Chemical Integrity: Phosphorus
Why is this a Water Quality problem?
The bottom waters of the Lake Erie central basin become anoxic (without oxygen) annually, creating what is known as a dead zone. The anoxic zone is a natural phenomenon. However, the area affected by anoxia has been magnified by man-made inputs of NPS pollutants, particularly phosphorus. The increased size of the anoxic zone led to the existing Lake Erie phosphorus concentration targets and a goal in the Great Lakes Water Quality Agreement (GLWQA) that there be a "substantial reduction in the present  level of algal biomass to a level below that of nuisance condition in Lake Erie."
Although the long-term trend in magnitude of central basin anoxia is downward, there hasn't been any improvement since the late 1980s, whereas there have been significant decreases in phosphorus concentrations. The influence of the zebra mussels and quagga mussels (a close relative) on nutrient cycling and subsequently dissolved oxygen depletion in the Central Basin of Lake Erie is not well understood. They could have an ecological effect that obscures the rate of oxygen depletion from decomposition of algal organic matter. Therefore, scientists from Canada and the US have collaborated through the Lake Erie Millennium Network to further research the factors that trigger and control the magnitude of the central basin anoxic zone.