The Gulf of Mexico hypoxic zone, located west of the mouth of the Mississippi River, is a mass of water containing so little oxygen that it cannot support life other than a few bacteria. There are several reasons for the development of this hypoxic zone, e.g., stratification of salt water from the Gulf and freshwater from the river and a large influx of nutrients. This influx of nutrients has been identified as one of the primary causes of the extreme hypoxic events now found in the Gulf. Under the right conditions, these nutrients cause algal blooms, some of which can be harmful or toxic to people and marine life. When the algae die or are eaten by zooplankton, they fall or are excreted to the bottom and decompose, a process that depletes oxygen from the bottom waters. Because the waters are stratified, no new oxygen can reach the bottom and hypoxia (low oxygen) results in the lower strata of water. In addition to causing most marine life to either die or move out of the area, hypoxia in some waters can cause chemical reactions to occur which release contaminants stored in the bottom sediments. Therefore, reduction of the input of nutrients is critical to preserving a healthy ecosystem along the Louisiana Gulf Coast.
Inputs of additional nutrients can reach the Gulf of Mexico as runoff coming down the Mississippi River and other tributaries or as wet or dry deposition from the air. Less is know about the role of atmospheric inputs than those from runoff. To begin to answer questions about the role of atmospheric inputs, the Ecological Society of America (ESA) hosted a one-day workshop on September 26, 1999, in New Orleans, LA, with support from the U.S. Environmental Protection Agency?s (EPA) Office of Wetlands, Oceans, and Watersheds, Office of Air Quality Planning and Standards, and Gulf of Mexico Program. It was one of a series of workshops focused on atmospheric inputs to coastal waters being undertaken jointly by EPA Offices of Water and Air and Radiation and ESA.
The purpose of the workshop was to review our knowledge about the contribution of atmospheric deposition to the Gulf hypoxic zone and identify additional information needed to fully understand the significance of the relationship. This workshop report is intended only to summarize the presentations and discussions of the workshop on what we know and what additional knowledge is needed to better understand the role of atmospheric deposition to the Gulf of Mexico hypoxic zone. In addition, appendices are included with information about major models currently used in the watershed, the workshop agenda, and a list of workshop participants.
Nitrogen (N) occurs in many different forms. Its transport and deposition rates depend on which form, as may its biological activity. The family of nitrogen compounds are:
How Does Atmospheric Deposition Contribute to the Gulf of Mexico Hypoxic Zone?
To improve our understanding of the role atmospheric deposition plays in the Gulf of Mexico hypoxic zone, we need to better define the problem which requires a better understanding of the impacts of atmospheric deposition on coastal eutrophication in general. These impacts can include increased hypoxia/anoxia, fish and shellfish kills, toxic algal blooms, and secondary effects of eutrophication (Paerl and Whitall 1999). We need more information on the connection of atmospheric deposition to primary production in coastal and marine waters and then how that is connected to eutrophication and hypoxia.
When investigating atmospheric deposition, we need to be concerned about wet vs. dry, over water (direct) vs. over land (indirect), and over freshwater vs. over saltwater. Wet:dry ratios (the proportion of deposition that falls as wet divided by the proportion that falls as dry) for NO3 and NH4 are very different and are influenced by salt water and temperature. Wet deposition rates tend to depend more on solubility than aqueous chemistry, but dry deposition rates are very sensitive to form (whether the nitrogen is a gas or particle). Over water, aerodynamic resistance of dry deposition increases and laminar resistance decreases. In the Chesapeake Bay, the aerodynamic resistance change dominates and the deposition velocity is roughly two-to-three times smaller over water. But sea salt increases HNO3 deposition–sea salt reacts with HNO3 creating large particles that have a deposition velocity similar to that over land (reversing the water effect). The warmth of the Gulf of Mexico, and resulting mixing of air over it, may make deposition velocities more equivalent to those that generally occur over land, possibly resulting in more deposition over the water than expected.
Are the ecological/biological effects of air deposited nitrogen different from those of other sources? How can these sources and impacts be distinguished? We are beginning to develop methods to track atmospheric nitrogen that has been deposited and separate it from other sources using stable isotopes. One method of doing this is to measure the amount of 15N in runoff, algae or other places. The major sources of atmospheric nitrogen are depleted in 15N compared to other sources, so algae using atmospherically-deposited nitrogen will also be depleted in 15N. But, stable isotopes alone are not enough to determine partitioning by sources. Methods or combinations of methods need to be developed and improved to identify specific sources of atmospheric nitrogen.
Other questions to be investigated which can help us better define the problem include:
These questions should be addressed using monitoring of atmospheric deposition in the Gulf and along the coast, direct experimental observation, extrapolation from laboratory observations, models factoring in isotope signals of sources, and good observational work (including remote sensing of phytoplankton response to events/inputs). A Gulf-wide physical-biological model is needed to evaluate the problem and recommend nutrient management actions. It should couple atmospheric and watershed models with biological response.
