The dead zone is caused when spring rains wash fertilizer from northern farmland into the Mississippi River. That nutrient-rich water winds up in the Gulf, where the fertilizer that once helped grow corn in Kansas or wheat in Missouri also helps oxygen-hungry algae bloom.

The algae sucks up all the oxygen that fish and other marine life need to survive. The organisms that can flee survive, and the ones that can't die.

http://www.truthout.org/article/midwest-flooding-may-worsen-dead-zone

Gulf dead zone may worsen because of flood

Midwest Flooding May Worsen Gulf Dead Zone

Houma, Louisiana - Flooding in the Midwest is swelling the Mississippi River, sending increased water through Louisiana and into the Gulf of Mexico - a phenomenon that has left researchers confident that their dire dead-zone predictions are right on track.

Scientists have predicted that this year's dead zone - an area of the Gulf of Mexico that lacks enough oxygen to support marine life - will be the largest ever.