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Hypoxia: The Forming of Dead Zones

By Rosanna Zhao, V Form, Mary Hoffman, VI Form, and Claire O’Brien, VI Form

Hypoxia: The Forming of Dead Zones

Infographic: https://infograph.venngage.com/ps/l35FghyQ4DM/dead-zone-project

A “Dead Zone” is a region in the ocean in which oxygen concentrations are too low to support healthy marine life. This phenomenon of insufficient oxygen levels is known as hypoxia. Hypoxia is associated with the overabundance of algae which can lead to oxygen deficiency when they cover the water surface and disallow photosynthesis to occur within plants beneath the water. Once a vast body of water becomes hypoxic and oxygen levels drop below 2 ppm DO, a dead zone is formed.

Dead zones occur near coastal regions because the cause of formation is primarily linked with eutrophication. Eutrophication is defined by an excess of nutrient pollution in an open body of water, causing death of animal life due to a deficiency of oxygen. Although nutrients are good for fertilizing plants, nutrient pollution is detrimental to the ocean because it causes chances in the marine ecosystem, resulting in deaths of animals. During the spring and the summer, heavy rain washes nutrients containing nitrogen and phosphorus that farmers use to fertilize their land into streams and rivers. Once these nutrients flow into coastal areas, they stimulate the growth of algae.

Algae blooms, or large growths of algae, are detrimental to aquatic ecosystems when they spread across the ocean surface. The algae blocks sunlight from reaching the bottom of the ocean. Since sunlight is an important reactant of photosynthesis, the plants sitting under the ocean surface cannot undergo this process.

C6H12O6 + 6O2 + light energy → C6H12O6 + 6O2


Without the fundamental process needed for plant life, the plants underwater and drop to the bottom. As a result, bacteria reproduce rapidly due to an increase in food source as they feed on the dead plants. As aerobic bacteria decompose the waste, they use up oxygen in order to perform cellular respiration and provide energy for themselves.

C6H12O6 + 6O2 → C6H12O6 + 6O2 + ATP


Since bacteria is using up the oxygen supply and little additional oxygen is provided from the dead plants, dissolved oxygen concentration in the water decreases. Without oxygen, less marine animals inhabit the area because they need to perform cellular respiration. As the number of living organisms performing cellular respiration decrease, the amount of oxygen and glucose as a byproduct decreases as well, resulting in a smaller possibility for photosynthesis. Furthermore, only the algae at the very top of the ocean surface that can reach the sunlight is able to go through photosynthesis. As a result, the algae that is not exposed to sunlight, which also requires photosynthesis to survive, dies and sinks to the bottom of the ocean, allowing the growth of more bacteria as they feed on both the dead plants and the dead algae. As bacteria continue to use oxygen to perform cellular respiration and they decompose the waste and the bottom of the ocean and no additional oxygen is produced through photosynthesis, the oxygen level drops below 2 ppm DO. With a deficiency of oxygen, there is a lack of dissolved oxygen available for the fish and other marine animals to undergo cellular respiration, creating an unproductive aquatic ecosystem. By this state, a dead zone is formed.

Currently, the largest dead zone in the world is in the Gulf of Mexico, with a shocking area of 8,776 square miles or the size of New Jersey. This size is the largest the dead zone has ever been as it had been at an average of 5,806 square miles for the past 5 years. The sharp increase in size may be due to the development of agriculture, the extreme heavy rains, and the rapidly melting snows.


The main contribution to the growth of the dead zone in the Gulf of Mexico is the Mississippi River. The Mississippi River runs from Lake Itasca through ten states – Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana – all the way into the Gulf of Mexico. The river drains 41% of the continental United States and carries a greater amount of water than any other American river. With a significant increase in the amount of nitrogen in fertilizers since 1940, more nutrients have been washed into the Mississippi River and thus contaminating the Gulf of Mexico to worsen the dead zone.

The increase in the use of fertilizers has a direct correlation to the human population growth in the United States since 1940s. From 1940 to 2010, population in the United States more than doubled in size, beginning from 132.2 million people and growing to 308.7 million people. With more people to feed, farmers and agricultural businesses must accommodate by growing food at a faster rate, thus requiring more fertilizers to trigger more plant growth in a shorter amount of time. The fertilizers used by farmers contain nitrogen and phosphorus. Since nitrogen is a key component of the plants’ chlorophyll molecules and phosphorus is a part of the nucleic acid structure of plants, these two nutrients are needed to sustain healthy plant growth. The direct correlation between population growth and use of fertilizers containing nitrogen and phosphorus since 1940 is shown in the graph below from the United States Geological Survey.

Noting the increased use of fertilizers containing nitrogen and phosphorus, scientists have conducted research to evaluate trends of nutrient levels in marine environments in order to determine other causes of nutrient pollution. Reported by the Watershed Nutrient Task Force in a 2015 Report to Congress, a study was conducted in which nitrate levels of the Mississippi River was recorded over a period of three years. Based on the results, it can be seen that nitrate concentrations have seasonal patterns.

Looking at the peaks on the graph, it can be inferred that nitrate levels are higher in the summers, specifically during the period of June to August. This seasonal pattern of the dead zone in the Gulf of Mexico occurs due to environmental factors. During the summer, heavier precipitation and temperature levels occur in the Gulf of Mexico Seaplane Base.

