Abstract
The Great Lakes have tremendous ecological and economic importance, and they are subject to a wide array of human stressors. Humans have altered the Great Lakes ecosystems directly through nutrient pollution and introductions of invasive species, and indirectly through climate change. This chapter examines how nutrient pollution interacts with hydrology, climate, and species invasions to determine water quality in Lake Erie and Lake Superior, which represent two ends of a continuum of biological, chemical, and physical conditions. Lake Erie is much shallower than the others and is divided into three distinct basins: the shallow Western Basin, the Central Basin, and the deepest Eastern Basin. Water quality is typically quantified in terms of chlorophyll concentration, an indicator of phytoplankton biomass. Invasion by zebra mussels in the late 1980s, followed shortly by the closely related quagga mussels, led to further dramatic reductions in phytoplankton density due to the filter-feeding bivalves.