the lower fox river watershed monitoring program    
University of Wisconsin Green Bay
University of Wisconsin Milwaukee


Nutrients such as phosphorus and nitrogen are essential to maintain stream life. However, nutrient overloading into streams and lakes is one of the major challenges facing the Fox River Basin today.

A limiting nutrient is the nutrient which is in shortest supply in a particular ecosystem. Plant and animal growth will occur only to the point where that nutrient is no longer available. In freshwater systems, phosphorus is typically the limiting nutrient for plant growth, whereas nitrogen is the limiting nutrient in oceanic environments. Excessive amounts of these nutrients throw off the ecological balance of the aquatic system, with far-reaching impacts on biota. For example, phosphorus additions to Green Bay cause huge summer algal blooms. Similar conditions exist in the Mississippi River delta, except that it is nitrate causing phytoplankton blooms. Large amounts of detritus from these blooms settle to the bottom of the water column, where microbial decomposition depletes dissolved oxygen levels for fish and invertebrates.

Typical sources of excess nutrients in streams are wastewater treatment facility discharges and storm water runoff. Much progress has been made over the past 20 years in reducing phosphorus in wastewater, especially through the elimination of phosphates from laundry detergents. However, both manure and inorganic fertilizer inputs from agriculture and lawn-care are still widely uncontrolled nonpoint sources of nutrients. Fertilizers contain highly soluble compounds because plants need nutrients to be in solution for uptake by roots. However, these forms of nitrate, ammonia, and phosphorus are more able to migrate from the soil to nearby streams during precipitation events. In addition to fueling eutrophication in streams and lakes, both ammonia and nitrate can reach toxic levels under certain conditions.