Texas A&M University, with support from EPA?s Gulf of Mexico Program, is developing a program to assess the importance of direct atmospheric deposition of pollutants to the near shore waters of the Gulf of Mexico. They suspect that larger inputs of nitrogen, organic pollutants, and mercury in estuarine and coastal waters may be due to increasing population densities along the coast and emissions from chemical, petroleum, and other industries. While this is would be a concern in any area, it is thought that this region may be more susceptible to nitrogen due to the high surface area to water volume ratios of coastal habitats.
Why are the Gulf of Mexico Estimates of Atmospheric Nitrogen
Lower than those of the U.S. East Coast and Europe?
Various studies looking at basins along the North Atlantic Ocean have estimated the contribution of nitrogen from the atmosphere to be 10-40% for most coastal areas. Goolsby et al. (1999) estimate direct atmospheric deposition of nitrogen to the Gulf of Mexico at approximately 1%. Alexander et al. (2000) estimated that atmospheric deposition represents 18% of the nitrogen reaching the Gulf from sources in the Mississippi River basin (on the lower end of the range of what other areas receive). Possible reasons for this include:
- a lack of data–there are fewer monitoring stations around the Gulf than in areas in the eastern U.S.;
- differences in measurement methods;
- characteristics of the Gulf and its watershed;
- differences in watershed processing;
- differences in human population densities; or
- that inputs from agricultural runoff are so great that atmospheric inputs are dwarfed.
A Case Study from North Carolina
The Albemarle-Pamlico Estuary in coastal North Carolina is the site of some of the most intensive research on atmospheric deposition of nitrogen in the country. The system is similar to the Louisiana coast in the Gulf of Mexico in some ways?they are both relatively shallow, they both have intensive agriculture upstream, and they both become hypoxic each summer. There are some differences as well; the Albemarle-Pamlico system is enclosed by barrier islands, and the Mississippi is a far larger river than the Neuse, Tar, or Pamlico rivers that feed the Albemarle-Pamlico Estuary. However, there is enough similarity between the systems to ask the question: ?Is the research taking place in North Carolina relevant to the Gulf of Mexico hypoxic zone??
The answer is: Probably (at least for the estuaries).
For example, researchers working at the mouth of the Neuse River have found that they receive a large amount of nitrogen through atmospheric deposition from agricultural activities upwind, which in North Carolina is primarily animal feedlots. In these operations, manure is stored in open lagoons rather than being treated in a sewage treatment plant or discharged into a body of water. These lagoons are thought to prevent water pollution, but in fact they contribute to huge emissions of ammonia which soon deposit to the landscape and make their way into river, streams, and estuaries. In other cases the same agricultural practices that cause direct water pollution also cause indirect water pollution through atmospheric deposition. Agricultural practices need to be examined holistically for their effects on land, water, and air and the relationships between them.
Atmospheric deposition from a variety of sources (automobile, industrial, and agricultural) have been shown to cause algae growth in laboratory experiments. Researchers in North Carolina have demonstrated that more algae grow in estuarine water samples spiked with rainwater collected at the mouth of the Neuse River than those without rainwater, indicating that rain contributes to algal blooms in the estuary. This is likely to be true of other estuaries as well. Rain falling on the Gulf of Mexico can also be relatively enriched with nitrogen. That nitrogen is immediately available to algae; it doesn?t pass through soil, wetlands, or river ecosystems before becoming available for algae growth in the Gulf. In addition, some portion of the nitrogen in the rain falling throughout the Mississippi River basin also reaches the Gulf. Together, these direct and indirect sources of atmospheric nitrogen emissions have the potential to significantly impact the ecology of the Gulf of Mexico.
How Have Atmospheric Deposition Rates Changed over Time?
Inorganic nutrients in the Mississippi River have doubled since 1950. In the U.S, there was a doubling of NOX emissions between World War II and the 1970s, but it has flattened out since. We don?t know how emissions of NH3 have changed over time, except that deposition measurements have been increasing in recent years at some National Atmospheric Deposition Program (NADP) sites across the country. What will happen in the future? Trends for the U.S. show that NOX emissions are not decreasing and agriculture in many areas is not moving toward lower emission practices.
How Can Estimates of Nitrogen Deposition to the Gulf of Mexico be Improved?
Most estimates of nitrogen deposition directly to the Gulf of Mexico have been extrapolated from other areas and have large errors associated with them. To obtain more accurate estimates of nitrogen deposition, direct deposition to the Gulf needs to be measured rather than extrapolated. Estimates of wet deposition to the Mississippi River basin have not been extrapolated because of the availability of monitoring data throughout the basin suitable for interpolation. The greatest uncertainty in the basin exists with respect to dry deposition for which few monitoring data are available. Dry deposition of nitrogen (and other pollutants) is difficult to measure accurately. There are several technologies currently in use (annular denuders, filterpaks, and bulk buckets) and each has significant limitations, especially the latter. New methods or protocols must be developed to better quantity this significant portion of the nitrogen deposited from the air.
In terms of relative wet vs. dry deposition of nutrients other than nitrogen, we know that dryfall is 5-12 times greater than wet for phosphorus in most agricultural areas (John Downing, personal communication). Dust mobilizes high concentrations of silicon, nitrogen, and phosphorus, as well as calcium. A 1997 EPA Emissions Update looked at sources of dust: nationally, 20% is from agriculture, forestry, and other combustion and 50+% is from construction and unpaved roads. However, currently there is not much monitoring of large particles (> 2.5 mm) and their composition. African dust is also a source of nutrients and minerals to the Southeast US. During the summer it provides concentrations of iron and silicon greater than local sources.
