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1 Introduction The northern Gulf of Mexico hypoxic zone was first recorded on the continental shelf in the early 1970s and has remained persistent since sustained data collection on its distribution and dynamics was begun in 1985. Hypoxia is the term that describes conditions in a waterbody with levels of dissolved oxygen low enough to harm fish and other aquatic species. The existence of this seasonal “dead zone” derives from excess inputs of nutrients from the Mississippi and Atchafalaya rivers into the northern Gulf of Mexico. These inputs result in nutrient overenrichment in the northern gulf, which contributes to high levels of algal biomass production. When these algae die, the process of decomposition depletes dissolved oxygen from the water column and leads to these hypoxic conditions. Efforts to remedy hypoxia are complicated by many factors, including the numerous sources and actions across the vast Mississippi River basin that generate nutrient yields, and the large time lags between nutrient inputs to the northern Gulf of Mexico and subsequent changes in the hypoxic zone. The hypoxic zone has been the subject of extensive research and many studies and initiatives, some of the more recent and prominent of which are summarized in Box 1-1. The reader interested in further details of Mississippi River water quality, nutrient loadings across the river basin, and the science of hypoxia is encouraged to consult these reports. The U.S. Environmental Protection Agency (EPA), through its Clean Water Act authorities and responsibilities, plays a key role in the monitoring and management of northern Gulf of Mexico water quality and hypoxia. To obtain advice on Mississippi River basin nutrient control strategies, the EPA requested that the National Research Council (NRC) and its Water Science and Technology Board (WSTB) convene a committee to consider and advise in three broad topic areas. In making this request to the National Research Council, the EPA also sought to build upon an earlier, 2008 report from the NRC on Mississippi River water quality and the Clean Water Act (summarized as part of Box 1-1). The three topic areas addressed in this report (abbreviated here and found in full in the committee statement of task in Appendix A) are: 1. Given the state of scientific knowledge and associated uncertainties applicable to reducing the hypoxic zone in the Gulf, how might existing loading estimates and targets be used to initiate pollutant control programs? 7

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8 NUTRIENT CONTROL ACTIONS FOR IMPROVING WATER QUALITY BOX 1-1 Recent Studies and Reports on Mississippi River Basin Nutrient Loadings, Water Quality, and Northern Gulf of Mexico Hypoxia Northern Gulf of Mexico hypoxia has been the subject of extensive scientific research over the past two decades. The period of 2008 and late 2007 saw the release of many prominent reports on these topics. This box summarizes four of these reports. 2008 Gulf Hypoxia Action Plan This 2008 report from the federal interagency Mississippi River/Gulf of Mexico Watershed Nutrient Task Force (MS River/Gulf of Mexico Watershed Nutrient Task Force, 2008) follows up and builds upon a 2001 report from the task force, which was the first “action plan” for gulf hypoxia. That 2001 report listed a goal of reducing the 5-year running average areal extent of the hypoxic zone to less than 5,000 square kilometers by the year 2015 (MS River/ Gulf of Mexico Watershed Nutrient Task Force, 2001). This goal was restated in the 2008 task force report. SPARROW Model Results This 2008 paper presents results from a spatially referenced regression on watershed attributes (SPARROW) water quality model. This paper was co-authored by six scientists, most of whom are USGS staff and are affiliated with its National Water Quality Assessment program (Alexander et al., 2008). The paper presents geographic differences in nitrogen and phosphorus yields from across the Mississippi River basin as determined in the SPARROW model results. It also shows geographic differences in the percentage of stream nutrient load that eventually are delivered to the Gulf of Mexico. NRC Study on Mississippi River Water Quality and the Clean Water Act This 2008 report from a previous National Research Council committee addresses four broad topics: 1) Mississippi River corridor water quality problems, 2) data needs and system monitoring, 3) water quality indicators and standards, and 4) policies and implementation. The report finds that at the scale of the entire Mississippi River basin and into the gulf, nutrients and sediment are the two primary water quality problems. It concludes that as a result of limited interstate coordination, the Mississippi River is an “orphan” from a water quality monitoring and assessment perspective. It also finds that the EPA has failed to use its Clean Water Act authorities to provide adequate interstate coordination and oversight of state water quality activities. It recommends that the EPA develop water quality criteria for nutrients in the Mississippi River and the northern Gulf of Mexico; ensure that states establish water quality standards and TMDLs such that they protect water quality; and, develop a federal TMDL, or its functional equivalent, for the Mississippi River and the northern Gulf of Mexico. Report from the EPA Science Advisory Board This 2007 report from the EPA Science Advisory Board (SAB) Hypoxia Advisory Panel (HAP) summarizes and evaluates the most recent science on the hypoxic zone and the potential options for reducing its size. Among the report’s many conclusions is an affirmation of the basic scientific understanding that contemporary changes in the hypoxic area in the northern Gulf of Mexico are related primarily to nutrient fluxes from the Mississippi and Atchafalaya rivers. The report also finds that a significant reduction in the hypoxic zone “is not likely to be achievable over the next eight years” (EPA, 2007). Finally, if the size of the hypoxic zone is to be reduced, the SAB report finds that “a dual nutrient strategy is needed that achieves at least a 45% reduction in both riverine total nitrogen flux and riverine total phosphorus flux” (USEPA, 2007).

