Coastal Dead Zones: Killing coastal ecosystems worldwide

When you think of coastal ecosystems, a region of thriving life generally comes to mind. This image, however, is being slowly corrupted because of human activities. The “life” out of these zones is being sucked out, creating what is referred to as “dead zones”.

Can coasts die? In a way, the life associated with coasts, human activities dependent on coasts are directly because of the organisms that live there. Take these away, and not only does the natural ecosystem collapse, but human economic activities like fishing and tourism will also fall apart. That may not seem like a big deal, but coasts play a huge role in our economies (see here and here for more).

Dead zones: What are they and how are they caused?

Coastal dead zones are the parts of the seas and oceans that cannot and do not support any form of life. This is a popular term given to hypoxic zones; zones devoid of dissolved oxygen. Almost every living organism living in the coastal ecosystem required dissolved oxygen to survive (barring anaerobic bacteria, if any). Without this, nothing can survive.

Such zones can occur naturally over a period of time. By the process of succession, for instance, it is possible for an aquatic ecosystem to reach a stage where there is presence of organisms that put an unrealistic demand on dissolved oxygen, depleting it severely.

However, the current dead zones of the world are mostly a consequence of discharge of industrial effluents, agricultural waste water and human sewage. All three of these sources contain huge amounts of nutrients in the form of organic and inorganic compounds. The heavy influx of these nutrients (like nitrogen, phosphorus and potassium, apart from others) can promote the growth of massive amounts of microorganisms and algae. Commonly, this is called algal bloom and the process is called eutrophication.

algal bloom के लिए चित्र परिणाम
Imagine going for a swim in waters like this…

These organisms increase the demand for dissolved oxygen called Biological Oxygen Demand or BOD. Further, when they die, they are decomposed by anaerobic bacteria, further depleting oxygen levels. The result of this is that other naturally occurring organisms of that ecosystem, like fishes, corals, birds, reptiles, plants are deprived of the oxygen they need. Mobile organisms like fishes will simply swim to other places, while plants are suffocated and killed.

A slow and drawn-out death…

I say this, because the algal carpet formed on the surface of the water doesn’t instantly kill organisms. First off, they cut the availability of sunlight to the plants and corals living under water. Without sunlight, photosynthesis is impossible, and the food chain underwater slowly starves.

The algae and microbes themselves are generally photosynthetic, but that’s not good news. When they undergo photosynthesis, they take up massive amounts of nutrients and carbon dioxide, significantly altering the pH of the water body.

Dead zones are known to cause problems in spawning of fishes and the reproduction of other shellfish and reptiles of that ecosystem. This is one of the main reasons why the shrimp industry of the Gulf of Mexico. Fishes migrating into dead zones don’t know they are dead; they come in and are trapped. Most fishes are simply suffocated, unless they find a way to leave.

Dead zones around the world

A 2008 study published in Science counted 405 dead zones worldwide. These are both small, and large, ranging from a few square meters to huge areas of several square kilometers. Some of the most infamous dead zones around the world are in the the Dead Sea, Black Sea, North Sea, Gulf of Mexico, Chesapeake Bay, San Francisco Bay and the Hudson River.

coastal dead zones nasa
Source: Wikepedia

Almost every one of these have a source pointing to human activities. The Gulf of Mexico Dead zone, especially, has been subject to much study because it has been caused by discharges from the farms of the mid-west, along with the major cities along the south-west coast of America. It has had a huge impact on the economic activities of the region.

Can these dead zones be revived?

Unlike real death, dead zones of the coast can be revived and made alive again. This can be done if the reason for the dead zone is recognized and rectified immediately. This happened inadvertently in the Black Sea, after the fall of the Soviet Union. The cost of fertilizers rose so much after the fall, that its use reduced. This revived the dead zone off the coast there.

Such efforts have been taken up voluntarily in the North Sea, Rhine River, Chesapeake Bay and Hudson Bay, and has seen promising revival. Goes to show, most of our pollution is reversible, only if we tried hard enough to figure out the necessary solution.

We know that dead zones happen because of human pollution; it is time to take action before more dead zones crop up around the world.


  1. Scientific American—Coastal Dead Zones
  2. National Geographic—Dead Zone

Author: Saurab Babu

Usually found sitting with a good book, nibbling on a piece of dark chocolate. Always ready for a good story.

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