Phosphorus in the Minnesota River Basin

 
 



Excess phosphorus in the Minnesota River is a concern because it can stimulate the growth of algae. Excessive algae growth, death, and decay can severely deplete oxygen supply in the river, endangering fish and other forms of aquatic life.



Photo: Algae growth in the Blue Earth River, July 2002		    

 


What is Phosphorus?


Phosphorus is an important nutrient for plant and animal growth. Total phosphorus is the measure of the total concentration of phosphorus present in a water sample. Both point and nonpoint sources of phosphorus are important contributors. Point-source phosphorus comes mainly from municipal and industrial discharges to surface waters. Nonpoint-source phosphorus comes from runoff from urban areas, construction sites, agricultural lands, manure transported in runoff from feedlots and agricultural fields, and human waste from noncompliant septic systems. There are two primary types of phosphorus: sediment attached and bio-available.

Bio-available (Not Sediment-Attached)
Phosphorus from wastewater treatment plants, feedlot runoff and failing septic systems are in a soluble form that is readily available to algae, and so exerts an immediate impact on the growth of algae and subsequent dissolved oxygen depletion.

Sediment Attached
By contrast, phosphorus runoff from cropland is mainly attached to sediment particles, and is not immediately available to support algae growth. However, as sediment particles move down the river in a cycle of runoff, deposition, resuspension and secondary deposition, etc., it is likely that the particles will release the phosphorus ions attached to their surface, and later become reattached to phosphorus ions under certain conditions. Once phosphorus enters the riverine system, it undergoes a series of complex biochemical transformations, and through this process is likely to stimulate several growth cycles of algae. Thus, although bio-available phosphorus entering the river has a more immediate impact than particular phosphorus, in the long run both sources are important to manage to reduce the total amount of algae-producing phosphorus in the river.

 

Why is phosphorus important?


Excess phosphorus in the river is a concern because it can stimulate the growth of algae. Excessive algae growth, death, and decay can severely deplete oxygen supply in the river, endangering fish and other forms of aquatic life (see dissolved oxygen).

What is the standard for Phosphorus?


Currently, there are no statewide standards for total
phosphorus in rivers or streams. The US Environmental Protection Agency states a desired goal of 0.10 milligrams per liter (mg/L) for prevention of nuisance plant growth in streams. An analysis of algal productivity and total phosphorus (TP) concentration data for the Minnesota River has shown that algal productivity will not start to diminish until TP concentrations fall below approximately 0.26 mg/L. Based on this information, the Minnesota River mainstem will continue to experience undesirable levels of algal growth until TP concentrations are reduced to below this level.
Algae growth on the Blue Earth River, July 2002.  

What is the status of phosphorus in the Minnesota River Basin?


Low dissolved oxygen rates are of concern particularly during low-flow times or in slow-flowing areas such as reservoirs and the lower reaches of the Minnesota River. Large total phosphorus loads can have major impacts on downstream receiving waters such as Lake Pepin.

The map and chart at right illustrate the total phosphorus levels in 2002 for selected Minnesota River Basin watersheds. The illustrations show flow-weighted mean concentrations (FWMC) of total phosphorus. This is equivalent to routing all of the flow that passed a monitoring site during a specific timeframe into a big, well-mixed pool, and collecting and analyzing one sample from the pool to give the average concentration.

Total phosphorus is an issue in all major tributaries. During 2002, the bulk of total phosphorus loading from tributaries occurred in the watersheds located in the middle and lower Minnesota River Basin (i.e. downstream of Morton).

Total phosphorus FWMC values in the Minnesota River mainstem, six major tributaries, and five minor tributaries were greater than 0.30 mg/L during 2002. These TP concentrations are well above the USEPA desired goal of 0.10 mg/L for preventing nuisance plant growth in streams (see chart).

Because total phosphorus loading is strongly correlated with stream flow, year-to-year variability in runoff quantities explains much of the variability in TP loading.


  


 

References:
State of the Minnesota River: 2002 Surface Water Quality Monitoring.
Minnesota River: Basin Information Document. Minnesota Pollution Control Agency. November, 1997.
MPCA Glossary website - http://www.pca.state.mn.us/gloss/index.shtml
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For more information about other nutrients, see orthophophorus, nitrate-nitrogen.
For more information about nitrate-N in the Minnesota River Basin, see publications.

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This page was last updated 9/15/04