Humans have a particular interest in the quality of lake habitats, because lakes are by far our most important source of drinking water. In 1991, 70% of Illinois' public water supply was drawn from lakes, with Chicago and its suburbs alone pulling 1.1 billion gallons from Lake Michigan. Lakes are also an important source of food--fish--and Lake Michigan in particular supports large commercial and sprot-fishing industries.
The principal threats to lake ecosystems are exotic species and pollution. The former exert great competitive pressures on native species, while the latter can slowly poison them, either directly or by sapping much needed oxygen from the lake. Pollution can take the form of eroded soils that silt up lakes, plant fertilizers or nutrients that wash in with these soils, and industrial wastes and sewage that are dumped into the lakes. Fortunately, industrial dumping has fallen considerably over the last two decades.
The upshot of all these pressures is most evident in Lake Michigan, where one in seven native fish species has been extirpated or suffered a severe population crash.