Subscribe to RSSGuest post on gulf hypoxia (GH)
Len Bahr
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
3146 TAMU
College Station, TX 77843-3146
Phone: 979-845-7211
Fax: 979-845-6331
Email: piers.chapman@tamu.edu
* 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.
NASA image of central Louisiana coast April 7 2009
6 Comments
2009-05-16
21:09:02
Actually Don, we didn't say this. Of course it is true to say that if there is no nutrient supply, there is no production, but also, without the stratification, there is no hypoxia. So respiration of organic matter is the prime "cause" of hypoxia. What we did say was that we are essentially arguing over the importance of different controls of hypoxia, and as I said above, the relative importance of the stratification becomes greater the further west you go, since this allows respiration below the pycnocline to continue for longer before oxygen can be replenished from the surface layer.
2009-05-01
21:20:25
Having reread our article in EOS, I'm still trying to see how Don manages to find support for his statement "It was Piers and his co-authors in their Eos forum who offered the statement that “nutrients, and only nutrients, control everything to do with Gulf of Mexico hypoxia,” not those accused of this dogma. " Gil Rowe and I suggested several years ago that the importance of surface stratification increases the further west you go, because there are essentially no nutrients in the surface layer in summer once you get west of Atchafalaya Bay, and nothing we have seen in numerous cruises since then has caused us to change our minds on this.
2009-05-09
14:12:16
Piers, you're right, it was not Bianchi et al., but the paper by you and and Gil that invented the straw man fallacy that nutrients, and only nutrients, control hypoxia. You reprised it again here in your guest post. My point is that I know of no scientists who ever suggested this to be the case in the first place. However, just because one can easily knock down this straw man by showing that stratification is also necessary does not mean, logically, that nutrients play no role in hypoxia farther west of the Atchafalaya. In our Eos article we address this issue: "Stratification alone is insufficient to deplete the oxygen inventories of waters beneath the density discontinuity; the consumption of oxygen faster than its resupply is also required [Hetland and DiMarco, 2007]. . . . the labile organic matter supporting benthic respiration must be derived predominantly from planktonic production either from the overlying surface water or, through advection, from highly productive waters just inshore." This surface planktonic production is clearly supported by riverine nutrients, particularly by recycled nutrients that would not be reflected in high surface concentrations.
2009-04-29
21:07:29
The Gulf of Mexico is a complicated place, but a number of excellent scientists have worked hard to unravel the causes of the Gulf hypoxia for a couple of decades now, with some of the finest efforts led by Nancy Rabalais, Don Boesch, and their colleagues. As a member of the EPA Science Advisory Board's Panel on Gulf Hypoxia, I agree totally that the solution to hypoxia is to reduce the flows of nitrogen and phosphorus down the Mississippi. Most of these nutrients come from agriculture in the upper midwest, so the solution must begin there. Unfortunately, as Pier alludes, the rapid increase in production of biofuels in the US is aggravating the problem. Models suggest that if the US meets its Congressionally mandated target for producing ethanol from corn, nitrogen loads down the Mississippi will increase by more than 30%, significantly increasing the size of the hypoxia area. For more details on this and other environmental issues with producing biofuels, see our report published earlier this month from the International SCOPE Biofuels Project, available on line at http://cip.cornell.edu/biofuels/
Bob Howarth, Chair of the International SCOPE Biofuels Project, Cornell University, Ithaca, NY
2009-04-29
16:11:48
The take-home message as I see it is that the only real way to reduce the size of the dead zone is to reduce the nitrogen and phosphorus pollution getting into the Mississippi-Atchafalaya River system. As Dr. Chapman sates, agricultural interests,and thus agricultural states have been and will continue to be very resistant to any reduction to nitrogen and phosphorus if this would have any impact on row-crop, fertilizer intensive agriculture. Because of resistance coming from upper basin states, Louisiana needs to become a real advocate for reducing the size of the dead zone. In my view, Louisiana has completely failed in this advocacy. If Louisiana, lead by our Governor, does not make this a national issue, we will continue to see massive dead zones off our coast...
2009-04-29
13:06:03
Wow, I was just fine with Piers' post up until the point he began accusing those (including, presumably, the EPA Hypoxia Advisory Panel http://epa.gov/msbasin/tfproducts.htm ) who advocate, based on the scientific evidence, action to control nutrient pollution in order to reduce hypoxia as guilty of politics and dogma. I couldn't agree more with his closing statement and would hope all parties resist the temptation to sow confusion about the causes of and solutions for hypoxia. We can see how such denialist-sown confusion has delayed action on global climate change.
From the earliest publications in the 1980s, the effects of water column stratification and winds on the severity, distribution and duration of hypoxia were well recognized. It was Piers and his co-authors in their Eos forum who offered the statement that “nutrients, and only nutrients, control everything to do with Gulf of Mexico hypoxia,” not those accused of this dogma. But, as Piers points out, there is "absolutely no doubt" that without the large nutrient input (mainly nitrogen) from the rivers there would be no hypoxia in the northern Gulf. Further, he states that because we have no hope of controlling either freshwater inflows or winds, the only practical way to reduce Gulf hypoxia is through reducing the nutrient concentrations in the Mississippi and Atchafalaya by a pretty large percentage. Amen, that's really all we so-called political dogmatists have ever said.
In our recent Eos article discussed in Len's April 9 blog, we clarified several of the scientific issues that Piers lists as unclear. The one issue that I would agree remains highly relevant to integrated environmental management of Louisiana coastal ecosystems is the effect on hypoxia of river diversions. Bill Mitch, John Day and colleagues showed in 2001 (BioScience 51: 373) that diversions through wetlands could conceivably reduce nitrogen loads by about 10%, but every little bit helps. However, diverting large flows to different parts of the inner shelf could either increase or decrease hypoxia rather considerably, depending on where the fresh water and nutrients are injected. It seem to me that this would be a very productive area of scientific discourse and assessment, rather that pointless debates about whether nutrient loads must be reduced to shrink hypoxia. At this point, this is a no brainer.