What are the best deposition velocities to use in estimating dry deposition? Past efforts (calculating dry deposition as a fraction of wet deposition) have probably underestimated dry deposition amounts by as much as 20-40% (Robin Dennis, personal communication). Another possible approach is to base calculations on 10-year averages of NADP data and calculate wet:dry ratios over the long-term.
Other questions need to be investigated to improve dry deposition measurements: Can dry deposition be extrapolated from where it is measured to surrounding areas? How can it be measured better? How can we improve methods for more accurate partitioning dry deposition between gas and particles?
The suggestion was made that oil platforms in the Gulf could be incorporated in a sampling program if contacts and collaborative agreements were made with oil companies to provide space on the platform and on-board personnel to conduct the sampling. However, care must be taken in the sampling design as activities on the oil platforms can also be sources. The Department of Interior?s Minerals Management Service is conducting a project to conduct sampling on oil platforms, mainly for sulfur dioxide and meteorological data. The focus is on transport, not deposition, but it may be possible to coat-tail activities on to this project.
How Can Transport Mechanisms of Atmospherically-Deposited
Nitrogen through the Watershed be Quantified?
As it is difficult to precisely determine nitrogen sources, it is likely that some atmospherically-deposited nitrogen, especially that which falls as dry deposition, is being counted as fertilizer loss through runoff. The processing of nitrogen compounds on highly fertilized watersheds is probably small and most atmospherically-deposited nitrogen excess is not taken up before it reaches the water. It is still not clear how much of that nitrogen is used by aquatic plants in the waterways, partially because this depends on the growth rate of aquatic plants, which depend on phosphorus concentrations in freshwater as well and light and temperature. (Estimates of annual mean rates of nitrogen removal in streams are presented in Alexander et al. (2000) which could be used to account for the processing by aquatic plants under steady-state conditions).
The transport, transformation, and processing of all forms and sources of nitrogen within the Mississippi River watershed needs to be better understood. What is the difference between nitrogen emission and deposition? How much nitrogen generated in the basin stays in the basin vs. how much leaves? How does this vary over time? Where is the processing of nitrogen deposition occurring and where is it ultimately ending up? What is the contribution of groundwater? Retention/transport in watershed will partially determine where control is most effective (e.g., stream order and size seems to determine transport efficiency).
Estimates of denitrification rates are needed in order to complete the nitrogen budget. One approach could be to look at different Gulf of Mexico estuaries with different levels of inputs and see how they respond. Denitrification rates over open waters are important, as well as processing during in-stream transport.
What Agricultural Activities Contribute to Atmospheric Emissions of Nitrogen?
The agricultural input to the hypoxic zone has an air component that has not been included in conventional wisdom. We need to further explore the agricultural air deposition sources and how they impact the Gulf of Mexico.
The Mississippi River basin is a primarily agricultural basin containing 41% of the US land area and 52% of all US farms. Much of the nitrogen in the watershed comes from agricultural processes. Nitrogen is applied to fields as fertilizer and is produced in the form of manure from livestock farms (some of which is also applied to cropland as fertilizer or to fallow fields as a disposal method). It is also added from air emissions through the process of atmospheric deposition. This nitrogen is initially either bound in the soil or taken up by plants. Whatever is not bound or taken up can be considered “excess.” This excess nitrogen dissolves and is washed into streams and rivers. The amount of atmospheric deposition that reaches the waterways depends on the amount of nitrogen already in the field. If it is at or near saturation, most of the atmospheric nitrogen will be delivered to streams or rivers. It is important to understand this part of the nitrogen transport cycle because it means that atmospheric deposition could increase the amount of fertilizer that washes off the fields. Any estimate of the impact of atmospheric deposition on over-enrichment of nutrients must take these soil dynamics into account.
Nitrogen becomes airborne in agricultural settings in several ways. It can volatilize directly from the surface of fields or animal waste lagoons, or it can be blown off fields or roads as particles. Some agricultural activities that emit nitrogen into the atmosphere include: spraying of wastes from livestock operations; wind erosion; denitrification mediated by oxygen; fertilizer loss/ volatilization based on application; and NHX emissions by growing plants. In agricultural environments, losses of NH3 are important, but how well are we looking at these? There may be as much mobilized atmospherically as coming out of the pipe, particularly in cases of large livestock operations.
The potential of nitrogen fertilizer loss from farm fields through dust and the resulting deposition needs further investigation. Little is known about the quantity, characteristics, or impacts of agricultural dust. We do know that soil nutrients are mobile and the soil surface mixing zone, which is subject to wind erosion, has a high concentration of applied nutrients and other agricultural chemicals.
The loss of nitrogen from agriculture in the Mississippi River basin is not helping anyone, including the farmers. Farmers view nitrogen as cheap insurance for a successful crop, but they would just as soon not pay for something that is not doing them any benefit. This makes the agricultural sector a very attractive place to find ways to reduce the loss of nitrogen. There is substantial support for this in the agricultural community. In Wisconsin, for example, the Cooperative Extension agents cannot train enough trainers to handle the requests for agricultural nitrogen reduction education programs.
The Tampa Bay experiences point out the importance of working with constituents to find flexible ways to meet common goals. Generally, quotas, regulations, or penalties are not well-accepted. Producers will usually accept reasonable, attainable, relevant regulations or voluntary actions in which they see benefits, economic and noneconomic.
Programs to reduce agricultural sources of atmospheric nitrogen would have to be carefully designed to meet the needs and situations in the Mississippi River Basin. For example, Denmark uses a nitrogen tax to control emissions. However, in economic studies of the Mississippi basin, such a tax would need to be very large to result in even a small reduction of nitrogen usage. In Holland, nitrogen from atmospheric deposition is factored in to the total nitrogen application for each field, both reducing the total amount of nitrogen added to each field and making it easy to see the contribution of atmospheric deposition.
A Case Study of Tampa Bay
Today, we know that direct atmospheric deposition to Tampa Bay contributes 27% of the annual loads of total nitrogen to the Bay. However, when Tampa Bay National Estuary Program (NEP) began the process of developing a nutrient reduction strategy, there was little local information available on the contribution of atmospheric deposition.
In 1995, the Tampa Bay NEP started an atmospheric deposition monitoring program that included measurements of daily wet deposition and integrated 24-hour measurements once weekly of dry deposition. Over the next few years the program developed a relatively good estimate of the nitrogen being deposited from the air, but questions about the completeness of the monitoring, location of sources, and watershed transport remained.
Despite these uncertainties, it was important to begin to reduce nitrogen loading to the Bay. Seagrass restoration was chosen as the program goal because seagrass is visible, important to the Bay ecosystem, and closely tied to nitrogen loadings. Participants could identify with the goal and progress was easy to see. A nitrogen management goal necessary to allow seagrass restoration was established that took into account population growth, which has been increasing dramatically. This target requires reductions of 17 tons of nitrogen per year every year.
To accomplish the nitrogen management strategy, the Nitrogen Management Consortium was established. Partners receive reduction credits for participating in projects to reduce nitrogen loadings to Tampa Bay. Projects include: storm water upgrades, land acquisition, wastewater reuse, emission reduction, habitat restoration, agricultural best-management practices, education/public involvement, and industrial upgrades.
To deal with the initial uncertainties surrounding atmospheric deposition of nitrogen, the partners agreed to ?cover? those sources through other reduction projects. Once there is information on the amount of nitrogen being deposited, sources have been determined and their contribution allocated, the Consortium will include nitrogen deposition in its reduction plans. Management has begun by requesting local electric utilities to participate in the Consortium. One company has already promised a voluntary reduction of 20%. Tampa is unique in that a large percentage of the sources appear to be within the local area. For sources outside the watershed, it may be necessary to go to a state or regional level organization to obtain the necessary reductions.
The Tampa Bay Nitrogen Management Consortium works because it includes agreement on the goals, flexibility to meet those goals within the legal rules, non-regulatory voluntary participation, participants who are willing to work together, measurable goals considered worthy by the participants and the public, a strong technical basis, and a commitment to implement actions in writing.
How Do Non-Agricultural Sources Contribute to Atmospheric
Deposition in the Mississippi Watershed and Hypoxic Zone?
Industry, particularly the petrochemical industry along the Gulf Coast, may contribute significantly to direct atmospheric deposition, but there are no measurements of deposition over the hypoxic zone. We do know that there are a large number of sites that release large amounts of NOx along the Louisiana coast (for example, see map of large NO2 point sources in the U.S.).
Although nitrogen emissions are used for ozone modelling, less is known about nitrogen deposition from urban and suburban areas. Automobile exhaust is a large contributor of atmospheric nitrogen and wet deposition on paved surfaces runs off quickly without any absorption by soils. Deposition over urban and suburban areas is likely contributing a lot of nitrogen which is being transported long distances to coastal waters. We need more information on the amounts and sources of these urban inputs. More regional modeling of nitrogen deposition is needed.
What will be the Impact on the Gulf of Mexico if Atmospheric Deposition is Reduced?
In the Gulf of Mexico, the basic cause and effect relationship between atmospheric deposition and eutrophication or hypoxia is not as concrete as it is in other areas. The link is there, but it needs to be strengthened.
In smaller systems, we can show that reductions in atmospheric nitrogen deposition will have an impact, but will a larger-scale system like the Gulf respond to such reductions? What do we know (and not know) about the Gulf of Mexico system that will enable use to determine if reducing atmospheric nitrogen will make a difference? How will the Gulf respond to reductions in deposition directly to the Gulf and to reductions to the Mississippi River watershed? Can we monitor or model the system to see a signal of change? It may be possible for existing models to be used for some sensitivity analyses to see how reductions will impact the Gulf hypoxic zone.
Other factors to need to be considered in looking at the hypoxic zone. Research is needed to better determine the role of terrestrial carbon in hypoxia. What is its role in oxygen consumption dynamics? It can be an important source for hypoxia, but is overshadowed by large algal production and die off. In catastrophic events, hypoxia can be driven by carbon from the watershed deposited by storms. We need to look at C:N:P ratio of the Mississippi River plume. The role of light in the development of hypoxia is another area of future research. The optical properties of the water column affect photosynthesis and, therefore, algal growth.
