COOL Classroom 2.1

Discover Dead Zones

Why are organisms dying in the dead zones?


Fish need to breath, and they breathe oxygen that is dissolved in the water. In order to increase the amount of dissolved oxygen in the water for your pet fish you should add aquatic plants or an aerator (that thing that makes bubbles in the water) to your aquarium. If you just leave a goldfish in a small bowl for a long time the fish can use up all of the dissolved oxygen in the water and suffocate. This lack of dissolved oxygen can also happen in the ocean. Dead zones are areas in the ocean where the water at the bottom is almost completely devoid of oxygen. Without oxygen the marine life suffocates just like your pet goldfish would in its bowl. Many times fish can swim to areas with more oxygen, but less mobile marine life such as shellfish and plankton do not have that option!

Dead zones are occurring in many areas along the coasts of major continents. Right now 146 dead zones exist in the world and most of them are located in Europe or on the East Coast of the United States. The largest dead zone in the world encompasses the entire bottom of the Baltic Sea. Others dead zones occur in the Chesapeake Bay, off the coast of Oregon, Lake Erie, and the most famous dead zone is located in the Gulf of Mexico.

What causes the dead zones?

Credit EPA



Algae are small, chlorophyll containing organisms that live in the ocean. They are different from plants because they lack true roots, stems, and leaves. However, like plants, algae need nutrients such as phosphorus and nitrogen in order to photosynthesize. When it rains fertilizers from farms and yards, sewage from waste treatment plants, and even detergents you use to wash your car in your driveway all have to go somewhere. If these nutrients make it to a stream or a river (which they usually do), they can eventually make it out to the ocean! The amount of available nutrients in the ocean usually keeps the algae from growing out of control, just like food limitations can keep a population of animals such as deer from growing out of control. So when all of these extra nutrients are washed into the local river by the rainfall, and the river flows out to the sea, it brings with it such a large concentration of nutrients that the algae grows out of control.

The process in which excess nutrients cause increased algal growth is called eutrophication. Eutrophication is a natural process, but excessive amounts of additional nutrients can cause large blooms. This can happen in a bay, a lake, or even the ocean. Increased algae growth can initially cause an increase in the amount of oxygen in the water. However, algae have short life spans, some algae only live for a few days. When the algae die, bacteria decompose the algae and in the respiration process use up large amounts of oxygen. This decrease in oxygen can create areas where the oxygen levels are so low that marine life cannot survive ~ creating dead zones.

What is the deadliest zone of all?

Credit USGS



The dead zone in the Gulf of Mexico is the second largest in the world and is located at the mouth of the Mississippi River. This vast expanse of ocean can cover 7,000 square miles (about the size of New Jersey) and is completely devoid of most marine life. Unfortunately, slow moving organisms, like oysters and crabs, are unable to flee to areas with more oxygen so they die. If you are wondering why the dead zone in the Gulf of Mexico is bigger than most, consider the length of the Mississippi River. The River almost covers the United States from top to bottom. Imagine all the farmland (a lot of the Midwest is farmland) and all of the industrial cities that the Mississippi River runs by as it travel down to the Gulf of Mexico. Consider the magnitude of the amount of nutrients that make their way into the river in the form of farm fertilizers, treated sewage waste and many other sources it picks up along the way. All of that is being dumped into the Gulf of Mexico! The dead zone in the Gulf of Mexico has grown bigger over the years and is a dramatic case of what can happen. Unfortunately, according to many scientists, dead zones are becoming more common in the coastal waters of the United States.

What are some effects of dead zones?


In the ocean, hypoxic areas (which are spots with low amounts of oxygen) support less marine life and can result in major fish kills. This can be detrimental to the fishing industry. The Gulf of Mexico alone provides 1.7 billion pounds of fish and shellfish which is worth $26 billion to the fishing industry. That is no small change and would have drastic effects on the economy! Other significant effects include altering the food web and decreasing biodiversity (the total amount of different species in an ecosystem). Unfortunately, what is happening in the Gulf of Mexico is symptomatic of what is happening along coasts all over the world.

 
 

Deadly Quiz Zone!

Test your knowledge on the above material:

1. Dead zones are areas at the bottom of the ocean that have very low levels of ________________.

2. Areas where dead zones have occurred include all of the following EXCEPT:

a. Coast of Oregon
b. Chesapeake Bay
c. Gulf of Mexico
d. Lake Superior
e. Lake Erie

3. What is one factor that limits the growth of algae in the ocean?
(Hint – Rearrange the following letters: URSNNEITT)

4. The process by which excess nutrients in a system causes increased algal growth is:

a. Decomposition
b. Photosynthesis
c. Eutrophication
d. Hypoxia

5. All of the following are effects of dead zones EXCEPT:

a. Alters the food web
b. Beneficial to the fishing industry
c. Kills marine life
d. Decreases biodiversity
e. Hurts the economy