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
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
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.
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
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
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.
more information, click here
to the Spring 2002 Water Line
Brought to you by the Lakes of
Missouri Volunteer Program