic macrophytes as indicators of hydrologic disturbance. Natural Areas Association. Bloomington, IN, USA.

Wilcox, D. A. and J. E. Meeker. 1992. Application of the intermediate disturbance hypothesis to management of wetlands in regulated lakes. Botanical Society of America. Honolulu, HI, USA.

Meeker, J. E., D. Waller, and D. A. Wilcox. 1993. Variability in biomass, stand density, and population structure of riverine wild rice in northern Wisconsin. Society of Wetland Scientists, Edmonton, AB, Canada.

Wilcox, D. A., J. E. Meeker, and J. Elias. 1993. The effects of water-level regulation on wetland plant communities in Lake Ontario. Society of Wetland Scientists, Edmonton, AB, Canada.

Meeker, J. E. and D. A. Wilcox. 1994. Sedimentation and wild rice productivity associated with seiche action in the Kakagon Sloughs of Lake Superior. International Association of Great Lakes Research, Windsor, ON, Canada.

Wilcox, D. A. 1994. Wetlands of Lake Huron: role, function, and management. Aquatic Ecosystem Health and Management Society. Windsor, ON, Canada.

Meeker, J. E., D. A. Wilcox, J. E. Elias, and S. G. Spickerman. 1995. Factors influencing the diversity in Lake Superior coastal wetlands. Society of Wetland Scientists, Boston, MA, USA.

Wilcox, D. A. 1995. The response of coastal wetlands to Great Lakes water-level fluctuations. Freshwater Coastal Wetlands—Issues and Applications. Cambridge, ON, Canada.

Wilcox, D. A. and L. Maynard. 1996. Coastal Wetlands. State of the Lakes Ecosystem Conference. Windsor, ON, Canada.

Wilcox, D. A. 1999. Use of latitude, geomorphic setting, elevation, and wave exposure to differentiate and cluster Great Lakes wetlands. International Association for Great Lakes Research. Cleveland, OH, USA.

Wilcox, D. A. 1999. Interaction between water levels and wetlands. IJC Cumulative Impacts of Water Withdrawals, Commissioners Meeting. Windsor, ON, Canada.

Wilcox, D. A. 1999. Cumulative impacts of water withdrawals on Great Lakes wetlands. IJC Cumulative Impacts in the Great Lakes-St. Lawrence River Ecosystem: Experts Workshop. Windsor, ON, Canada.

Wilcox, D. A. 1999. Effects of Great Lakes water-level fluctuations on wetlands. Eastern Michigan University Dept. of Biology Seminar Series. Ypsilanti, MI, USA.

Wilcox, D. A. 1999. Environmental considerations for the Lake Michigan Potential Damages Study. Lake Michigan State of the Lakes ‘99. Muskegon, MI, USA.

Wilcox, D. A. 1999. Aquatic macrophytes and wetlands in Lake St. Clair: the role of water-level fluctuations. Lake St. Clair: Its Current State and Future Prospects. Port Huron, MI, USA.

Wilcox, D. A. 2000. Water-level changes–climatic or man-made? Michigan Lakes and Streams Association. Gaylord, MI, USA.

Wilcox, D. A. 2000. Interactions between water levels and wetlands. Michigan State University, Kellogg Biological Station Seminar Series. Hickory Corners, MI, USA.

Wilcox, D. A. 2000. Effects of Great Lakes water-level fluctuations on wetlands. U.S. EPA Great Lakes National Program Office Seminar Series. Chicago, IL, USA.

Wilcox, D. A. 2000. Water-level fluctuations: the driving force behind Great Lakes coastal wetlands. Society of Wetland Scientists/INTECOL. Quebec, PQ, Canada.

Wilcox, D. A. 2000. Dynamic Great Lakes water levels and wetlands. Upper Mississippi River and Great Lakes Region Joint Venture Management Board Meeting. Bay City, MI, USA.

Financial support for this study was provided through an interagency agreement between the U.S. Army Corps of Engineers and the Great Lakes Science Center.