Sustainable Biosphere Initiative Project Office Workshop Report Atmospheric Nitrogen Deposition to Coastal Watersheds 1997
This document reports the recommendations of a diverse group of eminent scientists, coastal managers, and national policymakers assembled to discuss a key emerging policy issue the impacts of atmospheric nitrogen deposition to coastal waters and their watersheds. The gathering was convened by the Sustainable Biosphere Initiative Project Office of the Ecological Society of America from June 2-4, 1997 at the University of Rhode Island Coastal Institute. The report includes specific policy relevant conclusions based on current scientific knowledge, short to medium-term information needs for future policy and management actions, and long-term research needs to fully understand and address the issue.
What is the problem?
Normally in short supply, nitrogen plays an important role in controlling the productivity, dynamics, biodiversity, and nutrient cycling of estuarine and marine ecosystems. During the past century, human activities have doubled the amount of nitrogen available annually to living organisms. In many coastal waters, human sources of nitrogen now rival or exceed natural inputs of nitrogen. From 10-45% of the anthropogenic nitrogen reaching estuarine and coastal ecosystems is transported and deposited via the atmosphere.
Why is atmospheric nitrogen deposition a problem?
Increased supplies of nitrogen usable by plants and animals have resulted in ecological impacts with significant economic, political, social, and cultural consequences. One of the best documented and understood results of increased nitrogen is the eutrophication of estuaries and coastal waters. Eutrophication involves an increase in the rate of supply of organic matter to an ecosystem. Consequences of eutrophication include massive die-offs of estuarine and marine plants and animals; loss of biological diversity; growth of nuisance algae potentially toxic to humans and marine animals; loss of seagrass and other habitats important for healthy coastal ecosystems; and, negative effects on the sustainability of desired fisheries.
Where is atmospheric nitrogen deposition a problem?
The extent of nitrogen inputs from atmospheric sources varies locally and regionally throughout the coastal areas of the United States. The seriousness of the problem depends on vulnerability of coastal systems, the amount of atmospheric deposition, and the relative importance of other sources of nitrogen. Some regions of the United States do not have nitrogen overenrichment problems and for some regions experiencing such problems atmospheric sources may be relatively unimportant when compared with other sources.
What should be done about atmospheric nitrogen deposition?
The ecological and economic impacts of nitrogen deposition can only worsen if nothing is done. Many point and non-point nitrogen sources have been dealt with successfully in recent years. Addressing the sources and consequences of atmospheric nitrogen challenges existing regulatory programs and assessment efforts. However, atmospheric deposition of nitrogen must be included in policy and management actions to act successfully on coastal eutrophication issues.
How do we measure the effectiveness of policy and management actions?
Present monitoring methods and facilities must be improved on regional scales in order to successfully measure the status, trends, and effectiveness of controls on atmospheric sources of nitrogen. For example, present atmospheric monitoring networks should be supplemented by coastal sites currently conducting complementary monitoring. Environmental monitoring systems also must be better integrated and protected from the vagaries of the budget process.
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During the past century, human activities have doubled the annual amount of nitrogen available in a form usable by living organisms. In many coastal waters, human sources of nitrogen now rival or exceed natural inputs of nitrogen. The major human activities increasing supplies of usable nitrogen include production and use of nitrogen fertilizers; cultivation of crops that host symbiotic nitrogen-fixing bacteria; land uses that release nitrogen stored in forests, grasslands, wetlands, and other ecosystems; and, combustion of fossil fuels. Of the nitrogen produced by these activities, from 10-45% of the nitrogen reaching estuarine and coastal ecosystems is transported and deposited via the atmosphere. By increasing supplies of nitrogen usable by plants and animals, human activities have resulted in ecological impacts with significant and far-reaching economic, political, social, and cultural consequences.
This document reports the results of a workshop convened by the Sustainable Biosphere Initiative Project Office of the Ecological Society of America with support from the Environmental Protection Agency's (EPA) Office of Wetlands, Oceans, and Watersheds and the National Oceanic and Atmospheric Association's (NOAA) Office of Ocean and Coastal Resource Management. The workshop took place from June 2-4, 1997 at the University of Rhode Island Coastal Institute. Participants were concerned with one key piece of this broad issue: input of nitrogen to coastal waters and their watersheds via the atmosphere. A diverse group of scientists, managers, and policymakers gathered to consider several questions:
- What is the problem?
- Why is this issue a problem?
- Where is it a problem?
- What should be done about it?
- How do we measure the effectiveness of policy and management actions?
What is the problem?
Atmospheric deposition refers to the flux of nitrogen from the atmosphere to land and water surfaces. Nitrogen may be deposited directly to water surfaces and/or indirectly to land surfaces in the watershed, with subsequent runoff transporting nitrogen to waterbodies. Nitrogen may reach land and water surfaces through precipitation, referred to as "wet deposition," or as "dry deposition" in the absence of precipitation.
Atmospheric deposition is one of the most rapidly growing anthropogenic sources of fixed nitrogen in marine and coastal ecosystems, both in terms of amount and geographic scale. In many locations, atmospheric nitrogen is the single largest source of new nitrogen (as opposed to nitrogen recycled within aquatic systems) impacting the coastal zone. Depending upon the location, from 10% to more than 40% of new nitrogen inputs into coastal waters along the east coast are of atmospheric origin. Virtually all of this new nitrogen is attributable to growing fossil fuel emissions from power generation, transportation, and industrial sources, which are the primary sources of NOx emissions (nitrogen oxide plus nitrogen dioxide); and, agricultural practices such as volatization and airborne particles from animal waste and fertilizers, which are the primary sources of ammonia (NH3) and ammonium (NH4). The relative contribution of atmospheric sources to coastal nitrogen inputs is projected to increase substantially as we enter the next century, when nearly 70% of the United States population will reside within 50 km of coastal areas.
Atmospheric sources of nitrogen are difficult to address. They span broad geographical areas, cross environmental media and management authorities, and involve diverse scientific disciplines. For example, long range atmospheric transport of nitrogen from mid-continental regions is recognized as a significant source of nitrogen inputs to surface waters on the east coast. Moreover, few laws and policies regulating atmospheric sources of nitrogen specifically address water quality problems. Finally, action (reductions of emissions) and response (improvement of coastal water quality) relationships are not simple and straightforward.
Why is atmospheric nitrogen deposition a problem?
Nitrogen is one of the four most common chemical elements in living tissue, and is an essential component of key organic molecules; in short, all organisms require nitrogen in order to live. However, most plants and animals cannot utilize nitrogen directly from the air, which is composed of 78% dinitrogen gas (N2). Instead, they require nitrogen to be bonded to hydrogen molecules through a process called "fixation," creating biologically usable compounds such as ammonium and nitrate. Generally, this short supply of usable forms of nitrogen plays an important role in controlling the productivity, dynamics, biodiversity, and nutrient cycling of estuarine and marine ecosystems.
As nitrogen is normally a nutrient of limited availability, it is an important factor in regulating the rate at which biomass is produced in many estuarine and marine ecosystems. Greater amounts of available nitrogen have increased productivity in many ecosystems where nitrogen availability historically has been low (e.g., Chesapeake Bay, Waquoit Bay), leading to changes in species dynamics and losses in biological diversity. One of the best documented and understood results of increased nitrogen is the eutrophication of estuaries and coastal seas. Eutrophication involves an increase in the rate of supply of organic matter to an ecosystem, often with many undesirable consequences:
- Increases in available nitrogen stimulates organic production in ecosystems, leading to increased potentials for hypoxia (low dissolved oxygen) or anoxia (no dissolved oxygen); production of toxic hydrogen sulfide; and, ultimately, massive die-offs of estuarine and marine higher plants and animals.
- Eutrophication stimulates growth of algae that may become nuisance species, such as blue-greens (cyanobacteria), dinoflagellates, and macroalgae. Some of these algae are toxic to estuarine and marine mammals and can devastate shellfish and aquaculture industries, causing harm to recreational fishing and tourism industries in coastal areas.
- Nuisance blooms pose human health risks as well. For example, paralytic shellfish poisoning results from eating shellfish exposed to toxic algae and there are emerging concerns related to Pfisteria in Maryland and North Carolina.
- Eutrophication and greater algal growth decrease light levels in the water column, resulting in degradation of seagrass and other habitats important for healthy coastal ecosystems.
- Changes in available nitrogen also play an important role in shifting food webs, which can have dramatic effects on the sustainability of desired fisheries.
- The economic impact of this issue is only recently becoming clear, but is potentially enormous and promises to grow.
The extent of nitrogen inputs from atmospheric sources varies locally and regionally throughout the coastal areas of the United States. The seriousness of the problem depends on vulnerability of coastal systems as affected by physical characteristics, the amount of atmospheric deposition, and the relative importance of other sources of nitrogen. Some regions of the United States do not have nitrogen overenrichment problems and for some regions experiencing such problems atmospheric sources may be relatively slight when compared with other sources. In addition, changing demographic and settlement patterns in the United States will effect the relative contribution of atmospheric and other nitrogen sources to coastal waters over time.
The specific role of atmospheric transport as a path for nitrogen entering coastal waters was first raised in the late 1980s, with a focus on the Chesapeake Bay. Since that time, there have been efforts to quantify atmospheric nitrogen inputs to Chesapeake Bay and many other estuarine areas in the United States, primarily on the Atlantic coast and in parts of the Gulf of Mexico. The attached table from a report published by Valigura et al in 1996 (see suggested reading) presents summary results of a number of studies performed along the east and Gulf coasts that are comparable in broad terms.
What should be done about nitrogen deposition?
Nitrogen deposition is a trans-boundary issue, crossing watershed, legal, ownership, administrative, and other jurisdictional boundaries. Given the compartmentalized nature of agencies and institutions at all governmental levels, addressing atmospheric sources of nitrogen will challenge existing regulatory structures and assessment programs. Moreover, the response of ecological systems to atmospheric nitrogen reductions may not be immediate, directly obvious, nor the same in every area.
What is clear, however, is that increased atmospheric
deposition of nitrogen has ecological and economic impacts. The consequences
associated with these impacts can only worsen if nothing is done. A number
of point and non-point nitrogen sources have been dealt with successfully
in recent years through policy and management actions. However, atmospheric
sources of nitrogen and their ultimate consequences are neither well understood
nor addressed, and atmospheric deposition of nitrogen must be considered
in future policy and management actions. Workshop participants felt that
current scientific information and understanding supported several conclusions
with direct relevance for current policy issues and implementation.
|Policy relevant conclusions:
The issues involved in linkages between air and water resources and the role of atmospheric nitrogen deposition in coastal water quality are still evolving and emerging on the policy agenda. Workshop participants identified various short to medium-term information needs that must be fulfilled, using existing and emerging information, if policymakers and managers are to successfully understand and address these issues.
|Technical assessment needs:
Finally, a great deal is known about the relationship
between the sources and deposition of atmospheric nitrogen and the health
of coastal and estuarine waters. Yet, significant uncertainties remain,
and various research issues must be addressed over the medium to long-term
in order to inform policymakers and managers about issues surrounding atmospheric
How do we measure the effectiveness
of policy and management actions?
Present monitoring methods and facilities must be improved on regional scales in order to successfully measure the status, trends, and effectiveness of controls on atmospheric sources of nitrogen. In order to capitalize on current strengths and make monitoring cost effective, for example, present atmospheric monitoring networks should be supplemented by the addition of coastal sites currently conducting complementary monitoring. Environmental monitoring systems must be better integrated and protected from the vagaries of the budget process. In addition, socioeconomic indicators are required to evaluate societal progress in understanding and addressing environmental goals and developing an understanding and assessment of short and long-term impacts of management actions. Workshop participants made several specific suggestions in this regard.
For Further Reading
- Howarth, Robert W. 1997. Nitrogen Cycling in the North Atlantic Ocean and its Watersheds. Boston: Kluwer Academic Publishers.
- Nixon, Scott W. 1995. Coastal Marine Eutrophication: A Definition, Social Causes and Future Concerns. Ophelia 41: 199-219.
- U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards. June 1997. Deposition of Air Pollutants to the Great Waters: Second Report to Congress.
- Valigura, Richard A., Winston T. Luke, Richard S. Artz, and Bruce B. Hicks. Atmospheric Nutrient Input to Coastal Areas: Reducing the Uncertainties. 1996. Decision Analysis Series No. 9.
- Vitousek, Peter M., John D. Aber, Robert W. Howarth, Gene E. Likens, Pamela A. Matson, Davaid W. Schindler, William H. Schlesinger and David G. Tilman. 1997. Human Alteration of the Global Nitrogen Cycle: Causes and Consequences. Ecological Applications 7 (August): 737-750.
Copies of this report are available from: