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Since 1985, scientists have been documenting a hypoxic zone in the Gulf of Mexico each year. The hypoxic zone, an area of low dissolved oxygen that cannot support marine life, generally manifests itself in the spring. Since marine species either die or flee the hypoxic zone, 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. Since 2001, the hypoxic zone has averaged 16,500 km2 during its peak summer months1, an area slightly larger than the state of Connecticut, and ranged from a low of 8,500 km2 to a high of 22,000 km2. To address the hypoxia problem, the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force (or Task Force) was formed to bring together representatives from federal agencies, states and tribes to consider options for responding to hypoxia. The 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 (Integrated Assessment), which formed the scientific basis for the Task Force’s Action Plan for Reducing, Mitigating and Controlling Hypoxia in the Northern Gulf of Mexico (Action Plan, 2001). In its Action Plan, the Task Force pledged to implement ten management actions and to assess progress every five years. This reassessment would address the nutrient load reductions achieved, the responses of the hypoxic zone and associated water quality and habitat conditions, and economic and social effects. The Task Force began its reassessment in 2005.
In 2006 as part of the reassessment, EPA’s Office of Water, on behalf of the Task Force, requested that the Environmental Protection Agency (EPA) Science Advisory Board (SAB) 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 Mississippi-Atchafalaya River basin (MARB). The Task Force was particularly interested in scientific advances since the Integrated Assessment and issued charge questions in three areas: characterization of hypoxia; nutrient fate, transport and sources; and the scientific basis for goals and management options. The SAB Hypoxia Advisory Panel (SAB Panel) began its deliberations in September of 2006 and completed its report in August of 2007 while operating under the “sunshine” requirements of the Federal Advisory Committee Act, which include providing public access to advisory meetings and opportunities for public comment. This Executive Summary summarizes the SAB Panel’s major findings and recommendations.
Since publication of the Integrated Assessment, scientific understanding of the causes of hypoxia has grown while actions to control hypoxia have lagged. Recent science has affirmed the basic conclusion that contemporary changes in the hypoxic area in the northern Gulf of Mexico (NGOM) are primarily related to nutrient fluxes from the MARB. Moreover, new research provides early warnings about the deleterious long-term effects of hypoxia on living resources in the Gulf.
The SAB Panel was asked to comment on the Action Plan’s goal to reduce the hypoxic zone to a five-year running average of 5,000 km2 by 2015. The 5,000 km2 target remains a reasonable endpoint for continued use in an adaptive management context; however, it may no longer be possible to achieve this goal by 2015. In August of 2007, the hypoxic zone was measured to be 20,500 km2 (LUMCON, 2007), the third largest hypoxic zone since measurements began in 1985. Accordingly, it is even more important to proceed in a directionally correct fashion to manage factors affecting hypoxia than to wait for greater precision in setting the goal for the size of the zone. Much can be learned by implementing management plans, documenting practices, and measuring their effects with appropriate monitoring programs.
To reduce the size of the hypoxic zone and improve water quality in the MARB, the SAB Panel recommends a dual nutrient strategy targeting at least a 45% reduction in riverine total nitrogen flux (to approximately 870,000 metric tonne/yr or 960,000 ton/yr) and at least a 45% reduction in riverine total phosphorus flux (to approximately 75,000 metric tonne/yr or 83,000 ton/yr). Both of these reductions refer to changes measured against average flux over the 1980 1996 time period. For both nutrients, incremental annual reductions will be needed to achieve the 45% reduction goals over the long run. For nitrogen, the greatest emphasis should be placed on reducing spring flux, the time period most correlated with the size of the hypoxic zone. While the state of predictive and process models of NGOM hypoxia has continued to develop since 2000, models similar to those in place at that time are still the best tools for producing dose response estimates for nitrogen (N) reductions, with most recent model runs showing a 45 – 55% required reduction for N in order to reduce the size of the hypoxic zone. A number of studies have suggested that climate change will create conditions for which larger nutrient reductions, e.g., 50 – 60% for nitrogen, would be required to reduce the size of the hypoxic zone.
New information has emerged that more precisely demonstrates the role of phosphorus (P) in determining the size of the hypoxic zone. Contrary to conventional wisdom that N typically limits phytoplankton production in near-coastal waters, the NGOM exhibits an unusual phenomenon whereby P is an important limiting constituent during the spring and summer in the lower salinity, near-shore regions. Phosphorus limitation is now occurring because over the past 50 years excessive N loadings have dramatically altered nitrogen to phosphorus ratios. Taken together, N and P both contribute to excess phytoplankton production and the hypoxia associated with such production, and they will need to be reduced concurrently to make progress in reducing the size of the hypoxic zone. The SAB Panel’s best professional judgment is that phosphorus reductions will need to be comparable (in percentage terms) to nitrogen reductions to reduce the size of the hypoxic zone.
