Oceanic Dead Zones Part 1

Updated: Jan 29

Oceanic dead zones. Have you heard of them? If you are very familiar with environmental and ocean-specific issues, you may have. For many, oceanic dead zones remain a mystery.


In this two part series, we’ll explore the topic of dead zones. What are they? Where did they come from? Can we reverse this problem and, if so, how? Where do we go from here?


What is an oceanic dead zone?

A “dead zone,” a common term for hypoxia, is an area of water that is so oxygen deleted it cannot support marine life. Lack of oxygen in the water forces marine life to migrate. Fish and organisms that can leave flee the area, while clams and other bottom-feeders die because they are unable to relocate.


Dead zones can occur in any body of water. The Arabian Sea’s bottom waters, the world’s largest dead zone, remain oxygen-free year-round. The second largest dead zone is located in the Gulf of Mexico near Louisiana at the base of the Mississippi River. To gain a larger perspective of global dead zones, review Scientific American’s map below!

Source: Scientific American


What causes them?

While dead zones can occur naturally, they are often exacerbated by human activity. According to the US EPA, hypoxia can be caused by excess nitrogen that promotes algae overgrowth. Fertilizer overuse from agricultural fields, golf courses and lawns, soil erosion, and sewage treatment plants discharge runoff into rivers and streams and eventually the ocean. Nutrient pollution stimulates an overgrowth of algae which decompose in the water. The decomposition process is oxygen intense, depleting the supply of oxygen for existing marine life. Lack of oxygen turns areas that would normally be full of fish and marine life into “biological deserts” (NOAA).

Source: EPA


In addition to fertilizer overuse, burning fossil fuels emits nitrogen-oxides, better known as smog, into the atmosphere. When it rains, the nitrogen-oxides fall and wash into the ocean and are often buried underneath the sediment. While algae cannot absorb the nitrogen beneath the sediment, nitrogen is slowly released from the sand, perpetuating the cycle. Greenhouse gas emissions contribute to and help sustain the universal challenge of dead zones.


Why are they bad?

When dead zones occur, fish move further away from the coast into more oxygen-filled areas of water. Fishermen thus have to travel further out to sea to catch fish. As fishermen struggle to catch the same number of fish as previous years, the price of fish increases. Likewise, while the supply of fish decreases, the demand for fish remains about the same. Dead zones result not only in environmental losses but economic ones.


Can dead zones be reversed?

Unfortunately, very few dead zones have been recovered or reversed. In the 1990s, when the Soviet Union greatly reduced fertilizer runoff, the Black Sea rebounded. However, for many, fertilizer reduction is not a simple choice. Denitrification, while slightly effective, cannot meet the necessary requirements to make a difference. With that in mind, it is best to focus on prevention rather than reversal efforts.


Check out Part 2 where we explore the denitrification process as well as alternative solutions and treatments to responding to the global challenge of dead zone mitigation.



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