Agricultural Pollution in the Mississippi River 

"A dead zone is a large body of water that doesnt have enough oxygen to support life.  The cause of such “hypoxic” (lacking oxygen) conditions is usually eutrophication, an increase in chemical nutrients in the water, leading to excessive blooms of algae that deplete underwater oxygen levels. Nitrogen and phosphorous from agricultural runoff are the primary culprits, but sewage, vehicular and industrial emissions and even natural factors also play a role in the development of dead zones." 

Background information

 

This image above depicts the location of the Mississippi delta dead zone. The dead zones boarders change seasonaly depending on a number of variables such as ocean temprature, rain fall and wind paterns.