What additional research is needed on the coupling and decoupling of nutrient fluxes (specifically coupling of nitrogen and phosphorus reductions)? Decreasing only the nitrogen concentration could lead to a drop in the N:P ratio in the Mississippi River below an optimum level for growth, thereby exacerbating the problem. How do other elements, nutrients, and chemicals play a part in the system and processing of nitrogen? Atmospheric deposition of toxics (especially mercury) is also important.
What Process Should be Followed to Craft a Management Plan that has Practical, Realistic Goals that the Entire Watershed will Contribute Towards Meeting?
For a successful voluntary program, participants need to see the benefits to themselves. These benefits can include: participating and attaining compliance so that if regulations change, they will be prepared (participants have more time to meet standards and receive assistance in doing so); allowing them to provide input to regulators in rule development; and financial savings of using less nitrogen.
We need to set a goal that is MEASURABLE, VISIBLE, and MEANINGFUL. To do so, it is important to distinguish some of the reasons why we should care about hypoxia (e.g., fisheries, perturbations to the natural nitrogen cycle, and possibly functioning as a “canary in the coal mine” alerting us to other environmental problems). We also need to determine and collect the information needed to set the appropriate goals. These can include sediment control, subsurface flow, runoff, and volatilization. We need to pick a goal for which we can garner support and choose a parameter to measure to see if we have reached this goal. Can this be done for the Gulf? Is there enough information to choose an appropriate goal?
What should be the target for a management goal? What can we change through management and what is inherent to the system? Scientific models and methods need to be developed to assess the potential impacts of management options on nitrogen load and its dynamics.
We need to show how nitrogen reduction works. This will require evidence that an x% reduction will result in a y% improvement. Coastal managers and policy makers need clear scientific information to help them decide on what to concentrate their efforts.
The geographic extent of the issue in the Mississippi River basin is unique in that it is larger than that of many other places. The idea that what happens in Iowa impacts the Gulf of Mexico is not intuitive for many people and makes it difficult for voluntary actions to work. In addition, different people in different parts of the basin have different goals and the challenge will be to come up with set of goals to which everyone can agree. The workshop participants agreed that targets should be set on a regional scale that allow for local control. Regional efforts are often needed to allow local level control to succeed (e.g., reducing background and transport). Educational efforts should stress that nutrient management within the watershed will also have benefits within the watershed and not just for the Gulf of Mexico.
Questions to be addressed in developing a management strategy include:
We need to link atmospheric deposition impacts on coastal waters to the air quality regulations coming in the near future. It is imperative to link air and water regulators and policy makers by increasing the communication between the two groups and piggy-backing on each other?s efforts. It is also important to increase communication between science and management.
Alexander, R.B., R.A. Smith, and G.E. Schwarz (2000). Effect of stream channel size and the delivery of nitrogen to the Gulf of Mexico. Nature, 403:758-761.
Alexander, R.B., R.A. Smith, G.E. Schwarz, S.D. Preston, J.W. Brakebill, R. Srinivasan, and P.A. Pacheco (In Press). Atmospheric nitrogen flux from the watersheds of major estuaries of the United States: an application ofthe SPARROW watershed model. In R. Valigura, H.W. Paerl, T. Meyers, M.S. Castro, R.E. Turner, R. B. Alexander, D. Brock, and P.E. Stacey (eds). Assessing the Relative Nitrogen Inputs to Coastal Waters from the Atmosphere, American Geophysical Union Water Monograph.
Alexander, R.B., P.S. Murdoch, and R.A. Smith (1996). Streamflow-induced variations in nitrate flux in tributaries to the Atlantic coastal zone. Biogeochem, 33:149-177.
Brezonick, P.L., V.J. Bierman, et al. (1999). Effects of Reducing Nutrient Loads to Surface Waters within the Mississippi River Basin and the Gulf of Mexico, Topic 4 Report of the Committee on Environment and Natural Resources Hypoxia in the Gulf of Mexico. NOAA Coastal Ocean Program Decision Analysis Series No. 17. NOAA Coastal Ocean Program, Silver Spring, MD.
Committee on Environmental and Natural Resources Hypoxia Work Group for the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force (1999). Gulf of Mexico Hypoxia Assessment. NOAA Coastal Ocean Program Decision Analysis Series No. 17. NOAA Coastal Ocean Program, Silver Spring, MD. http://www.nos.noaa.gov/products/pubs_hypox.html.
Diaz, R., A. Solow, et al. (1999). Ecological and Economic Consequences of Hypoxia, Topic 2 Report of the Committee on Environment and Natural Resources Hypoxia in the Gulf of Mexico. NOAA Coastal Ocean Program Decision Analysis Series No. 17. NOAA Coastal Ocean Program, Silver Spring, MD.
Dinnel, S.P. (1995). Estimates of Atmospheric Deposition to the Mississippi River Watershed. In Proceedings of the First Gulf of Mexico Hypoxia Management Conference. December 1995. U.S. Environmental Protection Agency, Gulf of Mexico Program Office, Stennis Space Center, MS. http://pelican.gmpo.gov.
Doering, O.C. et al. (1999). Evaluation of Economic Costs and Benefits of Methods for Reducing Nutrient Loads to the Gulf of Mexico, Topic 6 Report of the Committee on Environment and Natural Resources Hypoxia in the Gulf of Mexico. NOAA Coastal Ocean Program Decision Analysis Series No. 17. NOAA Coastal Ocean Program, Silver Spring, MD.
Downing, J.A., J.L. Baker, R.J. Diaz, T. Prato, N.N. Rabalais, R.J. Zimmerman (1999). Hypoxia in the Gulf of Mexico: Land and Sea Interactions. Task Force Report 134. Council for Agricultural Science and Technology. Ames, IA. http://www.cast-science.org/hypo/hypo.htm or http://www.cast-science.org/pdf/hypo.pdf.
Ecological Society of America (1997). Atmospheric Nitrogen Deposition to Coastal Watersheds. Workshop Report. 1990 M Street, NW, Suite 700, Washington, DC 20036. http://www.esa.org/science/publications/sbindep1.htm.
Ecological Society of America (1999). Acid Deposition: The Ecological Response. Workshop Report. 1707 H Street, NW, Suite 700, Washington, DC 20036. http://www.esa.org/science/publications/aciddep.htm.
Goolsby, D.A., W.A. Battaglin, G.B. Lawrence, R.S. Artz, B.T. Aulenbach, R.P. Hooper, D.R. Keeney, and G.J. Stensland (1999). Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin, Topic 3 Report of the Committee on Environment and Natural Resources Hypoxia in the Gulf of Mexico. NOAA Coastal Ocean Program Decision Analysis Series No. 17. NOAA Coastal Ocean Program, Silver Spring, MD.
Goolsby, D.A. (1994). Flux of herbicides and nitrate from the Mississippi River to the Gulf of Mexico. In M. Dowgiallo (ed.) Coastal Oceanographic Effects of Summer 1993 Mississippi River Flooding. NOAA Special Report, Washington, DC.
Lynch, J.A., J.W. Grimm, and V.C. Bowersox (1995). Trends in precipitation chemistry in the United States: a national perspective, 1980-1992. Atmospheric Environment 29(11):1231-1246.
Mitsch, W.J., J.W. Day Jr., J.W. Gilliam, P.M. Groffman, D.L. Hey, G.W. Randall, and N. Wang (1999). Reducing Nutrient Loads, Especially Nitrate-Nitrogen Loads to Surface Water, Ground Water, and the Gulf of Mexico, Topic 5 Report of the Committee on Environment and Natural Resources Hypoxia in the Gulf of Mexico. NOAA Coastal Ocean Program Decision Analysis Series No. 17. NOAA Coastal Ocean Program, Silver Spring, MD.
National Atmospheric Deposition Program (NRSP-3)/National Trends Network (1998). NADP Program Office, Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820. http://nadp.sws.uiuc.edu/.
Paerl, H.W. and D.R. Whitall (1999). Anthropogenically-derived atmospheric nitrogen deposition, marine eutrophication and harmful algal bloom expansion: is there a link? Ambio 28(4):307-311.
Public Comments (1999a). Public Comments on the Six Assessment Reports. NOAA Coastal Ocean Program, Silver Spring, MD. http://www.nos.noaa.gov/products/pubs_hypox.html#pubcomm.
Public Comments (1999b). Public Comments on the Draft Integrated Assessment. NOAA Coastal Ocean Progam, Silver Spring, MD. http://www.nos.noaa.gov/products/pubs_hypox.html#pubcommdia.
Rabalais, N.N., R.E. Turner, J. Dubravko, Q. Dortch, and W.J. Wiseman, Jr. (1999). Characterization of Hypoxia, Topic 1 Report of the Committee on Environment and Natural Resources Hypoxia in the Gulf of Mexico. NOAA Coastal Ocean Program Decision Analysis Series No. 17. NOAA Coastal Ocean Program, Silver Spring, MD.
Science Meeting Notes (1999). Gulf Hypoxia Science Meeting Notes. NOAA Coastal Ocean Program, Silver Spring, MD. http://www.nos.noaa.gov/pdflibrary/gulfhypoxscimtg.pdf.
Seitzinger, S.P. and R.W. Sanders (1999). Atmospheric inputs of dissolved organic nitrogen stimulate estuarine bacteria and phytoplankton. Limnology and Oceanography 44(3):721-730.
Smith, R.A, G.E. Schwarz, and R.B. Alexander (1997). Regional interpretation of water-quality monitoring data. Water Resources Research 33(12):2781-2798.
This report was written by Lori Hidinger and Tamara Saltman of the Ecological Society of America based on notes of the presentations and discussions at the workshop. We would like to thank all those who contributed to the development and review of the report, specifically, Nancy Rabalais, Louisiana Universities Marine Consortium, Jay Pinckney, Texas A&M University, Richard Alexander, USGS Water Resources Division, Gary Stensland, Illinois State Water Survey, Deborah Martin, USEPA Coastal Management Branch, and Mary Barber, ESA, for their review and input. We would also like to thank the National Atmospheric Deposition Program for the use of the nitrogen deposition isopleth map and the USEPA AIRS Database for the use of the map of point source facilities with NO2 emissions. Finally, we would like to thank the U.S. Enivornmental Protection Agency Office of Wetlands, Oceans, and Watersheds, Office of Air Quality Planning and Standards, and Gulf of Mexico Program for supporting this workshop and subsequent report.
