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Student Activity Lesson 5
Phytoplankton blooms are sometimes visible in satellite
imagery as swaths of blue and green. Blooms like those
pictured below off the coast of Namibia (Africa, Nov 8,
2007) have caused dead zones in the Atlantic Ocean.
Photo: NOAA
Who Will Survive The Dead Zone?
A dead zone is an area that has little or no oxygen and cannot support marine life. One large
dead zone (about the size of New Jersey) exists off the Gulf of Mexico. Similar dead zones
exist in marine and fresh waters around the world (see photo below), though the one in the
Gulf of Mexico is particularly significant due to its large size. A dead zone is one example of
eutrophication, which means an ecological imbalance that occurs as a result of excess
nutrients.
It was in August 1972 that scientists
began investigating the dead zone in
the Gulf of Mexico, finding severe
oxygen depletion at shelf depths of 10-
20 meters. Many studies since then
have revealed dissolved oxygen (DO)
levels lower than the normal seawater
level of about 6 milligrams per liter
(mg/L) (See Figure 2). Marine life
needs DO to live. When conditions
become hypoxic (this means DO is less
than 2mg/L) shrimp, crabs, fish and
other marine life suffocate and die. In
some cases, there is no DO (0mg/L).
When there is no DO, conditions are
known as anoxic. As DO drops very
low, large fish and marine mammals
will quickly swim out of the area.
Several factors can cause a dead zone.
Excess nutrient input to rivers and the
ocean is a primary factor. Nutrients,
such as nitrogen and phosphorus, are
found in sources including fertilizers,
animal wastes (pets and farms) and
wastewater treatment. When it rains,
these nutrients are flushed into rivers and the ocean. In the Gulf of Mexico, the Mississippi
River is the primary source of freshwater and nutrient input. These nutrient sources can
trigger and feed large algae or phytoplankton blooms. As the algae die from these blooms
and fall to the bottom, bacteria begin to decompose them. Decomposition requires large
amounts of DO. The more phytoplankton that bloom, the greater the number of
phytoplankton that will die, and the more bacteria will use DO to decompose them. The end
result after a bloom is water with little to no oxygen. Hypoxic and anoxic conditions can
result in massive fish die-offs and long-lasting dead zones as we’ve seen in the Gulf of
Mexico.