Estuaries and Coasts Vol. 30, No. 5, p. 753–772 October 2007 Hypoxia in the Northern Gulf of Mexico: Does the Science Support the Plan to Reduce, Mitigate, and Control Hypoxia? N. N. RABALAIS1,*, R. E. TURNER2,B.K.SEN GUPTA2,D.F.BOESCH3,P.CHAPMAN4, and M. C. MURRELL5 1 Louisiana Universities Marine Consortium, Chauvin, Louisiana 70344 2 Louisiana State University, Baton Rouge, Louisiana 70803 3 University of Maryland Center for Environmental Science, Cambridge, Maryland 71613 4 Texas A&M University, College Station, Texas 77843 5 U.S. Environmental Protection Agency, Gulf Ecology Division, Gulf Breeze, Florida 32561 ABSTRACT: We update and reevaluate the scientific information on the distribution, history, and causes of continental shelf hypoxia that supports the 2001 Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 2001), incorporating data, publications, and research results produced since the 1999 integrated assessment. The metric of mid-summer hypoxic area on the Louisiana- Texas shelf is an adequate and suitable measure for continued efforts to reduce nutrients loads from the Mississippi River and hypoxia in the northern Gulf of Mexico as outlined in the Action Plan. More frequent measurements of simple metrics (e.g., area and volume) from late spring through late summer would ensure that the metric is representative of the system in any given year and useful in a public discourse of conditions and causes. The long-term data on hypoxia, sources of nutrients, associated biological parameters, and paleoindicators continue to verify and strengthen the relationship between the nitrate- nitrogen load of the Mississippi River, the extent of hypoxia, and changes in the coastal ecosystem (eutrophication and worsening hypoxia). Multiple lines of evidence, some of them representing independent data sources, are consistent with the big picture pattern of increased eutrophication as a result of long-term nutrient increases that result in excess carbon production and accumulation and, ultimately, bottom water hypoxia. The additional findings arising since 1999 strengthen the science supporting the Action Plan that focuses on reducing nutrient loads, primarily nitrogen, through multiple actions to reduce the size of the hypoxic zone in the northern Gulf of Mexico. Introduction tribal governments. The Action Plan calls for a long- The development, extent, and persistence of low term adaptive management strategy coupling man- oxygen concentrations in bottom waters of the agement actions with enhanced monitoring, mod- continental shelf of the northern Gulf of Mexico eling and research, and reassessment of accomplish- were unknown until the first systematic mapping ments and environmental indicators at five-year and monitoring of oxygen began in 1985 (Rabalais intervals. et al. 1991). At that time, shelf hypoxia (defined This paper is a component of the first reassess- here as dissolved oxygen levels at or below 2 mg l21) ment, updating the earlier reports (Rabalais et al. was recorded as localized and ephemeral (Turner 1999, 2002a,b) that considered publications and and Allen 1982). Since then, a large volume of data observations made through 1998. We review herein hasbeencollectedandnumerouspapersand new information on the spatial and temporal reports have been published that increased our (seasonal, annual, and long term) characteristics understanding of the seasonal and interannual of hypoxia on the northern Gulf of Mexico shelf distribution of hypoxia and its variability, history, developed since the Integrated Assessment and and dynamical causes. summarize the current understanding of the onset, An Integrated Assessment (CENR 2000) of the extent, and duration of the hypoxic zone. We also causes, consequences, and actions needed to reduce address the relationships between long-term trends hypoxia was completed and an Action Plan for in the extent, duration, and severity of hypoxia and Reducing, Mitigating, and Controlling Hypoxia in both increased nutrient loading and other environ- the Northern Gulf of Mexico (Mississippi River/ mental factors that may influence hypoxia, evaluat- Gulf of Mexico Watershed Nutrient Task Force ing the degree to which these relationships are 2001) was endorsed by federal agencies, states, and understood and quantified. We then answer two questions: Do new research contributions and un- * Corresponding author; tele: 985/851-2801; fax: 985/851- derstanding of the science supporting the Action 2874; e-mail: [email protected] Plan justify the prior conclusions of the Integrated ß 2007 Estuarine Research Federation 753 Estuaries and Coasts estu-30-05-02.3d 20/11/07 16:02:36 753 Cust # 4289 754 N. N. Rabalais et al. Assessment concerning increased nitrogen loads, vicinity of the river discharges as well as across the eutrophication, and worsening hypoxia and man- broader Louisiana and upper Texas continental agement scenarios to reduce nitrogen loads to the shelf. The flux of fixed carbon in the form of northern Gulf of Mexico? and Do new research senescent