The Dead Zone

Today’s post will be a practical application of yesterday’s lesson on photosynthetic bacteria, just in case you were thinking aquatic photosynthesis was all fun and games. Each year, the northern Gulf of Mexico experiences a ‘Dead Zone,’ an area of the aquatic ecosystem depleted of oxygen where hardly any life can exist. The hypoxic zone of the Gulf of Mexico peaks in the summer and is on the order of magnitude of several thousand square miles. Many factors influence the size of the dead zone, but it is largely the fault of aquatic photosynthesizers like algae, cyanobacteria, and phytoplankton.

Surprised? Confused? Haven’t I said many times that photosynthesis produces oxygen? How do aquatic photosynthetic organisms deplete the oxygen in the system? Don’t worry, I haven’t been lying to you. This is how it works out…

Over the course of the spring and the growing season in the Mississippi River drainage basin, farmers apply fertilizers to their soils. Only some of this fertilizer is used by their crops and the rest is washed away by rains and eventually empties into the Mississippi River. Also, urban areas use the Mississippi as a waterway for releasing treated sewage. This is quite the tasty combination of nutrients for aquatic photosynthetic microorganisms. When this buffet converges on the Gulf of Mexico, the perfect conditions exist for an extreme proliferation of photosynthetic life. These conditions create another nutrient bubble for fish feeding on the photosynthetic organisms as well as bigger fish that eat those fish. All of this time, these organisms are creating waste that falls to the bottom of the Gulf which is consumed by other bacteria. These bacteria rely on oxygen to decompose this organic matter and quickly deplete the warm water on the bottom. The difference in salinity between the fresh Mississippi river water and the salty Gulf of Mexico creates an additional mixing barrier for oxygen making it easier to deplete the area for oxygen. At this point, many nutrients are used up and the whole system collapses*. Organisms either die, further contributing to the hypoxic zone caused by the decomposers at the bottom, or they leave to find areas with enough oxygen to survive. Check out this video for a visual explanation.

This year, scientists predict the dead zone could be more than 7000 square miles, one of the top ten since recording began in the 1980s. The official survey cruise is set for the end of this month, and researchers from the Louisiana Universities Marine Consortium (LUMCON)** are on top of it. Of course, hurricane storm activity before that time could definitely drop the size down to ~5000 square miles. So, you know, there’s that to possibility look forward to. Just like the drought of 2012 resulted in fewer nutrients contributing to the problem, and one of the smallest dead zones on record (map from LUMCON press release shown below), which was just under 3000 square miles. With those numbers, it’s hard to wonder why everyone was so up in arms about the drought, right? Keep up-to-date with measurements and research by LUMCON researchers on Gulf Hypoxia here.


The scope of this problem spans the fields of environmental sciences, marine biology, wildlife management, meteorology, engineering, agriculture, and hydrology. As you can probably guess from the diverse parties involved, this problem is multifaceted. Possible interventions must be balanced with the economics of agriculture in the Mississippi River drainage basin and that of the Fishing industry in the Gulf of Mexico. Both of which are no small potatoes. Experts are still trying to quantify the precise economic impacts of such a large seasonal dead zone in order to appropriately calculate the value of any upstream interventions. While hypoxic conditions exist in a vast area creating a dead zone, the jury is still out on the extent to which fisheries are taking a hit or not (there may be the same amount or more shrimp and fish in the Gulf, just not in the dead zone area).

Nonetheless, seasonal dead zone areas have to be a strain on the Gulf of Mexico ecosystem. You can be sure that scientists are collecting annual data on the Gulf’s hypoxic zone and working on new initiatives to decrease its size and nail down its ecological and economic impacts. It is not as simple as telling Midwestern farmers to stop using fertilizer; we still need them to grow lots of corn and corn takes a considerable amount of fertilizer. Likewise, introducing chemical herbicides in the Gulf to literally stem the tide of algae and cyanobacteria in the summer is associated with its own set of unexplored hazards. It will take a concerted effort on many fronts to combat the dead zone problem. Since it’s my blog and I can, I’d just like to reiterate the importance fundamental plant research can play in resolving this issue. If plant scientists can find a way to make crop plants more efficient at uptake and use of nutrients, then farmers would not have to spend so much money on extra chemicals and there would be less waste feeding into the hypoxia problem. That would be a win-win situation for both industry (agriculture and fishery) and the environment.


*It is tempting to make a parallel between another species trending toward overpopulation when there is a convenient energy-dense nutrient source available, but staving off that collapse is the topic for another day.

**LUMCON is stationed in Cocodrie, LA, which is also the site of the numerous mandatory family fishing trips of my childhood. It truly is a special place for me, and worth checking it out if you are a fisherman. If you don’t catch your own lunch, I recommend the Sportsman’s Paradise restaurant. Try the bronzed shrimp and the gumbo any day of the year and you won’t be disappointed.


5 thoughts on “The Dead Zone

  1. Pingback: Connecting the dots | New Under The Sun Blog

  2. Pingback: Visualizing Photosynthesis | New Under The Sun Blog

  3. Pingback: 5800 square miles | New Under The Sun Blog

  4. Pingback: Toxic Cyanobacteria | New Under The Sun Blog

  5. Pingback: Dangerous Photosynthesizers | New Under The Sun Blog

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