Chapman, Piers

Rowe, Gilbert T.

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Gulf of Mexico ScienceVolume 21Number 2 Number 2 Article 72003A Response to "Continental Shelf Hypoxia: SomeCompelling Answers" by Donald F. Boesch, ThisIssueGilbert T. RoweTexas A&M University, GalvestonPiers ChapmanLouisiana State UniversityDOI: 10.18785/goms.2102.07Follow this and additional works at: https://aquila.usm.edu/gomsThis Article is brought to you for free and open access by The Aquila Digital Community. It has been accepted for inclusion in Gulf of Mexico Scienceby an authorized editor of The Aquila Digital Community. For more information, please contact Joshua.Cromwell@usm.edu.Recommended CitationRowe, G. T. and P. Chapman. 2003. A Response to "Continental Shelf Hypoxia: Some Compelling Answers" by Donald F. Boesch,This Issue. Gulf of Mexico Science 21 (2).Retrieved from https://aquila.usm.edu/goms/vol21/iss2/7COMMENTARY Gulf of Ml'xiro Sdl'nre, 2003(2), pp. 205-207 © 2003 by the .Marine Environmental Sciences Consorlitlm of Alabama A RESPONSE TO "CONTINENTAL SHELF HYPOXIA: SOME COMPELLING ANSV11£RS" BY DONALD F. BOESCH, THIS ISSUE.-Boesch (2003) takes us to task for our sugges-tion (Rowe and Chapman, 2002) that physical factors on the Texas-Louisiana shelf are as im-portant as nitrate loading as a cause of hyp-oxia. It was never our intention to deny that total nitrogen is important, and we firmly be-lieve, as we stated in our article, that better management of nitrogen upstream can only benefit the system. The point of our argument was that the physical controls and especially the timing of the nitrogen delivery are equally important and must be considered when re-mediation strategies are being considered. Boesch apparently acknowledges this when he states "How humans affect the timing and lo-cation of the delivery of freshwater to the Gulf also matters a lot." However, to our knowl-edge, the effects of subsidiary peaks in flow on local stratification, and hence the extent of the hypoxic region, have not been addressed. It was this idea of physics, chemistry, and biology acting together that led us to propose our three-zone system. This schematic picture was not designed to include smaller-scale features such as the coastal current but to indicate var-iability that needs to be better defined. Simi-larly, the position of the boundary between our brown and green zones (where turbidity con-trol gives way to biological control) is not fixed-it will certainly move during the year as river discharge varies. V\Te put it at the western edge of the anticyclonic gyre that is frequently visible in satellite imagery of the region. We do believe, however, that the stratified region (the blue zone in our scheme) is a major control on the extent of hypoxia. This was borne out well in the summer of 2003, when two tropical storms in June and July stirred up the water so much that the annual hypoxia survey carried out by Rabalais and coworkers showed only about 8,000 km~ affected instead of the more than 20,000 km 2 affected in 2001 and 2002 (for the latest information see http:/ /www.cop. noaa.gov /FacLSheets/NGOMEX.html), V1Te (and several others) have recently been funded by National Oceanic and Atmospheric Administration (NOAA) to address the inter-action of physics, chemistry, and biology in controlling the extent of hypoxia on the Lou-isiana shelf. Our colleague Robert Hetland will be making use of a Regional Ocean Modeling System (ROMS) model of the region, which can resolve the physical domain on a 1-km scale and take account of both the prevailing wind forcing and the freshwater input. We be-lieve that this model will improve predictions of the extent of hypoxia and allow us to predict better the effects of changes in the amount of nitrate delivered to the system-even the au-thors of the Connnittee on the Environment and Natural Resources report on the hypoxic region have admitted that the model used in their estimates was inadequate for the job (Bre-zonik et al., 1999). In truth, both we and Boesch are essentially arguing the same points, that nitrate