that contains very low concentrations of oxygen, and therefore cannot support any life forms living there. Dead zones are present along many coastlines around the world, but can also develop in lakes and rivers given the right conditions.
Dead zones are typically formed in a situation by which a layer of warmer freshwater is present on top of cooler and more dense saltwater in coastal regions, where it becomes difficult for oxygen in the surface water to mix with the deeper saltwater layers. When excessive nutrients, such as nitrogen and phosphorous from agricultural fertilizers, are brought by the flow of rivers and streams to the freshwater layer, algae that is present in the water will reproduce at a very high rate, leading to an “algal bloom.”
When all of this excess algae dies, it decompose after sinking into the denser saltwater layer below, using up all of the oxygen. With little oxygen remaining in the lower water layer, no organisms are able to live there, and they either move away to a more hospitable environment, or die. Thus, a “dead zone” is created where no organisms can live anymore.
Although nitrogen and phosphorous from agricultural fertilizer runoff are the primary causes of dead zones around the world, sewage, automobile and industrial emissions, and some natural factors can also cause dead zones to develop. Essentially all that is needed for dead zones to form is for there to be an excess of nutrients in the water that algae can feed from, reproduce, and then use up all of the oxygen present in the water, leading to the die off of aquatic life.
The size of a particular dead zone varies from year to year, depending on a number of factors, including drought, flooding, sun, wind, amount of rain, riverflow, saltwater/freshwater ratios, and temperature. All of these factors impact the amount of nutrients that are carried into coastal areas and other water bodies where algal blooms occur, forming dead zones.
It has been estimated that there are more than 400 dead zones around the world, including in North and South America, China, Japan, and Australia. One of the most well-known dead zones is the U.S. dead zone in the Gulf of Mexico that spans approximately 8, 500 miles2, and receives excessive nutrients from a network of rivers throughout much of the Continental United States by way of the Mississippi River basin¹.
The impacts of dead zones
In addition to killing off all of the aquatic life in a dead zone, the ecology of an entire foodweb and ecosystem of a region might be decimated, because prey species that other species depend on for food have likely been severely depleted. Local human communities and economies that depend on fishing for their livelihoods can also be negatively impacted by dead zones.
Some possible solutions for dead zones
While dead zones are a serious problem, they can be reversed if their causes are reduced or eliminated entirely. Industrial and sewage emissions reductions and their sustainable treatment will lead to a reduction in nitrogen levels present in freshwater bodies. Sustainable farming practices that restore the soil and reduce or eliminate the need to till and use chemical fertilizers can go a long way in reducing excessive nutrients in runoff. Wetland ecosystems can also be restored along rivers, lakes, streams, and coasts that help to capture and filter out pollutants that would otherwise end up in water bodies and reach coastlines.
As with the other environmental issues of our time, solutions to the problem of dead zones lie in a re-examination of our relationship to nature and how we treat and use natural resources. The old viewpoint of “out of sight, out of mind” with our pollution and waste is no longer acceptable. There truly is no “away” when it comes to where our waste goes. It will always impact something or someone else downstream.