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Mighty Mississippi Chokes on CO2

Farming practices around the Mississippi River have changed its chemistry, as well as the chemistry of the Gulf of Mexico, which it empties into. (Image credit: Jerry Ting)

Midwestern farming practices have left the mighty Mississippi River chock full of carbon dioxide, acidifying the downstream Gulf of Mexico and harming coral and other marine life, a new study finds.

Using data from as far back as 100 years ago, researchers tracked changes in the water level and in certain chemicals in the river, and found its chemistry has been significantly altered.

The researchers concluded that farming practices such as liming (adding calcium to the soil), changes in drainage and crop rotation have caused an increase in bicarbonate and the amount of water running off into the Mississippi.

The result is an injection of the equivalent of five Connecticut Rivers' worth of carbon dioxide into the Mississippi each year over the last 50 years, said study leader Peter Raymond of Yale University. (The Connecticut River is the largest river in New England, at 407 miles (655 kilometers) long. Its source is in northern New Hampshire and it empties into the Long Island Sound.)

"It's like the discovery of a new large river being piped out of the Corn Belt," Raymond said. "Agricultural practices have significantly changed the hydrology and chemistry of the Mississippi."

The research, funded by the National Science Foundation, is detailed in the Jan. 24 issue of the journal Nature.

The carbon dioxide build-up starts when water in the farmlands surrounding the river dissolves soil minerals. The important mineral in the carbon equation is bicarbonate, which forms when carbon dioxide in the farm water runoff dissolves the soil minerals.

Bicarbonate is a key player in the absorption of atmospheric carbon dioxide. When the Mississippi waters eventually run out into the Gulf of Mexico, these chemical changes cause the ocean waters to absorb more carbon dioxide and therefore become more acidic. (Carbon dioxide dissolved in water also accounts for the acidity of soft drinks.)

Ocean acidification can have critical impacts on marine organisms, as it decreases the amount of carbonate ion, a key component of the calcium carbonate that makes up shells and coral reef structures.