Algae assault. Wisconsin's Lake Mendota is undergoing eutrophication due to over-fertilization in the area.

Fertilizer's Legacy of Slime

Chemical fertilizers along with animal feed and manure are causing damage to many lakes that could take a thousand years to resolve, even if farmers stopped using them tomorrow, a new study suggests.

The biological productivity of lakes is typically limited by the amount of phosphorus and nitrogen they contain. When the waters become enriched in these nutrients, aquatic plants and algae thrive and can transform lakes into weed-choked pools of slime. And when the algae decays, microbes suck oxygen from the water, which can kill fish. This is called eutrophication. Traditionally, sewage and industrial discharges have been the main source of phosphorus pollution. But with agriculture intensifying in the second half of the twentieth century, fertilizers have become a major driver for eutrophication in many regions through the accumulation of soil phosphorus and nitrogen, which leach into aquatic ecosystems.

This week, the news got worse. A study published online this week in the Proceedings of the National Academy of Sciences suggests that some soils in intensively farmed lands have become so rich in phosphorus that they are likely to cause serious environmental problems in the future. University of Wisconsin-Madison limnologist Stephen Carpenter mathematically modeled the eutrophication of Wisconsin's Lake Mendota and simulated two approaches to mitigating the damage already done.

In the first scenario, farmers reduce their use of fertilizers and animal feed so that phosphorus stops building up in soils. But the soils are already so enriched that phosphorus will keep leaching into the lake--ruining any chances of recovery for the next 2000 years.

The second simulation looks a bit more promising. Here, farmers stop using fertilizers and animal feed altogether, leaving plants and animals to live on phosphorus in the soil. Over time, soils will become progressively less enriched, and the leaching of phosphorus slows down. The lake eventually recovers, although it still takes at least 1000 years.

The model is a "reality check," says Robin Tyler of the Delaware Department of Natural Resources and Environmental Control. "It suggests that foreseeable future generations will continue to be challenged by eutrophication." But Jean-Marcel Dorioz, director of the INRA limnology research unit at Thonon-les-Bains, France, feels that the "model was based on a very simplified view" of phosphorus leaching. Still, he believes the study is valuable for showing that soil phosphorus management is imperative in highly-fertilized regions.

Related sites
Carpenter's home page
International Lake Environment Committee Foundation for sustainable management of world lakes and reservoirs
The U.S. Environmental Literacy Council information on eutrophication
U.S. Environmental Protection Agency information on how to prevent eutrophication