Demystifying gulf hypoxia, the “dead zone”

in Technical Issues by Admin — April 9, 2009 at 8:58 am | 3 comments

summer chlorophyll a concentration along northern gulf coast

Three incidents prompted me to write this post.

First, my weekly happy hour conversation on religion, politics and science ended on a sour note last thursday. I steadfastly refused to discuss the dead zone issue with an old friend, on the grounds that the subject was too complicated to deal with in a few minutes – especially over alcohol and without a blackboard!  Since then I’ve been thinking that my personal perception of the causes, effects, solutions – and uncertainties re the dead zone may be of general interest.  

Second, I received an email from Walter Sikora, a long term coastal colleague now living in Georgia, who recounted a story about the origin of the phrase “dead zone.” Here are the relevant (expurgated) quotes from his message:

…the first time the term “Dead Zone” appeared in print with regard to the Louisiana coast was in the Morning Advocate in an article by Bob Anderson about the dead areas we encountered in (Lake) Pontchartrain in September.  The article was in the paper around Thanksgiving, either 1981 or 1982 as I recall… Anderson asked me if the areas could be described as a dead zone, and I told him that sounded like a pretty good description…

Boesch, et al. present a point of view that contrasts sharply with that of another group of equally credible coastal scientists** who also published in EOS in June 2008. The senior co-author of the latter reference is Tom Bianchi, formerly at Tulane, now at Texas A&M.  

I strongly recommend reading both the Boesch, et al. and Bianchi, et al. papers and comparing points of disagreement.

In the spirit of saving space the following text abbreviates gulf hypoxia to “GH.”  A considerable amount of time and effort has been devoted to describing GH, but typical government reports tend to gloss over complexities and uncertainties and they inevitably underplay possible connections between GH and the collapse of the Mississippi River delta.    

The standard mantra of the GH phenomenon is that the spring floodwaters of the Mississippi River spread out over the shelf, floating on the denser ocean water.  Eutrophication from excess dissolved nutrients – especially nitrogen – in the westward drifting plume of river water stimulates algal blooms along the nearshore zone.  As the rapidly proliferating surface algae die they sink to the bottom and undergo decomposition by bacteria and fungi, causing a spike in the respiration rate and consuming most of the available dissolved oxygen.  A persistent summer-long rain of this organic matter sets up a long term oxygen deficit in the bottom sediments, such that a huge bottom area that would otherwise be vital habitat for benthic and bottom feeding organisms becomes depauperate or virtually lifeless.  

EPA graphic

GH has been observed and monitored along the Continental shelf of Louisiana each summer since the early 1980′s, from the Mississippi River to Texas and beyond.  There is strong evidence that GH was occurring long before the 1980′s but these episodes seem to have been far less severe than today.  

GH has expanded in recent years, largely in lockstep with nitrogen loading by the river. The onset of severe episodes of GH began in synchrony with a global proliferation of hypoxic zones, especially in the vicinity of major river deltas.  Not surprisingly, the worldwide spread of hypoxia coincided with a dramatic global increase in the application of nitrogen fertilizer, beginning in the 1970′s.

EPA graphic

With respect to the Mississippi delta, the 1970′s time period also coincided with the peak in the breakup and inundation of Louisiana coastal wetlands, a fact that raises questions about a possible causal relationship between coastal land loss and GH. An intriguing question is whether much of the organic matter being released from disappearing marshes is helping to fuel the offshore biological oxygen demand that causes GH. 

Uncertainties and Controversies

Even after twenty-some years of study, serious scientific uncertainties surround GH and noted researchers still disagree about some fundamental assumptions re causes and consequences of GH. These uncertainties tend to be downplayed or ignored in the government documents, however. 

Unfortunately, GH has been addressed in a totally different context and under a different federal program*** than the collapse and possible restoration of the Mississippi River deltaic ecosystem. GH has been overseen by EPA with help from NOAA, the USGS and USDA, whereas the deltaic restoration program is primarily managed (at least on the federal level) by the Corps of Engineers. This has created a false dichotomy between the two issues, a problem that persists even today.

