Phosphorus is an essential nutrient for plant and aquatic life; however, elevated concentrations of phosphorus in rivers and lakes primarily from agricultural runoff can lead to nuisance algal blooms impacting aquatic life and recreational activities.  High phosphorus levels cause algal blooms in Lake DuBay, Big Eau Pleine, Pentenwell and Castle Rock Reservoirs that deplete oxygen levels which stresses aquatic life and fish communities, and on occasion, cause large fish kills. 

The Wisconsin Department of Natural Resources maintains a list of surface waters that do not meet specific water quality standards in the state, and is required to update the list every two years by the United States Environmental Protection Agency.  The 2019 Wisconsin River Total Maximum Daily Load Report identifies the number of river miles in Marathon County considered to be impaired by high levels of phosphorus concentrations.