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INTRODUCTION 9 2. What are the alternative methods to allocate load reductions to upstream tributaries, states, land uses, and other source classifications? 3. How should the effectiveness of pollutant loading reduction strategies be documented, and how much time would be required to determine if reductions in nutrient and sediment loadings are resulting in a reduction of Gulf of Mexico hypoxia? Two topics mentioned in the full statement of task to this committee deserve elaboration at this point. They are Section 303(d) of the Clean Water Act, and the roles of nutrients and sediment in northern Gulf of Mexico hypoxia. These are mentioned here because although both topics are referred to in this committee’s task statement and are important in Mississippi River and northern Gulf of Mexico water quality issues, neither topic is explored in great detail in this report. Regarding Section 303(d), the previous 2008 NRC report on Mississippi River water quality and the Clean Water Act discusses EPA authority to act under Section 303(d) and other provisions of the act. It is explained that Section 303(d) requires states to “…identify those waters within its boundaries…” where technology-based controls are insufficient for meeting water quality standards. For each water quality segment so identified, 303(d) requires a state to establish a Total Maximum Daily Load (TMDL) for pollutants that have been identified by EPA as being appropriate. Language within the Clean Water Act makes it clear that the TMDL process is predominantly intrastate in focus. Nevertheless, as that report importantly notes, TMDLs also must deal with cross-border effects: the Clean Water Act, as interpreted by EPA, imposes obligations on upstream states to protect downstream water quality through the adoption of their own water quality standards … Section 303(d) effectively requires an upstream state to adopt a TMDL at a level such that it will prevent interference by its point and nonpoint sources with attainment of downstream state water quality standards (NRC, 2008). The report goes on to state that: EPA has the authority to establish TMDLs with both downstream and upstream interstate effects. … the Clean Water Act requires the EPA to set TMDLs when states fail to do so (Section 303(d)), and the federal courts have upheld the EPA's authority to set federal TMDLs even when only nonpoint source pollutants are contributing to water quality impairment (NRC, 2008). Thus, if EPA chooses to pursue Section 303(d) authority to develop an implementation plan for the Mississippi River Basin, it apparently has the

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10 NUTRIENT CONTROL ACTIONS FOR IMPROVING WATER QUALITY authority to do so. However, implementation of Section 303(d) TMDLs depends upon waterways' non-compliance with state water quality standards. Most states along the Mississippi River have not set nutrient water quality standards for the river’s mainstem. For states that have set such standards, they have relied primarily on narrative, rather than numeric, water quality criteria. EPA's development of recommended nutrient water quality criteria pursuant to Section 304 of the Clean Water Act, and the states' adoption of nutrient water quality standards pursuant to Section 303, are thus legal prerequisites to the use of Section 303(d) and TMDLs. Moreover, those legal prerequisites depend in turn on the development of a water quality database adequate to support numeric nutrient water quality criteria. This report recommends some initial steps necessary to develop the information necessary for the EPA and the states to establish numeric nutrient water quality criteria. Specifically, the Nutrient Control Implementation Initiative (NCII) recommended in this report will provide basic information needed for states to set water quality standards (and which, in turn, could lead to the establishment of a basinwide TMDL, if ever it was decided to do that). Regarding the roles of nutrients and sediment fluxes, forms of nitrogen and phosphorus are contained in excess levels in Mississippi River discharge into the Gulf of Mexico. Both nutrients contribute to overenrichment of the northern gulf’s coastal waters, large algae blooms, and subsequent hypoxic conditions. It is beyond the scope of this committee’s charge and report to analyze and present in detail the respective roles of these nutrients; further, these types of analyses have been performed by many other scientists and groups of scientists and a large body of literature is available to the interested reader. An excellent starting point is the 2007 report from the EPA Science Advisory Board entitled, Hypoxia in the Northern Gulf of Mexico (USEPA, 2007), which provides a detailed and up-to-date review of the roles of nitrogen and phosphorus in gulf hypoxia. Nevertheless, some explanation of the roles of nitrogen and phosphorus in gulf hypoxia is appropriate here. There is scientific consensus that nitrogen (its nitrate form, more specifically) is causing the northern Gulf of Mexico hypoxic zone in the largest areas and for the longest period (USEPA, 2007). Phosphorus also is a factor, but only in localized areas in the gulf. Phosphorus also is causing impairments in the upper river basin, such as in Lake Pepin on the Mississippi River. Comprehensive nutrient control actions for water quality improvements across the Mississippi River basin and into northern Gulf of Mexico therefore will include both nitrogen and phosphorus control measures. This rationale underpins the EPA SAB recommendation for a “dual nutrient strategy” to reduce the size of the hypoxic zone (USEPA, 2007). Through the rest of this report, references to “nutrients” or “nutrient control” can be considered as referring to both nitrogen and phosphorus management unless specified otherwise. Sediment transport affects hypoxia primarily through downstream transport

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INTRODUCTION 11 of phosphorus that is attached to fine sediment particles. Sediment transport dynamics across the Mississippi River basin have changed markedly in the past two centuries; for instance, much less sediment is transported down the Missouri River system and to the Gulf of Mexico compared to 200 years ago, and sediment deprivation is a significant problem in coastal Louisiana. Soil conservation actions across the river basin could further reduce sediment loadings in some areas and thus reduce phosphorus loadings somewhat. Sediment management actions by themselves, however, would not likely have a large effect on downstream phosphorus transport or on gulf hypoxia. Although phosphorus limitation of phytoplankton production does occur closer to the delta and under high discharge conditions, the driver for this phosphorus limitation is the high nitrogen loads compared to phosphorus. Overall, nitrogen loadings to the gulf are primarily responsible for the severity and extent of hypoxia, which has changed in parallel with increasing nitrogen inputs. This report contains five following chapters. Chapter 2 is entitled “Nutrient Inputs and Water Quality Effects.” It presents and discusses the scientific understanding and nature of northern Gulf of Mexico hypoxia and various efforts to manage this water quality problem better. It is presented as fundamental background information on key water quality issues and problems as they relate to this committee’s statement of task. Chapter 3 is entitled “Getting Started: A Nutrient Control Implementation Initiative.” It presents recommendations for a program to help better monitor and control nutrient yields across the Mississippi River basin. It addresses point 1 in this committee’s statement of task. Chapter 4 is entitled “Allocating Nutrient Load Reduction Targets.” It discusses factors to be considered in allocating load reduction targets and provides advice to be used in making these decisions across the Mississippi River basin. It addresses point 2 in this committee’s statement of task. Chapter 5 is entitled “Monitoring the Effectiveness of Nutrient Control Actions and Policies.” It offers advice for a more formal and structured program for evaluating changes in water quality across the river basin and into the northern Gulf of Mexico. It addresses point 3 in this committee’s statement of task. Chapter 6 is entitled “Overcoming Perceived Obstacles to Action.” It identifies several possible objections to decisive actions for improving water quality and nutrient pollutant control, and reasons why these objections need not delay implementation of this report’s recommendations.

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