The Dead Zone: The Deepwater Horizon oil spill versus the dead zone in the northern Gulf of Mexico – which is worse?

The Dead Zone: The Deepwater Horizon oil spill versus the dead zone in the northern Gulf of Mexico – which is worse?

By By Nathaniel E. Ostrom, PRAIRIE FIRE NEWS
September 2010

Oil rigs on the Gulf of Mexico. (Ben Kamphuis)

A few weeks ago I received an e-mail message from Gabriella Belmarez, a high school student at Lamar Academy in McAllen, Texas, that inspired me to think about the oil spill in the Gulf of Mexico in a new light. Gabriella wrote that I gave her “…the impression that you felt the public should have been as concerned with the dead zone in the Gulf as with BP’s recent oil spill…”. Her insightful inquiry caused me to consider which situation is worse, the oil spill or the presence of a large body of oxygen-poor or hypoxic water that develops along the coast of the northern Gulf of Mexico each year. This “dead zone” was first observed in 1972 and has now become a regular feature that at times encompasses an area the size of the state of New Jersey. The dead zone is associated with strong declines in fish and shrimp fisheries. The ultimate cause of the dead zone is the excess delivery of nitrogen from the Mississippi River that stimulates excessive growth of phytoplankton, the plants of the ocean that respond to nitrogen in much the same way that corn responds to fertilizer. As the phytoplankton die and settle to the bottom, bacterial decay depletes oxygen resulting in hypoxic (< 2 part per million

[ppm] O2) or anoxic conditions. We now know that the source of the excess nitrogen is human activity and largely the result of fertilizers washing off of agricultural fields and entering the tributary waters of the Mississippi. Between the 1950s and mid-1990s the nitrate load in the Mississippi River increased by 300 percent. The ultimate solution is to decrease nitrogen runoff or use of fertilizers, but these are much more challenging tasks than it initially seems.

The occurrence of dead zones around the world is steadily increasing. Prior to the 1940s there were less than 25 known dead zones and now there are over 400. The number of reported dead zones has been rising since the 1960s and is currently doubling each decade. There does not appear to be an end in sight as only 4 percent of these dead zones are showing signs of recovery. Climate change is expected to make the situation worse as warm water holds less oxygen than cold water. Further, as climate warms, rainfall will increase, which means that runoff from land will increase as well and carry with it the excess nitrogen that stimulates phytoplankton growth. Climate change is also expected to increase the strength and duration of tropical-storm and hurricane winds and may introduce more oxygen by stirring the waters than would occur otherwise. A very extensive dead zone was predicted in 2005 but this was the year of hurricanes Cindy, Dennis, Katrina and Rita. What was very harmful to land was actually quite beneficial to the waters.

The Deepwater Horizon oil spill has added a new dynamic to the dead zone. The hope is that natural bacteria, perhaps enhanced by dispersants, will degrade large portions of the hydrocarbons that are being released into the Gulf of Mexico. Indeed, a recent report by the National Incident Command1 indicates that while response efforts addressed 33 percent of the spilled oil, approximately 50 percent of the residual or dispersed oil is being naturally degraded. But in so doing, oxygen will be consumed and may strengthen the development of hypoxia. With funding from the National Science Foundation’s RAPID program, Zhanfei Liu from the University of Texas at Austin’s Marine Science Institute and I proposed that this might be the case. We proposed that the presence of petroleum would block sunlight and reduce phytoplankton growth, increase oxygen consumption by bacteria and reduce the introduction of oxygen from the atmosphere. We are a few of many investigators that have been funded by this program and have been greatly assisted by ship time provided by the National Oceanic and Atmospheric Administration. While it is too soon to reach a definitive conclusion, there is no doubt that oxygen is consumed during the bacterial degradation of petroleum and preliminary data suggests lower than expected oxygen levels in some areas impacted by the spill.

While the Deepwater Horizon spill is certainly one of the worst oil spills in history, it is surprising similar to the Ixtoc 1 spill that occurred off the coast of Mexico in 1979. The events are remarkably familiar. On June 3, 1979, the Ixtoc well blew out, the blowout preventer failed, the drilling rig caught fire and sank, dispersants were extensively used and the spill was ultimately stopped by a relief well. The Ixtoc well flowed for over nine months with tarballs washing up on the beaches of Texas during that time. Both Ixtoc 1 and the Deepwater Horizon are listed among the top five oil spills in history. Yet we seem to have largely forgotten about the Ixtoc spill. Scientific research on the Ixtoc spill was not extensive, but recent inquiries have reported that shrimp catches in nearby waters were back to normal within two years. Fishermen have reported that catches improved substantially within three to five years following the spill. Not all ecosystems fared well. Oil from Ixtoc can still be found in mangroves and marshes, and oysters in some coastal areas have never recovered for reasons that are largely unknown. If we can extend these observations to the Deepwater Horizon spill, then the fisheries may recovery fairly quickly and may even be enhanced by the moratorium that has been in effect in oil-impacted waters. But we can expect oil to persist in the coastal marshes for decades to come. What the ecological impacts of oil in the coastal marshes will be is anything but certain.

The future of the marshes in Louisiana is very concerning. Louisiana has the highest rates of coastal erosion than any state and currently loses 25 square miles of wetlands each year. By 2050 an area of land equivalent to the state of Rhode Island will be lost. This has happened in part due to sea-level rise, which has recently increased from approximately 2 to 3 millimeters per year. This rise is largely a consequence of the thermal expansion of seawater in response to global warming. Estimates for continued increases in sea-level rise range anywhere between about 0.5 meters to as much as 2 meters by 2100. On top of this, the Mississippi River delta is sinking at a rate of approximately 5 millimeters per year, which has been exacerbated by hydrocarbon withdrawal. Thus it is quite likely that the collective actions of many companies withdrawing oil from the Louisiana coastline has increased the exposure of New Orleans to tropical storms and hurricanes and, perhaps, contributed to the severity of the Katrina flooding. And yet, there is no mechanism to recover the costs associated with land subsidence. If the trends of sea-level rise and subsidence continue at current rates (3 and 5 millimeters per year, respectively), then New Orleans will be exposed to the open sea by 2090. We may not need to worry about oil contamination in wetlands that might not be here in a few decades. My sincerest hope is that we will pursue means to re-create new wetland areas for the protection of New Orleans and restoration of valuable ecological habitats. A number of ideas have been proposed; now is the time to act.

2010 map showing dissolved oxygen in the Gulf of Mexico dead zone (NOAA)

To return to the question of which human-induced disaster is worse, the northern Gulf of Mexico dead zone or Deepwater Horizon oil spill, my inclination is the dead zone. I have no desire to excuse an incredibly harmful event that very likely could have been prevented with appropriate safety measures. My expectation, however, is that in 30 years we will likely have forgotten about the Deepwater Horizon oil spill as we have forgotten about Ixtoc. I am cautiously optimistic that fisheries will recover. Marshes will be heavily impacted and oil will be present in 30 years, but the marshes may succumb to a different ecological disaster: sea-level rise and land subsidence. But unless we can find a way to reduce nitrogen loading to the Mississippi River (by approximately one-half), the dead zone will still be with us in 30 years and will very likely increase in size and duration.


1. The National Incident Command report may be found at 

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