What does phosphorus do in the river?

Little things cause big problems

Tiny plant-like organisms called algae live in river water. They use sunlight to make sugar, and serve as food for fish and other aquatic animals.

Like all living things, algae needs phosphorus to grow. Normally, river water has little phosphorus, which keeps algae in check. But when large amounts of phosphorus enter the water, algae grows explosively.

This has several negative effects:


Fast-growing blue-green algae, which is not very nutritious, out-competes and replaces green algae and diatoms, which are better food sources.

Algae blooms may cover the surface of the water, making it unsuitable for boating and swimming.

Photo courtesy MPCA


With the surface covered, rooted plants on the river bottom, can't get any sun and die. These plants provide important food and shelter for fish, like the Northern Pike and Bowfin pictured above.

Photographs by Konrad Schmidt


The huge numbers of algae eventually die and sink to the bottom, where bacteria decompose them. This takes oxygen out of the water—bad news for fish, freshwater mussels, and other aquatic critters.

The St. Croix River is home to several endangered species of fresh-water mussels. As the decaying algae removes oxygen from the water it suffocats bottom-dwelling animals like the mussel seen here.

Photo by William Smith, courtesy Wisconsin DNR

Learn more about how excess nutrients in the water cause problems in the Gulf of Mexico's "Dead Zone".

To learn more about algae in Minnesota's lakes and rivers, and what you can do about it, read this article from the Minnesota Pollution Control Agency.