This satellite image of summer conditions in the Gulf of Mexico south of Texas, Louisiana and Mississippi (US) is from the MODIS/Aqua satellite. Red and orange colors indicate the large amounts of phytoplankton that have multiplied because of nitrogen-rich water entering the Gulf at the Mississippi River Delta. When the phytoplankton die and decompose, oxygen is taken from the water and other marine life can not survive. This is known as a dead zone.

Fertilizing the Earth with Nitrogen

Plants need nitrogen. It is a nutrient that allows them to grow larger and faster. Plants are not able to make use of the nitrogen gas in the atmosphere, two nitrogen atoms bonded together. It is not a useable form of the nutrient. So they get the nitrogen they need from the soil where its form has been converted by soil bacteria. In natural conditions, plant growth is limited in many areas by the amount of usable nitrogen available in the soil. In an effort to grow more crops, people have been transforming nitrogen from the atmosphere into nitrogen fertilizers for crops. This has been very successful over the past century, allowing people to farm on lands that had not been as productive in the past, and allowing areas of the world with less fertile lands to produce enough food for growing populations. However, fertilizers are often overused, and that can cause problems.

Nitrogen from fertilizers sinks into soils, often creating conditions that favor the growth of weeds rather than native plants. Nitrogen then washes into waterways causing a surplus of nutrients, a situation called eutrophication. In freshwater lakes, rivers, and streams eutrophication causes aquatic weeds to grow unchecked up from the bottom. They sometimes fill the entire lake, river, or stream. Algae cloud the water green and slimy algal scum coats shallow rocks.

As the nitrogen-rich waters make their way downstream to the ocean, they cause even more problems. For example, every summer for more than 30 years high nitrogen levels at the Mississippi River Delta have caused a dead zone in the Gulf of Mexico where the water empties into the ocean. This dead zone, in which oxygen levels are too low for animals to survive, covered more that 8000 square miles (more than 20,000 km2) of ocean in 2001. The dead zone forms when excess nitrogen causes algae to grow and reproduce very rapidly. As the huge algae population begins to die and decompose, oxygen that is usually dissolved in the seawater is used up. Animals can not survive without oxygen. They flee to another part of the ocean if they can, or they die. Compounding the low oxygen problem, some of the algae that are multiplying rapidly are toxic. Known as “red tides”, blooms of toxic algae can cause major fish kills, and render clams, scallops and other shellfish highly dangerous for human consumption.

Although it is one of the larger dead zones and is relatively well-studied, the dead zone in the Gulf of Mexico (see image at left) is not the only one of its kind. There are about 150 dead zones in the world’s oceans. Almost all of them are located at the mouths of rivers where nitrogen fertilizers and other nutrient sources like sewage and livestock waste are added to the seawater. Scientists who have studied the Gulf of Mexico dead zone say that if the amount of fertilizers used on American crops were decreased by 12-14%, the dead zone would shrink to about a quarter of its current size without impact on the growth of the crops.

Last modified January 19, 2010 by Randy Russell.

You might also be interested in:

Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Kingdom Plantae

Though not the largest kingdom, with a mere 300,000 species catalogued, many might argue that the Kingdom Plantae just may be the most important group of living organisms. In the process known as "photosynthesis",...more


Nitrogen is a chemical element with an atomic number of 7 (it has seven protons in its nucleus). Molecular nitrogen (N2) is a very common chemical compound in which two nitrogen atoms are tightly bound...more

Changing the Nitrogen Cycle, Changing the Planet

Look up into the sky and you look through millions of air molecules, eighty percent of which are nitrogen molecules, two atoms of nitrogen bonded together. Nitrogen is found all over the planet, not just...more

World Leaders Developing a New Plan to Help Earth’s Changing Climate

Leaders from 192 nations of the world are trying to make an agreement about how to limit emissions of heat-trapping greenhouse gases, mitigate climate change, and adapt to changing environmental conditions....more

What is Climate?

Climate in your place on the globe is called regional climate. It is the average weather pattern in a place over more than thirty years, including the variations in seasons. To describe the regional climate...more

Earth's Greenhouse Gases

Less than 1% of the gases in Earth's atmosphere are called greenhouse gases. Even though they are not very abundant, these greenhouse gases have a major effect. Carbon dioxide (CO2), water vapor (H2O),...more

Space Missions to study Earth's Atmosphere & Climate

Television weather forecasts in the space age routinely feature satellite views of cloud cover. Cameras and other instruments on spacecraft provide many types of valuable data about Earth's atmosphere...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA