Fertilizer feeds humanity, and it's creating a gigantic dead zone in the Gulf of Mexico
The use of fertilizers by American farmers to grow crops results in an oxygen-depleted dead zone in the Gulf of Mexico that kills fish, shrimp and other marine creatures. But that's not the only environmental problem that fertilizer is responsible for. It can also pollute drinking water, become smog, and contribute to climate change. How can we stop a pollutant that we depend on to survive?
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 in August 2017 that the largest dead zone ever recorded in the Gulf of Mexico - 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, the Oregon coast, and 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, particularly 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 in the Mississippi River Valley, including Minnesota, Iowa, Illinois, Wisconsin, Missouri, Tennessee, Arkansas, Mississippi, and Louisiana. Nitrogen and phosphorus enter the river through upstream runoff from fertilizers, soil erosion, animal waste, and sewage.
In a natural system, these nutrients are not important factors in algae growth because they are depleted by plants in the soil. However, with the anthropogenic supply of nitrogen and phosphorus, the growth of algae 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 (link to USGS definition), which has been shown to reduce benthic biomass and biodiversity (link to definition). Hypoxic water supports fewer organisms and has been linked to mass 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 severely affected.
What can be done to remedy the problem?
The key to minimizing the Gulf dead zone is addressing it at the source. The solutions include:
- Use less fertilizer and adjust fertilizer application time to limit runoff of excess nutrients from farmland.
- Control of animal wastes so that they are not allowed to enter the 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 phosphorus into the Gulf of Mexico. A similar approach has been used successfully in the recovery of the Great Lakes from eutrophication.
The government is also funding efforts to restore wetlands along the Gulf Coast to naturally filter water before it enters the Gulf.