Nitrogen and phosphorus are essential nutrients - yet too much of a good thing is not always a good thing. Scientists are investigating nutrient pollution down the Mississippi River. Each spring, water flows approximately 2,300 miles down the Mississippi River, beginning its journey at Lake Itasca in northern Minnesota and emptying into the Gulf of Mexico. Along the way, streams and rivers accumulate nutrients that run off the land and into the waterways, and eventually these nutrients enter the Gulf of Mexico. Spring pulses of nutrients to the Gulf contribute to the second largest hypoxic - or low oxygen - zone in the world.