Algae blooms like this one are caused by excess nitrogen from fertilizer and manure runoff, killing fish by reducing the oxygen in the water.
updated 6/14/2005 9:42:25 AM ET 2005-06-14T13:42:25

Farmers’ routine application of chemical fertilizers and manure to the land poses a far greater environmental problem to freshwater lakes than previously thought, potentially polluting the water for hundreds of years, according to new research.

Phosphorus in those substances has built up in the soil and will slowly end up in many lakes, where the nutrients lead to plant and algae growth. The environmental problem, known as eutrophication, can turn pristine lakes into smelly, weed-filled swamps with lots of dead fish.

In a paper published Monday in the Proceedings of the National Academy of Sciences, a University of Wisconsin-Madison expert blames the buildup largely on industrial agriculture’s use of fertilizer and manure since the 1940s.

The concentration could cause the eutrophication of lakes for centuries as the treated soil slowly washes into lakes and streams, writes Stephen Carpenter, a professor of zoology and a leading expert on freshwater lakes. The problem leads to fish kills and the growth of toxic algae that can make lakes unsuitable for swimming.

“A very small percentage of the phosphorus moves into the lake each year and that small amount is sufficient to cause a great deal of water pollution,” Carpenter said.

Farm view: Dams make it worse
Paul Zimmerman, executive director of governmental relations for the Wisconsin Farm Bureau Federation, said he hadn’t seen the report, but he defended farmers, saying they’ve improved soil conservation over the last two decades to make sure more dirt remains in place. Dams have made lakes more stagnant, exacerbating pollution, he said.

The light green water color in Southern California’s Salton Sea (lower right) is an algae bloom caused by farm fertilizer runoff.
“You can’t blame all eutrophication on agriculture,” Zimmerman said. “Each year we’re getting better and better. The eutrophication of lakes didn’t happen overnight and it’s not going to be solved overnight either.”

The study concludes that major changes in soil management are needed to reverse the trend. It may add urgency to government efforts to stop phosphorus from fouling up lakes and streams.

Carpenter studied Lake Mendota, an urban lake in the Madison area that is a popular recreation and fishing spot, as a model for all freshwater lakes in rich farming areas. He said the lake’s water quality has declined in recent decades, which will continue if left unchecked.

'Digesters' and buffers advised
In his study, Carpenter wrote that"this type of eutrophication is not reversible unless there are substantial changes in soil management."

Carpenter said machines called manure digesters should be used to convert phosphorus into a sludge that can be put in landfills or transferred to phosphorus-deficient areas. He also said buffer strips should be developed to protect waterways from runoff and new technologies found to remove phosphorus from soil.

“If we don’t do something,” Carpenter said, “the water quality will get considerably worse, the lake will smell bad, there will be algae blooms all summer long, and more and more of those blooms will be the toxic kind.”

Zimmerman said few farmers can afford manure digesters, which can run about $1 million apiece. Most in Wisconsin are found only on farms with 1,000 or more cows. “It’s way too expensive,” Zimmerman said.

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