Satellite image of the Gulf of Mexico. Light colored water samples, often nutrient-rich sediments flowing into deeper ocean water. These nutrients facilitate the growth of phytoplankton blooms, which can lead to hypoxic conditions.
What is the Dead Zone of the Gulf of Mexico?
The Gulf of Mexico Dead Zone is an area of hypoxic waters (link to USGS definition) (less than 2 ppm dissolved oxygen) at the mouth of the Mississippi River. Its area varies in size, but it can cover up to 6,000-7,000 square miles. The zone lies between the inner and middle continental shelf in the northern Gulf of Mexico, beginning in the Mississippi River delta and extending west to the upper Texas coast.
National Geographic News reported the largest dead zone ever recorded in the Gulf of Mexico in August 2017 - the New Jersey-sized dead zone is the largest in the Gulf of Mexico.
Where are the dead zones?
Dead zones can be found all over the world (link to NASA's dead zones page). The dead zone in the Gulf of Mexico is one of the largest in the world. Marine dead zones can be found in the Baltic Sea, the Black Sea, off the coast of Oregon, and in the Chesapeake Bay. Dead zones can also be found in lakes, such as Lake Erie.
What causes the dead zone?
The dead zone is caused by the enrichment of nutrients from the Mississippi River, especially nitrogen and phosphorus. Watersheds within the Mississippi River Basin drain much of the United States, from Montana to Pennsylvania and extend south along the Mississippi River. Most of the nitrogen input comes from the major agricultural states of the Mississippi River Valley, such as Minnesota, Iowa, Illinois, Wisconsin, Missouri, Tennessee, Arkansas, Mississippi, and Louisiana.
Nitrogen and phosphorus enter the river through fertilizer runoff, soil erosion, animal waste, and sewage. In a natural system, these nutrients are not significant factors in algae growth because they are depleted in the soil by plants. However, with the anthropogenic increase in nitrogen and phosphorus, algae growth is no longer limited. As a result, algal blooms develop, the food chain is disrupted, and dissolved oxygen in the area is depleted. The size of the dead zone fluctuates seasonally, as it is exacerbated by agricultural practices. It is also affected by weather events such as floods and hurricanes.
What are the effects?
Nutrient overload and algal blooms lead to eutrophication, which has been shown to reduce benthic biomass and biodiversity (link to definition). Hypoxic water supports fewer organisms and has been linked to massive fish kills in the Black Sea and the Gulf of Mexico.
The Gulf of Mexico is an important source area for the fishing industry. The Gulf supplies 72% of the shrimp harvested in the US, 66% of the oysters harvested, and 16% of the commercial fish (Potassium and Phosphate Institutes of the US and Canada, 1999). Consequently, if the hypoxic zone continues or worsens, fishermen and the economies of coastal states will be greatly impacted.
What can be done to fix the problem?
The key to minimizing the Gulf Dead Zone is to address it at the source. The solutions include:
- Use less fertilizer and adjust the timing of fertilizer applications to limit runoff of excess nutrients from farmland
- Control of animal waste so that they are not allowed to enter waterways.
- Monitoring of septic systems and wastewater treatment facilities to reduce the discharge of nutrients to surface and groundwater
- Careful industrial practices, such as limiting the discharge of nutrients, organic matter, and chemicals from manufacturing facilities
These solutions are relatively simple to implement and would significantly reduce the entry of nitrogen and phosphorous into the Gulf of Mexico. A similar approach has been used with success in recovering from the eutrophication of the Great Lakes.
The government is also funding efforts to restore wetlands along the Gulf Coast to filter the water naturally before it enters the Gulf.
Created by Monica Bruckner, Montana State University