Our Lh'hii; Kcsiiiiii c\ — Aiiualic Eciisystcnis 



249 



substantially. More than 275 taxa were recoixled 

 in a sampling of 18 wetlands along the U.S. 

 shoreline. 216 of which were obligate (sec glos- 

 sary) or facultative [see glossary) wetland 

 species. Vegetation mapping showed the most 

 prevalent vegetation types to be those dominat- 

 ed by submersed aquatic vegetation or shrubs, 

 both of which were present in all sites and aver- 

 aged about 2y/( of the cover. Vegetation types 

 dominated by cattails ( Typha sp. or other taxa 

 plus cattails) occuned 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 reg- 

 ulated 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 niid-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 dewa- 

 tered conditions at the upper and lower edges of 

 the wetlands resulted in establishment of exten- 

 sive stands of cattail (Typha sp.) and domination 

 of other areas by purple loosestrife (Lythnim 

 salicaria). reed canary grass (Plmlaris aruiuli- 

 luicea), and various shrubs. Although more than 

 250 taxa were recorded in a sampling of 1 7 wet- 

 lands along the U.S. shoreline, only 151 were 

 obligate or facultative wetland plants. 

 Vegetation mapping showed the cattail-domi- 

 nated vegetation type to be most prevalent, 

 occurring at all sites and averaging about 329^ 

 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 dif- 

 ferent vegetation types were mapped. 



Habitat Structure 



Differences in the species and structural 

 types of plants at different elevations in wet- 

 lands of regulated Lakes Superior and Ontario 

 result in different habitats for faunal organisms 

 because the greater diversity of taxa and vegeta- 

 tion types in Lake Superior wetlands provides 

 more niches for fauna than in Lake Ontario wet- 

 lands (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). 



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 sedi- 

 ments, 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 regula- 

 tion, the processes for rejuvenating wetland 

 plant coininunities are lost and habitat values 

 decrease. 



References 



Engel, S. 1985. Aquatic community interactions of sub- 

 merged macropliytes. Wisconsin Department of Natural 

 Resources Tecli. Bull. 156. 79 pp. 



Weller. M.W.. and C.S. Spatcher. 1965. Role of habuat m 

 the distribution and abundance of marsh birds. Iowa 

 Agricultural and Home Economics Experiment Station. 

 Ames. lA. Special Rep. 43. 



Wilco.x. 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 fau- 

 nal habitat related to altered macrophyte structure in reg- 

 ulated lakes in northern Minnesota. Wetlands 12:192- 

 2(13. 



For further information: 



Douglas A. Wilcox 



National Biological Service 



Great Lakes Science Center 



1451 Green Rd. 



Ann Arbor. MI 48105 



The historical freshwater gastropod fauna of 

 the Mobile Bay basin in Alabama, Georgia, 

 Mississippi, and Tennessee was the most 

 diverse in the world, comparable only to the 

 diversity repotted for the Mekong River in 

 Southeast Asia. This fauna was represented by 9 

 families and about 1 18 species. Several families 

 have genera endemic to the Mobile Bay basin: 

 Viviparidae: Tiilotoma: Hydrobiidae; Clappia. 

 Lepyriiim: Pleuroceridae: Gywtoma: and 

 Planorbidae; Amphigyra and Neopkmorbis. The 

 greatest described species diversity was in the 

 Pleuroceridae (76 species). The pleurocerid 

 genera Pleurocera, Leptoxis, and Elimia had 

 their greatest radiation in the Coosa River 

 drainage. 



Although this extremely diverse aquatic gas- 

 tropod fauna received little attention in the past 

 50 years, it was actively studied during the sec- 

 ond quarter of this century (Goodrich 1922, 

 1924, 1936, 1944a, 1944b). During the last 60 

 years, this unique gastropod fauna has declined 

 precipitously (Table 1; Atheam 1970; Heard 

 1970; Stansbery 1971). More recent documen- 

 tation of the decimation of this fauna was pre- 

 sented by Stein (1976) and Palmer ( 1986). The 

 endemic genus Tidotoma (Figs. 1 and 2). for- 

 merly widespread in the main channel of the 

 Alabama and Coosa rivers, was presumed 

 extinct until recently rediscovered (Hershler et 

 al. 1990). The pleurocerid genus Gywtoma, 

 restricted primarily to the shoals of the Coosa 



Decline in the 

 Freshwater 

 Gastropod 

 Fauna in the 

 Mobile Bay 

 Basin 