Generally, this increase in amount of precipitation and temperature is present through the United States during the summer. From 2015 to 2016 in Bushland, Texas, rainfall levels during the months of July were more than five times than that during the months of autumn, measuring at 227 mm compared to an average autumn rainfall level at 42.5 mm. Coupled with heavy rains, springtime is the time of the year when farmers prepare their soils for planting; thus, they need to augment their use of fertilizers to ensure soil fertility. With increased rainfall levels and use of fertilizers, nutrients from fertilizers are washed into the Gulf of Mexico at higher rates and greater amounts, raising the nitrogen and phosphorus levels of the water. The nutrients support the growth of algae blooms in the gulf, blocking sunlight and in turn disallowing plants in the ocean to undergo photosynthesis. Referring back to the general formation of a dead zone, this phenomenon directly links to a greater depletion of oxygen concentration, causing the size of the dead zone in the Gulf of Mexico to increase.

Multiple issues have arisen from the large hypoxic zone in the Gulf of Mexico. First of all, the decreased number in shrimp population increased the price of shrimp around the United States, creating economic ripples. The Gulf of Mexico produces 72% of the shrimp in the entire United States. Shrimp live on the seafloor of coasts and estuaries all around the world; therefore, since they inhabit the water near the Gulf of a Mexico, the lack of oxygen is causing a decrease in the shrimp population. In an environment with dissolved oxygen levels below 4.0 mg/L, shrimp cannot grow normally and may die off. Currently, in the Gulf of Mexico, the dissolved oxygen level is only 2 mg/L. With a lack of oxygen in the gulf, shrimp cannot perform cellular respiration and therefore large amounts cannot survive in this region of water. As a result, shrimpers are unable to catch as many shrimp in the past years. According to the National Broadcasting Company, “Shrimpers caught more than three million pounds [of shrimp] less this past December than in Decembers past.” Furthermore, in 2016, the total catch of shrimp in Louisiana was 30% below the historic average, and the total catch in Texas was 25.9% below the average.

In addition to harming the aquatic life in the Gulf of Mexico, the nutrient runoff in the Mississippi River has also affected public health in areas across America. In Ohio, Grand Lake St. Mary’s connects to the Mississippi River, and the nitrogen and phosphorus pollution in the lake has caused illnesses in animals and even humans. In 2009, nutrients washed away into the lake by rain produced toxic algae blooms. As a result, there had been an increase in the death of fish, birds, dogs, and severe illnesses in seven people. In the United States, 22% of community water systems, which serve more than 200 million people, use surface water as their major water supply. Surface water is in streams, rivers, lakes, and reservoirs, bodies of water that are susceptible to toxic algae blooms as heavy rain can wash nutrients into them. As a result, the water that many people of this nation consume are high in nitrogen and phosphorus levels. According to the Nutrient Innovations Task Group, “high nitrate levels in drinking water have been linked to methemoglobinemia, a decrease in the oxygen-carrying capacity of red blood cells, which causes serious illness and sometimes death in infants” (DeSimone 2009). Since each nitrate molecule is composed of one nitrogen atom, the high nitrate levels have been a result of the nitrogen runoff into bodies of water. Furthermore, consumption of marine animals that have lived in water with high nitrogen concentrations can result in health issues. According to the Hypoxia Task Force, food poisoning from consumption of shellfish and ciguatera fish from the Gulf of Mexico costed “$33.9-81.6 million in public health expenditures.”

While alleviating the situation, perspectives of stakeholders must be considered as they will be the ones affected by the solution. Assuming the position of a farmer near the Mississippi River or the Gulf of Mexico, farmers cannot stop farming in order to minimize fertilization usage. Yeoman farmers must continue farming in order to support their own families, while farmers for large scale industries need to continue farming so that businesses can still provide food for customers. Farming also relies heavily on a fresh water supply, or a water supply that is not contaminated by the toxic algae blooms and bacteria in the Gulf of Mexico. In fact, agriculture takes up more than 70% of the freshwater usage world wide. As a result, 30 trillion gallons of water are utilized to irrigate 55 million acres of farmland according to the United States Department of Agriculture. Unfortunately, in recent years, over 75% of cropland in the United States experienced a severe drought, causing drought-related insurance and disaster payments to average at $4 billion per year, while it averaged at $1.3 billion three decades ago. In order to compensate for the loss of money, farmers are desperately fertilizing their crops in hopes for faster and healthier growth to produce more profit.

Recognizing the cause of this issue, a solution to solve this problem would be to implement efficient and effective drip irrigation systems on all farmland to minimize the waste of water. As a result, farmers would no longer be in debt due to the need to pay heavily for drought-related insurances and disaster payments, which causes the increased need for a surplus of fertilization. During recent years in the United States, 51% of farmland was watered using sprinkler systems, 42% was watered using flooding systems, and only 7% used drip irrigation systems. The sprinkler system and the flooding system sprinkle and flood the farmland, respectively, as their names suggest, while the drip irrigation use drip emitters that drip droplets of water instead of spraying it in large amounts. General sprinkler systems and flooding systems both use more water than drip irrigation systems since the latter is more precise and emits only droplets of water rather than sprinkles or floods. In order to decrease the amount of water used by farmers to maintain their crops, drip irrigation systems can be implemented and thus allow farmers to save money on drought-related issues or water usage.

Furthermore, the drip system minimizes the usage of water on cropland, allowing for less floods of water with a high nitrate concentration into rivers and oceans. As a result, there will be a decrease in the amount of nitrogen and phosphorus in the Gulf of Mexico, causing less algae blooms as they will not be fertilized by nutrients. Without a vast layer of algae blooms on the ocean surface blocking sunlight, photosynthesis of plants under water will continue to occur, thus creating enough oxygen to support the marine ecosystem in the gulf.

Recognizing a shortcoming of this proposal, the drip irrigation system utilizes 13% more energy than the flood system as it requires water pumps to release only droplets of water.




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