What Causes a Dead Zone

Dead zones occur in fresh and saltwater coastal areas around the nation with no part of the country or the world being immune to this environmental phenomenon. What causes a Dead Zone?

Dead Zones are caused by an increase of nutrients mostly nitrogen & phosphorous in agricultural fertilizers & sewage runoff with natural factors playing a role in the development of excessive algae blooms that block out the sun & deplete underwater oxygen levels (eutrophication) killing aquatic life.

Algae blooms can smell bad, block sunlight, and even release toxins in some cases. When the algae die, they are decomposed by bacteria—this process consumes the oxygen dissolved in the water and is needed by fish and other aquatic life to breathe. If enough oxygen is removed, the water can become hypoxic, where there is not enough oxygen to sustain life, creating a “dead zone”.


What Causes a Dead Zone


Dead zones are low-oxygen or hypoxic areas in oceans and lakes of the world. It’s because most organisms on earth need oxygen on which to live, and few organisms can survive in these low-oxygen conditions. This is the reason these areas are called dead zones. 

Dead zones happen because of a process called eutrophication, which happens when a body of water holds an abundance of nutrients such as phosphorus and nitrogen. At normal levels, these nutrients or chemical elements are used to feed the growth of naturally occurring microscopic organisms called cyanobacteria better known as blue-green algae.

When too many nutrients are involved in oceans, rivers, and lakes to small streams where cyanobacteria’s population grows out of control, which ends up being detrimental to the water they live in. Human activities are the main cause of these excess nutrients being washed into the body of water.

According to the EPA-Dead zones happen throughout the world but are more often located near inhabited areas of coastlines. The presence of excess nutrients that happen in the air and water can affect human health, the environment, and the economy. Federal, state, and local governments spend billions of dollars per year to minimize these effects.


What is Eutrophication


Like all living organisms on earth nutrients like phosphorous and nitrogen are building blocks and are needed to sustain life as the most substantial food source in the simplest form in order for plants and organisms to exist and grow. These nutrients are readily available and occur naturally in nature.

Unfortunately, an overabundance of nutrients in a body of water can have harmful health and negative effects on the aquatic life in that environment. The overabundance of nutrients primarily nitrogen and phosphorus in water start a process called eutrophication.

Algae that feed on nutrients grow and will suddenly appear on the surface of the water, decreasing the appearance and health. Decaying mats of dead algae brought on by bacteria can produce foul tastes and odors in the water consuming dissolved oxygen from the water, and the end results in fish kills. 

Human activities can accelerate eutrophication by increasing the uprising rate at which nutrients enter the water. Dead Zones are hypoxia areas in an ecosystem that occurs when dissolved oxygen concentration falls to or below 2 mg of O₂/liter. 

Eutrophic events in the world have increased over the years because of the rapid use of intensive agricultural practices, industrial activities, and population growth. These three processes emit large amounts of nitrogen and phosphorous. These nutrients can enter our air, soil, and water near the source of their emission.


Happening Now: Dead Zone in the Gulf 2020 - YouTube


Dead Zones in Water


Dead zones in water are areas of water bodies where aquatic life cannot survive because of low oxygen levels. Dead zones are generally caused by excessive nutrient pollution and are a primarily huge problem for bays, lakes, and coastal waters since they receive excess nutrients from upstream sources where the human population is located.

The overgrowth of algae blooms uses up available oxygen while blocking needed sunlight for underwater plants. When the algae eventually die, the oxygen in the water is depleted. The lack of oxygen makes it impossible for aquatic life to survive in water.

Some types of algae blooms are large and produce chemicals or toxins in the body of water they inhabit for any length of time. The event is called a harmful algal bloom. This algal bloom threatens large areas of a coastline and even closes it down to the general public. Harmful algal blooms or (HABs) can occur in lakes, reservoirs, rivers, ponds, bays, and coastal waters throughout the world.

The Chesapeake Bay experiences dead zones every year. Runoff from industrial and agricultural activities contributes to the dead zones in the Chesapeake Bay, an estuary shared by Maryland, Virginia, Delaware, and Washington, D.C. A 2008 study found more than 400 dead zones worldwide, including in South America, China, Japan, southeast Australia and elsewhere.

Elevated nutrient levels along algal blooms can also cause problems in drinking water for communities nearby and upstream from dead zones. Harmful algal blooms release toxins that contaminate drinking water, causing illnesses for animals and humans.


What is a Dead Zone in the Ocean


Dead zones are not just unique to freshwater rivers, streams lakes, and ponds. Dead Zones are also prevalent in areas of deep ocean saltwater worldwide. The causes of Dead zones are the same as in the smallest pond or stream that also exposes plankton to an excess of chemical nutrients like nitrogen and phosphorus. The main causes are of this event are human population.

An algal bloom forms in ocean water the algae population quickly increases in density. As the exploding population of algae dies, a large amount of dead organic matter is created, which becomes food for more bacteria growth. The end result is robbing oxygen from ocean waters that are needed by all aquatic life in the area.

Scientists have known about Dead Zones in the oceans but with new technical instruments and record-keeping researchers have discovered that Dead Zones are more prevalent than what was suspected. About 10x more than what was suspected. 

Usually, Dead Zones in deep water oceans occur in warmer and more acidic waters that had deadly effects on coral reefs, crabs, sponges, and sea urchins but more reduced controls on pollution, agricultural runoff, and sewage disposal could offset the events.


How Can Wastewater Be Reused?

Wastewater can be Reused first by:

  • Harvesting the Graywater with a process called Phytoremediation Treatment that uses plants to naturally purify sewage
  • Standard treatments-Primary-Secondary & Tertiarily
  • Microfiltration
  • Reverse Osmosis-(RO)
  • UV light
  • High Heat Systems that purify water & generate energy ………………………………………………………………………………………………………… Read more



What is Water Availability?

Water availability is the quantity of water that can be used for human purposes without significant harm to ecosystems or other users. Consideration is given to demands from human and ecosystem needs, equitable apportionment of water among uses, and indicators of stress to the water resource ……………………………………………………………………… Read more

Gulf of Mexico Dead Zone


The biggest environmental hazard threatening aquatic life in the Gulf of Mexico is not an oil spill but is considered the largest Dead Zone in the US and is located at the mouth of the Mississippi River.

Scientists say the Dead Zone is directly the cause of fertilizer and sewage that drain from farmland into the gulf and then onward to the sea. It then feeds algal blooms that pull oxygen out of the water destroying sea life. 

Once the excess nutrients reach the Gulf they stimulate an overgrowth of algae, which eventually die, then sink and decompose in the water. This event has long-term impacts on living marine resources that are unable to leave the area.

The resulting depleted oxygen levels near the bottom are insufficient to support most marine life and have long-term impacts on living marine resources that are unable to leave the area. Considered one of the world’s largest, the Gulf of Mexico dead zone occurs every summer.

In the Gulf of Mexico, the changes underwater from the lack of human-caused Dead Zones are astounding as green turns to black as light from above can no longer penetrate water. The Gulf is fast becoming depleted of oxygen ruining the seafood industry and the void stretches over a vast area that is the Gulf of Mexico.


Scientists this year are predicting the Dead Zone in the Gulf of Mexico to decrease in its size, nevertheless, at 6,700 square miles, this year’s Dead Zone is still larger than the State of Connecticut. Also, other good news is that a severe hurricane season could help mix ocean waters, reducing the size of the dead zone and the algal blooms that are depriving the waters in the area.



What is Dissolved Oxygen in Water?

Dissolved oxygen in H2O (DO) is the amount of oxygen available to all living aquatic organisms necessary to survive, living in natural waters, usually measured in parts per million (ppm) used as an indicator of the health of lakes/streams.
5 ppm & higher-healthy
Below 3 ppm-concern
Below 1 ppm-hypoxic …………………………………………………………………………………………………………………….. Read more



JimGalloway Author/Editor



EPA-The Effects of Dead Zones and Harmful Algal Bloom

USGS-Nutrients and Eutrophication



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