Mechanisms Controlling Hypoxia – Integrated Causal Modeling of the Oceanographic Processes that Cause the Dead Zone in the Northern Gulf of Mexico
Project Status: This project began in January 2009 and was completed in December 2014
We’re examining the complex physical and biogeochemical relations that control and maintain the low-oxygen dead zone in the northern Gulf of Mexico (nGOM). We are combining field data from moored observatories, shipboard samples, satellite observations, and autonomous underwater vehicle samples with models to understand the interaction of physical, biological, and geochemical processes controlling hypoxia. Our early results show that the causes of hypoxia vary depending on the location.
Why We Care
The dead zone of low oxygen in the northern Gulf of Mexico (nGOM) occurring from Texas to Louisiana is one of the most intractable environmental issues facing the United States. Far-reaching corrective measures that have taken place since hypoxia developed in the early-1980s have been unsuccessful. Year after year, the dead zone varies in size but persists, seriously degrading the ecological health of the region. Examining the interplay and variability of the physical, biological, and geochemical processes occurring throughout the nGOM shelf are crucial to deepen our understanding of the complex mechanisms controlling the hypoxia dead zone.
What We Are Doing
Our current objective builds on previous work that yielded our proven, realistic, coupled hydrodynamic-biological-geochemical-sediment numerical model developed for the nGOM. The model can resolve the dominant oceanographic processes that control the timing, duration, and severity of hypoxia of the region. For this project, new observations and modeling activities will be coordinated with existing federal and state sampling programs and ship cruises of opportunity. The new data will permit us to make advances in wind, river discharge, and currents affecting freshwater plume dispersal; vertical mixing processes; stratification over the Texas-Louisiana shelf; biogeochemical and transport processes affecting the load of biologically available nutrients and organic matter to the Gulf of Mexico; the role of phosphorus relative to nitrogen in regulating phytoplankton production; and the linkages between inshore primary productivity, offshore production, and the fate of the carbon produced. To achieve these objectives, we will expand on our integrated multidisciplinary approach by utilizing a combination of moorings, detailed process-oriented surveys, remote sensing observations, and enhanced realistic coupled numerical modeling.
This work is part of the Gulf of Mexico Ecosystems & Hypoxia Assessment (NGOMEX) program. The project team is led by Dr. Steven DiMarco of Texas A&M University with co-investigations from thirteen other investigators from Texas A&M University, Louisiana Universities Marine Consortium, Virginia Institute of Marine Science, Louisiana State University, Coastal Carolina University, and Dalhousie University.
What We Are Finding
We are finding that the causes of hypoxia in the nGOM vary depending on the location. Close to the Mississippi River Delta, the mechanisms that maintain and sustain the hypoxia are mostly driven by biological processes (e.g., nutrients and phytoplankton growth). Further downstream (farther offshore and westward toward Texas), the dominant controlling processes are physical (e.g., currents and winds). Currents and winds in the regions farther from the Mississippi River combine to break down the vertical stratification necessary to sustain the low dissolved oxygen. These factors produce two distinct hypoxic regions:
An eastern region of the shelf, between 91°W and 89°W that is almost always hypoxic in mid-summer.
A variable western region between 91°W, the Louisiana-Texas border and westward toward Brownsville, Texas.
The variable westward region largely controls the total size of the nGOM hypoxic area in a given year.
Regions of Study: Gulf of Mexico, Louisiana, Texas
Primary Contact: David Scheurer
Related NCCOS Center: CSCOR
- DiMarco, S.F., J. Strauss, N. May, R.L. Mullins-Perry, E. Grossman, and D. Shormann, 2012. Texas Coastal Hypoxia Linked to Brazos River Discharge as Revealed by Oxygen Isotopes. Aquatic Geochemistry 18(2):159-181. doi:10.1007/s10498-011-9156-x.
- Feng, Y., S.F. DiMarco, and G.A. Jackson. 2012. Relative role of wind forcing and riverine nutrient input on the extent of hypoxia in the northern Gulf of Mexico. Geophysical Research Letters 39: L09601. doi:10.1029/2012GL051192.
- Fennel, K., R. Hetland, Y. Feng, and S. DiMarco. 2011. A coupled physical-biological model of the Northern Gulf of Mexico shelf: model description, validation and analysis of phytoplankton variability.
Biogeosciences 8:1881-1899. doi:10.5194/bg-8-1881-2011.
- Forrest, D., R. Hetland, and S.F. DiMarco. 2011. Multivariable statistical regression models of the areal extent of hypoxia over the Texas–Louisiana continental shelf. Environmental Research Letters 6 045002. doi:10.1088/1748-9326/6/4/045002.
- Hetland, R.D. and S.F. DiMarco. 2012. Skill assessment of a hydrodynamic model of circulation over the Texas- Louisiana continental shelf. Ocean Modelling 43-44:64-76. doi:10.1016/j.ocemod.2011.11.009.
- Laurent, A., K. Fennel, J. Hu, and R. Hetland. 2012. Simulating the effects of phosphorus limitation in the Mississippi and Atchafalaya River plumes. Biogeosciences 9:4707-4723. doi:10.5194/bg-9-4707-2012.
- Li, X., T.S. Bianchi, Z. Yang, L.E. Osterman, M.A. Allison, S.F. DiMarco, and G. Yang, 2011. Historical trends of hypoxia in Changjiang River estuary: Applications of chemical biomarkers and microfossils. Journal of Marine Systems 86(3-4):57-68.
- Mullins-Perry R.L., S.F. DiMarco, X. Zhang, and N.L. Guinasso, Jr. 2011. Interdisciplinary ocean observing applications for investigating coastal hypoxia in the Gulf of Mexico; OCEANS ’11, MTS/IEEE Kona, Program Book, Article no. 6107127.
- Reese, B.K., H.J. Mills, S.E. Dowd, and J.W. Morse. 2013. Linking molecular microbial ecology to geochemistry in a coastal hypoxic zone. Geomicrobiology Journal 30(2):160-172. doi 10.1080/01490451.2012.659331.
- Strauss, J., E.L. Grossman, and S.F. DiMarco. 2012. Stable isotopes in mollusk shells as indicators of benthic respiration and freshwater penetration on the Texas-Louisiana Shelf. Bulletin of Marine Science 88(4):817-842. doi:10.5343/bms.2011.1047.
- Xu, K.H., C.K. Harris, R.D. Hetland, and J.M. Kaihatu. 2011. Dispersal of Mississippi and Atchafalaya Sediment on the Texas-Louisiana Shelf: Model Estimates for the Year 1993. Continental Shelf Research 31:1558–1575. doi:10.1016/j.csr.2011.05.008.
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