Lentis/Hypoxic Zones


Hypoxic zones (dead zones) are areas in bottom and near-bottom water with depleted oxygen levels below what is necessary to support most marine life.[1]


Hypoxic zones are caused by eutrophication, which refers to the enrichment of bodies of water by plant nutrients. This process results from nutrient pollution.[2]

Nutrient SourcesEdit

Pollution sources can be divided into point and non-point sources. Point sources include fossil fuel combustion specifically from power plants, which contributes to atmospheric deposition of nitrogen, industrial plants that release nitrogen and phosphorous in effluents, and sewage treatment processes, which release oxides of nitrogen and phosphorous effluents. Non point sources include fertilizer runoff from agriculture and direct discharge of sewage not connected to treatment plants.[3]

Some contributors of eutrophication and some associated effects of hypoxic zones.

Anthropogenic nutrient discharge sources, such as agricultural runoff, development runoff, and sewage, increase the flux of both inorganic nutrients and organic substances into terrestrial, aquatic, and coastal marine ecosystems.[4] This nutrient discharge, particularly nitrogen and phosphorous, contributes to cultural eutrophication, which is characterized by excess plant growth, particularly algal blooms. The plant growth blocks out sunlight and inhibits subaquatic plants' ability to photosynthesize in littoral zones. Additionally, predators relying on light face greater difficulty catching prey.[5] Eutrophication also leads to the destruction of organisms crucial to ecosystems, such as coral and sea grass.[6][7] Algal blooms can be toxic and decomposition may drive down pH levels in bottom waters, causing acidification. Algal blooms are temporary and eventually die out, but the consequent decomposition increases biological oxygen demand while decreasing levels of dissolved oxygen, resulting in hypoxic zones.[8]


Nutrient pollution is the source of eutrophication, but a number of other factors facilitate the process of eutrophication and the consequent development of hypoxic zones. For example, urbanization results in increased areas of impervious surfaces, which reduces infiltration and increases runoff. Roads, sidewalks, roofs, and driveways are examples of such impervious surfaces. The increased runoff facilitates delivery of nutrient pollution.[9] A number of organisms provide ecological services including the filtration of nitrogen and phosphorous, but human activity can decrease populations of such organisms. For example, oyster reefs remove nitrogen from the water column, limiting the potential for eutrophication and development of hypoxic zones.[10] In the Chesapeake Bay, the oyster population has decreased to less than 1% of what it once was. Damage done by harvest, increased disease, and falling salinity from runoff associated with urbanization have significantly contributed to population declines over several decades. Estimations indicate initial populations of oysters could filter all of the water in the Bay in a week. The current population now requires a year to filter the same amount of water.[11]

Economic EffectsEdit

Hypoxia and anoxia resulting from eutrophication damage ecosystems important to commercial and recreational fisheries. Cyanobacteria from algal blooms is toxic and results in poisoning. The estimated cost of damage done by eutrophication in the United States alone is $2.2 billion.[5] Hypoxia can lead to reproductive impairment, immunosupression, and pathogen-related mortality in aquatic organisms. Depressed immune function leads to mortality from bacterial function in shrimp and crabs. In fish, even short-term exposure can reduce bactericidal activity and antibody levels.[12] Dead zones cost United States fisheries and tourism industries $82 million each year.[13]

Norway Fishery CollapseEdit
The European lobster.

Hypoxia resulted in the collapse of the Norwegian lobster fishery.[14] Initially, an oxygen concentration decrease to 40% saturation resulted in hypoxic conditions that led lobsters to emerge from their burrows, making them more susceptible to fishing gear and consequently doubling landings.[15] The increased landings were only temporary as lobster populations diminished. The Norwegian lobster fishery, at one point supplying 24% of total landings supplied to the European market, saw its landings fall dramatically from 1960 . Many lobster dealers were forced to close down.[16]

