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Phosphorus and the Lower Rock River

Phosphorus pollution is an important environmental issue affecting Wisconsin’s waterways. While some phosphorus is essential for aquatic ecosystems, in excess it can lead to serious ecosystem damage (3). Phosphorus is washed into lakes and rivers from runoff from both agricultural and urban areas (1). The main source of phosphorus is from animal waste. Each year, 9.5 billion gallons of manure are produced in Wisconsin (1). Farmers spread the manure on their fields for fertilizer (1). Because storage of manure is difficult farmers often use more manure fertilizer than needed (1). Rain and snow then wash both dissolved and sediment phosphorus in to rivers and lakes (1,3). The main urban sources of phosphorus are lawn and garden fertilizers and construction sites (1). On average, construction sites contribute thirty tons of sediment to Wisconsin’s waterways per year (1). Because urban areas tend to have large areas of nonporous surfaces, runoff is quick and efficient (3).
Phosphorus in lakes and rivers spurs growth of aquatic plants and algae (1,3). Unfortunately, because too much phosphorus is present in Wisconsin’s waterways, it is the leading cause of eutrophication in the state (1). Eutrophication is “the over-enrichment of our rivers and lakes by nutrients” (1). When excess phosphorus is deposited into waterways it causes the rapid growth of aquatic plants and algae (1,3). When this vegetation dies, it decays consuming large amounts of oxygen (1). Because there is so little oxygen in the water where decay occurs, a dead spot is created (1). A dead spot is an area of a waterway that does not have enough oxygen to support aquatic plants and animals (1). Thus, due to eutrophication caused by excess phosphorous aquatic plants and animals suffocate in their own environments(1).

The average annual phosphorus load in the Lower Rock River, measured at the City of Beloit Treatment Facility, is 37,596 lb/yr (2). The average measured at the Town of Beloit Treatment Facility is lower at 2230 lb/yr (2). This suggests that phosphorus runoff is worse in the city as opposed to the surrounding areas. The Basin Education Program believes that it is both healthy and possible to lower these numbers to 29,507 lb/yr and 882 lb/yr respectively (2). WisPIRG suggests that .033milligrams per liter is a healthy level of phosphorus for Wisconsin’s rivers and lakes (1). Currently, the state’s average phosphorus load is .64 mg/l (1).
Possible solutions for controlling eutrophication by phosphorus generally involve the regulation of soil erosion, manure wastes and urban runoff. The Runoff Management Plan (RMP), part of the Wisconsin Department of Natural Resources, currently monitors larger farms in WI. Runoff, however, is not well regulated in small to mid-sized cities like Beloit because higher density cities tend to be the focus of environmental policies.


1. WisPIRG Foundation. “Phosphorus in Runoff Pollution in Wisconsin”

2. Basin Education. “Production of Phosphorus Loads in the Rock River Basin, WI”

3. Pierzynski, Gary, J. Thomas Sims and George F. Vance. Soild and Environmental
Quality. London: CRC Press. 2000.