11-19-07 Science Advisory Board (sab) Hypoxia Panel Draft Advisory Report Do Not Cite or Quote

Название11-19-07 Science Advisory Board (sab) Hypoxia Panel Draft Advisory Report Do Not Cite or Quote
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The SAB Panel’s Approach

The NOAA, CENR, and Task Force documents (see Section 1.2 above) provide a comprehensive scientific review of hypoxia causes, and potential mitigation and control actions through about 1999 to 2000. Further, more recent science and management information on the Gulf and MARB has been captured in the Task Force sponsored symposia, literature search, MART reports, and CEAP activities. Accordingly, the SAB Panel initiated its deliberations by reviewing these documents. The SAB Panel invited the chairs of the four symposia to present summaries of key findings, and also invited selected researchers (see acknowledgements) currently working on hypoxia issues to present their recent work. The SAB Panel also relied on the individual and collective experience and expertise of its members to provide additional relevant publications and information to assist its deliberations. The SAB Panel convened four public face-to-face meetings and 15 public teleconferences to deliberate and develop this state-of-the-science report (background and other materials for the meetings may be found at: http://www.epa.gov/sab/panels/hypoxia_adv_panel.htm).

The SAB Panel recognized the inherent complexity and connectivity between the Mississippi –Atchafalaya River basin and Gulf of Mexico and agreed that a systems perspective within an adaptive management framework was needed. The systems approach allowed understanding of feedback loops so that perturbations in one part of a system affect the interrelationships and stability of the system as a whole. Adaptive management seeks to maximize flexibility in management so that learning and adjustments can occur. Adaptive management employs six basic operating principles: 1) resources of concern are clearly defined; 2) conceptual models are developed during planning and assessment; 3) management questions are formulated as testable hypotheses to guide inquiry; 4) management actions are treated like experiments that test hypotheses to answer questions and provide future management guidance; 5) ongoing monitoring and evaluation is necessary to improve accuracy and completeness of knowledge; and 6) management actions are revised with new cycles of learning.

This report considers models as essential for understanding the inherent complexities of the MARB and the NGOM. Additionally, the collection of critical data at appropriate spatial and temporal scales is absolutely necessary to optimize future research and management actions. Data collection should be based on a well-defined conceptual model of the overall system. Monitoring programs will often provide data for existing models and assist with broader interpretations of data and information. In summary, a systems perspective combined with an adaptive management approach will greatly enhance scientific understanding and management of hypoxia in the MARB and the NGOM.

This report deals largely with the review of research and findings since the Integrated Assessment. Background material and findings prior to 2000 are used when appropriate or when instrumental to understanding the relative importance of more recent work. However, those interested in the details of the Integrated Assessment and the six topical reports that provided the scientific basis for the assessment are referred directly to those documents.

  1. Characterization of Hypoxia

    1. Processes in the Formation of Hypoxia in the Gulf of Mexico.

The hypoxic region along the northern Gulf of Mexico (NGOM) extends up to 125 km offshore and to 60 m water depth, has substantial variability with an average mid-summer areal extent of 16,500 km2 (2001-2007), and extends in some years from the Mississippi River mouth westward to Texas coastal waters (Rabalais et al., 2007). This hypoxic region (Figure 1) occurs along a relatively shallow, open coastline with complex circulation and water column structure typical of many coastal regions and includes massive inputs of freshwater, weak tidal energies, seasonally varying stratification strength, generally high water temperature, wind effects from both frontal weather systems and hurricanes, and mixing of river plumes from the Atchafalaya and Mississippi Rivers and other smaller sources (DiMarco et al., 2006; Hetland and DiMarco, 2007). The plumes of the Mississippi and Atchafalya Rivers can be observed as areas of highly turbid low salinity surface water. The limits of these plumes have been defined in different ways, but in satellite imagery their boundaries can be clearly observed as sharp color discontinuities. Since the release of the Integrated Assessment and the Action Plan in 2001, the measured areal extent of the hypoxic region has averaged 16,500 km2, with a range of 8,500 to 22,000 km2. Many reports from both the Integrated Assessment and post-Integrated Assessment periods concluded that physical and morphological characteristics such as these make the NGOM prone to hypoxic conditions.

      1. Historical Patterns and Evidence for Hypoxia on the Shelf

An important question regarding hypoxia on the Mississippi River shelf is how far back in time has hypoxia been observed? Is it a recent phenomenon or has hypoxia been a regular natural feature of a productive shelf region? Unfortunately the monitoring data are not entirely sufficient to address this question, for only a limited number of measurements are available prior to the time when wide-spread hypoxia was first observed on the Louisiana shelf in the mid-1980s (Rabalais et al., 1999a). However, a limited number of additional paleoecological studies have been carried out on the Mississippi River shelf since the Integrated Assessment. All studies from dated sediment cores show recent increases in low oxygen concentrations with time, although the precise timing and response varies depending upon the proxy studied and the dating of cores. The accumulated body of evidence shows that the pattern of change is concomitant with recent (since the 1960s) increases in nutrient loading from the Mississippi River causing increasingly severe hypoxia on the shelf. The spatial distribution of reliably dated sediment cores, with most cores taken on the southeastern Louisiana shelf just west of the Mississippi River delta, is not sufficient to determine the increases in the spatial extent of hypoxia with time.

A limiting factor in all paleoecological studies is the availability of undisturbed sediment cores to provide an accurate picture of changes through time. This is a particular challenge in a hydrologically dynamic, relatively shallow environment as found on the Mississippi River shelf with resuspension processes, movement of fluid muds, mixing by benthic organisms, and more recently sediment disturbance of upper sediment layers through bottom trawling. Despite these challenges, a number of reasonably dated sediment cores, primarily within the Louisiana bight, have provided a coherent picture of changes in hypoxia with time.

Bacterial pigments measured in sediments at one location on the Louisiana shelf were characteristic of anoxygenic phototrophic sulfur bacteria and have their highest concentrations between 1960 and the present (Chen et al., 2001). These bacteriopigments were not present prior to 1900. Further evidence of increased hypoxia is provided by Chen et al. (2001) using algal pigments, which show increases in the 1960s. The increase in these pigments reflects enhanced preservation with hypoxia as well as nutrient-driven increases in production. Rabalais et al. (2004, 2007) also report increases in algal pigment concentrations over time from a number of sediment cores, with gradual changes from 1955 to 1970, followed by a steady increase to the late 1990s. However, the patterns observed by Rabalais et al. (2004, 2007) are confounded by the rapid degradation of carbon and algal pigments in upper surface sediments with most studies of sediment pigments correcting for diagenesis by normalizing pigments with organic carbon (Leavitt and Hodson, 2001). In addition, there is some evidence for spatial increases in hypoxic extent through time: increases in pigment concentrations from one sediment core from west of the Atchafalaya River outflow suggests that nutrient-driven increases in production occurred later at this location than in the Mississippi River Bight (Rabalais et al., 2004). There has been an increased accumulation of total organic carbon and biogenic silica in recent sediments near the mouth of the Mississippi River (Turner and Rabalais, 1994; Turner et al., 2004), although the spatial and temporal variations observed between dated sediment cores are large.

Several studies have examined changes in the benthic foraminiferal community in dated sediment cores (Platon and Sen Gupta, 2001; Osterman et al., 2005; Platon et al., 2005). Different species of bottom living benthic foraminifera are particularly sensitive to changes in bottom water oxygen concentrations, and the abundance of these species is a widely used indicator of hypoxia. Significant changes in the composition of the benthic foraminiferal community have occurred in the past century. Several indicators, e.g., the PEB index (the relative abundance of three low-oxygen tolerant species of benthic foraminifers; Pseudononin altlanticum, Epistominella vitrea, and Buliminella morgani) (Osterman et al., 2005) and the A/P ratio (agglutinated to porcelaneous orders) (Platon et al., 2005) indicate that increases in the occurrence of low oxygen events have occurred over the past 50 years (Figure 3). In addition, the porcelaneous genus Quinqueloculina, an organism that occurs where dissolved oxygen concentrations are higher than 2 mg/l, was present but has disappeared from the foraminiferal community since 1900, indicating that prior to this time there was sufficient oxygen at the sediment-water interface to enable survival of such species (Rabalais et al., 2007). Osterman et al. (2005) have shown that several probable low oxygen events that occurred in the past 180 years are associated with high Mississippi River discharge rates, although the recent changes in foraminiferal communities are more extreme than any that occurred in the past. The data support the interpretation that hypoxia is a recent phenomenon and has been amplified from an otherwise naturally occurring process.
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