Gulf of Mexico Hypoxic Zone Is Largest in Western Hemisphere

June 18, 1999

Gulf of Mexico Hypoxic Zone Is Largest in Western Hemisphere


Scientists Recommend Decreasing Hypoxia in Gulf of Mexico

Scientists offer recommendations to decrease hypoxia in the Gulf of Mexico. The Council for Agricultural Science and Technology (CAST), an international consortium of 38 scientific and professional societies, released a report Gulf of Mexico Hypoxia: Land and Sea Interactions in which a CAST task force of 6 scientists provides recommendations to help better understand all aspects of hypoxia in the Gulf of Mexico and other areas of the world, and to decrease the Gulf hypoxic zone. The task force chaired by Dr. John A. Downing, Department of Animal Ecology, Iowa State University, Ames, includes individuals with expertise in agricultural economics, agricultural engineering, agronomy, animal ecology, biogeochemistry, biological sciences, environmental research, limnology, marine science, oceanography, and soil science.

Characteristics of Gulf of Mexico Hypoxia

The Gulf of Mexico hypoxic zone, a bottom area with dissolved oxygen levels too low to sustain animal life, is the largest zone of human caused coastal hypoxia in the Western Hemisphere. Mid-summer coastal hypoxia in the northern Gulf was first recorded in the early 1970s. Persistent coastal hypoxia has been most widespread during the 1990s. Mid-summer surveys show the hypoxic zone has covered as much as 18,000 square kilometers (km2) (1993-1996), covering 12,400 km2 (4,800 square miles) in 1998: about the size of Connecticut.

Causes of Gulf of Mexico Hypoxia

Gulf hypoxia results from

  • decomposition of organic matter growth stimulated by Mississippi River nutrients and
  • stratification of marine waters due to Mississippi River water inflow.

Nitrogen (N) is the principal nutrient yielding excess organic matter sedimentation to the Gulf hypoxic zone. Nitrogen export from the Mississippi River Basin has increased 2- to 7-fold over the last century. Silicon (Si) and phosphorus (P) also play a role, and the changing balance of N, Si, and P can affect marine food chains.

Social and Economic Dimensions of Hypoxia

Advanced anthropogenic hypoxia elsewhere in the world has had serious impacts on fisheries. Short-term economic costs impact

  • commercial fishing,
  • consumers of seafood,
  • tourism,
  • recreational fishing, and
  • nonusers discouraged by perceived pollution.

Commercial and recreational fisheries in the Gulf generate $2.8 billion annually. Signs consistent with impact on Gulf of Mexico fisheries are

  • reduced food sources for fish in hypoxic areas,
  • reduced abundance of fish and shrimp in hypoxic waters, and
  • declines in shrimp catch and catch efficiency since hypoxia expanded.

    Sources of Mississippi River Nitrogen

    The majority of Mississippi River N originates from agricultural practice, while smaller fractions arise from human sewage, nonagricultural fertilizer use, and precipitation. Agriculture's share of this lost nitrogen has a fertilizer value of about $410 million.

    The Mississippi River Basin

    • covers 41% of the contiguous United States,
    • is home to 47% of the nation's rural population,
    • generates 52% of U.S. farm receipts ($98 billion annually),
    • comprises 52% of U.S. farms, and
    • creates 33% of all U.S. farm-related jobs.

    Nitrogen Export from Agricultural Landscapes

    Nitrogen moves from agricultural land to surface waters by air, surface runoff, sediment transport, and subsurface drainage. Inexpensive management options can reduce N losses from agricultural lands, e.g., alterations of fertilizer application methods to decrease runoff losses, alterations of tillage regimes to decrease sediment bound nutrient transport, and fine-tuning application rates to decrease losses through subsurface drainage.

    Cost and Benefits of Decreasing Agricultural Nutrient Flux

    Decreasing agriculture's contribution to Gulf of Mexico nutrients will require changes in crop and livestock management practices to reduce nutrient losses. Several benefits would accrue to agriculture and society:

    • decreased risk to marine fishing communities,
    • more efficient use of nutrients and energy,
    • lower fertilizer costs,
    • decreased health risk from contamination, and
    • improved aquatic habitats and recreation.

    What Do the Experts Recommend?

    • Control, retain, and monitor nutrients leaving agricultural and key Mississippi River Basin lands.
    • Create, enhance, and distribute information on cost-effective agricultural nutrient management methods.
    • Set and achieve goals of nutrient flux reduction tied to downstream water quality improvement.
    • Seek cost-effective solutions to enhance the security of agricultural and coastal communities.
    • Gauge effectiveness of solutions by societal and private costs and benefits.
    • Implement policies favoring long-term, broad strategies that enhance life and environment in the Mississippi River Basin and the Gulf.
    • Monitor changes in hypoxia, its potential causes, and the impacts of marine eutrophication on society and environment.

    Gulf of Mexico Hypoxia: Land and Sea Interactions, 44 pages, is available for $20.00 from CAST.