Wetlands in Regulated Great Lakes

Wetlands in Regulated Great Lakes
		by *Douglas A. Wilcox* National Biological Service *James E. Meeker* Northland College, WI
Water levels in the Great Lakes are affected by variations in precipitation, evaporation, ice build up, internal waves (seiches), and human alterations that include modifying the connecting channels between lakes and regulating the water levels of Lake Superior and Lake Ontario. Fluctuations in water level promote the interaction of aquatic and terrestrial systems, thereby resulting in higher quality habitat and increased productivity. When the fluctuations in water levels are reduced through stabilization, shifting of vegetation types decreases, more stable plant communities develop, and species diversity and habitat value decrease (Wilcox and Meeker 1991, 1992). Although water levels in Lake Superior are regulated by structures at the outlet, water-level cycles and patterns remain fairly similar to natural conditions. Lake Ontario water levels are also regulated, but high and low water extremes have been eliminated since the mid-1970's. The effects of water-level history on wetland plant communities under the two regulation regimes were investigated by studying wetlands on each lake.
Seventeen sites on Lake Ontario and 18 on Lake Superior were sampled. Vegetation was mapped and then sampled along transects that followed elevation contours with specific water-level histories (number of years since last flooded or last dry). The histories and elevations differed between lakes. 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; these data were used to create schematic cross-sections depicting the structural habitat provided by the plant communities characteristic of each lake.
Vegetation and Water Level
At study sites on both Lakes Ontario and Superior, wetland plant communities differed at different elevations; these plant communities developed as a result of the water-level history of each elevation that was sampled. 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 other nonwoody plants.

		Small unnamed bay near Bete Grise, Lake Superior, August 1991. Scattered lilies (Nuphar variegata) with submersed plants adjacent to a floating bog mat. Courtesy J. Meeker, Northland College
The plant communities at elevations that were flooded periodically at 10- to 20-year intervals and dewatered for successive years between floods had the greatest diversity of wetland vegetation. Dominants included grasses, sedges, rushes, short emergent plants, and submersed aquatic vegetation. At elevations that were rarely or never dewatered, submersed and floating plants were dominant, with emergent plants also occurring at some sites.

Lake Superior
Water levels on Lake Superior have been regulated for much of this century, although the range of fluctuations and the cyclic nature of high and low lake levels have not been altered substantially. More than 275 taxa were recorded in a sampling of 18 wetlands along the U.S. shoreline, 216 of which were obligate (see glossary) or facultative (see glossary) wetland species. Vegetation mapping showed the most prevalent vegetation types to be those dominated by submersed aquatic vegetation or shrubs, both of which were present in all sites and averaged about 25% of the cover. Vegetation types dominated by cattails (Typha sp. or other taxa plus cattails) occurred in about half the sites but averaged only about 6% of the cover. Across all sites, 27 different vegetation types were mapped.
Lake Ontario
Water levels on Lake Ontario have been regulated since 1960, when the St. Lawrence Seaway began operation. Before regulation, the range of fluctuations during the 20th century was about 2 m (6.6 ft). After regulation, the range was reduced slightly between 1960 and 1976, but low water-supply conditions in the mid-1960's and high supplies in the mid-1970's maintained much of the range. Regulation reduced the range to about 0.9 m (2.9 ft) in the years after 1976.
The lack of alternating flooded and dewatered conditions at the upper and lower edges of the wetlands resulted in establishment of extensive stands of cattail (Typha sp.) and domination of other areas by purple loosestrife (Lythrum salicaria), reed canary grass (Phalaris arundinacea), and various shrubs. Although more than 250 taxa were recorded in a sampling of 17 wetlands along the U.S. shoreline, only 151 were obligate or facultative wetland plants. Vegetation mapping showed the cattail-dominated vegetation type to be most prevalent, occurring at all sites and averaging about 32% of the cover. The submersed aquatic vegetation type occurred at 75% of the sites and averaged about 30% of the cover. Across all sites, 20 different vegetation types were mapped.
Habitat Structure

Differences in the species and structural types of plants at different elevations in wetlands of regulated Lakes Superior and Ontario result in different habitats for faunal organisms because the greater diversity of taxa and vegetation types in Lake Superior wetlands provides more niches for fauna than in Lake Ontario wetlands (Figure; Engel 1985; Wilcox and Meeker 1992). The prevalence of dominant cattail stands in Lake Ontario wetlands reduces habitat value there (Weller and Spatcher 1965).

Figure. Schematic cross-sections depicting the structural habitat provided by plant communities characteristic of regulated Lakes Superior and Ontario. Elevations at which vegetation sampling was conducted are shown beneath each cross-section (benchmark: International Great Lakes Datum 1955).

Periodic high waters are necessary to reduce dominant emergent vegetation in Great Lakes wetlands; low waters are necessary to reduce dominant submersed vegetation. High waters followed by low-water years allow a diversity of plants to grow from seed on the exposed sediments, reproduce, and replenish the seed bank. Although competitive species such as cattails will again become dominant, the next high-water year will eliminate them again. When water-level fluctuations are reduced by regulation, the processes for rejuvenating wetland plant communities are lost and habitat values decrease.
		For further information: Douglas A. Wilcox National Biological Service Great Lakes Science Center 1451 Green Rd. Ann Arbor, MI 48105

References
Engel, S. 1985. Aquatic community interactions of submerged macrophytes. Wisconsin Department of Natural Resources Tech. Bull. 156. 79 pp. Weller, M.W., and C.S. Spatcher. 1965. Role of habitat in the distribution and abundance of marsh birds. Iowa Agricultural and Home Economics Experiment Station, Ames, IA. Special Rep. 43.	Wilcox, D.A., and J.E. Meeker. 1991. Disturbance effects on aquatic vegetation in regulated lakes in northern Minnesota. Canadian Journal of Botany 69:1542-1551. Wilcox, D.A., and J.E. Meeker. 1992. Implications for faunal habitat related to altered macrophyte structure in regulated lakes in northern Minnesota. Wetlands 12:192-203.