Nitrates

• Nitrates enter the Mississippi River from several sources, both organic and inorganic.
• Organic Sources ~manure from animals, human sewage, and nitrogen products produced by plants
• Inorganic Sources ~fertilizers (nitrogen, phosphates, and silica), industrial waste, and airborne nitrogen oxides from fossil fuel burning
• Most of these nitrates are coming from the farmland that the Mississippi Basin drains.  The Mississippi Basin encompasses 41% of the continental United States.

http://www.usgsquads.com/images/ms_watershed.jpg

• The United States Geologic Survey (USGS) has done studies and determined that 60% of the Mississippi Rivers nitrates are coming from north of the Ohio River, which is an area known as the corn belt.
• The nitrates that are entering the river are being carried all the way to the Gulf of Mexico.  Once they reach the Gulf they are helping create huge algal blooms.  The organic matter forms at the surface and then sinks to the bottom where bacteria begins to break it down and release carbon dioxide.  The bacteria thrive off the excess organic matter and being using up all the oxygen.  This is the same oxygen that all the other animals need to survive.

Density and Temperature

• Density and temperature are another factor in the creation of the dead zone.
• The Mississippi River is a fresh water source which makes it lighter than the Gulf which is a salt water source.  When the fresh water of the Mississippi flows into the Gulf the fresh lighter water stays on top of the heavier salt water.
• The Mississippi River is also warmer than the Gulf water so the warmer water stays on top of the colder water.
• This process forms a stratified layer of the warmer, fresher, lighter water on top of the colder, saltier, heavier water.
• This doesn't allow the water to mix to help oxygenate the water below the surface.

Wind and Waves

• Wind and waves contribute to the dead zone as well.
• Although the dead zone can form as early as February and persist to as late as October, it generally stays from May to September.
• During these summer months the ocean waves are calmer than they are in the winter months. 
• Without waves there isn't much turbidity to help oxygenate the water and to help mix the stratified layers.