Models are tools that allow us to learn about and manage systems and processes. Both water and air models can contribute to our knowledge of the impacts of atmospheric deposition on the Gulf of Mexico hypoxic zone. Models currently being used to investigate this issue include SPARROW, RADM and Extended RADM, Models-3/CMAQ, and REMSAD (Table 1).
Table 1. Models for Determining Atmospheric Deposition Input to the Gulf of Mexico.
The SPARROW (SPAtially Referenced Regression On Watershed Attributes) watershed model divides a watershed up using river reaches and models mean annual total nitrogen yield by looking at upstream sources and computing the mass-balance between sites (Smith et al. 1997; Alexander et al. 2000; http://water.usgs.gov/nawqa/sparrow/). SPARROW predictions of total nitrogen flux for the Mississippi Basin were based on the calibrations of the model to a national set of 374 stations, including 123 watersheds with monitoring locations. The model was used to look at the contributions of different sources to the Gulf of Mexico. The percent contribution of different sources in the Mississippi basin showed that approximately 60% of the nitrogen delivered to the Gulf originates from agricultural sources (fertilizer and livestock wastes) and approximately 18% from atmospheric deposition (the large error bars on the estimate yield a range of 6-28% for atmospheric deposition). The model was also used to look at where within the region the atmospheric contribution was originating. Most, nearly 50%, of the atmospheric nitrogen was emanating from the Ohio and upper Tennessee River basins. Agricultural sources seem to be the dominant feature in most of the Mississippi basin watersheds, except in the western reach of the basin. Atmospheric input makes its largest contribution in the eastern portion of the basin.
SPARROW predictions of in-stream loss and nitrogen loads reflect long-term mean conditions. SPARROW is not a dynamic model; it addresses the issue of retention by assuming a steady-state and looking at the concentrations over a long time period. SPARROW can make seasonal predictions and for many management decisions, mean seasonal and annual estimates are satisfactory. Refinements will be required to make the model dynamic and these are planned for the future. Also, SPARROW currently has no way of handling sources of nitrogen stored in the system.
Enhancements to the model were made to refine the in-stream delivery term to give a better estimate of in-stream loss in large rivers. The next generation of SPARROW will expand on finer spatial resolution and land to water delivery. The developers are also adding output from topographic models to get more information on subsurface flow. Future improvements to SPARROW will include explicit quantification of atmospheric inputs from dry deposition and descriptions of the types and location of watershed sinks (e.g., ground water storage, subsurface transport), and will account for temporal variability in flow, source inputs, and nitrogen transport within watersheds.
The Regional Acid Deposition Model (RADM) and Extended RADM models are process air quality models. These models can be used to look at a source region and see where atmospheric deposition of nitrogen, both oxidized and reduced, is falling. In the model NH3 travels about 2/3 as far as NO3 but still farther than considered by conventional wisdom. The model can also estimate the percent oxidized nitrogen deposition to a watershed explained by local airshed NOx emissions vs. that from long range transport. RADM has been operational since 1990. It models oxidized nitrogen deposition (wet and dry) in the eastern U.S. for terrestrial areas (watersheds) and coastal estuaries. It has been used by the Chesapeake Bay Program to help address atmospheric issues and used to define oxidized-nitrogen airsheds for coastal estuaries. The Extended RADM became operational in 1999. It models oxidized and reduced nitrogen deposition (wet and dry) in the eastern U.S./terrestrial area (watersheds) and coastal estuaries. The Extended RADM has been used to define oxidized nitrogen airsheds and now will be used to define reduced nitrogen airsheds for selected estuaries and in the Chesapeake Bay and North Carolina programs.
The Models-3/CMAQ model is EPA?s latest 1-atmosphere process model. It became available for testing in 1999 and will be operational in 2001. CMAQ will model oxidized and reduced nitrogen deposition (wet and dry deposition), and will include sea salt influence, updated dry deposition information, and in a few years mercury deposition. EPA is in the process of undergoing model evaluation on CMAQ and expects to apply it to Gulf Coast studies for year 2000 measurement campaigns in Tampa Bay (nitrogen deposition and ozone) and Houston (ozone and particle formation).
To improve these and other models, more extensive characterization of the bias in NADP ammonia estimates and in weekly data (e.g. CASTNet) is needed. Other data issues include a lack of ammonia data (air concentration and deposition), which results in an inability to check models on NH3/NH4 split; a lack of data over water, particularly the Gulf of Mexico; and the need for good sea surface temperature data over the Gulf.
Models help characterize the problem?how much (help interpolate data or fill in for data gaps), from where (determine airsheds), from whom (which sector of nitrogen oxide emissions and which sector of NH3 emissions); and what to expect of management options. There are, however, some things that air quality models cannot tell us, or cannot tell us yet. It is difficult to get the deposition details (i.e., the actual deposition to a specific location or to a small watershed). The organic fraction of nitrogen atmospheric deposition is still beyond us by several years. Other challenges for future modeling efforts are modeling individual, multiple years of simulated nitrogen deposition and modeling the actual indirect nitrogen load attributable to the atmosphere. The biggest challenge is that we can?t measure everything yet.
The REgulatory Modeling System for Aerosols and Deposition (REMSAD) models atmospheric transport and deposition of nitrogen and mercury. The REMSAD platform is based on the UAM-V regional air quality model which was extended to treat nitrogen transport and several toxics (mercury, dioxin, atrazine, cadmium) and particulate matter. The model was extended vertically to the tropopause to look at longer range transport. The model inputs include emissions, meteorological data, land uses, photolysis rates, and hydroxyl radical concentrations (for parameterized chemistry).
REMSAD can be used to assess the magnitude and patterns of total nitrogen deposition and resulting data can be fed into watershed models to derive the nitrogen loadings into water bodies. The toxic deposition module within REMSAD simulates the atmospheric chemical and physical processes leading to mercury deposition and includes in-cloud transformation of mercury. REMSAD is being used in current EPA projects to model atmospheric deposition of nitrogen and mercury in the US, atmospheric concentrations of particulate matter, and deposition of pollutants including total oxidized nitrogen (NOx), reduced nitrogen (NH3), and acid species. It is also being used in an evaluation of nitrogen deposition comparing annual and monthly depositions with observations from the NADP.
The Total Maximum Daily Load (TMDL) evaluation in Wisconsin is using REMSAD for a management application. It will assess the effect of changes in the mercury emission levels on its deposition in the Great Lakes area. The model inputs will include meteorological input produced from the RUC model output from NOAA and the MM5 model and the latest emission inventory from EPA which includes recent estimates of heavy duty diesel NOx, air conditioning NOx from light duty vehicles, and toxic emissions estimates. The assessment will aid EPA in determining the need to promulgate more stringent mobile source emission standards and evaluating the environmental consequences of alternative control strategies to reduce mercury deposition to designated area. It will also look at the contribution to watersheds of mobile and other sources of nitrogen deposited on the Mississippi river basin and estuaries along the coasts.
Where Air and Water Meet:
The Role of Atmospheric Deposition
in the Gulf of Mexico Hypoxic Zone
Sunday, September 26, 1999
LaSalle Ballroom B, Hotel Inter-Continental, New Orleans
|7:30 – 8:00am
8:00 – 8:15am
8:15 – 8:35am
8:35 – 8:45am
10:40 – 11:10am
11:40 – noon
2:00 – 2:20pm
2:20 – 3:30pm
3:30 – 3:45pm
5:15 – 5:30pm
Welcome?Darrell Brown, Chief, Coastal Management Branch, USEPA
Overview of the Issues: Atmospheric Deposition of Nitrogen to Coastal Waters?
Hans Paerl, University of North Carolina
Questions and Discussion
Atmospheric Deposition of Nitrogen and the Gulf of Mexico?Luis Cifuentes, Texas A&M University
Questions and Discussion
Sources of Nitrogen to the Gulf and Loading from the Air?Gary Stensland, Illinois State Water Survey
Questions and Discussion
Deposition on the Mississippi Watershed and Transportation to the Gulf?
John Downing, Iowa State University
Questions and Discussion
Models and What They Can and Can?t Tell Us: SPARROW, RADM and REMSAD
Panel: Richard Alexander, USGS; Robin Dennis, USEPA; and Tom Myers, ICF Kaiser Consulting
Questions and Discussion
Digestion: Now that you?ve had some time to think about this, what do you think??
Discussion Facilitator: Bill Cure, North Carolina Department of Environment and Natural Resources,
Division of Air Quality
Case Study: How we have dealt with some of these issues on a smaller scale in Tampa?
Holly Greening, Tampa Bay Estuary Program
Experiences and Extrapolation: What are other people?s experiences and how can these be
transferred to the Gulf??Discussion Facilitator: Holly Greening, Tampa Bay Estuary Program
What We Don?t Know and Need to Find Out: Brainstorming and Priority Setting
Discussion Facilitator: Debora Martin, Coastal Management Branch, USEPA
3:45 – 4:15pm Assignments and break-out groups
4:15 – 4:30pm Break-out group reports
4:30 – 5:15pm Compilation, compromise and consensus
Where Air and Water Meet:
The Role of Atmospheric Deposition
in the Gulf of Mexico Hypoxic Zone
Sunday, September 26, 1999–New Orleans
U.S. Environmental Protection Agency
Gulf of Mexico Program
Building 1103, Room 202
Stennis Space Center, MS 39529-6000
228-688-7015; 228-688-2709 fax
University of North Carolina Institute of Marine Sciences
3431 Arendell Street
Moorehead City, NC 28557
Kerry St. Pé
U.S. Geological Survey Water Resources Division
12201 Sunrise Valley Drive
Reston, VA 20192
Louisiana Department of Environmental Quality
P.O. Box 82135
Baton Rouge, LA 70884-2136
Joseph M. Prospero
Copies of this report are available from: