August 23, 2010 - On Monday Louisiana state biologists investigated whether a large fish kill at the mouth of the Mississippi River was caused by oil or dispersants from the BP spill in the Gulf of Mexico. The fish were found in the Mississippi River Gulf Outlet, a 76-mile shipping shortcut from the Gulf of Mexico to New Orleans that was dug by the U.S. Army Corps of Engineers in the 1960s. The shortcut is like a dead-end canal. Preliminary testing by the state indicated that the cause of the fish kill was "hypoxia" or lack of oxygen. Hypoxia is most often caused by an excess of nitrogen and phosphorus from agricultural fertilizer or human waste, but it could also be caused by chemical dispersants, which were used extensively after the oil spill. More from Margot Roosevelt LATimes

The cause of the fish kill may be be related to the dead zone at the mouth of the Mississippi caused by agricultural run-off and other industrial activities. The massive dead zone is caused primarily by nitrogen and phosphorous run-off from fertilizers, soil erosion, animal wastes, and sewage. The Mississippi River Basin drains much of the United States, from Montana to Pennsylvania and includes the Mississippi River Valley, Minnesota, Iowa, Illinois, Wisconsin, Missouri, Tennessee, Arkansas, Mississippi, and Louisiana. In a natural system, these nutrients aren't significant factors in algae growth because they are depleted in the soil by plants. However, excess nitrogen and phosphorus from human activities overwhelms the system such that algae growth is no longer limited - algal blooms develop, the food chain is altered, and dissolved oxygen in the area is depleted. The size of the dead zone fluctuates seasonally, as it is exacerbated by farming practices. It is also affected by weather events such as flooding and hurricanes. More from Monika Bruckner Montana State University.