PrintThe NOAA Environmental Visualization Lab Presents...
The Dead Zone: Nutrient Runoff Creates Hypoxia In the Gulf of Mexico
Data provided by: Maria A. Faust (Smithsonian Institution), Katja Fennel (Dalhousie Univ.), and Robert Hetland (Texas A&M)
Did you know that half of the oxygen we breathe comes from tiny organisms that live in the ocean? It's true! These microscopic marine organisms, called phytoplankton, produce oxygen just like land plants.
*images of phytoplankton*
But phytoplankton are not plants, they are Protists, single celled organisms. They are so small that thousands of them can fit in a single drop of water.
In order to study phytoplankton, scientists often use microscopes...
...or satellites.
From space we see Earth like this...
But some satellites see Earth like this...a dance of rainbow colors. In this case the colors represent the concentration of phytoplankton in the ocean: red is high concentration; blue is low concentration.
Phytoplankton depend on nutrients and the proper temperature and light conditions to grow and reproduce. Coastal areas are extremely rich in nutrients, which have been washed off the land by rivers
Areas such as the open ocean have lower concentrations of phytoplankton because of the limited amount of nutrients there.
The mouth of the Mississippi River is a perfect example of how nutrient run-off creates plankton blooms.
41% of the United States drains into the Mississippi River and then out to the Gulf of Mexico. That's a total of 3.2 million square kilometers of land, or about 600 million football fields.
About 12 million people live in urban areas that border the Mississippi, and these areas constantly discharge treated sewage into rivers.
However, the majority of land in the Mississippi's watershed is farm land. Each spring as farmers fertilize their lands preparing for crop season, rain washes fertilizer off the land and into streams and rivers.
All of the urban and farm discharge includes nutrients such as nitrogen and phosphorus that are very important for the growth of phytoplankton.
Incredibly, about 1.7 million tons of these nutrients are dumped into the Gulf of Mexico every year.
Once the Gulf of Mexico receives this huge influx of nutrients, massive phytoplankton blooms occur.
These blooms result in an area called the Dead Zone–areas with such low oxygen concentration that few organisms can survive there.
But if phytoplankton blooms produce oxygen, then why does a Dead Zone occur?
For animals, such as microscopic zooplankton and fish, phytoplankton blooms are like an all-you-can-eat buffet. Small animals eat the phytoplankton, and are in turn eaten by bigger fish.
There lurk bacteria that decompose the waste, and in the process use up the oxygen, creating hypoxic conditions. The different densities of fresh water from the Mississippi and salt water from the Gulf...
...create barriers that prevent mixing between the surface and deep waters. Soon there is not enough oxygen for other organisms to use.
The Dead Zone has arrived.
But as summer turns to fall, winds help to stir up the water, allowing the layers to mix and replenish oxygen throughout the water. Eventually the Gulf and its fish populations return to normal.
Until next year..