Phosphorus

Excess phosphorus is harming Minnesota waters.

Dark green algae covers the surface of a lake near the shore between a boat in a hoist and a pontoon.A quarter of Minnesota lakes have high levels of phosphorus, which means that they don't meet water-quality standards for recreation. Excess phosphorus feeds algae growth. Algae-covered water is less attractive for swimming and other aquatic recreation — highly valued pastimes in Minnesota — and degrades the conditions that fish, bugs, wildlife, and desired plants need to thrive. In addition, phosphorus can fuel toxic blue-green algal blooms, which are harmful to people and pets.

The MPCA designates bodies of water that have excess phosphorus as "impaired." The agency both identifies the sources of phosphorus and the reductions needed to meet water quality standards for impaired waters. Reducing phosphorus pollution requires continual long-term strategies, but all Minnesotans can take action to protect our lakes and streams.

Even if a body of water has high levels of phosphorus, it can still be suitable for recreation. Algae generally "blooms" in hot, calm weather and may be disrupted by rain and winds.

How phosphorus gets in the water

Muddy water runs off the edge of a corn field

Phosphorus comes from both regulated and non-regulated sources. The nutrient is a common element in agricultural fertilizers, manure, and organic wastes in sewage and industrial discharges. Rain and snowmelt can wash fertilizers and manure off ag land and into ditches, streams and lakes. It can also be in discharges of treated wastewater, from communities and businesses.

Erosion of soil and other particles is another source of phosphorus, which binds to sediment. Reducing any runoff, from hard surfaces in urban areas and from cropland in rural areas, is an important strategy for reducing phosphorus in lakes and streams.

Government entities and individuals can take steps to reduce phosphorus pollution:

  • Using less fertilizer on lawns, cropland, and other areas and using management practices such as buffer strips that filter runoff
  • Following feedlot operation and manure application rules to prevent runoff, such as mandatory setbacks from bodies of water and avoiding applications during rainy weather
  • Using phosphorus-free lawn fertilizer
  • Complying with phosphorus discharge limits at wastewater treatment facilities
  • See the Reducing phosphorus pollution web page for more

The problems with phosphorus

A thick mat of bright green algae covers part of a lake near the shore.

Lakes, rivers, and streams with excess phosphorus can grow algae, which sets off a vicious cycle:

  • Algae blooming can hinder recreation such as swimming while also damaging habitat for fish and other aquatic species
  • As algae decay, they use up oxygen in the water, leaving less to sustain fish and other aquatic species
  • In addition, algae release more phosphorus into the water as they decay, providing more fuel for additional algal blooms
  • Some algae contain a bacteria that can be toxic to people and pets

Downstream effects

Minnesota is a headwaters state, meaning major river systems start here and flow into other states and Canada. The Mississippi River empties into the Gulf of Mexico, the Red River flows into Lake Winnipeg in Canada, and several rivers in northeast Minnesota run into Lake Superior. Our nutrient pollution impacts millions of people downstream, including those who depend on rivers for drinking water. To help waters here and downstream, Minnesota has adopted a nutrient reduction strategy that outlines goals for reductions in phosphorus and nitrogen, and ways to get there.

Impact of climate change

Minnesota is receiving more precipitation in intense storms that lead to more runoff, which in turn leads to more phosphorus in lakes and rivers. Also, longer periods of hotter temperatures are raising lake water temperatures, increasing the potential for algal blooms, including toxic blooms.

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