Typically, though, anoxic conditions are observed during warmer seasons as the Lake waters form an epilimnion and a hypolimnion. Reduced nutrient loading from decreased phosphorus loading and increased predation from zebra mussels have severely affected phytoplankton stocks, pushing the Lake to an oligotrophic state. Thus, community structure and function change accordingly. It is difficult to predict the final outcome. The Great Lakes serve many functions.
Toxins are another confounding factor.
Lake Erie has also been subjected to exotic species, specifically the zebra mussel, Dreissena polymorpha . As these molluscs are extremely fecund and natural predation removes only a small percentage of larvae, the population has surged in the recent past. As well, nutrient-rich waters have provided, until recently, an abundant food supply. As filter-feeding organism, D. polymorpha removes particulates and other nutrients from the water column. Combining high fecundity and low predations yields uncontrollable population growth until the carrying capacity of the ecosystem is passed.