Scientific advances have improved our understanding of the physical factors that contribute to hypoxia. One physical factor that has changed substantially over the past century is river hydrology. The hydrologic regime of the Mississippi and Atchafalaya Rivers and the timing of freshwater inputs to the continental shelf are critical to mixing and hypoxia development. The most important hydrological change over the past century has been the diversion of a large amount of freshwater from the Mississippi River through the Atchafalaya River to the Atchafalaya Bay, and maintenance of this diversion by the U. S. Army Corps of Engineers. The major injection of freshwater into Atchafalaya Bay, some 200 kilometers to the west of the Mississippi River Delta, has profoundly modified the spatial distribution of freshwater inputs, nutrient loadings and stratification on the Louisiana-Texas continental shelf.
Methods used by the U.S. Geological Survey (USGS) to calculate nutrient fluxes in the MARB have changed since the Integrated Assessment. The latest USGS estimates show that total N flux averaged 1.24 million metric tonne/yr (1.37 million ton/yr) from 2001 – 2005 (65% of the flux is nitrate), and the total P flux averaged 154,000 metric tonne/yr (170,000 ton/yr). This change represents a 21% decline in total N flux and a 12% increase in total P flux when compared to the averages from the 1980 – 1996 time period. The spring (April – June) flux of nutrients appears to be an important determinant of hypoxia, for that is when the river is disproportionately enriched with both N (especially nitrate) and P. Spring total N flux has declined since the 1980s; whereas total P flux shows a 9.5% increase (when average total P flux for 2001-2005 is compared to the 1980 – 1996 average). USGS data also show that during the last 5 years, the upper Mississippi and Ohio-Tennessee River subbasins contributed about 82% of nitrate-N flux, 69% of the TKN flux, and 58% of total P flux, although these sub-basins represent only 31% of the entire MARB drainage area.
The SAB Panel’s estimates of point source discharge show that point sources represented 22% of total annual average N flux and 34% of total annual average P flux discharged to the NGOM during the last five years. New methods also have been used to calculate nutrient mass balances (net anthropogenic N inputs, NANI). NANI for the MARB has declined in the past decade because of increased crop yields, reduced or redistributed livestock populations, and little change in N fertilizer inputs. From 1999-2005, NANI calculations show 54% of non-point N inputs in the MARB were from fertilizer, 37% from nitrogen fixation, and 9% from atmospheric deposition.
The SAB Panel finds that the Gulf of Mexico ecosystem appears to have gone through a regime shift with hypoxia such that today the system is more sensitive to inputs of nutrients than in the past, with nutrient inputs inducing a larger response in hypoxia as shown for other coastal marine ecosystems such as the Chesapeake Bay and Danish coastal waters. Changes in benthic and fish communities with the change in frequency of hypoxia are cause for concern. The recovery of hypoxic ecosystems may occur only after long time periods or with further reductions in nutrient inputs. If actions to control hypoxia are not taken, further ecosystem impacts could occur within the Gulf, as has been observed in other ecosystems.
Certain aspects of the nation’s current agricultural and energy policies are at odds with the goals of hypoxia reduction and improving water quality. Since the Integrated Assessment, an emerging national strategy on renewable fuels has granted economic incentives to corn-based ethanol production. The projected increase in corn production from this strategy has profound implications for water quality in the MARB, as well as hypoxia in the NGOM. Recent energy policies, combined with pre-existing crop subsidies, tax policies, global market conditions and trade barriers all provide economic incentives for conversion of retired and other cropland to corn production for use in ethanol production. Such conversions are projected to lead to corn production on an additional 6.5 million ha (16 million ac) in coming years with the majority of this increase occurring in the MARB. Without some change to the current structure of economic incentives favoring corn-based ethanol, N loadings to the MARB from increased corn production could increase dramatically in coming years, rather than decreasing, as needed for the NGOM.
Recommendations for Monitoring and Research
Most of the research and monitoring needs identified in the Integrated Assessment have not been met, and fewer rivers and streams are monitored today than in 2000. The majority of monitoring recommendations in the Integrated Assessment remain relevant and should be heeded. The SAB Panel affirms and reiterates the CENR’s call to improve and expand monitoring of the temporal and spatial extent of hypoxia and the processes controlling its formation; the flux of nutrients, carbon, and other constituents from non-point sources throughout the MARB and to the NGOM; and measured (rather than estimated) nitrogen and phosphorus fluxes from municipal and industrial point sources.
The SAB Panel affirms the need for research in the following areas identified in the Integrated Assessment: ecological effects of hypoxia; watershed nutrient dynamics; effects of different agricultural practices on nutrient losses from land, particularly at the small watershed scale; nutrient cycling and carbon dynamics; long-term changes in hydrology and climate; and economic and social impacts of hypoxia.
A suite of models is needed to simulate the processes and linkages that regulate the onset, duration and extent of hypoxia. Emerging coastal ocean observation and prediction systems should be encouraged to monitor dissolved oxygen and other physical and biogeochemical parameters needed to continue improving hypoxia models.
To advance the science characterizing hypoxia and its causes, the SAB Panel finds that research is also needed to:
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