Imagine living in an area where suddenly the oxygen level in the surrounding atmosphere drops so low that it cannot sustain life. Breathing is next to impossible, other animals and inhabitants that are able to flee from the area, seeking better conditions. There are some however that cannot leave. Ultimately, those left behind are unable to survive the low oxygen and will die.
While a concept that seems highly unlikely if near impossible in a terrestrial environment, low-oxygen, or hypoxic conditions, can occur easily in marine habitats. The dissolved oxygen content of water is measured in parts per million. Water is classified as being hypoxic when it's dissolved oxygen content is less the 2 parts per million.
There are two classifications of marine hypoxia, named after the over-arching drivers that contribute to the condition. These are human induced and natural hypoxic zones. Hypoxic zones are often referred to as "dead zones".
Both kinds of hypoxia have similar results, and that is the decline of overall biodiversity and in many cases the death of a majority of animals in the area because of the low oxygen content. The more motile species tend to leave the area, but those who are sessile are at the most risk since they can't escape.
Natural Hypoxic Zones: Created by a Breakdown in Upwelling
Naturally occurring hypoxic zones are influenced mostly by the variations in upwelling that is driven by wind. Upwelling is a process of deep ocean water that is nutrient rich moving from the bottom of the ocean up to the near-shore surface. These nutrient rich waters are what drive the ultra productive waters most notably on the west coast of North America and South America.
Problems arise when those winds die down or subside and the nutrient flow from deep water stops. Much of the plankton life that was blooming will then begin to die-off. Bacteria and other decomposers kick into high drive and deplete the oxygen content from the water. Conditions like these can be reversed if the winds pick up again and upwelling reinstates itself.
This kind of natural hypoxia has been a problem on the Oregon Coast, especially in the last few years.
Hypoxia can also happen in fjords or lakes, where the turnover of water to increase oxygen content is very slow or not existent.
Human Induced Hypoxia: The Result of River Runoff
Human induced hypoxia is most notable in the Gulf of Mexico, an area that is highly influenced by runoff from the Mississippi River.
Because of its massive delta, the river dumps a heavy amount of runoff into the Gulf. Much of this runoff is excess nutrients from fertilizers used in the farmlands. When those fertilizers hit the water, they create a cycle that is much the same as that seen in the natural zones where the nutrients are input from upwelling.
The hypoxic state is perpetuated when there is sufficient die-off from the blooms created by the excess nutrients to sustain the decomposer bacteria that suck up the oxygen in the water.
The major difference between the two is that while the natural cycle can come and go, the situation in the Gulf will continue as long as the nutrients continue to be in excess of the normal levels because of input from the land.
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