Activity #6: Gulf of Mexico - Dead Zone

How Do Dead Zones Form?

Dead zones are a result of excess nutrients in the water. Once in the water, nutrients help small plant-like organisms called phytoplankton to grow. Excess nutrients cause phytoplankton populations to increase quickly. While these organisms are a very important part of the ecosystem, too many of them can cause problems. As the phytoplankton die and sink, they feed the bacteria on the bottom of the ocean. Bacteria, like most organisms, use oxygen. When the phytoplankton population increases, the bacteria population also increases. If the bacteria population gets big enough, there is little or no oxygen left for other organisms.

Dead zones were first noticed in the early 1900s. Starting in 1985, researchers began gathering samples in the area where water flows out of the Mississippi River into the Gulf of Mexico (called the Mississippi Delta). This gave convincing evidence that the Gulf of Mexico contained very high amounts of nutrients.

The data collected by scientists studying the area showed that the excess nutrients in the Gulf of Mexico came from human activities, such as the use of fertilizer and fuel. Fertilizer contains nutrients like nitrogen and phosphorus, which plants need to grow. Farmers apply fertilizer to crops to help them grow. The fertilizer not absorbed by plants remains in the soil. When it rains, the nutrients are carried away by the water into nearby lakes, rivers, and oceans. This effect is increased if the fertilized soil is also washed away. Dead zones form where nutrient-rich river water enters a lake or ocean and slows down. When the faster-flowing water slows down, the nutrients tend to drop out of the water and become concentrated near the shore.

Where Do the Nutrients Come From?

The Mississippi River has a huge river basin, which is an area of land where water drains downhill on the surface or through the ground into a river or series of rivers. The Mississippi drains water from 41% of the land in the United States. The waterways in the basin are all connected. This means that fertilizer put on fields in Minnesota can travel all the way to the Mississippi Delta. This also means that there is no single source of pollution causing the Gulf of Mexico dead zone. As water runs downhill toward the ocean, it picks up nutrients from the soil along the way. The nutrients in the Mississippi Delta can come from 31 different states!

Given the vast size of the basin, it is not surprising that the dead zone at the mouth of the Mississippi can also be vast. It has been as large as 22,730 km² (8,776 miles²). This dead zone is seasonal, growing in size during the warmer months of the year when bacteria can grow rapidly. The recent average size is about 14,150 km² (5,500 miles²), about the size of the state of Connecticut. The size of the dead zone has grown over the years. This correlates with an increase in the amount of nitrogen in streams that drain into the Mississippi Basin. This amount has nearly tripled since the late 1950s.

Since agriculture covers nearly one-half of the continental United States, it is not surprising that much of the nitrogen in the Gulf of Mexico comes from farmland. However, there are other sources, such as the burning of fossil fuels, storm water, runoff, wastewater, and some home products. This graph (on pg. 29) shows the sources of two kinds of nutrient pollution: nitrogen and phosphorus.

Dead zones have occurred naturally in certain environments throughout time. Any condition that results in lower oxygen levels can result in a dead zone. For example, the dead zone in the lower part of the Black Sea in northern Europe is due to a natural lack of mixing of water rich in oxygen with water poor in oxygen. This area of the Black Sea is a permanent dead zone.

However, over the past several decades, the number of dead zones throughout the world has increased in size and number. Currently there are over 500 dead zones in the world. The recent increase is a direct result of human activities. Rising populations coupled with advances in science and technology have resulted in a large increase in the release of nitrogen and phosphorus into the environment. These nutrients have entered our air, soil, and water mainly through agricultural and industrial activities. Human activities add almost twice as much nitrogen and three times as much phosphorus as natural emissions.

What Is the Impact?

Drinking or swimming in nutrient-rich water can cause serious health problems for animals and humans. Aquatic ecosystems have changed as a result of dead zones. Areas with dead zones have a lower diversity of organisms, smaller populations, and smaller-sized organisms compared with well-oxygenated water. Some organisms that have been heavily impacted are shrimp, eels, and crabs. Ecologists still do not fully understand the consequences of a dead zone over a long time period.

Nutrient pollution also has an impact on people’s jobs. Because of many factors, including nutrient pollution, some fisheries are already overfished or in danger of becoming overfished. The commercial fishing industry in the Gulf of Mexico provides millions of jobs and catches billions of pounds of finfish and shellfish each year. The economic gain of this industry has been negatively impacted by steady and significant declines in the aquatic population that has led to smaller fishing catches.

What Can We Do?

There is evidence that dead zones can be reversed. After decades of nutrient inflow from commercial farming, the northwestern Black Sea developed a large dead zone. In the late 1980s, the economy of the area collapsed, commercial farming decreased, and nutrient runoff dropped by 50 percent. Over the next several years, the water quality improved. Since then, careful management and continued reductions in emissions have helped the Black Sea recover. Fish populations have now increased to high enough levels that fishing has again become part of the region’s economy.

While commercial practices cause the most nutrient pollution, our everyday actions can make a difference. We can help by reducing our use of fertilizers and detergents and properly disposing of yard and pet waste. Other ways to help combat nutrient pollution is to reduce our consumption of fossil fuels by using public transportation, using more energy-efficient vehicles, and reducing home energy use through more efficient heating, cooling, and lighting systems.

Broader things that communities can do are to manage land and water sensibly. This could include encouraging development that allows people to walk or take public transportation to jobs and shopping. Better engineering and landscape design can also help. When water runs off across hard surfaces, like parking lots, rooftops, sidewalks, and roads, it often carries pollutants down storm drains and into local waterways. Making sure that the runoff gets properly treated and replacing paved surfaces with materials that allow water to drain (such as small stones) or absorb the water (such as plants) can help keep nutrients out of our streams, lakes, and oceans. Restoration of wetlands can also help. Wetlands are a natural filter that can remove contaminants from water, but as populations have grown around the world, many wetlands have been neglected or destroyed.