Guest post on gulf hypoxia (GH)
An earlier posting on this website (April 9, 2009) discussed two recent papers on hypoxia in the Gulf and the infamous “dead zone,” one by Don Boesch and others and a second by Tom Bianchi and his colleagues. As a member of the second group of authors, Len asked me to write something that tries to draw together what we do agree on. I thought we had tried to do this in a paper* that resulted from a meeting in New Orleans a couple of years ago, but arguments continue, mainly because of trying to equate science and politics.
First, on the science side, there is absolutely no doubt that without the large nutrient input (mainly nitrogen) from the Mississippi and Atchafalaya rivers there would be much less primary production on the shelf and thus no hypoxia in the northern Gulf of Mexico.
However, for hypoxia to occur, there needs to be also a fairly high flow of freshwater, which sits on top of the denser saltwater on the shelf and effectively insulates it from the atmosphere, a process known as stratification. We know this is true because in low-flow years (e.g., 1988, 2000), the area affected by hypoxia is also very much smaller than normal. Similarly, when a tropical storm or hurricane comes through the region immediately before the hypoxia monitoring cruise in July and stirs up the water (e.g., 2003, 2005), the hypoxic area seen during the cruise is also reduced.
What else is needed for hypoxia to occur? Basically, slow currents on the bottom, so that the organic matter produced in the surface water has time to decay once it sinks without being removed from the area adjacent to the river mouths. This occurs in summer in the Gulf because of the way the local wind system shifts from season to season; in summertime the wind is predominantly from the west, in opposition to the natural movement of the river water. So we need the combination of nutrient input, which leads to phytoplankton growth, stratification, and slow bottom currents to produce hypoxia in the region. This is the uncontroversial basic science that produces water containing low levels of oxygen, and both groups agree on this.
So what is the problem, and why is there this argument between two groups of scientists? The basic reason is the desire to manage the system and reduce hypoxia off Louisiana and Texas. The aim is to reduce the area affected each year from the present 20,000 square km to 5,000 square km, although there is no real scientific reason for choosing 5,000, rather than 3,000 or 7,000. To do this we need to control either the wind, the amount of water coming down the river, or the dissolved nutrient concentrations. Since we have no hope of controlling either of the first two, the only practical way to make any impression on Gulf of Mexico hypoxia is through reducing the nutrient concentrations in the Mississippi and Atchafalaya by a pretty large percentage.
And this is where we switch from science to politics. The agriculture industry has been fingered as the main culprit for hypoxia because of runoff from fields in the corn belt and animal feedlots along the river. (While we don’t know how much comes from sewage works, this is likely only a relatively small percentage of the total nutrient load).
The farming community has a huge impact on Congress, and is doing its best to prevent any action on nutrient reduction other than through voluntary activities by individual farmers. Congress, through its adoption of mandatory requirements for biodiesel production has only exacerbated the situation, as the corn used for biodiesel needs large amounts of nitrogen fertilizer, much of which runs off into feeder streams for the Mississippi (the economics of biodiesel are also terrible, but that’s another story).
Don Boesch, Nancy Rabalais and their colleagues have been pushing the idea of the need for nitrogen reduction as a management tool for many years, and I don’t have a problem with this. But there are plenty of legitimate scientific issues which are not clear, such as how the extent and intensity of hypoxia varies throughout the year and how quickly it can change as the wind changes, whether there is any real effect on the local fishery, how much additional carbon can be supplied by e.g., coastal estuary production and wetland erosion, how much of the organic matter can be removed via the Mississippi Canyon in near-bottom mud transport, or whether river diversions into the swamps of the delta would increase or decrease the area affected by hypoxia.
For some reason it seems impossible to have a sensible discussion on these because everything has to be subservient to the political idea that says “nutrients, and only nutrients, control everything to do with Gulf of Mexico hypoxia.” That isn’t science, it’s dogma, and it’s this confusion of science with politics that has led to the ridiculous situation where the different groups no longer talk to each other.
As Len says in the final sentence of his April 9 post: “Scientific uncertainties about GH [Gulf hypoxia] have been misused politically, primarily by some agricultural interests, to delay serious congressional action to offset or reverse GH. I am anxious that this post not be used to fuel or give comfort to the advocates of postponing action.” Confusing science with politics and arguing among ourselves only plays into the hands of the agriculture lobby and delays action.
Piers Chapman, Ph.D., FRSC, C. Chem.
Head, Department of Oceanography
Texas A&M University
College Station, TX 77843-3146
* N. N. Rabalais, R. E. Turner, B. K. Sen Gupta, D. F. Boesch, P. Chapman, and M. C. Murrell, Hypoxia in the Northern Gulf of Mexico: Does the Science Support the Plan to Reduce, Mitigate, and Control Hypoxia? Estuaries and Coasts Vol. 30, No. 5, pp. 753–772, October 2007.