Lake Erie is the most productive of the Great Lakes . Located along the southern fringe of the Precambrian Shield of North America, Lake Erie has been ecologically degraded by a variety of anthropogenic stressors including nutrient loading ; extensive deforestation of its watershed that caused severe siltation and other effects; vigorous commercial fishing ; and pollution by toxic chemicals .
The watershed of Lake Erie is much more agricultural and urban in character than are those of the other Great Lakes. Consequently, the dominant sources of phosphorus (the most important nutrient causing eutrophication) to Lake Erie are agricultural runoff and municipal point sources. The total input of phosphorus to Lake Erie (standardized to watershed area) is about l.3 times larger than to Lake Ontario and more than five times larger than to the other Great Lakes.
Because of its large loading rates and concentrations of nutrients, Lake Erie is more productive and has a larger standing crop of phytoplankton , fish, and other biota than the other Great Lakes. During the late 1960s and early 1970s, the eutrophic western basin of Lake Erie had a summer-chlorophyll concentration averaging twice as large as in Lake Ontario and 11 times larger than in oligotrophic Lake Superior. However, since that time the eutrophication of Lake Erie has been alleviated somewhat, in direct response to decreased phosphorus inputs with sewage and detergents . A consequence of the eutrophic state of Lake Erie was the development of anoxia (lack of oxygen) in its deeper waters during summer stratification . In the summer of 1953, this condition caused a collapse of the population of benthic mayfly larvae (Hexagenia spp.), a phenomenon that was interpreted in the popular press as the "death" of Lake Erie.
Large changes have also taken place in the fish community of Lake Erie, mostly because of its fishery, the damming of streams required for spawning by anadromous fishes (fish that ascend rivers or streams to spawn), and sedimentation of shallow-water habitat by silt eroded from deforested parts of the watershed. Lake Erie has always had the most productive fishery on the Great Lakes, with fish landings that typically exceed the combined totals of all the other Great Lakes. The peak years of commercial fishery in Lake Erie were in 1935 and 1956 (62 million lb/28 million kg), while the minima were in 1929 and 1941 (24 million lb/11 million kg). Overall, the total catch by the commercial fishery has been remarkably stable over time, despite large changes in species , effort, eutrophication, toxic pollution, and other changes in habitat.
The historical pattern of development of the Lake Erie fishery was characterized by an initial exploitation of the most desirable and valuable species. As the populations of these species collapsed because of unsustainable fishing pressure, coupled with habitat deterioration, the fishery diverted to a progression of less-desirable species. The initial fishery focused on lake white fish (Coregonus clupeaformis ), lake trout (Salvelinus namaycush ), and lake herring (Leucichthys artedi ), all of which rapidly declined to scarcity or extinction . The next target was "second-choice" species, such as blue pike (Stizostedion vitreum glaucum ) and walleye (S. v. vitreum ), which are now extinct or rare. Today's fishery is dominated by species of much smaller economic value, such as yellow perch (Perca flavescens ), rainbow smelt (Osmerus mordax ), and carp (Cyprinus carpio ).
In 1989 an invasive species—the zebra mussel (Dreissena polymorpha —reached Lake Erie and began to have a significant ecological impact on the lake. Zebra mussels are filter feeders, and each adult mussel can filter a liter of water per day, removing every microscopic plant (phytoplankton or algae) and animal (zooplankton ) in the process. Zebra mussel densities in Lake Erie have reached such a level that the entire volume of the lake's western basin is filtered each week. This has increased water clarity up to 600 percent and reduced some forms of phytoplankton in the lake's food web by as much as 80 percent. In addition, the increased clarity of the water allows light to penetrate deeper into the water, thus facilitating the growth of rooted aquatic plants and increasing populations of some bottom-dwelling algae and tiny animals. Zebra mussels also concentrate 10 times more toxins than do native mussels, and these contaminants are passed up the food chain to the fish and birds that eat zebra mussels. Since bioaccumulation of toxins has already led to advisories against eating some species of Great Lakes fish, the contribution of zebra mussels to contaminant cycling in lake species is a serious concern.
[Bill Freedman Ph.D. ]
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Freedman, B. Environmental Ecology. 2nd edition San Diego: Academic Press, 1995.
Regier, H. A., and W. L. Hartman. "Lake Erie's Fish Community: 150 Years of Cultural Stresses." Science 180 (1973): 1248–55.
"Zebra Mussels and Other Nonindigenous Species." Sea Grant Great Lakes Network. August 15, 2001 [June 19, 2002]. <http://www.seagrant.wisc.edu/greatlakes/glnetwork/exotics.html>.