Gulf of Mexico Ecosystems and Hypoxia Assessment

ISSUE

The Gulf of Mexico contains almost half of the nation's coastal wetlands and supports approximately 20% (by dollar) of its commercial fishery landings, as well as important recreational fisheries. The vast majority of the Gulf's commercial fishery landings come from the area directly affected by the Mississippi River. This northern portion of the Gulf of Mexico ecosystem has undergone profound changes due to increased nutrient enrichment from the Mississippi River which is carrying higher nutrient loads from land-based sources. Nutrient loadings from land-based sources can "over-enrich" coastal ecosystems, often leading to adverse impacts. A common effect of this nutrient over-enrichment is excessive production of algae, which can kill benthic marine organisms by reducing the concentration of dissolved oxygen in bottom waters through the decomposition of the large amounts of organic material. More than one million tons of nitrogen, the major nutrient controlling algal production in the northern Gulf of Mexico, makes its way into the Gulf via the Mississippi River system each year. It is important to understand, predict, and assess the influence of this massive river load on the development of low oxygen (hypoxic) conditions and the functioning of the northern Gulf of Mexico ecosystem.

APPROACH

Through a combination of field and modeling studies, Coastal Ocean Program (COP) continues to develop an understanding of the northern Gulf of Mexico ecosystem in the region affected by Mississippi River inputs with a focus on the causes and effects of the hypoxic zone over the Louisiana continental shelf and the prediction of its future extent and impacts. COP supports studies documenting the dynamics of the hypoxic zone over the Louisiana continental shelf and studies to better define the relationships among nutrients, phytoplankton, carbon production and flux, physical properties, and hypoxia development. The carbon (i.e. organic material) flux fuels the growth of the hypoxic zone while the physical properties enable its growth. COP also supports studies to assess oxygen dynamics and extend modeling efforts to predict changes in oxygen budgets and severity of hypoxia under altered hydrologic scenarios. Studies investigating the effects of the hypoxic zone on fisheries are also being supported.

ACCOMPLISHMENTS

COP investigations have documented the zone off the Louisiana continental shelf with seasonally depleted oxygen levels (< 2 mg/l) since 1990. In the summer of 2002, the largest hypoxic zone ever recorded was measured at 22,000 km2, an area larger than the size of Massachusetts. The hypoxic zone covered just 8,560 km2 in July 2003. This was smaller than what was predicted by COP investigators based upon freshwater and nutrient loads to the northern Gulf of Mexico. Although hypoxia was well established and widespread off Terrebonne Bay in mid-June, two large storms passed through the region just prior to the July mapping cruise. Both storms generated 10 to 15 foot waves that mixed well-oxygenated waters from the surface down through the water column, reducing the extent of waters severely depleted in oxygen.

COP studies have produced considerable evidence that increased nutrient loading from the Mississippi and Atchafalaya River systems is the dominant factor in creating this hypoxia problem and have made hindcasts and forecasts of hypoxia presence given past and potential future nutrient and freshwater inputs. Studies have also documented the fine-scale distributions of fish and shrimp with respect to hypoxia and effects of hypoxia on the ecosystem.

COP published six technical reports and a final integrated report that form part of an assessment of hypoxia in the northern Gulf of Mexico. These reports support efforts of the White House Office of Science and Technology Policy's scientific assessment of the causes and consequences of Gulf hypoxia through its Committee on Environment and Natural Resources (CENR). Part of the COP Decision Analysis Series, these reports document the state of knowledge of the extent, characteristics, causes, and effects (both ecological and economic) of hypoxia in the northern Gulf of Mexico.

MANAGEMENT AND POLICY IMPLICATIONS

Knowledge gained through COP's northern Gulf of Mexico research program will lead to predictive models of the circulation, hypoxic zone, and species-interactions in this region. This will allow for the assessment of alternative management strategies within the context of long-term changes in eutrophication and hypoxia and the impacts on commercially and ecologically important species.

Project Abstracts and Related Links

For more information, contact:

Kenric Osgood
CSCOR/Coastal Ocean Program
phone: 301-713-3338
e-mail: coastalocean@noaa.gov

Last Updated: August 13, 2003