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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.
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
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
Minnesota River Basin Data Center | Minnesota State University, Mankato
184 Trafton Science Center S, Mankato, MN 56001 | Phone: (507)389-5492
| FAX: (507)389-5493 | Email: mrbdc@mnsu.edu