1. Phosphorus Cycling 

2. Nitrogen Cycling

The "dead zone" in the Gulf of Mexico is caused by nitrogen and phosphorus, which stimulates the growth of microscopic plants called phytoplankton. This will be eaten by small animals called zooplankton, zooplankton will be eaten by small fish and small fish will be eaten by large fish and it goes on up into the food web. The problem is there are too much nitrogen and phosphorus right now, too much phytoplankton falling to the bottom and decomposed by bacteria that use up the oxygen, that's the biology. The nitrogen that is put in fertilizers and the phosphorous goes on the land and drains off into the Mississippi River, and ends up in the Gulf of Mexico. There is three times more nitrogen in the water, in the Mississippi now than there was in the 1950s. What that means are more phytoplankton and more sinking sails and lower oxygen. this was not a natural feature of the Gulf; it has been caused by human activities.