Can the ocean run out of oxygen? - Kate Slabosky
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Dead zone forms when dissolved oxygen becomes a limiting factor for the marine life in the area. Actually, the process that forms the dead zone is a long tale of one limiting factor after another. Before the spring run-off of nutrient rich waters, nutrients such as nitrogen and phosphorus are the limiting factors for algae growth. After a large input of these limiting nutrients, the algae blooms. The next factor that limits algae growth is sunlight. So much algae grows that the algae aboves blocks the algae below from recieving sunlight, and a large die-off occurs. The dead algae is decomposed, a process which consumes a large amount of dissolved oxygen, and now dissolved oxygen becomes the limitng factor for marine life growth.
The nitrogen cycle and the invention of nitrogen based fertilizer
Many dead zones around the world, including the Gulf of Mexico dead zone, can be traced back to large amounts of fertilizer run-off. Fertilizer is anything you put on plants to increase the amount of nutrients available to them. Manure and compost have been used as fertilizer for thousands of years. Synthetic fertilizer has only been around for a little over 100 years and has significantly changed how we feed Earth’s growing human population.
Modern synthetic fertilizer contains one or more of the following nutrients: nitrogen, phosphorus and potassium. These nutrients are very important for plant growth. Nitrogen naturally enters the soil in small amounts through the nitrogen fixation step of the nitrogen cycle. Unlike oxygen and carbon dioxide, plants cannot readily use the nitrogen available in the atmosphere, but “nitrogen fixers” like bacteria on the roots of legumes and lightning transform the nitrogen molecules in the air into nitrogen containing compounds in the soil that plants can aborb through their roots.
The nitrogen that is naturally added to the soil through the nitrogen cycle is often not enough plants growing in some of the land intensive farms around the world, and using manrure and compost as fertilizer can be labor intensive. In the early 1900s a scientist name Franz Haber changed everything when he developed what is now known as the Haber process, a method for carrying out artificial nitrogen fixation. This process was used to create the first synthetic fertilizer. This invention has been both praised and criticized. On one hand, the invention of fertilizer enabled the world’s agricultral systems to grow much more food. Today the world has over 7 billion people and without synthetic fertilzer we would only be able to feed about 4 billion. On the other hand, the increase in food production drove up world population growth significantly and many scientists believe the world cannot support much more population growth. Furthermore, nutrient run-off from excess fertilizer harms ecosystems in way you learned about in this video.
Gulf of Mexico
The Gulf of Mexico dead zone affects “one of the most ecologically and economically productive ecosystems in North America.” The Gulf of Mexico is home to a 20 billion dollar fishing industry, half of the United State’s domestic supply of oil and natural gas, important shipping ports and countless recreational activities. Dead zones are not the only threat to the Gulf of Mexico. Eutrophication in the Gulf of Mexico can also lead to toxic algae blooms, also known as a “red tide.” Unsustainable fishing practices, the increased frequency of intense hurricanes associated with global climate change, and oil spills are also responsible for a great deal of problems in its ecosystems.
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Lesson duration 25:45