supply, local biological production, and physical forcing (wind and stratification) are all important in determining the extent of hypoxia in this region. \>\There we differ is in trying to establish the relative importance of each. This will change from year to year and even from nwnth to month depending on the local conditions. Of course, there is also the question of whether the goal of reducing the extent of the hypoxic region to less than 5,000 kln2 by 2015, as recommended by the :Mississip-pi River/Gulf of Mexico V\Tatershed Nutrient Task Force (2001), is actually attainable, given the excessive organic loading in the sediments close to the Louisiana coast, but this is another subject entirely. Of course, what is really im-portant is the effect of hypoxia on organisms. Although the benthos will certainly be affected by long periods of hypoxia, the overall effect on the ecosystem is unclear. The Committee on Environment and Natural Resources (2000) was unable to document any obvious effect of hypoxia on commercial catches, for example, and it must be re1nemberecl that the most pro-ductive fisheries in the world are situated in those regions suqjected to coastal upwelling, where hypoxia is an almost constant fact of life (see, e.g., Parrish et al., 1983; Crawford et al., 1987). Regardless of whether we or Boesch is correct, there are still plenty of questions to answer (such as the apparently constant respi-ration rates across the shelf mentioned in our earlier article) before we can confidently pre-dict the demise of the hypoxic zone in the Gulf of Mexico. LITERATURE CITED BOESCH, D. 2003. Continental shelf hypoxia: some compelling answers. Gulf Mex. Sci. 21 (2) :202-205. 1Rowe and Chapman: A Response to "Continental Shelf Hypoxia: Some Compelling AnswersPublished by The Aquila Digital Community, 2003COMMENTARY 207 BREZONIK, P. L., V. J. BIERMAN, R. ALEXANDER, J. AN-DERSON, J. BARKO, M. DORTCH, L. HATCH, G. L. HITCHCOCK, D. KEENEY, D. MULIA, V. SMITH, C. V\TALKER, T. WHITLEDGE, AND W. j. WISEMAN. 1999. Effects of reducing nutrient loads to surface wa-ters within the Mississippi River basin and the Gulf of Mexico: topic 4 report for the integrated as-sessment on hypoxia in the Gulf of Mexico. NOAA Coastal Ocean Program decision analysis series No. 18. NOAA Coastal Ocean Program, Silver Spring, MD. COMMITTEE ON ENVIRONMENT AND NATURAL REsOURC-ES. 2000. Integrated assessment of hypoxia in the northern Gulf of Mexico. National Science and Technology Council, Washington, DC. CRAWFORD, R. ]. M., L. V. SHANNON, AND D. E. POL-LOCK. 1987. The Benguela Ecosystem. Part IV. The major fish and invertebrate resources. Oceanogr. Mar. Bioi. Anml. Rev. 25:353-505. MISSISSIPPI RIVER/GULF OF MEXICO v\TATERSHED Nu-TIUENT TASK FORCE. 2001. Action plan for reduc-ing, mitigating and controlling hypoxia in the northern Gulf of Mexico. Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, \"''ash-ington, DC. PARIUSH, R. A., A. BAKUN, D. M. HUSBY, AND C. S. NELSON. 1983. Comparative climatology of select-ed environmental processes in relation to eastern boundary current pelagic fish production. In: Pro-ceedings of the expert consultation to examine changes in abundance and species composition of neritic fish resources; April 1983; San Jose, Costa Rica. G. D. Sharp and]. Csrike (eds.). FAO Fish. Rep. 291(3):731-777. RowE, G. T., AND P. CHAPMAN. 2002. Continental shelf hypoxia: some nagging questions. Gulf Mex. Sci. 20:155-160. GILBERT T. ROWE AND PIERS CHAP!I·IAN, (GTR) Department of Marine Biology, Texas A & M University at Galveston, Galveston, Texas 77551; and (PC) CREST Office, S C & E, Louisiana State University, Baton Rouge, Louisiana, 70803. 2Gulf of Mexico Science, Vol. 21 [2003], No. 2, Art. 7https://aquila.usm.edu/goms/vol21/iss2/7DOI: 10.18785/goms.2102.07

A Response to  Continental Shelf Hypoxia: Some Compelling Answers  by Donald F. Boesch, This Issue

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A Response to Continental Shelf Hypoxia: Some Compelling Answers by Donald F. Boesch, This Issue

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