The hypoxic zone in July 2007. Click to enlarge. Source: LUMCON

Marine species either die or flee the hypoxic zone, so the spread of hypoxia reduces the available habitat for marine species which are important for the ecosystem as well as commercial and recreational fishing in the Gulf.

To address this problem, the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force asked the White House Office of Science and Technology Policy to conduct a scientific assessment of the causes and consequences of Gulf hypoxia through its Committee on Environment and Natural Resources (CENR). In 2000 the CENR completed An Integrated Assessment: Hypoxia in the Northern Gulf of Mexico, which formed the scientific basis for the Action Plan for Reducing, Mitigating and Controlling Hypoxia in the Northern Gulf of Mexico. The plan called for a reduction in the size of the zone to 5,000 km² by 2015. The task force committed to implement ten management actions and to assess progress every five years; it began its first reassessment in 2005.

As part of that reassessment, EPA’s Office of Water, on behalf of the task force, requested the SAB to convene an independent panel to evaluate the state of the science regarding hypoxia in the Northern Gulf of Mexico and potential nutrient mitigation and control options in the MARB.

Area of the mid-summer hypoxic zone. Click to enlarge.

According to the draft SAB report, while the scientific understanding of the causes of hypoxia have grown, actions to control hypoxia have lagged. Mapping by the Louisiana Universities Marine Consortium (LUMCON) in July 2007 determined that the size of the zone had increased to 20,500 square kilometers (or 7,900 square miles), similar to the size of New Jersey. The low oxygen waters extended from near the Mississippi River across the Louisiana/Texas border towards Galveston.

The long-term average since mapping began in 1985 increased with this year’s measurement to 13,500 square kilometers (or 5,200 square miles). The goal to reach the 5,000 km² target size continues to remain far off, concluded LUMCON.

In its report, the SAB panel found indications that the Gulf of Mexico has undergone a regime shift with hypoxia such that the system is more sensitive now to inputs of nutrients than in the past, with the resulting induction of a larger response in hypoxia.

To reduce the dead zone to the target size of 5,000 km², the SAB draft report recommends a dual nutrient strategy targeting at least a 45% reduction in riverine total nitrogen flux and at least a 45% reduction in riverine total phosphorus flux.

However, the report cautions, “certain aspects of the nation’s current agricultural and energy policies are at odds with the goals of hypoxia reduction and improving water quality”—specifically, the emerging national strategy on renewable fuels that grants economic incentives to corn-based ethanol production.

Existing incentives could lead to corn production on an additional 16 million acres, which would “dramatically” increase nitrogen loadings to the MARB. The SAB report identifies the most significant five opportunities for reducing nitrogen and phosphorus loading in the Mississippi:

• Conversion to alternative cropping systems (e.g., using perennials or alternative rotation systems) and promotion of environmentally sustainable approaches to biofuel production in targeted areas of the basin;

• Improved management of nutrients by emphasizing infield nutrient management efficiency and effectiveness to reduce losses;

• Construction and restoration of wetlands as well as criteria for targeting those wetlands that may have a higher priority for reducing nutrient losses;

• Introduction of tighter nitrogen and phosphorus limits on municipal point sources;

• Improved targeting of conservation buffers, including riparian buffers, filter strips 40 and grassed waterways, to control surface-borne nutrients.

The Science Advisory Board will vote on approval of the draft report in December.

Resources:

• Science Advisory Board (SAB) Hypoxia Panel Draft Advisory Report

• The Mississippi and Atchafalaya River Basin (EPA)

• Characterization & Long-Term Trends of Hypoxia in the Northern Gulf of Mexico (Rabalais, Louisiana Universities Marine Consortium)

• Hypoxia in the Northern Gulf of Mexico (LUMCON)