11-19-07 Science Advisory Board (sab) Hypoxia Panel Draft Advisory Report Do Not Cite or Quote




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Appendices


  1. Appendix A: Studies on the Effects of Hypoxia on Living Resources


The abstracts in this appendix all came from a workshop sponsored by the NOAA Center for Sponsored Coastal Ocean Research, held at Tulane University, New Orleans, LA held September 25-26, 2006.


Brouwer, Marius, 2006. “Changes in Gene and Protein Expression and Reproduction in Grass Shrimp, Palaemonetes pugio, Exposed to Chronic Hypoxia” Presentation at “Hypoxia Effects on Living Resource in the Gulf of Mexico” NOAA Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA. September 25 – 26, 2006.


Abstract: Hypoxic conditions in estuaries are one of the major factors responsible for declines in habitat quality. Previous studies examining the effects of hypoxia on crustacea have focused on individual/population-level, physiological or molecular responses but have not considered more than one type of response in the same study. The objective of this study was to integrate disciplines by examining the responses of grass shrimp to chronic hypoxia both at the molecular and whole animal level. Hypoxia-induced alterations in gene expression were screened using custom cDNA macroarrays containing 78 clones from a hypoxia-responsive suppression subtractive hybridization (SSH) cDNA library. Grass shrimp respond differently to moderate (2.5 ppm DO) versus severe (1.5 ppm DO) chronic hypoxia. The initial response to moderate hypoxia was down-regulation of genes coding for ribosomal proteins, HSP 70 and MnSOD. The initial response after short-term (3 d) exposure to severe hypoxia was upregulation of genes involved in oxygen uptake/transport and energy production, such as hemocyanin and ATP synthases. The major response by day 7 was an increase of transcription of genes present in the mitochondrial genome, together with upregulation of a putative heme binding protein and the iron storage protein, ferritin. By day 14 a dramatic reversal was seen, with a significant downregulation of transcription of genes in the mitochondrial genome. Both ferritin and the heme binding protein were downregulated as well. Levels of Hypoxia Inducible Factor (HIF1-alpha) remained unchanged. The macroarray data were validated using real-time qPCR. Changes in mitochondrial proteins were examined by separating proteins in 2 dimensions (IEF and reverse phase) followed by MS. At the organismal level, hypoxia exposure resulted in marked effects on shrimp egg production and larval survival, suggesting population-level implications of long-term hypoxia.


Baltz, Donald M., Hiram W. Li, Philippe A. Rossignol, Edward J. Chesney and Theodore S. Switzer, 2006. “A Qualitative Assessment of the Relative Effects of Bycatch Reduction,Fisheries and Hypoxia on Coastal Nekton Communities in the Gulf of Mexico”, Presentation at “Hypoxia Effects on Living Resource in the Gulf of Mexico” NOAA Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA. September 25 – 26, 2006.


Abstract: We applied qualitative mathematical models to develop an understanding of linkages that influence shrimp, fishes, and fisheries in coastal Louisiana where biotic communities face many natural and anthropogenic stressors, one of which is fishing activities related to the harvest of shrimp. Shrimp trawling ranks high in terms of impact on nekton and their habitats, and like most fishing gears catches non-target species or sizes that are not marketed. These individuals, termed 'bycatch', are often returned to the water in dead or dying condition. Numerous other individuals are not 'caught' per se but also suffer the 'effects of fishing', that can degrade habitats or cause injuries leading to mortality. Modeling was used to examine the effects of fishing and bycatch mortality on community structure in the 'Fertile Fisheries Crescent' and how major stressors interact with hypoxia to influence fisheries. We explored direct and indirect interactions between shrimp, their predators, bycatch species, and shrimp landings. A major finding was that bycatch reduction efforts may feedback on fisheries and shrimp populations in an unexpectedly negative manner. Another was that changes in community structure that might be attributed to hypoxia are also possible from fishing alone. To corroborate our models, we analyzed 15 years of quantitative data on National Marine Fisheries Service shrimp landings, Louisiana Department of Wildlife and Fisheries (LDWF) gillnet surveys, and LDWF shrimp trawl surveys from central Louisiana. Abundant bycatch and other species were summarized into several functional groups including small and large shrimp predators, non-shrimp predators, major bycatch consumers, minor bycatch consumers, and non-bycatch consumers. Factor and correlation analyses of quantitative data for functional groups on a bimonthly basis corroborated results from the qualitative models, and combined indicated that shrimp abundance and shrimp landings would likely suffer from increased natural mortality if the shrimp-fishery bycatch was substantially reduced.


Craig, J. Kevin and Larry B. Crowder, 2005. “Hypoxia-induced habitat shifts and energetic consequences in Atlantic croaker and brown shrimp on the Gulf of Mexico shelf” Marine Ecology Progress Series, Vol. 294, pp 79-94.


Abstract: This paper evaluates the effects of hypoxia-induced habitat loss on Atlantic croaker and brown shrimp. The compare spatial distributions and the relationship to abiotic factors, including temperature, dissolved oxygen and salinity across years with differing levels of hypoxia using 14 years of fishery-independent trawl data. They find that hypoxia results in considerable shifts in temperature and oxygen conditions that croaker and brown shrimp experience. Croaker typically occupy relative warm, inshore waters. During periods of hypoxia, croaker remain in the warmest inshore waters, but are also displaced to cooler offshore waters. Brown shrimp typically are distributed more broadly and further offshore. During periods of hypoxia, brown shrimp shift to warm inshore waters and cooler waters near the offshore edge of the hypoxic zone. The shifts in spatial distribution are reflected in decreases in water temperature for croaker that are displaced offshore the hypoxic region, and increases in water temperature for brown shrimp that are displace inshore of the hypoxic zone. Both species also face increased variance in water temperatures due to hypoxia-induced habitat displacement. Despite avoidance of the lowest oxygen waters, high densities of croaker and brown shrimp occur in areas of 1.6 to 3.7 mg/l near the offshore hypoxic edge. Shifts in spatial distribution during severe hypoxia may impact organism energy budgets. For example, laboratory studies indicate low oxygen impacts individual movement, growth, and mortality (Wannamaker & Rice 2000, Taylor & Miller 2001, Wu 2002). High croaker and shrimp densities near the hypoxic edge likely have implications for trophic interactions as well as the harvest of both target (brown shrimp) and nontarget (croaker) species by the commercial shrimp fishery. Croaker may benefit from high concentrations of brown shrimp at the edge of the hypoxic zone, while brown shrimp may become more susceptible to predation by croaker.


Craig, J. Kevin, Larry B. Crowder, and Tyrrell A. Henwood, 2005. “Spatial distribution of brown shrimp (Farfantepenaeus aztecus) on the northwestern Gulf of Mexico shelf: effects of abundance and hypoxia” Canadian Journal of Fisheries and Aquatic Science. Vol. 62 pp 1295-1308.


Abstract: This paper uses fishery-independent hydrographic and bottom trawl surveys from 1983–2000 used to test for density dependence and effects of hypoxia on spatial distribution of brown shrimp. The spatial distribution of shrimp was found to be positively related to abundance on the Texas shelf, but negatively related to abundance on the Louisiana shelf. Density dependence was weak, and may have been due to factors other than habitat selection. Large-scale hypoxia (up to ~20 000 km2) on the Louisiana shelf occurs in regions of typically high shrimp density, resulting in loss of up to 25% of shrimp habitat on the Louisiana shelf. They also find shifts in distribution and densities both inshore and offshore of the hypoxic region. Results placed in terms of the generality of density-dependent spatial distributions in marine populations. Potential consequences of habitat loss and associated shifts in distribution due to low dissolved oxygen. They note that shifts in spatial distribution may precede major stock declines, and thus could potentially serve as an early warning sign of future declines in abundance (Gomes et al.1995; Rose et al. 2000; Overholtz 2002).


Diaz, Robert, 2001. “Overview of Hypoxia around the World” Journal of Environmental Quality Vol. 30, No. 2. (March-April) 275-281.


Abstract: This paper summarizes effects of hypoxia in various locations around the world, which provides lessons for potential consequences of hypoxia in the Gulf of Mexico. They note that hypoxia was probably not a prominent feature of the shallow continental shelf in the Northern Gulf of Mexico prior to the 1920’s through 1950’s based on geo-chronology of sediment cores. A longer, 2000-year chronology in the Chesapeake indicates that early European settlement of the watershed was a key feature that set the stage for current oxygen problems. Improved water quality in Lake Erie is the best example in the US that large ecosystems can respond positively to nutrient regulation, but the time interval for recovery can be long. In Lake Erie, the extent of hypoxia was similar between 1970 and 1990 despite reduced nutrient loads. Delayed improvements in oxygen levels are argued to be consistent with mechanisms and processes that contribute to ecosystem’s resilience (Charlton et al, 1993), and as a consequence improvements in oxygen may not be noticed for decades following implementation of management actions.


Hendon, Laura A. Erik A. Carlson, Steve Manning, and Marius Brouwer, 2006. “Cross-talk between Pyrene and Hypoxia Signaling Pathways in Embryonic Cyprinodon variegates” Presentation at “Hypoxia Effects on Living Resource in the Gulf of Mexico” NOAA Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA. September 25 – 26, 2006.


Abstract: The aryl hydrocarbon nuclear translocator (ARNT) is a general dimeric partner for the aryl hydrocarbon receptor (AhR) and hypoxia-inducible factor one alpha (HIF1-α). The AhR/ARNT complex binds to promoters in target genes, such as CYP1A1, resulting in alterations in gene expression, while the HIF1-α/ARNT heterodimer binds to hypoxia response elements in target genes, such as VEGF. While AhR is activated by PAHs, such as pyrene, HIF1-α is activated by hypoxia. Since ARNT is a general dimeric partner for both AhR and HIF1-α, possible cross-talk may exist between the two pathways in which the activation of one results in inhibition of the other. The objective of this study was to determine if pyrene-activation of AhR2, or hypoxia-activation of HIF1-α could sequester the ARNT protein away from HIF1-α and AhR2, respectively, resulting in reduced developmental toxicity associated with hypoxia or pyrene alone in embryonic Cyprinodon variegatus. As a first step to examine this hypothesis, we cloned AhR2, CYP1A1 (PAH-activated gene) and VEGF (HIF-activated gene). Next, pyrene (20, 60, and 150 ppb) and hypoxia’s (1-2 ppm) individual developmental toxicity endpoints were determined, together with CYP1A1 and VEGF expression levels using real-time quantitative RT-PCR. Combined treatments of pyrene and hypoxia were examined in order to determine sequestration of the ARNT protein and developmental toxicity endpoints. Results demonstrate that pyrene-treated embryos alone develop toxicity endpoints such as pericardial edema and dorsal body curvature. Hypoxia-treated embryos alone display delayed hatching and less-developed characteristics in comparison to normoxic treatments. Under hypoxic conditions alone, real-time quantitative RT-PCR determined that VEGF was down-regulated significantly at 24 hpf, while at 14 dph, the HIF-activated gene was significantly up-regulated. Pyrene-treated embryos showed a dose-dependent and time-dependent response in CYP1A1 regulation with increasing expression over time of exposure. The combined effects of pyrene and hypoxia appeared to alter VEGF expression, while CYP1A1 remained unaffected in C. variegatus.


Montagna, Paul, Ben Hodges, David Maidment and Barbara Minsker, 2006. “Long-Term Studies of Hypoxia in Corpus Christi Bay: The Cybercollaboratory Testbed” Presentation at “Hypoxia Effects on Living Resource in the Gulf of Mexico” NOAA Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA. September 25 – 26, 2006.


Abstract: Corpus Christi Bay is a shallow (~3.2 m) enclosed bay with a level bottom. It experiences high wind speeds, temperatures, and receives a low amount of fresh water inflow. Hypoxia has been documented in the southeastern region of Corpus Christi Bay every summer since 1988. Hypoxia found in bottom waters, usually within 1 m from bottom, when the bay is stratified. Over the last 20 years, there has been increased surface water temperatures, but no change in nutrient concentrations, which are low. Ecosystem processes during salinity stratification likely drive the hypoxia, because respiration is stimulated and the surface and bottom water masses are not mixing. Hypoxia causes reduced benthos abundance, biomass, and diversity. The reduction is due to loss of deeper-dwelling organisms, and is likely a direct effect (stress or death), and not an indirect effect (increased predation by exposure to the surface). There is increased interest in developing real-time environmental forecasting and management to better monitor and understand large-scale, event-based environmental phenomena, e.g., hypoxia and flooding. A new project focuses on creating a new Corpus Christi Bay Observatory Testbed Project to demonstrate how cyberinfrastructure can enable real-time forecasting from a hydrographic information system. Although only a few months old, the testbed project has already created a few simple models and visualization tools that improved sampling designs to better identify hypoxic events, extent, and intensity.


O’Connor, Thomas and David Whitall, 2007. “Linking Hypoxia to Shrimp Catch in the Northern Gulf of Mexico”, Marine Pollution Bulletin Vol. 54, no. 4 (April), Pp 460-463.


Abstract: This study carries out updates the statistical analysis of Zimmerman and Nance () of the effect of hypoxia on commercial shrimp landings data for 1985 through 2004. This study uses commercial landings data, not the interview data, and is therefore does not use spatial data on the location of catch. The paper confirms the results of Zimmerman and Nance that there is no correlation of hypoxic area with landings of white shrimp or with landings of brown shrimp in Louisiana, but there is a significant correlation with the total combined landings in Texas and Louisiana. Unlike Zimmermann and Nance, they find a significant relationship between the hypoxic area and brown shrimp landings in Texas alone. Hypoxia explains about 32% of the variance in catch using data for catch in July and August, and about 27% of the variance in catch using annual data.


Perez, Amy N., Leon Oehlers and Ronald B. Walter, 2006. “Detection of Hypoxia-related Proteins in Medaka (Oryzias latipes) by Difference Gel Electrophoresis and Identification by Sequencing of Peptides using MALDI-TOF Mass Spectrometry” Presentation at “Hypoxia Effects on Living Resource in the Gulf of Mexico” NOAA Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA. September 25 – 26, 2006.


Abstract: Multidimensional separation techniques combined with matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry (MALDI-TOF/TOF-MS) were used to identify hypoxia-related biomarker proteins in tissues of medaka fish (Oryzias latipes) and medaka cultured cells. The multidimensional protein/peptide separation methods used included two-dimensional difference gel electrophoresis (2D-DIGE) using fluorescent cyanine dyes, and gel electrophoresis combined with reversed phase liquid chromatrography of tryptic peptides isotopically labeled with 16O or 18O (geLC-MS). In both methods, control and hypoxia-treated tissue or cell protein extracts were differentially labeled, combined in 1:1 mass ratios, and subjected to separation and MALDI-TOF/TOF-MS analysis of tryptic peptides derived from proteins exhibiting significant changes in expression upon hypoxia exposure. Prior to MALDI-TOF/TOF-MS analysis, the peptides were N-terminally sulfonated using the derivatizing reagent 4-sulfophenyl isothiocyanate (SPITC) to enhance the post-source decay (PSD) fragmentation spectra of the peptides in MALDI-TOF/TOF-MS, which was shown to dramatically improve de novo sequencing of labeled peptides. The methods described here were used to monitor and analyze the changes in protein resulting from exposures of both cultured medaka cells and medaka fish to hypoxic conditions (0.8-1.0 mg/L dissolved oxygen) for periods up to 120 hours. We have identified a number of potential candidate biomarker proteins differentially-regulated upon exposure to hypoxia, including carbonic anhydrase, hemoglobin, calbindin, aldolase, glutathione-S-transferase, succinate dehydrogenase, and lactate dehydrogenase.


Rabalais, Nancy N. 2006. “Benthic Communities and the Effects of Hypoxia in Louisiana Coastal Waters” Presentation at “Hypoxia Effects on Living Resource in the Gulf of Mexico” NOAA Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA. September 25 – 26, 2006.


Abstract: The responses of the benthic fauna to decreasing concentration of dissolved oxygen follow a fairly consistent pattern of progressive stress and mortality as the oxygen concentration decreases from 2 mg l-1 to anoxia (0 mg l-1). Motile organisms (fish, portunid crabs, stomatopods, penaeid shrimp and squid) are seldom found in bottom waters with oxygen concentrations less than 2 mg l-1. Below 1.5 to 1 mg l-1 oxygen concentration, less motile and burrowing invertebrates exhibit stress behavior, such as emergence from the sediments, and eventually die if the oxygen remains low for an extended period. At minimal concentrations just above anoxia, sulfur-oxidizing bacteria form white mats on the sediment surface, and at 0 mg l-1, there is no sign of aerobic life, just black anoxic sediments. The composition of the benthic communities reflects differences in sedimentary regime, seasonal input of organic material and seasonally severe hypoxia/anoxia. Decreases in species richness, abundance and biomass of organisms are dramatic when bottom-waters are affected by severe hypoxia/anoxia. Some macroinfauna, the polychaetes Ampharete and Magelona and a sipuculan Aspidosiphon, are capable of surviving extremely low dissolved oxygen concentrations and/or high hydrogen sulfide concentrations. Macroinfauna, primarily opportunistic polychaetes, increase in the spring following flux of primary produced carbon, and increase to a lesser extent in the fall following the dissipation of hypoxia. Fewer taxonomic groups characterize the severely affected benthos, and long-lived, higher biomass and direct-developing species are mostly excluded. Suitable feeding habitats (in terms of severely reduced populations of macroinfauna that may characterize substantial areas of the seabed) are frequently removed from the foraging base of demersal organisms, including the commercially important penaeid shrimps.


Switzer, Theodore S., Edward J. Chesney, and Donald M. Baltz, 2006. “Habitat Selection by Flatfishes along Gradients of Environmental Variability: Implications for Susceptibility to Hypoxia in the Northern Gulf of Mexico” Presentation at “Hypoxia Effects on Living Resource in the Gulf of Mexico” NOAA Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA. September 25 – 26, 2006.


Abstract: Although eutrophication in the northern Gulf of Mexico contributes to the high fisheries productivity characteristic of the region, nutrient over-enrichment leads to the seasonal formation of hypoxic (< 2 mg L-1 O2) bottom water along the Louisiana-Texas continental shelf. Despite an increase in the magnitude and duration of hypoxic episodes in recent decades, fisheries landings have remained high; nevertheless, hypoxia remains a persistent threat to the long-term sustainability of regional fisheries production. The greatest threat to mobile nekton is likely the influence of reduced dissolved oxygen concentrations on habitat quality, potentially forcing the movement of individuals and/or prey from generally favorable habitats. At the population level, these movements may result in altered spatial distributions that reflect selection of resources along gradients of environmental variability. To unravel the potential influence of hypoxia on the distribution of nekton, we examined patterns of habitat use by several abundant flatfishes based on data collected during summer SEAMAP groundfish surveys from 1987 to 2000. Results from habitat suitability analyses indicated that most flatfishes selected a restricted range of suitable depths, temperatures, and salinities. Although most flatfishes were tolerant of moderately-low dissolved oxygen concentrations, hypoxic environments were generally avoided, indicating that hypoxia likely renders large areas of the Gulf of Mexico unsuitable. In comparisons of spatial habitat suitabilities between years of moderate (< 15,000 km2) and severe hypoxia (>15,000 km2), all flatfishes exhibited a reduction in the suitability of areas immediately west of the Mississippi River and a concomitant increase in suitability within adjacent areas. Altered spatial distributions corresponded to species-specific suitabilities along depth, temperature, and salinity gradients, indicating that habitat suitability analyses may be effective in predicting population-level responses to hypoxic episodes.


Wells, Melissa C., Zhenlin Ju, Sheila J. Heater and Ronald B. Walter, 2006. “Microarray Gene Expression Analyses in Medaka (Oryzias latipes) Exposed to Hypoxia” Presentation at “Hypoxia Effects on Living Resource in the Gulf of Mexico” NOAA Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA. September 25 – 26, 2006.


Abstract: We are investigating the genomic and proteomic effects of hypoxia exposure using the Japanese medaka (Oryzias latipes) aquaria fish model as a tool for biomarker discovery. We have developed a hypoxia exposure system allowing programmable exposure scenarios and have initiated experimental assessment of changes in gene expression and protein abundance using microarray and 2D-DIGE gel analyses of hypoxia exposed fish. We present the design, construction, validation, and subsequent use of a medaka 8,046 (8K) unigene oligonucleotide microarray to begin the study of hypoxia exposure. Array performance was validated via self-self hybridization. Optimization of sample size needed for robust array data, based upon the number features detected and the signal intensity, suggest 2 µg total RNA as a starting template for amplification is sufficient. For treatment, adult medaka are exposed to a hypoxic environment of 4% dissolved oxygen (DO) for 2 days and then the DO lowered to 2% for an additional 5 days. Upon sacrifice, changes in gene expression in brain, liver, skin, and gill tissues of these fish were assessed in conjunction with matched control fish exposed similarly to 18% DO. Analyses of array results identified 501 features from brain, 442 from gill, and 715 features from liver that exhibit statistically significant changes in transcript abundance upon hypoxia exposure. Nine features were found to exhibit common expression patterns between all three tissues. Data mining of the array results suggest hypoxic exposure results in a general slowdown of metabolic function. Real-time PCR was then employed to support the microarray results and this independent validation agreed well with the microarray findings. Overall these results indicate the medaka microarray will be a sound diagnostic tool for changes in gene expression due to hypoxia exposure.


Zimmerman, Roger J. and James M. Nance, 2001. “Effects of Hypoxia on the Shrimp Industry of Louisiana and Texas” Chapter 15 in Rabalais, N.N. and R.E. Turner, Coastal Hypoxia: Consequences for Living Resources Coastal and Estuarine Studies, 58 pp 293-310.


Abstract: This study carries out a statistical test for effects of hypoxia on commercial catch of shrimp in the Gulf of Mexico for 1985-97. The analysis combines landings data and interview data on fishing effort, catch and location of each trip. The analysis is spatially explicit, based on catch in 9 statistical subareas in Louisiana and Texas, with each subarea divided into 10 depth zones. Zimmerman and Nance found no correlation of hypoxic area with landings of white shrimp or with landings of brown shrimp in Louisiana, but they found a statistically significant relationship between hypoxia and combined landings in Texas and Louisiana. The finding of no relationship for white shrimp is consistent with prior expectations, because white shrimp are less sensitive to hypoxia (Renaud, 1986), and because white shrimp habitat is mostly in-shore the hypoxic region. In comparison, brown shrimp travel from inshore areas to offshore in order to spawn. Since brown shrimp migrate through the hypoxic region, they are more likely to be effected by hypoxia. The absence of a significant relationship between the size of the hypoxic region and catch of brown shrimp in Louisiana may be explained by the fact that much of the catch in Louisiana occurs in-shore of the hypoxic region, while catch in Texas occurs offshore.


Zou, Enmin, 2006. “Impacts of Hypoxia on Physiology and Toxicology of the Brown Shrimp Penaeus aztecus” Presentation at “Hypoxia Effects on Living Resource in the Gulf of Mexico” NOAA Center for Sponsored Coastal Ocean Research, Tulane University, New Orleans, LA. September 25 – 26, 2006.


Abstract: The brown shrimp, Penaeus aztecus, in the northern Gulf of Mexico is faced with dual stresses of environmental hypoxia, which occurs as a result of oxygen depletion from microbial decomposition of organic materials from algal blooms, and pollution from polycyclic aromatic hydrocarbons (PAHs) from petroleum and gas production on the continental shelf of the northern Gulf of Mexico. This study aimed to address the questions of 1) whether the presence of PAH contamination makes penaeid shrimps more susceptible to hypoxia and 2) whether hypoxia can promote PAH bioaccumulation in penaeid shrimps. The susceptibility of shrimps to hypoxia was represented by the oxyregulating capacity, a physiological parameter that describes how well an animal regulates its oxygen consumption when subjected to hypoxia. It was found that acute exposure to naphthalene significantly reduced the oxyregulating capacity of Penaeus aztecus. An ensuing consequence of a decrease in oxyregulating ability is that the stress from the lack of oxygen would set in sooner in the presence of PAH contamination than when shrimps are in the clean environment. Hypoxia was found to have no significant effect on naphthalene bioaccumulation in Penaeus aztecus. The absence of a significant effect was attributed to increased naphthalene metabolism in the brown shrimp subjected to hypoxia.

  1. Appendix B: Mass Balance of Nutrients


Atmospheric deposition



The Integrated Assessment concluded that atmospheric deposition as a new nitrogen input to the Mississippi River basin was not as important as agricultural sources but that deposition nonetheless was a significant source (Goolsby et al., 1999). Atmospheric deposition of nitrogen generally shows a trend of increasing from west to east in the Mississippi basin, and deposition was a particularly important source of nitrogen in the Ohio River basin (Goolsby et al., 1999). The Integrated Assessment followed the net anthropogenic nitrogen input (NANI) budgeting approach established by the International SCOPE Nitrogen Project in assuming that deposition of oxidized nitrogen (NOy) is a new input of nitrogen while the deposition of ammonium is not but rather is a recycling of nitrogen emitted to the atmosphere from agricultural sources within the basin (Howarth et al., 1996). The oxidized nitrogen is presumed to come largely from fossil-fuel combustion and, thus, is not accounted for in any other input to the budget (Howarth et al., 1996; Goolsby et al., 1999). The Integrated Assessment further considered that the deposition of organic nitrogen was a new input of nitrogen (Goolsby et al., 1999).


The Integrated Assessment used monitoring data to estimate NOy deposition and made a very rough guestimate for the magnitude of deposition of organic nitrogen. They used data from the NADP for wet deposition and from CASTnet for dry deposition. This yielded an average estimate of NOy deposition for the Mississippi River basin for the time period 1988 to 1994 of 3.4 kg N/ha/yr (3 lb N/ac/yr), of which 2 kg N/ha/yr (1.8 lb N/ac/yr) was nitrate in wet deposition and 1.4 kg N/ha/yr (1.25 lb N/ac/yr) was NOy dry deposition (Goolsby et al., 1999). The assessment estimated the deposition of organic nitrogen as 1 kg N/ha/yr (0.89 lb N/ac/yr), yielding a total estimate for new nitrogen deposition of 4.4 kg N/ha/yr (3.9 lb N/ac/yr) (Goolsby et al., 1999). This can be compared with an estimate for NOy deposition derived from the GCTM model, which estimates deposition rates from data on emissions to the atmosphere and on rates of reaction and advection within the atmosphere (Prospero et al., 1996). For the Mississippi River basin for essentially the same time period used in the Integrated Assessment, the GCTM model suggested a total NOy deposition of 6.6 kg N/ha/yr (5.9 lb N/ha/yr), with 6.2 kg N/ha/yr (5.5 lb N/ac/yr) of this input being attributable to new inputs from fossil-fuel burning and 0.4 kg N/ha/yr (0.36 lb N/ac/yr) originating from natural sources (Howarth et al., 1996).


Holland et al. (1999, 2005) noted that deposition estimates based on monitoring data are typically lower than those from emission-based models across most of the United States. For the northeastern United States from Maine through Virginia, the estimates from the GCTM model (Howarth et al., 1996) are again almost twice as high as are estimates from NADP and CASTnet monitoring data (Boyer et al., 2002). There are many possible reasons for this discrepancy, but probably at least part of the problem lies with an underestimation of dry deposition by the CASTnet program (Holland et al., 1999; Howarth et al., 2006b; Howarth, 2006). Most CASTnet monitoring stations are purposefully located away from emission sources, and deposition is likely to be higher near these emission sources, creating a bias in the network. Further, the CASTnet program only estimates deposition of nitrogen in particles and deposition of nitric acid vapor. The deposition of several other gases (including NO, NO2, and nitrous acid vapor) is not measured. Deposition of these gases, which would be included in the estimates from the emission-based models, is likely to be particularly high near emission sources (Howarth, 2006). Both the GCTM and TM3 models only estimate deposition at coarse spatial scales, but a new emission-based model (CMAQ) shows promise for estimation at relatively fine spatial scales (Robin Dennis, NOAA, personal communication). Note that this model suggests very high NOy deposition rates near urban centers in the eastern US and associated with power plant emissions in the Ohio River basin (Figure 54).





Figure 54: Annual average deposition of NOy across the United States (kg N /hectare-year) based on beta-testing runs of the CMAQ model. Note the very high rates of deposition in the Ohio River basin. Courtesy of Robin Dennis, NOAA.



In the mass balance presented in Section 3.2, deposition was estimated as in Goolsby et al. (1999). Organic N was not included, however, as there it is unclear what the importance is of this form of N, or what an appropriate estimate would be (Keene et al., 2002). A comparison was made of deposition inputs by region of the NOy estimate used in the mass balance and deposition from the CMAQ model for 2001. For the upper Mississippi basin, NOy deposition was 4.2 kg N/ha/yr (3.8 lb N/ac/yr), the same as the CMAQ model1. For the Missouri basin both methods again gave similar estimates, with NOy deposition of 2.2 N/ha/yr (2 lb N/ac/yr), and CMAQ modeled deposition 2.1 kg N/ha/yr (1.9 lb N/ac/yr). For regions with more fuel combustion, the pattern was different, with an Ohio basin NOy estimate of 5.0 N/ha/yr (4.5 lb N/ac/yr), and the CMAQ model estimate of 8.8 kg N/ha/yr (7.8 lb N/ac/yr). For the lower Mississippi River basin, NOy was 3.7 kg N/ha/yr (3.3 lb N/ac/yr), and the CMAQ estimate 5.1 kg N/ha/yr (4.6 lb N/ac/yr). Overall, this supports mass balance analysis that for the upper Mississippi basin, atmospheric deposition is a small component of N inputs (about 8% of N inputs) and is more important in the Ohio region (about 16% of N inputs using the CMAQ model for 2001).


1CMAQ model unpublished results courtesy of Robin Dennis, NOAA, with analysis by states provided by Dennis Swaney, Cornell University; unpublished.

  1. Appendix C: EPA’s Guidance on Nutrient Criteria


In 2000, EPA recommended criteria to States and Tribes for use in establishing their water quality standards consistent with section 303(c) of the Clean Water Act (CWA). Under section 303(c) of the CWA, States and authorized Tribes have the primary responsibility for adopting water quality standards as State or Tribal law or regulation. The standards must contain scientifically defensible water quality criteria that are protective of designated uses. On its website at http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/, EPA provides recommended criteria for nutrients in four major types of waterbodies – lakes and reservoirs, rivers and streams (U.S. EPA, 2006b), estuarine and coastal areas, and wetlands – across fourteen major ecoregions of the United States. The SAB Panel asked EPA for a comparison of the SAB Panel’s proposed 45% reductions for TN and TP flux to the nutrient levels that would correspond to EPA’s recommended ecoregional criteria.


Before presenting that preliminary analysis, the following caveats are stressed.


    • EPA’s recommended ecoregional nutrient criteria are not laws or regulations; they are guidance that States and Tribes may use as a starting point for developing criteria for their water quality standards. Final criteria developed by States and Tribes may have concentrations higher or lower than EPA ecoregional recommendations, or, if scientifically defensible, not include a nutrient if an impact on “designated use” was not found.




    • EPA’s recommended ecoregional nutrient criteria do not take into account local site-specific conditions and “designated uses” for particular water bodies (e.g., recreation, water supply, aquatic life, agriculture).




    • EPA’s guidance for ecoregional nutrient criteria are based on ambient concentrations of nutrients (expressed in mg/L or ug/L) in various ecoregions. By contrast, the SAB Panel’s recommended reductions of TN and TP are based on flux (expressed in million metric tons of TN and TP discharged at the mouth of the Mississippi River). A direct comparison of concentrations to flux necessitates the simplifying assumption that percentage reductions in concentrations have a one-to-one correspondence with percentage reductions in flux.




    • EPA’s guidance for ecoregional criteria is based on estimated “reference conditions” i.e., reference sites chosen to represent the least culturally impacted waters of the class existing at the present time. The estimated reference conditions are based on the 25th percentile of the frequency distribution of nutrient concentration data available for each ecoregion. This assumption lends uncertainty to EPA’s guidance for ecoregional nutrient criteria.


Given these caveats, the following analysis by EPA Office of Water’s Office of Science and Technology and EPA’s Office of Research and Development allows some comparison between EPA’s guidance for ecoregional nutrient criteria and the SAB Panel’s proposed 45% nutrient reductions.



Comparison of SAB Nitrogen and Phosphorus Recommendations with EPA Nitrogen and Phosphorus Criteria Recommended Reference Conditions – Submitted by EPA’s Office of Water, 8-24-07.


Question: How do the 45% recommended reductions in nitrogen (N) and phosphorus (P) at the mouths of the Mississippi and Atchafalaya Rivers compare with the 25th percentile of TN and TP concentration data from ecoregions draining the Mississippi-Atchafalaya River Basin (MARB)?


Answer: This question is addressed with a preliminary approach. A more thorough approach is needed, but this would require a longer period of time.


The preliminary approach was developed by staff from the EPA Office of Research and Development’s Gulf Breeze Lab and the EPA Office of Water’s Office of Science and Technology using USGS loading estimates from the lower Mississippi River at St. Francisville, LA and the Atchafalaya River at Melville, LA over the past 20 years. This approach compares the 45% reduction in nitrogen and phosphorus recommended by the SAB, to the 25th percentiles of the distribution of data in EPA’s National Nutrient Database for total nitrogen (TN) and total phosphorus (TP) in each aggregate nutrient ecoregion of the MARB. These 25th percentiles represent EPA’s approximated reference conditions for those ecoregions.


It is important to note that these 25th percentile values are not intended to be implemented or promulgated directly as criteria. Rather, EPA developed the nutrient criteria recommendations with the intent that they serve as a starting point for States and Tribes to develop more refined criteria, as appropriate, to reflect local conditions. States and Tribes may adopt criteria that are higher or lower than these 25th percentiles. Text in two EPA documents help clarify the use of the ecoregional reference condition values. See introductions to the ecoregional criteria documents at http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/rivers/index.html and EPA’s Nutrient Criteria Technical Guidance Manual for Rivers and Streams (http://www.epa.gov/waterscience/criteria/nutrient/guidance/rivers/chapter_1.pdf).


Given this description, one can compare a 45% reduction in N and P measured in two locations to the estimated reference conditions in each of the MARB ecoregions to obtain a rough estimate of whether a 45% reduction could be more or less stringent than what could result if EPA’s recommended reference conditions were adopted without further modification, as state water quality standards.


Data Sources: River flow and nutrient flux are monitored and computed by the U.S. Geological Survey’s (USGS) National Stream Quality Accounting Network (NASQAN) program at numerous river gauge stations in the Mississippi River Basin. A description of the USGS NASQAN program, the flux estimation methodology and the downloadable data records are available at http://toxics.usgs.gov/hypoxia/. Monthly average nutrient concentrations were calculated from the USGS data as


The Monthly Average Nutrient Concentration = USGS Monthly Load/ Monthly Average discharge rate where monthly average discharge rates for the mainstem Mississippi River were calculated from daily discharge rates obtained at the Tarbert Landing, MS gauge (ID = 01100).

Nitrogen. The median monthly nitrate concentration for the combined Mississippi River at St. Francisville and Atchafalaya River at Melville over the period 1979 – 2007 is 1.24 mg/L. In comparison, historical data from the Mississippi River at St. Francisville indicate that the median nitrate concentrations during the period 1955-1970 was 0.6 mg/L.


Nitrate, as a component of TN is about 60% on average (based on USGS nutrient load data); thus 1.24 mg/L nitrate would extrapolate to 2.07 mg/L TN.


A proposed 45% reduction of 2.07 mg/L TN would yield a concentration of 1.14 mg/L TN.


The relevant EPA recommended ecoregional reference conditions for TN are:


Ecoregion IV - 0.56 mg/L

Ecoregion V - 0.88 mg/L

Ecoregion VI – 2.18 mg/l

Ecoregion VII - 0.54 mg/L

Ecoregion IX - 0.69 mg/L

Ecoregion X - 0.76 mg/L

Ecoregion XI - 0.31 mg/L


These values range from 27% to 191% of the estimated 1.14 mg/L TN that would result from a 45% reduction, with all but one value below 100% (the Corn Belt and northern Great Plains ecoregion VI). This suggests that a 45% reduction of estimated median monthly TN concentrations to 1.14 mg/L would likely be less stringent than could be obtained if states adopted EPA’s recommended reference condition values into state water quality standards for TN.


Phosphorus. Using the same data (Mississippi River at St. Francisville and Atchafalaya River at Melville, 1979-2007, monthly means), the median monthly concentration of TP is 202 ug/L. Thus a 45% reduction of 202 ug/L TP would yield a concentration of 111 ug/L.


The relevant EPA recommended ecoregional reference conditions for TP are:


Ecoregion IV – 23.00 ug/L

Ecoregion V – 67.00 ug/L

Ecoregion VI – 76.00 ug/L

Ecoregion VII – 33.00 ug/L

Ecoregion IX – 36.00 ug/L

Ecoregion X – 128.00 ug/L

Ecoregion XI – 10.00 ug/L


These values range from 9% to 115% of the estimated 111 ug/L TP that would result from a 45% reduction, with all but one value below 100% (the Texas-Louisiana Coastal and Mississippi Alluvial Plains ecoregion X). This also suggests that a 45% reduction of estimated median monthly TP concentrations to111 ug/L would likely be less stringent than could be obtained if states adopted EPA’s recommended reference condition values into state water quality standards for TP.


A More Comprehensive Approach


A thorough comparison of the distribution approach to reference condition estimation and the 45% reduction in TN and TP could be made by calculating the nutrient concentrations from the USGS loading estimates at river gauge stations at each of the nine subbasins. The USGS provides monthly or annual nutrient flux estimates and river flow data from which nutrient concentration data can be derived (http://toxics.usgs.gov/hypoxia/.) These data provide values over many years for 9 subbasins located within the MARB. The data could be used in the following steps to compare the two sets of values:


  • Use the USGS nutrient loading data to compile a TN and TP concentration dataset for each subbasin;




  • Calculate the median TN and TP concentrations at each of the nine subbasin river gauge stations;




  • Overlay nutrient ecoregions on subbasins and extract nutrient ecoregional data from subbasins. From this refined data set, calculate the median value of the seasonal 25th percentiles of TN and TP for the ecoregion-subbasin.


These data can be used for the following comparisons:


1) Calculate the concentrations resulting from a 45% reduction in the median concentration for each subbasin.


2) Compare these to the EPA 25th percentiles (ecoregional reference conditions) in each subbasin, or specific subbasins of interest.


Submitted by EPA’s Office of Water, 8-24-07.


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