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Coastal & Estuarine Science News (CESN)

Coastal & Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries & Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bimonthly basis.

October 2007


Gulf of Mexico Hypoxia: Revisiting the 2001 Action Plan
Landscape, Land Use, and SAV in the Chesapeake
Lessons on Marsh Resilience Learned in the Bay of Fundy

Gulf of Mexico Hypoxia: Revisiting the 2001 Action Plan

A problem the size of New Jersey plagues the Gulf of Mexico most summers, when bottom waters become hypoxic and a “dead zone” averaging 20,000 km2 develops along the Louisiana/Texas continental shelf. Recognizing the seriousness of this problem, the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force developed an Integrated Assessment in 2000 and an Action Plan in 2001 to serve as a blueprint for addressing the issue. As part of a required reassessment to support adaptive management, the National Oceanic and Atmospheric Administration and the Environmental Protection Agency (EPA) convened a symposium in April 2006 calling on groups of authors to synthesize the most recent information on gulf hypoxia. Four papers in the current issue of Estuaries and Coasts were prepared for the symposium, reviewing the past ten years of published literature and additional data on gulf hypoxia. Two of the papers conclude that the major findings of the Integrated Assessment remain unchallenged, one is more cautious about the availability of appropriate data for drawing certain conclusions, and a fourth discusses modeling findings and future needs.

Turner et al. compare recent and older findings on nutrient and organic carbon loads to the gulf system, asking whether the quantity and form of those nutrients have changed in the past ten years, whether there are alternate sources not sufficiently considered in the 2001 plan, and whether current monitoring is adequate. In short, the authors state, “We know of no significant changes in the conclusion that the Mississippi River system is, by far, the major source of nutrients to the northern Gulf of Mexico where hypoxia is likely to develop.” There has been little change in river discharge or nutrient levels in the system: the Mississippi River supplies about 70% of the total nutrient load to the northern gulf, the distributary Atchafalaya River about 30%. While there has been recent speculation about the role of marsh erosion in contributing organic material to the gulf, these authors review the literature and find that, at most, marshes could account for only 10% of the organic load, a conclusion also drawn by the authors of the Rabalais et al. review in the same journal issue. Rabalais et al. also review the newest literature on gulf nutrient sources other than the Mississippi River (groundwater, atmospheric deposition, etc.) and conclude that the best current knowledge confirms that outflows of the Mississippi and Atchafalaya Rivers dominate the nutrient loads to the continental shelf. Finally, Turner et al. conclude that while water quality monitoring of the mainstem lower Mississippi and Atchafalaya Rivers has recently improved with the reestablishment of some previously abandoned monitoring stations, additional monitoring is needed in the upper portions of the watershed.

Rabalais et al. agree that the major scientific conclusions of the Action Plan have been supported and strengthened by research conducted in the five years since the plan’s publication. In particular, long-term data on hypoxia and nutrient sources and new paleoecological studies verify the relationship among the nitrate-nitrogen load contributed by the Mississippi, the extent of hypoxia, and changes in the coastal ecosystem. For example, examination of historical plankton assemblages reveals a recent increase in hypoxia-tolerant benthic foram species. These authors note that the most widely-known indicator of hypoxia is its overall extent across the bottom in mid-summer, but additional measurements, such as volume of hypoxic water and total oxygen deficiency, would also be valuable.

In contrast to these papers, a third review questions the adequacy of available data for drawing definitive conclusions about gulf hypoxia dynamics. Dagg et al. take a mechanistic perspective on the problem, reviewing research on the processes that contribute to hypoxia and finding that river contributions and many relevant biogeochemical processes are not fully understood. In particular, the authors call for more information on sources and rates of organic matter sedimentation and microbial recycling of nutrients. These authors dispute the primacy of the Mississippi River plume in hypoxia development, using results from an ecosystem model to conclude that sedimentation of organic matter from the Mississippi River plume accounts for only 23% of coastal hypoxia. Other sources may include the Atchafalaya River, recycling of nutrients from both rivers, and eroding coastal wetlands (a contribution discounted by the two reviews described above and the recent EPA Science Advisory Board hypoxia assessment). While Dagg et al. agree that both nitrogen and phosphorous are sometimes limiting to phytoplankton growth, they conclude that more research is needed to determine what causes changes in nutrient ratios. There is also a clear need to improve the evolving physical-biogeochemical models.

A fourth paper in the series, by Justić et al., explores the use of models in synthesizing knowledge about gulf hypoxia and makes recommendations for future modeling efforts. A survey of existing models, from simple regressions to complex mechanistic representations of the gulf ecosystem, reveals broad consensus that large-scale gulf hypoxia probably began in the 1970s and has intensified since then. The models also agree that hypoxia is caused mainly by loading of nitrate nitrogen. Nitrate inputs have increased 2.5-fold since the 1950s, coincident with increased fertilizer use in the watershed. These authors also set out to determine whether the Task Force’s goal of reducing the five-year running average hypoxic zone size to 5,000 km2 year-1 by 2015 is achievable by implementation of the plan’s suggested 30% decrease in nitrogen loading. The consensus of existing modeling studies is that the 30% reduction goal is probably insufficient, especially considering the likely impacts of global climate change. A nitrogen reduction goal of 40-45% is probably necessary, and during wet years, which may become more common with climate change, the needed reduction is probably closer to 50-60%. The authors also discuss what would constitute the “Cadillac” of gulf hypoxia models, developed within the framework of a larger hypoxia forecasting system that incorporates real-time data acquisition and tools for the general public.

Source: Turner, R. E., N. N. Rabalais, R. B. Alexander, G. McIsaac, and R. W. Howarth. 2007. Characterization of nutrient, organic carbon, and sediment loads and concentrations from the Mississippi River into the northern Gulf of Mexico. Estuaries and Coasts 30(5): 773-790. (View Abstract)

Rabalais, N. N., R. E. Turner, B. K. Sen Gupta, D. F. Boesch, P. Chapman, and M. C. Murrell. 2007. Hypoxia in the northern Gulf of Mexico: Does the science support the plan to reduce, mitigate, and control hypoxia? Estuaries and Coasts 30(5): 753-772. (View Abstract)

Dagg, M. J., J. W. Ammerman, R. M. W. Amon, W. S. Gardner, R. E. Green, and S. E. Lohrenz. 2007. A review of water column processes influencing hypoxia in the northern Gulf of Mexico. Estuaries and Coasts 30(5): 735-752. (View Abstract)

Justić, D., V. J. Bierman Jr, D. Scavia, and R. Hetland. 2007. Forecasting gulf’s hypoxia: the next 50 years? Estuaries and Coasts 30(5): 791-801. (View Abstract)

Additional Information: These four papers, among others, served as source materials for the Hypoxia Advisory Panel appointed by the EPA Science Advisory Board to assess the state of the science since 2000 related to causes of hypoxia. For their report, see http://www.epa.gov/sab/pdf/11-19-07_hap_draft.pdf. In addition, a new draft Action Plan is open to public comment until January 4, 2008. To view the draft, go to http://www.epa.gov/msbasin/taskforce/pdf/2008draft_actionplan.pdf.

Landscape, Land Use, and SAV in the Chesapeake

While it is clear that what happens on land impacts what happens in the water, that relationship is complex, requiring careful analysis to illuminate often elusive correlations. Adding to our knowledge of such relationships, a recent study of 101 Chesapeake subestuaries sheds light on the impacts of landscape characteristics and land use on spatial coverage of submerged aquatic vegetation (SAV) beds. The study used several statistical techniques to examine correlations between a variety of geomorphological and land use parameters and acreage of SAV beds. SAV abundance was enhanced by greater shoreline complexity, likely because those shoreline configurations provided more sheltered habitat for the plants. Land use was also significantly correlated with SAV abundance: SAV bed coverage was highest in subestuaries with forested watersheds and lowest in those with developed watersheds. In subestuaries with agricultural watersheds, lower SAV abundance was correlated with elevated rainfall, which may be the result of increased discharge of nutrients and sediments. SAV area exhibited some strong threshold responses, dropping precipitously above a total nitrogen loading of 16.7 kg m-2 d-1 and total phosphorus loading of 1.3 kg m-2 d-1. The best predictor of SAV cover was not a single variable, but a suite of characteristics incorporated into a regression model. The best model incorporated five variables related to the shape and complexity of the system; tidal range and wave height; and land use.

The value of this study for managers is two-fold: first, the results can be used in these particular systems to prioritize restoration sites or manage watersheds for SAV restoration. For example, special attention should be paid to reducing sewage inputs in subestuaries where nutrient inputs are higher than the thresholds discussed above. Second, the study has implications beyond the Chesapeake, as the statistical methods employed can be used wherever appropriate data sets are available.

Source: Li, X., D. E. Weller, C. L. Gallegos, T. E. Jordan, and H. Kim. 2007. Effects of watershed and estuarine characteristics on the abundance of submerged aquatic vegetation in Chesapeake Bay subestuaries. Estuaries and Coasts 30(5): 840-854. (View Abstract)

Lessons on Marsh Resilience Learned in the Bay of Fundy

Interest has been building in actively restoring salt marshes in the Bay of Fundy, some of which have been diked for centuries to create agricultural land. Some of these dikes have outlived their usefulness and have been breached intentionally or unintentionally (e.g., during storms). This paper uses observations from two such areas that were breached approximately 50-60 years ago to address questions such as: Will restoration be successful? and What does the trajectory of recovery look like?

Both of the recovering marshes evaluated in the study now resemble reference sites fairly closely. The successful recovery in these marshes seems to be related to marsh height when the dikes were breached, rapid rates of sediment deposition, and the large area available for plant growth in the observed sites. The resilience of these marshes indicates that active restoration efforts in this area are likely to be successful and could speed up that 50-60 year restoration horizon. These types of observations can inform future restoration projects. In particular, restoration has a high likelihood of success and will not require additional fill if undertaken at sites where the marsh surface elevation is at or above mean high water.

One of the study’s other findings challenges traditional views of the ecology of a common marsh plant. The vertical range of Spartina patens was found to increase with tidal range, a condition recognized for S. alterniflora but not previously reported for S. patens. This discovery indicates that conditions other than hydroperiod impact S. patens’ vertical distribution.

Source: Byers, S. E., and G. L. Chmura. 2007. Salt marsh recovery on the Bay of Fundy. Estuaries and Coasts 30(5): 869-877. (View Abstract)