phytoplankton, zooplankton fecal pellets, findings give us cause to alter the concepts that or aggregates to the lower water column and seabed support management goals intended to reduce provides a large carbon source for decomposition by nitrogen loading from the Mississippi River? aerobic bacteria. Its decomposition consumes dis- solved oxygen in the water column at a higher rate Background than resupply from the upper water column in The Mississippi River forms the largest watershed a stratified water column, leading to hypoxia in on the North American continent with an annual large portions of the water column for months at average discharge of 580 km3 into the northern a time from the spring to the fall (Rabalais and Gulf of Mexico through two main distributaries: the Turner 2001; Rabalais et al. 2002a,b). birdfoot delta southeast of the city of New Orleans, Demersal fishes, crabs, and shrimp will attempt to Louisiana, and the Atchafalaya River delta 200 km move away from oxygen concentrations less than to the west (Meade 1995). The Mississippi and 2mgl21, and few marine animals survive in pro- Atchafalaya rivers are the primary sources of fresh longed exposure to oxygen concentrations below water, nitrogen, and phosphorus to the northern that level. The hypoxic area is popularly known as Gulf of Mexico, delivering 80% of the freshwater the Dead Zone. The oxygen deficient waters are not inflow, 91% of the estimated annual nitrogen load, just confined to a bottom layer, but may extend well and 88% of the phosphorus load (Dunn 1996). If up into the water column; e.g., the hypoxic water only streams between and inclusive of the Trinity mass may occupy half of the lower water column River (Galveston, Texas) and the Mississippi River (Rabalais and Turner 2001). Hypoxia, as a symptom delta are considered—those most likely to influence of eutrophication due to nutrient enrichment, is the zone of hypoxia—the Mississippi and Atchafa- a growing problem around the world (Diaz and laya rivers account for 96% of the annual freshwater Rosenberg 1995; Boesch 2002; Diaz et al. 2004). The discharge, 98.5% of the annual nitrogen load, and extent and persistence of hypoxia on the Louisiana- 98% of the total annual phosphorus load. Texas shelf makes the Gulf of Mexico Dead Zone The fresh water, sediments, and dissolved and one of the most extensive manifestations of coastal particulate materials are carried predominantly eutrophication. westward along the Louisiana-Texas inner to mid continental shelf, especially during peak spring DIMENSIONS AND VARIABILITY OF HYPOXIA discharge (Rabalais et al. 1996; Smith and Jacobs 500-km Scale 2005). Although the area of the discharge’s in- fluence is an open continental shelf, the magnitude The extent of hypoxia primarily in July averaged of flow, annual current regime, and average 75- 13,500 km2 from 1985 to 2007, with a range from d residence time for fresh water all suggest that the negligible in 1988 (a summer drought year for the shelf receiving the fresh water behaves as an Mississippi River basin) to 22,000 km2 in 2002. An unbounded estuary stratified for much of the year average of 8,200 km2 from 1985 to 1992 increased to (Dinnel and Wiseman 1986). The stratification is an average of 15,900 km2 over the next 15 yr due primarily to salinity differences, and the (Rabalais et al. unpublished data). The maximal stratification intensifies in summer with thermal extent of bottom-water hypoxia measured was warming of surface waters (Wiseman et al. 1997). 22,000 km2 in 2002 (Rabalais and Turner 2006). The primary pycnocline depth varies seasonally with The five-year average size of bottom-water hypoxia discharge and physical mixing from 3–5 m to 10– in the Gulf of Mexico for 2003–2007 was 10,500 km2 15 m in a 20-m water column (Rabalais and Turner (Fig. 1; Rabalais et al. unpublished data), which is 2001; Rabalais et al. 2002c). A secondary tempera- well above the Action Plan goal of 5,000 km2. ture-controlled pycnocline at 10 to 15 m depth in The mid-summer hypoxic water mass is distribut- a 20-m water column often caps the hypoxic water ed across the Louisiana shelf west of the Mississippi mass (Wiseman et al. 1997). River and onto the upper Texas coast (examples in Seasonal hypoxia of bottom waters in this region Fig. 2; Rabalais and Turner 2001; Rabalais et al. is the result of the strong and persistent stratifica- 2002a). Hypoxia extends from nearshore to as tion coupled with the high organic production in much as 125 km offshore and in water depths overlying surface waters that is fueled by river- extending from the shore in 4–5 m up to 60 m derived nutrients (CENR 2000). The nutrients deep. Mid-summer hypoxia usually extends along delivered from the Mississippi River Basin support a single continuous zone, but may occur in distinct the primary productivity within the immediate areas west of the Mississippi and Atchafalaya River Estuaries and Coasts estu-30-05-02.3d 20/11/07 16:02:37 754 Cust # 4289 Hypoxia Advances in the Gulf of Mexico 755 Fig.