JEQ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Web of Science (10)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Whitall, D. R.
Right arrow Articles by Paerl, H. W.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Whitall, D. R.
Right arrow Articles by Paerl, H. W.
Agricola
Right arrow Articles by Whitall, D. R.
Right arrow Articles by Paerl, H. W.
Related Collections
Right arrow Water Quality
Right arrow Watershed and Landscape Processes
Right arrow Nutrient Management
Right arrow Air Pollution
Right arrow Water Pollution
Journal of Environmental Quality 30:1508-1515 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Atmospheric Pollutants and Trace Gases

Spatiotemporal Variability of Wet Atmospheric Nitrogen Deposition to the Neuse River Estuary, North Carolina

David R. Whitall* and Hans W. Paerl

University of North Carolina at Chapel Hill, Institute of Marine Sciences, 3431 Arendell St., Morehead City, NC 28557

* Corresponding author (drwhital{at}syr.edu)

Received for publication July 5, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Excessive nitrogen (N) loading to N-sensitive waters such as the Neuse River estuary (North Carolina) has been shown to promote changes in microbial and algal community composition and function (harmful algal blooms), hypoxia and anoxia, and fish kills. Previous studies have estimated that wet atmospheric deposition of nitrogen (WAD-N), as deposition of dissolved inorganic nitrogen (DIN: NO-3, NH3/NH+4) and dissolved organic nitrogen, may contribute at least 15% of the total externally supplied or "new" N flux to the coastal waters of North Carolina. In a 3-yr study from June 1996 to June 1999, we calculated the weekly wet deposition of inorganic and organic N at eleven sites on a northwest–southeast transect in the watershed. The annual mean total (wet DIN + wet organics) WAD-N flux for the Neuse River watershed was calculated to be 956 mg N/m2/yr (15026 Mg N/yr). Seasonally, the spring (March–May) and summer (June–August) months contain the highest total weekly N deposition; this pattern appears to be driven by N concentration in precipitation. There is also spatial variability in WAD-N deposition; in general, the upper portion of the watershed receives the lowest annual deposition and the middle portion of the watershed receives the highest deposition. Based on a range of watershed N retention and in-stream riverine processing values, we estimate that this flux contributes approximately 24% of the total "new" N flux to the estuary.

Abbreviations: AD-N, atmospherically deposited nitrogen • DIN, dissolved inorganic nitrogen • NADP, National Atmospheric Deposition Program • WAD-N, wet atmospheric deposition of nitrogen


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
THE N SOURCES for a water body are comprised of two components: endogenous or recycled N and exogenous or "new" N. "New" productivity is productivity that is attributed to exogenous N sources (Eppley and Peterson, 1979). In an estuarine system, "new" N is externally supplied N transported to the estuary in rivers. This includes natural sources of N such as N2 fixation, point sources of N (municipal and industrial discharges), agricultural and urban runoff, and other nonpoint sources of N (septic fields, atmospherically deposited nitrogen (AD-N) that fall on the watershed (indirect deposition) and fall directly on the estuary (direct deposition). Coastal waters, which make up only 15% of the world ocean surface area, account for up to one-half of the "new" global marine primary productivity (Mackenzie et al., 1991). This new productivity has been attributed in part to the increase in anthropogenic N inputs to coastal waters (Ryther and Dunstan, 1971; Nixon, 1986; Asman et al., 1993; Paerl, 1995). Atmospherically deposited nitrogen is a historically overlooked, yet potentially quite important source of N to coastal waters (Valigura et al., 1996). Precipitation contains a variety of forms of N, including inorganic and organic (amino acids, urea, organonitriles, N-heterocyclics) nitrogen (Timperley et al., 1985; Mopper and Zika, 1987; Duce et al., 1991; Paerl, 1995).

Previous studies have shown that AD-N is an important source of "new" nitrogen, contributing from 20 to >40% of the new N flux to U.S. east coast estuarine and coastal waters. This new N has been implicated in accelerating coastal productivity and eutrophication in N-sensitive coastal ecosystems (Paerl, 1985; Asman et al., 1993; Paerl, 1995; Paerl and Whitall, 1999). Eutrophication is often associated with altered phytoplankton community structure and function in coastal systems (Paerl, 1998). These perturbations can include the formation of nuisance algal blooms and cause hypoxia and anoxia, both causative agents in fish kills, resulting in ecological and economic losses. Such changes in biodiversity have been observed in the Neuse River estuary, which drains one of North Carolina's most productive and rapidly developing urban, industrial, and agricultural watersheds (Paerl et al., 1998).

Over the past 20 yr, symptoms of eutrophication, including nuisance cyanobacterial and dinoflagellate blooms, associated bottom water hypoxia and anoxia, fish kills, and altered food web structure, have plagued this productive and economically valuable system (Tedder et al., 1980; Paerl, 1983; Christian et al., 1986; Paerl et al., 1998).

This approximately 16000-km2 watershed is part of the Albemarle–Pamlico Estuary System, the USA's second largest estuarine complex. The Neuse has been ranked among the USA's 20 most threatened riverine–estuarine systems for three consecutive years by the American Rivers Foundation (American Rivers Foundation, 1997) and the population of the Neuse River watershed, which makes up a significant portion of eastern North Carolina's population, experienced a 6.7% increase from 1990 to 1996 (U.S. Census Bureau, 1996).

Wet atmospheric deposition of nitrogen has been documented to be increasing in eastern North Carolina (Paerl and Whitall, 1999). It is also important to note that not only is the total N flux increasing but the partitioning between dissolved inorganic nitrogen (DIN) species is changing as well; the relative proportion of NH+4 to NO-3 deposition is increasing (Paerl and Whitall, 1999). This can have profound effects on algal community structure because these DIN species have different bioreactivities to different algal groups (Collos, 1989; Stolte et al., 1994).

While natural sources of AD-N exist (plant senescence, lightning, decomposition of organic matter, volcanic emissions, surface water photolysis, dust generated by high winds [Paerl and Whitall, 1999]), anthropogenic sources tend to be much larger in magnitude, and more temporally variable. Unlike natural sources, anthropogenic sources may be managed through actions and regulations and thus are the focus of this study. Figure 1 shows the contribution of various anthropogenic N sources to the atmospheric N emissions inventory for North Carolina. Fossil fuel combustion (automobiles, point sources, and nonroad mobile sources in Fig. 1) is a large source of nitrogen oxides (NOx) (Likens et al., 1974; Levy and Maxim, 1987; Duce et al., 1991) and a smaller source of NH3 gas. Nitrogen oxide can be transported over relatively large distances (hundreds to thousands of kilometers) due its relatively long atmospheric lifetime (1–15 d) (Aneja et al., 1998). The Neuse River watershed may receive NOx plumes from metropolitan areas in the eastern USA (Atlanta, Charlotte, Richmond, Norfolk, Washington DC, Pittsburgh) in addition to within watershed urban (Raleigh–Durham) and localized sources.



View larger version (27K):
[in this window]
[in a new window]
  		Fig. 1. Relative contribution of various nitrogen (N) sources to the atmospheric N emissions budget for North Carolina (black) and coastal North Carolina (white). Statewide N emissions total 334748 Mg/yr (coastal plain total = 155931 Mg/yr). From North Carolina Department of the Environmental and Natural Resources Division of Air Quality (1996).

 
While oxidized N species are usually the dominant forms of DIN in rainfall, in areas with intensive agriculture, such as western Europe, NH3 (aq) and NH+4 (aq) can also be major DIN components (Buijsman et al., 1987). Anthropogenic sources of NH3 include stack emissions, sewage treatment plants, septic systems, and agricultural emissions (both from chemical fertilizers and animal waste). Stack emissions are unlikely to play a large role in the atmospheric NH3 budget for eastern North Carolina; there are only 19 point discharge sources in eastern North Carolina with a combined total annual flux to the atmosphere of less than 2000 Mg NH3–N/yr (Environmental Defense Fund, 1998), which is less than 2% of the total N emissions for the coastal zone of North Carolina (North Carolina Division of Air Quality, 1996; Fig. 1). Unlike NOx, atmospheric NH3 transport tends to occur over more local scales (100 km or less) (Aneja et al., 1998), although aerosolized NH+4 can travel farther. A likely source of new NH3 to eastern North Carolina is volatilization from animal waste, which has increased in scope with large increases in poultry and swine production since 1990 (Aneja et al., 2000; Walker et al., 2000). Furthermore, the majority of these swine operations are either within the Neuse River watershed or upwind from this N-sensitive watershed (North Carolina Department of Agriculture, 2000).

The primary objective of this study was to quantify the contribution of WAD-N to the total "new" nitrogen budget of the Neuse River estuary, and to characterize its spatiotemporal variability.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Sample Collection and Storage
Sampling sites were located along a longitudinal transect in the Neuse River watershed in eastern North Carolina (Fig. 2). They were chosen to achieve adequate spatial coverage of the watershed. Wet deposition was sampled using a wet/dry collector (Aerochem Metrics [Bushnell, FL] Model 301) on a weekly basis from July 1996 to July 1999.



View larger version (34K):
[in this window]
[in a new window]
  		Fig. 2. Site location map. Numbers show the locations of wet atmospheric deposition of nitrogen (WAD-N) collectors and rain gauges (see also Table 1). The WAD-N and rain gauge pairings are shown with like shapes. Zones shown are delineations for in-stream N degradation model (see Table 3). IMS is the University of North Carolina-Chapel Hill Institute of Marine Sciences in Morehead City.

 

View this table:
[in this window]
[in a new window]
  		Table 1. Sampling locations and site descriptions. North Carolina State Climate Office (NCSCO) sites are equipped with a rain gauge. National Atmospheric Deposition Program (NADP) sites are equipped with a rain gauge and an Aerochem Metrics Model 301 collector. University of North Carolina at Chapel Hill Institute of Marine Sciences (UNC-IMS) sites are equipped with a rain gauge and an Aerochem Metrics Model 301 collector. The predominant land use type for all stations is forest, followed by agriculture and urban (see Figure 2).

 

View this table:
[in this window]
[in a new window]
  		Table 3. In-stream nitrogen degradation factors for the Neuse River basin. Zones are delineated in Figure 2. Factors are from the North Carolina Department of Environment and Natural Resources Division of Water Quality.

 
Previous studies (Vet et al., 1989; Lamb and Comrie, 1993; Butler and Likens, 1998) have reported significantly lower NO-3 and NH+4 concentrations in co-located sampling stations sampled weekly versus those sampled on a daily basis. This has been attributed to biological utilization of deposited N in the wet bucket during the sample week (Sisterson et al., 1985; Ramundo and Seastedt, 1990; Vesely, 1990). To address this problem, thymol (C10H14O) was used as a biocide (Gillett and Ayers, 1991) in the wet bucket. Collectors have covered "wet" buckets that, in combination with the inherently low rain water pH (4 < pH < 5), minimize the potential for NH3 loss through volatilization. Sample buckets were cleaned weekly with 1% HCl and Nanopure water (Barnstead/Thermolyne, Dubuque, IA). All samples were filtered through precombusted Whatman (Maidstone, UK) GF/F filters and stored frozen (-20°C) in acid-washed HDPE bottles, which had been prerinsed with the sample, until analysis.

Chemical Analyses
The collected deposition samples were analyzed for NH+4, NO-3, and total Kjeldahl nitrogen (TKN), using a Lachat (Milwaukee, WI) QC8000 Autoanalyzer (NH+4 after Diamond and Huberty, 1996; NO-3 from Schetig, 1997; TKN from Wendt, 1997). Organic N was determined by difference (TKN minus NH+4).

Deposition Calculations
Annual wet atmospheric nitrogen deposition was calculated by:

Both N concentration in the rainfall and rainfall depth have exhibited large spatial variabilities on a weekly basis (Peierls and Paerl, 1997; National Atmospheric Deposition Program [NADP] data). Therefore, it was desirable to employ as many collection sites as possible. An array (six) of precipitation gauges, managed by the North Carolina State Climate Office (NCSCO), was used to complement the five locations in the watershed that have atmospheric deposition collectors and co-located rain gauges (see Fig. 2 and Table 1). Stand-alone (NCSCO) gauges were paired with concentration data from the closest wet/dry collector for deposition calculations. Two of the sites are parts of national monitoring programs; Site 2 is a NADP site and Site 11 was a Clean Air Status and Trends Network (CASTNet) site that is now part of the NADP network; because these monitoring programs do not measure organic N, data from the closest site measuring organics were used. In the case of the Beaufort site (Site 11), the closest location measuring organic deposition was the University of North Carolina at Chapel Hill Institute of Marine Sciences (IMS) in Morehead City, which is in close proximity to, but not in, the Neuse River watershed.

Atmospheric deposition may reach the estuary via two pathways: direct deposition to the estuary surface waters and deposition to the watershed itself, which can enter the waterways of the watershed in runoff (indirect deposition). In order to calculate watershed-level indirect AD-N inputs, the watershed was divided into polygons based on rain gauge locations using the Theissan method (Schwab et al., 1993, p. 41–42) (shown in Fig. 3). This allows for spatially weighted indirect WAD-N fluxes based on area (area of polygon A [m2] x deposition flux polygon A [mg/m2/yr] = total deposition for polygon A [mg/yr]). For direct deposition calculations, the estuary surface area was similarly divided with the Theissan method using the three WAD-N sites that bound the estuary (Sites 9, 10, and 11; Fig. 2).



View larger version (28K):
[in this window]
[in a new window]
  		Fig. 3. Spatial distribution of average annual wet atmospheric deposition of nitrogen (WAD-N flux). Watershed subdivision polygons were determined via the Theissan method and were used for spatially weighted flux calculations. The annual flux for Polygon 6 is significantly greater (one-way ANOVA post-hoc Bonferroni means analysis, {alpha} = 0.05).

 
Nitrogen Retention and In-Stream Degradation Models
In order to determine the relative importance of WAD-N fluxes to the N budget of the estuary, we have attempted to estimate how much of the indirect deposition would be retained by the landscape with the use of a nitrogen retention model. Wet atmospheric deposition of nitrogen deposited to land surfaces can be taken up by plants, be denitrified in soils, leach into ground water, or run off into streams and rivers. A nitrogen retention model estimates how much of WAD-N is "absorbed" by the land; the remaining flux is then assumed to reach a waterway. Developing a comprehensive watershed-specific N retention model for the Neuse is beyond the scope of this study. A simple N retention model using estimated average values for N retention for generalized land use types was employed in this study to estimate the contribution of indirect WAD-N to the "new" N budget of the Neuse River estuary. A problem with using average N retention values for the watershed is that true nutrient retention depends on a variety of land parcel–specific parameters such as soil N content, historical acid deposition, soil type, land slope, elevation, and vegetative type (Valigura et al., 1996). By using average values for an entire watershed, amounts of the N retained in the land parcel can be calculated, but these values are not absolute and must realistically be considered estimates.

In order to address these uncertainties, a range of nitrogen retention values for each land use type was compiled from published watershed retention values from the literature (Valigura et al., 1996; Tyler, 1988; Hinga et al., 1991; Fisher and Oppenheimer, 1991). Unfortunately, these studies reporting N retention values for watersheds are almost exclusively from the Chesapeake Bay watershed. While the Chesapeake and the Neuse may be comparable in some respects, there will obviously be some significant differences between the watersheds (see discussion below). Table 2 shows the three sets of input parameters for the model, representing the highest and lowest values reported in the literature for studies of the Chesapeake Bay, as well our "best estimate", which takes into account generalized patterns in watershed-specific variables such as slope, soil type, tillage, fertilizer application rate, extent of riparian buffers, and crop types for the Neuse River watershed.


View this table:
[in this window]
[in a new window]
  		Table 2. Three N retention models for estimating percent wet atmospheric deposition of nitrogen (WAD-N) retained by various land use types. High and low models are based on highest and lowest reported values for Chesapeake Bay watershed (Valigura et al., 1996) and serve to bound the "best estimate" model. See discussion in text.

 
The values for the "best estimate" model attempt to further refine the published range of values for U.S. east coast estuaries, which primarily come from studies in the Chesapeake Bay watershed. The parameters that influence N retention include soil N, ground water characteristics, historical acid deposition, soil type, land slope, elevation, and vegetative type. Some of these parameters differ between the Neuse region and the Chesapeake Bay region.

In general, the Chesapeake Bay watershed has larger slopes than the Neuse River watershed (Trapp and Horn, 1997). The larger slopes mean a higher runoff rate and the potential for more nitrogen to the transported from land to water when compared with areas of lower slopes (Schwab et al., 1993, p. 41–42). Therefore, based only on average slope, the Neuse River watershed should have relatively high retention values compared with the Chesapeake Bay watershed.

The extent to which forests retain N is largely determined by historical AD-N loading; specifically, forest systems that have had chronically high AD-N loading have been demonstrated to conserve N less (be more "leaky") than unaffected systems (Gundersen et al., 1998; Tietma, 1998). Based on NADP data for five sites in or near the Neuse River watershed and six sites in or near the Chesapeake Bay watershed, the Chesapeake Bay watershed has a statistically higher annual AD-N flux than the Neuse (Whitall, 2000). This suggests that forests in the Neuse may be expected to retain more N than forests in the Chesapeake.

One potential mechanism for N retention is loss to ground water. Ground water in the Chesapeake Bay watershed has a much longer retention time (Tres = 200 yr; Bohlke and Denver, 1995) compared with that of the Neuse River watershed (Tres = 50 yr; Reynolds and Spruill, 1995). Therefore, in general, the Chesapeake Bay watershed may be expected to have greater N retention based on ground water retention time.

Based on these general differences between the Neuse River watershed and Chesapeake Bay watershed, we chose N retention values that approach the maximum reported retention values for the Chesapeake (Table 2) as our "best estimate" parameters. Recognizing the uncertainty in these values, we have chosen to report three sets of parameters in an attempt to bound our "best estimate" model with published maximum and minimum retention values. Based on the discussed generalized differences between the Neuse and the Chesapeake, it is unlikely that either the minimum or the maximum reported retention values are exactly correct for this system; applying these values to the retention model will perhaps result in an unrealistically high or low contribution of WAD-N to the estuary. However, in the absence of comprehensive N retention studies in the Neuse River watershed, this range of values provides us with the best available method of estimating the indirect AD-N contribution to the estuary.

For each land use type a N retention value from 0 to 1 was assigned based on these published retention values. A value of 0.25 means that 25% of the flux would be retained by the land and 75% would reach the waterways. After selecting these three sets of retention values, the model was paired with 1996 GIS land use data for the 267 subwatersheds in the Neuse. After the amount of WAD-N reaching the streams from each subwatershed was determined, these N loadings were applied to a simple in-stream degradation model designed by the North Carolina Division of Water Quality (1993) (see Table 3) to determine how much of this N reached the head of the estuary. This model accounts for N losses in the streams and rivers due to denitrification and settling as particles; N losses are proportional to distance traveled to the estuary.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Results presented are from a 36-mo period starting in July 1996.

Speciation of Wet Atmospheric Deposition of Nitrogen
On an annual basis, nitrate is the dominant chemical species in wet AD-N, making up 35% of the total deposition, followed by organic N (33%) and ammonium (32%). The wet deposition of organic N has recently been shown to be a potentially important component of WAD-N in coastal North Carolina (Peierls and Paerl, 1997) and data from our current study support this assessment. Although the organic N fluxes reported here are higher than reported for coastal North Carolina in Peierls and Paerl (1997), they fall within published reports from other coastal areas (Timperley et al., 1985; Knapp et al., 1986).

Spatial Variability in Annual Wet Atmospheric Deposition of Nitrogen Flux
On an annual basis, the highest WAD-N flux occurs in the middle segment of the watershed (Fig. 3), with the lowest fluxes occurring in the upper watershed. Rainfall amount does not significantly vary across the watershed, although on an annual basis coastal sites receive slightly more annual rain than inland sites. Deposition of NH+4 and organics do not vary spatially, but NO-3 deposition is significantly higher at Site 6 (Goldsboro) than all sites except 7, 8, and 9.

Variations in Weekly Wet Atmospheric Deposition of Nitrogen Flux
Seasonally, the total weekly wet N deposition is highest in the spring (March–May) and summer (June–August) (significant at {alpha} = 0.01, one-way ANOVA with post-hoc Bonferroni means comparison; see Fig. 4). This does not mirror seasonal precipitation patterns; seasonally, spring has the lowest average weekly precipitation (Fig. 4). This suggests that other factors, including the direction from which a storm system originates and/or passes over N source regions and seasonal changes in sources, may be causing these differences.