North Carolina Brown ShrimpEdit

In terms of economic value, the North Carolina shrimp fishery is the state's second most important fishery behind the blue crab fishery. The North Carolina brown shrimp accounts for 66% of shrimp landings. Hypoxia has resulted in a 12.9% reduction in shrimp landings between 1999-2005. Producer surplus losses from hypoxia have been found to be approximately 25% of total revenue losses.[17]


The Green RevolutionEdit

Fertilizer has been used in agriculture since the beginning of farming, with ancient river valley civilizations using manure and wood ash. Modern fertilizer, however, is made using chemical processes, in particular the Haber Process and the Ostwald Process, developed in the early 1900s by Carl Bosch, Fritz Haber, and Wilhelm Ostwald.[18][19] The Haber Process produces ammonia, and then the Ostwald Process takes that ammonia and makes nitric acid, which is a main ingredient in a lot of modern fertilizers.

Shortly afterward, in about the 1930s, a worldwide movement known as the “Green Revolution” began, led by Norman Borlaug, sometimes known as “The Father of the Green Revolution” or “The Man Who Saved A Billion Lives”.[20][21] With a goal to maximize crop yield, these new fertilizers grew in popularity. While the Green Revolution ran its course through the 60s, fertilizer use continued to climb, and even afterward it kept going up as the world population continued to increase.[18] Fertilizer companies today tout their product’s abilities to make crops grow taller or fruit faster, helpful features quickly feeding the world.

Gulf of Mexico Dead Zone NASA NOAA
The Gulf of MexicoEdit

However, it was in the middle of this Green Revolution that a dead, hypoxic zone was first reported in the Gulf of Mexico, by shrimp trawlers.[22] Since hypoxic zones occur naturally as well, the size of the zone wasn’t considered large enough to cause concern. By the end of the Green Revolution in the 70s, the size of the zone had grown enough to raise alarm flags. Perhaps predictably, the increasing use of nitrous fertilizers during and after the Green Revolution coincides with the increasing presence of nitrates in the Mississippi River and Gulf of Mexico, and just as the nitrates help crops grow faster and bigger, they do the same for the algae blooms that threaten natural ecosystems.[23] The Gulf of Mexico in particular remains the largest recurring hypoxic zone in the US today.[24]

The Black SeaEdit

The results of hypoxia are not irreversible, as shown by the changes in the hypoxic zone of the Black Sea. The size of the dead zone increased dramatically following the Green Revolution, when nitrous and phosphorous fertilizer use was at its peak. But the fall of the Soviet Union in 1991 brought significant financial difficulties to the area, and using expensive fertilizers was no longer feasible. Fertilizer usage dropped off abruptly, and so did the size of the hypoxic zone. The Black Sea hypoxic zone had practically vanished by 2002, and fishing became a possible industry in that area once more.[25]

United States Regulatory AgenciesEdit

The contribution of point and nonpoint pollution from a wide area of land contributes to the development of hypoxic zones. Providing regulation over a wide area has proven to be difficult. Each region contributing to eutrophication does so on an individual basis based on many economical, agricultural, urban, and atmospheric factors. Broad scope regulation for specific practices contributing to a hypoxic zone are ineffective. To this purpose, agencies and boards like the Science Advisory Board (SAB) in the United States or the Helsinki Commission Baltic Sea Action Plan (BSAP) in the Baltics have been created at assess and advise political entities to aid in the eutrophication problem[26].

Environmental Protection AgencyEdit

The EPA which began operations in 1970 has aimed to improve public health by writing and enforcing regulations passed by Congress. The EPA's mission is in direct alignment with the management and regulation of point and nonpoint pollution produced in the United States. Therefore, the EPA was and is used to administer the Clean Water Act (CWA) to reduce water pollution[27]. The act itself is a revision of the Federal Water Pollution Control Act of 1948 mostly aimed at reallocating resources to achieve the same goal[28]. Under the CWA the government requires each state to cooperate with the EPA in the development of plans to prevent, reduce or eliminate pollution from navigable waters. Each state is also to develop and monitor its own pollution contributions and effects of pollution on the economic well being of their respective areas[29].

National Oceanic and Atmospheric AdministrationEdit

The National Oceanic and Atmospheric Administration (NOAA) is an organized agency dedicated to using science to enrich American lives through awareness of atmospheric conditions around them.[30]. They have been charged like the EPA to administer and enforce certain acts approved by congress. NOAA is the primary administer of the Coastal Zone Management Act (CZMA) of 1972. This act’s purpose is to preserve and protect coastal waters, estuarine lands, and the Great lakes. The scope of the CZMA includes appropriations and lays out a management structure for state and local entities to interact with NOAA to monitor, abate, and eliminate point and non-point pollution. This act would also help match state funding to purchase threatened coastal and estuarine lands. These lands indirectly aid in filtering eutropic waters before reaching coastal waters. [31]

Farming and Landscaping IndustriesEdit

In late 2014, Massachusetts officials tried restricting the sale and use of phosphorous fertilizers and was met with pushback from the farming and landscaping industries. Lee Corte-Real, director of the Division of Crops and Pest Services in the state Department of Agriculture, remarked “We’re trying to balance protecting the environment while allowing the agricultural industry to continue to grow food. It’s a challenge.”[32]

Current SolutionsEdit

Fertilizer Best PracticesEdit

The application of fertilizer varies widely based on geography, soil composition and climate. But over the last 40 years the agricultural industry has faced pressure by the EPA to lower N and P runoff by 45%. Because of the changing circumstances some general "best practices" when using fertilizer have been adopted. One of the most important is nutrient timing, where fertilizer is applied during a specific period of time where plant nutrient uptake is at its highest. The fertilizer must also not be applied during times of high rainfall or the nutrients will drain, hurting crop yields and N levels being discharged.[33] By using these best practices the largest amount of N is removed when compared to other methods. However, despite current efforts the goal of a 45% reduction has not been realized.[34]

Technical SolutionsEdit

Nutrient limitation and riparian buffers are used to prevent the formation of hypoxic zones. Riparian buffers are groups of low plants and grasses at the edges of water sources that help prevent runoff. [35]

Ultrasonic irradiation of blue-green algae can destroy parts of the bloom and control its growth without damaging the surrounding environment. The algae disappears in a semicircle reaching as far as the active irradiating device can. Alternatively, the algae could be filtered off the top of the water using a discfilter or drumfilter. Note that these solutions require the physical installation of a radiation or filtration device.

To avoid installing a device, clay can be scattered on the water surface. The clay sticks to the algae and causes it to sink. This is not yet permitted in the United States due to the currently unknown effect of bottom-dwelling life.[36]

Preventative solutions are also possible. Since shellfish filter nitrogen out of the water column, restoring or increasing shellfish populations can prevent or limit eutrophication. [10]


Hypoxic zones result in ecological, economic, and social damage, but no simple preventative solution exists. Due to the complexity of technical interactions that catalyze hypoxic zone-causing processes, finding a preventative solution necessitates a better understanding of the social interface of technology.

Eutrophication is a difficult problem to tackle. Lack of environmental consciousness in human development made possible the scale at which anthropogenic pollution drives eutrophication. From urban development to the destruction of plants that naturally filter nutrient runoff to the over-harvesting of shellfish, complex interactions of technology have disrupted the complex and delicate balance of ecosystems.

Hypoxic zone prevention requires first and foremost a desire for prevention. Given the economic toll that hypoxic zones take on the fishing and tourism industries, incentives to solve the problem clearly exist. But eutrophication prevention may appear to be a zero sum game. Regulations on fertilizer use can alleviate the situations faced by victims of hypoxic zones but take a toll on the agricultural industry and its ability to provide sustenance for the world's population. Regulations on emissions may be helpful, but regulating non-point sources of pollution requires agreement among individuals and societies to adopt inconvenient practices. Regulating nutrient pollution seems to be a working strategy for now, but accepting the complexity of sociotechnical interactions may be the key to ultimately solving the problem of hypoxic zones.


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