The Gulf of Mexico is dead. Well, part of it is anyway. The dead part was as big as New Jersey in 1999 and is as big as Massachusetts this year; over 8,000 square miles. It is a hypoxic region, also called a “dead zone”, so-called because the lack of oxygen within its borders has rendered it nearly devoid of aquatic life. And it’s all thanks to nutrient input from the Mississippi River basin. That’s us.

Oceans are typically nitrogen limited. This means that algal growth (the base of aquatic food chains) is limited by the amount of nitrogen in the water. Add more nitrogen and you get increased algal growth. If you were to add phosphorus, you would expect little to no response by the algae. By contrast, most Missouri lakes are phosphorus limited, Thus, algal growth in our lakes responds to increases in phosphorus concentration.
Gulf hypoxia is caused by the settling out and subsequent decomposition of organic matter in the water. The density differences cause the water to stratify, with the nutrient-rich and warmer river water above the colder, salty seawater. The nutrients fuel algal growth in the top layer. Dead and dying algae sink through to the more dense, saltier layer at the bottom and decompose. The decomposition consumes oxygen, which can’t be replaced in the deep layer, due to the stratification. In order for the stratification to be removed, a major storm event is required (e.g. tropical storm or hurricane). Then the water layers will mix, and the hypoxic zone will dissipate temporarily.

Much of the river’s natural meanderings have been removed, leaving well engineered, navigable channels. Wetland draining and channelization have left river water with fewer opportunities to shed nutrients through plant uptake and bottom deposition.
According to the USGS, 56% of the nitrogen entering the Gulf of Mexico comes directly from agricultural runoff.

The Mississippi River drains 40% of the US, or 12 ½ % of the continent of North America. On this very land lie 52% of the farms of the United States. These farms generate $98 billion in revenue each year.
Figuring out how to reduce nutrient inputs is a bit like balancing a budget. To communicate issues to a great number of people, environmental concerns must often be converted into monetary ones. Cost-benefit type analyses must be performed to determine the importance of the variables involved. Is the effect on the fisheries in the Gulf worth the overall cost of nutrient reduction in the Midwest?
Unfortunately, the benefits of a healthy ecosystem are difficult to quantify. How much would you pay each year to protect aquatic life hundreds of miles away? How much revenue will be lost if bottom-dwelling sponges die? What is the current market value of “clear water”? There is no correct answer, and yet these questions must be addressed.

It is quite likely that the Gulf of Mexico’s $690 million fishery will suffer greatly if the input of nutrients, specifically nitrogen, is not reduced. Some have argued that Gulf fisheries have benefited from the “dead zone”, as it has driven shellfish toward shore as they seek oxygen. This leads to “jubilee” events where harvesting is easier. Also, after fleeing the oxygen deprived waters, fish may congregate in high concentrations at the borders of the “dead zone”. This can lead to better fishing...for a short while.
As migration paths and spawning habitats are overtaken by hypoxic water, animals (and the humans who depend on them) can suffer horribly. Hypoxia has led to the collapse of several fisheries worldwide. Commercial bottom fishing simply doesn’t exist anymore in regions of the Black Sea, the Baltic Sea, Sweden, and Denmark, as their “dead zones” continue to grow.

Whatever course of action is chosen, recovery will be slow. Decomposition of organic matter in the sediments will continue to deplete oxygen, even after nutrient input to the Gulf is reduced. It could take years before results are seen from remediation efforts.

Tony Thorpe

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