For example, reintroducing water from the Mississippi River into its delta on a massive scale has been widely described as the keystone measure for achieving a sustainable coast.  Meetings and reports on GH rarely acknowledge, let alone address this subject, however.  

A new river management paradigm implied by the concept of diverting river water into the coastal interior, rather than the open gulf, could have huge consequences on the setup conditions for GH. For example, I believe that diverting the water upstream of head of passes would result in more mixing of river water with gulf water, reducing stratification (see below). Nevertheless, different scientists disagree about the risks and ecosystem effects of diverting river water on a big scale before a significant reduction in nitrogen concentration in riverwater has been accomplished.

There are also important differences of opinion about the primary source(s) of the organic carbon that fuels GH and about the importance of such physical (oceanographic) factors as stratification. Finally, there is a continuing dialogue about evidence for direct and indirect impacts of GH damage on Louisiana fisheries production and harvest.  

Don Boesch summarizes the opinion of his team as follows:

The attached paper was published online in today’s issue of Eos.  It was produced by a group of Gulf hypoxia experts, members of the EPA Science Advisory Board panel that completed last year’s reassessment, and international experts on eutrophication and hypoxia largely to address some misconceptions in an article published last year by a group of oceanographers mainly from Texas.  This is the Bianchi et al. paper referred to in the Jim Cowan piece on your blog (LaCoastPost).  In this paper we demonstrate that organic matter resulting from coastal wetland deterioration is relatively unimportant in fueling hypoxia and that its peak preceded the development of hypoxia by more than a decade, while the expansion of hypoxia closely tracked nitrate loads.  Secondly, stratification alone cannot cause hypoxia, it also requires a source of labile organic matter to support sufficient benthic respiration—that organic material is predominantly supplied by plankton production stimulated by nutrient delivered by the rivers and concentrated along the nearshore Louisiana Coastal Current.  We reinforce the conclusions drawn by the SAB panel that “contemporary changes in the hypoxic area in the northern Gulf of Mexico are primarily related to nutrient fluxes from the MARB.”  In short, there is no reason to accept Bianchi et al.’s argument that the region west of the Atchafalaya should be excluded from the management target for hypoxia reduction because “many other physical and biogeochemical factors complicate management projections” there.  Reducing nutrient inputs to the inner continental shelf is the only management action available that would reduce the extent and severity of hypoxia.  In this regard, the northern Gulf shelf is no different that the estuaries, bays, enclosed seas and other continental shelves experiencing anthropogenic hypoxia and a management strategy based on nutrient reduction is certainly not “doomed to failure.”

With respect to the disagreements referred to in the two papers and Boesch’s supplementary comments, I offer the following editorial comment: Scientific uncertainties about GH 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. 

Len Bahr

* D. F. BOESCH, W. R. BOYNTON, L. B. CROWDER, R. J. DIAZ, R. W. HOWARTH, L. D. MEE, S. W. NIXON, N. N. R ABALAIS, R. ROSENBERG, J. G. SANDERS, D. SCAVIA, AND R. E. TURNER

** THOMAS S. BIANCHI and STEVEN F. DIMARCO, Texas A&M University, College Station; E-mail: tbianchi@tamu.edu; MEAD A. ALLISON, University of Texas at Austin; PIERS CHAPMAN, Texas A&M Univer- sity, College Station; JAMES H. COWAN JR., Louisiana State University, Baton Rouge; ROBERT D. HETLAND and JOHN W. MORSE, Texas A&M University, Col- lege Station; and GIL ROWE, Texas A&M University, Galveston.

*** Sad to say, the State of Louisiana has not shown leadership on the GH issue.

Addendum: Frank Truesdale forwarded this piece on the relationship between eating red meat and the dead zone from the Wall Street Journal.