Dead Zones: Much scarier than the twilight zone!
Why are organisms dying in the dead zones?
Fish need to breath, and they breathe oxygen dissolved in the water. In order to get more
dissolved oxygen in the water for your pet fish the tank either needs some aquatic plants
or an aerator (that thing that makes bubbles). If you just leave a goldfish in a small bowl
for a long time he can use up all of the dissolved oxygen in his water and suffocate. 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
his 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 is the entire bottom of the
Black Sea below 150 m. Others occur in the Chesapeake Bay, off the coast of Oregon,
Lake Eerie, and the most famous one is in the Gulf of Mexico.
Pictures of a goldfish in a bowl, a fish tank with an aerator and aquatic plants, and of
marine fish in the ocean.
What causes the dead zones?
Algae are like small plants that live in the ocean and plants need nutrients, such as
phosphorous 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 or your dog on your lawn all have to go somewhere. If they make it to a
stream or a river (which they usually do), they can eventually make it out to the ocean!
The lack 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 allows the algae to grow out of control.
The process in which excess nutrients in a system (such as a bay, lake, or even the ocean)
cause increased algal growth is called eutrophication. One would think that increased
algae growth would increase the amount of oxygen in the water since a product of
photosynthesis is oxygen. But think about the fact that algae only have a short life span,
maybe of only a few days. What happens when something dies? Usually bacteria
decompose it and this process uses up oxygen. That is how dead zones are created!
Pictures of it raining on the water, an algae bloom in a body of water, and a dead fish
decomposing.
What is the deadliest zone of all?
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 marine life. All that
remains are the bodies of organisms such as crabs and oysters that litter the bottom
because they were unable to flee to areas with more oxygen. If you are wondering why
the dead zone in the Gulf of Mexico is bigger than most, consider how long the
Mississippi River is (almost the length of the United States from top to bottom) and then
think about all of that farmland (a lot of the Midwest is farmland) and all of those
industrial cities that the Mississippi River runs by as it travel down to the Gulf of Mexico.
Then imagine 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.
Pictures of the Gulf of Mexico and the Gulf of Mexico dead zone.
What are some effects of dead zones?
Hypoxic areas of the ocean support less marine life and so 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.
Picture of a dead zone, of the Chesapeake Bay, and of fishing boats bringing in fish.
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.
b.
c.
d.
e.
Coast of Oregon
Chesapeake Bay
Gulf of Mexico
Great Salt Lake
Coast of New Jersey
3. What is one factor that limits the growth of algae in the ocean?
URSNNEITT
4. The process by which increased nutrients in a system causes increased algae
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