View larger version (24K):
[in this window]
[in a new window]
  		Fig. 4. Seasonal variability in weekly wet nitrogen deposition (black bars) and precipitation depth (white bars) for 11 sites pooled. Letters show significant differences between groups (one-way ANOVA with post-hoc Bonferroni analysis, {alpha} = 0.05). Groups with common letters are not significantly different from each other.

 
Total Annual Deposition
Total N deposited from the atmosphere due to wet deposition to the land area of the Neuse River watershed was estimated to be 15026 Mg N/yr (standard deviation ±5266 Mg). Using a nitrogen retention model and in-stream riverine degradation model (see discussion above and Tables 2 and 3), we estimated the WAD-N retained in a land parcel based on land use type and how much of the N reaching the waterways was transported to the estuary. This flux ranged from 717 to 3358 Mg N/yr, depending on the nutrient retention values used (Table 4). Using our "best estimate" input values, this flux via indirect deposition was estimated to be 1412 Mg N/yr.


View this table:
[in this window]
[in a new window]
  		Table 4. Predicted relative importance of wet atmospheric deposition of nitrogen (WAD-N) to the total "new" nitrogen (N) flux to the Neuse River estuary. See Table 2 for N retention values for the three models.{dagger}

 
Direct deposition to the estuary surface (based on Sites 9, 10, and 11, which bound the estuary) was 385 Mg N/yr (standard deviation ±88 Mg), resulting in a total WAD-N flux to the estuary of between 1103 and 3743 Mg N/yr (best estimate = 1797 Mg N/yr). When compared with the total flux of nitrogen to the estuary of 7315 Mg N/yr (riverine loading [M. Lebo, personal communication, 1998] above New Bern, NC at Streets Ferry Bridge [SFB], which inherently includes indirect WAD-N, directly deposited WAD-N, point [North Carolina Division of Water Quality, personal communication, 2000] and nonpoint-source loading [North Carolina Division of Water Quality, 1993] below SFB), these atmospheric fluxes represent between 15 and 51% (best estimate = 24%) of the total "new" nitrogen flux to the estuary (see Table 4).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The data presented here indicate that WAD-N is fairly evenly distributed between the chemical species in wet deposition. From an ecological response perspective, however, the inorganic species may be more important on short time scales than the organic species due to their high degree of availability to phytoplankton (Peierls and Paerl, 1997). Although some organic nitrogen species (urea, amino acids, primary amines) can be utilized directly by phytoplankton (Antia et al., 1991), nutrient addition bioassays conducted with rain water have shown that only 20 to 30% of the organics in WAD-N can be utilized by phytoplankton on a time scale of hours to days (Peierls and Paerl, 1997). This contrasts phytoplankton's ability to completely utilize all of the inorganic N on short time scales. Previous studies (Paerl and Whitall, 1999) using National Atmospheric Deposition Program (NADP) data from eastern North Carolina (NC35) have indicated that NH+4 has contributed an increasingly larger percentage of the inorganic WAD-N budget over the past two decades. The short duration of our current study precludes us from making similar conclusions. Chemical speciation of WAD-N may be important from an estuarine phytoplankton community response perspective because different phytoplankton groups are able to utilize different N compounds differently (Collos, 1989; Stolte et al., 1994). This apparent historical shift in the forms of bioavailable N may therefore cause alterations in algal community composition, which is important both from a trophic perspective and a potential shift toward nuisance and/or toxic species.

The seasonal variability in weekly WAD-N flux cannot be explained by variability in precipitation amount alone (Fig. 4). Therefore, this seasonal pattern must be explained by other variables such as seasonal fluctuations in source emissions of atmospheric N and other meteorological factors. During the summer, elevated air temperatures lead to relatively high NH3 volatilization rates from animal waste stored in lagoons or applied to land (Aneja et al., 2000). The seasonal influx of tourists to coastal North Carolina, 94% of whom travel by motor vehicle (Carteret County Tourism Development Bureau, 2000), translates into periods of elevated fossil fuel emissions (containing both NOx and NH3) from mobile sources. Furthermore, ammonia and ammonium flux from plant senescence, the process by which maturing plants lose NH+3 and NH4 through their stomata, is highest during the summer, which may also help explain seasonal patterns in wet NH+4 deposition. The direction from which a storm system originates and the amount of time the system spends over land will relate to the potential for the air parcel to gain N. Air parcels that pass over relatively unpolluted areas (storms of marine origin, for example) should contain a small amount of N and the flux from such weather systems should be low. Conversely, air parcels passing over areas having high atmospheric N emissions (cities and areas of intensive agriculture) should contain relatively more N and flux from such systems should be higher. Previous studies (Walker, 1998; Walker et al., 2000) have shown NADP depositional data to be well correlated with storm back trajectories from the NOAA-ARL's HYSPLIT 4 model (National Oceanic and Atmospheric Administration Air Resources Laboratory, 2000).

There are no significant spatial differences in wet NH+4 or wet organic deposition in the watershed. Therefore, observed spatial differences in total wet N deposition are driven by differences in NO-3 deposition. In theory, enhanced NO-3 deposition in the middle to lower portion of the watershed may be due to interaction of small-fraction aerosols of NOx with sea salt aerosols (Harrison and Pio, 1983; Brimblecombe and Clegg, 1988), but this is unlikely to be the case because the site nearest to the ocean (Site 11) had significantly less NO-3 deposition than the inland Goldsboro site. No definitive explanation of this NO-3 depositional pattern is available at this time. However, prevailing meteorology and spatial distribution of source emissions must be considered as potential factors.

Future research directions include developing a watershed-specific nitrogen retention model to better estimate the contribution of indirect WAD-N, better quantifying the contribution of direct WAD-N to coastal waters and the quantification of dry depositional fluxes to the watershed. As discussed above, the largest source of uncertainty in this study is the nitrogen retention model. This uncertainty results in a large range of possible values for the contribution of indirect WAD-N to the estuary. From a management perspective it is critical to refine this model so that the relative importance of WAD-N be more accurately determined. This study did not address the dry deposition of nitrogen, which may be a significant additional flux of nitrogen to the system. Two of the USEPA's Clean Air Status and Trends Network (CASTNet) sampling sites are located in eastern North Carolina. Modeling efforts at these two sites estimate the dry deposition of particulate and gaseous inorganic N to be between 150 and 320 mg N/m2/yr (80% as oxidized N) for this time period (USEPA, 2000), which would represent up to a 33% increase over the watershed-wide average N deposition reported here. The sampling techniques employed by the CASTNet program exlude gaseous NH3 and may undersample gaseous NOx in coastal areas due to the association of HNO3 with sea salt aerosols (Zhuang et al., 1999), meaning that the dry depositional fluxes may be equal in magnitude to the wet depositional fluxes reported here (Valigura et al., 1996). The direct depositional component of WAD-N, which makes up 4 to 6% of the total "new" N loading, may be particularly important when considering WAD-N's contribution to the "new" N budget of Pamlico Sound, due to its large surface area (4500 km2).

This study is one of the first efforts to quantify the importance and patterns in WAD-N on a watershed-level scale for a nitrogen-sensitive coastal watershed. In conclusion, WAD-N clearly is an important component of the N flux to the Neuse River estuary, accounting for approximately 24% (a range of 15–51%) of the total "new" nitrogen flux to the estuary. This flux varies seasonally, with the highest fluxes occurring in the summer months, and spatially, with the highest fluxes occurring in the middle portion of the watershed.


   ACKNOWLEDGMENTS
 
Funding for this work was provided by the Environmental Defense Fund, the North Carolina Department of Environment and Natural Resources, the USDA, the USEPA STAR program, and the North Carolina College Seagrant Program (NOAA). We would like to thank Malia Go, Tom Nanni, Nathan Hall, and John Fear for technical assistance, Ben Peierls, Jay Pinckney, Wayne Robarge, and Joe Rudek for valuable discussions, and the National Atmospheric Deposition Program, Lenoir Community College (NC), and the U.S. Forest Service (Croatan Work Center) for their cooperation.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Web of Science (10)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Whitall, D. R.
Right arrow Articles by Paerl, H. W.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Whitall, D. R.
Right arrow Articles by Paerl, H. W.
Agricola
Right arrow Articles by Whitall, D. R.
Right arrow Articles by Paerl, H. W.
Related Collections
Right arrow Water Quality
Right arrow Watershed and Landscape Processes
Right arrow Nutrient Management
Right arrow Air Pollution
Right arrow Water Pollution

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome