Environ. Sci. Technol. 2001, 35, 3482-3487
Stable Nitrogen Isotopes in Waterfowl Feathers Reflect Agricultural Land Use in Western Canada C R A I G E . H E B E R T * ,† A N D LEONARD I. WASSENAAR‡ Environment Canada, Canadian Wildlife Service, National Wildlife Research Centre, 100 Gamelin Boulevard, Hull, Quebec, Canada K1A 0H3, and Environment Canada, National Water Research Institute, National Hydrology Research Centre, 11 Innovation Boulevard, Saskatoon, Saskatchewan, Canada S7N 3H5
Stable nitrogen and carbon isotope analysis was performed on secondary feathers collected from flightless, mallard ducklings (Anas platyrhynchos) from 17 locations across western Canada. The δ15N values of mallard feathers ranged from +6.1 to +23.7‰ (AIR). Mean δ15N feather values from the 17 locations were strongly correlated with the percentage of land under agricultural development. Higher δ15N values in waterfowl feathers collected from agricultural areas possibly reflected the entry of excess fertilizer nitrogen into local water bodies. However, other processes may have also been important. These results provide evidence that nitrogen isotope values in avian feathers may reflect long-term nitrogen additions to surface waters in agricultural areas and may also provide important clues in elucidating the origin of nonpoint source nitrogen inputs.
Introduction In terrestrial systems, anthropogenic sources of biologically available nitrogen are currently estimated to be at least equal to those produced naturally (1). Of those anthropogenic sources, the greatest single contribution is from nitrogen in fertilizers (1), most of which is used in agricultural applications. Nutrient loading is often considered to be one of the most important detrimental effects associated with agriculture (2). In Canada and the United States agriculture is the largest single source of nitrogen entering surface and groundwaters (3, 4). Research into the water quality impacts of agriculture necessitates both an examination of the levels of nitrogen in receiving waters and the sources of nutrients. Previous work has documented positive relationships between agricultural intensity and nitrate-nitrogen concentrations in surface waters (5, 6). For example, in Alberta, total nitrogen water quality guidelines, designed to protect aquatic life, were exceeded in 32%, 65%, and 87% of samples taken from low, medium, and high intensity agricultural areas, * Corresponding author phone: (819)953-3904; fax: (819)953-6612; e-mail:
[email protected]. † Environment Canada, Canadian Wildlife Service, National Wildlife Research Centre. ‡ Environment Canada, National Water Research Institute, National Hydrology Research Centre. 3482 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 17, 2001
FIGURE 1. Range of δ15N values in nitrate originating from different nitrogen sources (panels A-C, data taken from ref 8). Also shown is the range in baseline δ15N estimates that were calculated using mean feather δ15N values from each of the 17 waterfowl sampling sites (panel D). The numbers on the x-axis represent the maximum value within a particular category. respectively (5). Therefore, we would predict that in areas of high agricultural intensity, nitrogen inputs and surface water nitrogen concentrations would be greater than in nonagricultural settings. This may stem from the application of high amounts of nitrogen fertilizers and manure to agricultural lands, resulting in the entry of anthropogenic nitrogen into nontarget ecosystems such as local wetlands, lakes, and rivers (3, 4). The stable isotopic ratios of nitrogen (δ15N) have been used to elucidate nitrogen sources in the environment (7). Different sources of nitrogen have different stable isotopic values (see ref 8 for review) and so δ15N may be used to trace nitrogen sources. For example, mean δ15N values in synthetic fertilizers produced from atmospheric nitrogen vary around 0 ( 3‰, whereas nitrate derived from animal manure has δ15N values ranging from +10 to +25‰ (8) (Figure 1). Rodvang et al. (9) analyzed synthetic fertilizers commonly used in Alberta and confirmed that their δ15N values were around 0‰. δ15N values of nitrate from fertilized soils are somewhat more negative in their δ15N values than the nitrate found in natural soils (Figure 1). This likely reflects the addition of nitrogen from synthetic fertilizers. These differences in the range of δ15N values provide evidence that stable isotope measurements of environmental samples may provide insights into the sources of anthropogenic nitrogen entering ecosystems. Still, there are a number of factors that 10.1021/es001970p CCC: $20.00
Published 2001 by the Am. Chem. Soc. Published on Web 08/01/2001
FIGURE 2. Location of sampling sites where mallard duckling feathers were collected. Site numbers correspond to those in Table 1. complicate the use of stable isotope tracers in identifying nitrogen sources (see ref 8). In Canada, there is no national program to monitor water quality or to evaluate the specific effects of agricultural practices on water quality (2). Assessing the cumulative and long-term effects of agricultural activities on water quality is difficult and reflects the high cost of monitoring, the difficulty of tracing sources of chemicals and nutrients, and the wide variety of farming methods to be evaluated as well as the time-lag between application of chemicals and fertilizers and the observation of an effect (2). Therefore, additional information that may be gleaned from monitoring programs designed for other purposes may prove useful in improving our capacity to detect the cumulative effects of nutrient additions to the environment. In particular, the use of higher trophic level organisms to monitor the impacts of nutrients on surface waters may lead to useful insights. Because higher trophic level organisms integrate into their tissues the isotopic composition of dietary nutrients over a period of time, they will reflect the ambient isotopic environmental conditions. This is in contrast to traditional abiotic sampling matrices (e.g. water), which may reflect more short-term characteristics. Therefore, the use of higher trophic level organisms may provide a cost-effective means to monitor and assess the cumulative impact of nonpoint source nitrogen additions to watersheds and augment traditional water-quality monitoring techniques. In this study, we measured the stable nitrogen and carbon isotope values in feathers from flightless, mallard ducklings (Anas platyrhynchos) from a variety of sites in western Canada in order to examine the relationship between feather isotope values and land use (i.e. agricultural activities). Our hypothesis was that increased or long-term nitrogen additions to landscapes and watersheds, particularly in Western Canada through the addition of animal manure, would lead to progressively higher δ15N values in the organisms that live and grow in these environments. Mallard ducks were used because they are an integral part of aquatic food webs, and samples could be obtained during annual banding programs that are conducted in support of waterfowl population management activities (10).
Materials and Methods In 1999, young-of-the-year mallard ducklings were livetrapped during annual waterfowl banding activities (10). These young birds were flightless and thus reflected environmental conditions at their natal locations where they were sampled. The fourth secondary feather was removed from each wing; the birds were sexed, banded, and released. Mallards from 17 locations (n ) 77, Figure 2) were captured, and feather samples (see Table 1) were selected for stable isotope analysis. Feather samples were cleaned of surface oils by rinsing them three times with methanol, oven-dried at 80 °C for 24 h to remove adsorbed water and solvent. Between 1 and 1.5 mg of feather material was combusted online using a CarloErba NA1500 elemental analyzer. The resulting CO2 and N2 gas from the samples was separated chromatographically and introduced into a VG Optima triple collector isotoperatio mass-spectrometer via an open split. Stable nitrogen (δ15N) and carbon (δ15C) isotope ratios were expressed in delta (δ) notation, as parts per thousand (‰) deviation from the AIR (atmospheric nitrogen) and PDB (Pee Dee Belemnite) standards, respectively. Using internal laboratory and primary isotopic standards, sample repeatability for δ15N and δ15C was better than (0.2‰. Information on land use activities around the 17 sampling sites came from the classification of Advanced Very HighResolution Radiometer (AVHRR) satellite data collected in 1995 (11). Using a geographic information system (GIS) (12), each of the 17 sampling sites was categorized by ecodistrict. Ecodistricts represent the smallest geographic units included in Canada’s National Ecological Framework and are characterized by distinctive assemblages of relief, landforms, geology, soil, vegetation, water bodies, and fauna (13). Ecodistricts in Alberta and Saskatchewan (n ) 13) had a mean area of 5400 km2. The four northern ecodistricts were larger in size with average areas of 22 300 km2. Information from different GIS datasets was overlaid to examine land-use patterns. Ecodistrict polygon layers were cross-correlated with the AVHRR layer to determine the land cover composiVOL. 35, NO. 17, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Site-Specific Mean Feather δ13C and δ15N Values (( 1 SD), Baseline δ15N Estimates Calculated Using Feather Data (See Text for Details), and Site Information Regarding Location, Land Use, and Soil Characteristics site
location
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Horseshoe Slough, YT Willow Lk, NWT Stagg R., NWT Mills Lk, NWT Hay Lk, AB Cardinal Lk, AB Big Hay Lk, AB Samson Lk, AB Johnson Lk, AB Frank, AB Brooks, AB Medicine Hat, AB Teo Lk, SK Ford Lk, SK Stanges Lk, SK Foam Lk, SK Cumberland House, SK
mean feather mean feather baseline % medium % total latitude longitude N δ13C (( 1 SD) δ15N (( 1 SD) δ15N biomass crop agriculture 63.40 65.25 62.80 61.50 56.00 56.10 53.12 52.40 52.20 50.30 50.50 50.05 51.50 53.10 52.25 51.60 54.00
-135.00 -125.45 -115.50 -118.25 -118.30 -117.40 -113.20 -113.10 -112.40 -113.40 -112.00 -110.60 -109.30 -108.30 -104.80 -103.60 -102.25
6 3 6 5 5 5 2 1 5 6 6 4 2 6 6 3 6
-25.2 ( 2.9 -25.0 ( 1.5 -27.2 ( 2.1 -28.0 ( 1.7 -19.4 ( 0.4 -14.5 ( 0.7 -26.8 ( 1.9 -28.6 -26.4 ( 0.5 -25.6 ( 1.2 -20.3 ( 2.9 -20.0 ( 0.3 -27.6 ( 0.2 -23.7 ( 0.1 -24.0 ( 0.4 -25.8 ( 0.4 -24.5 ( 0.4
8.5 ( 1.6 7.3 ( 0.8 7.5 ( 0.5 8.4 ( 0.8 11.4 ( 1.2 12.9 ( 0.4 7.3 ( 0.2 9.3 7.1 ( 0.5 17.4 ( 3.3 9.7 ( 0.6 10.2 ( 0.2 9.3 ( 0.4 12.8 ( 0.5 10.7 ( 0.3 11.0 ( 0.4 11.3 ( 0.3
3.4 2.2 2.4 3.3 6.3 7.8 2.2 4.2 2.0 12.3 4.6 5.1 4.2 7.7 5.6 5.9 6.2
0 0 0 0 35 60 10 14 14 62 13 3 2 16 22 11 0.4
0 0 0 0 35 60 10 14 16 93 19 16 70 16 22 11 0.4
drainage class imperfect well well mod. well imperfect well well well well well well well well well well well well
tion of each ecodistrict. Land use data for each ecodistrict was used to represent the land use activities surrounding each sampling location. The proportion of land in each of four land use categories was estimated (11). These categories were as follows: (1) high biomass croplandscroplands dominated by crops with high biomass, designated using cover type (e.g. corn) or climate (adequate precipitation); (2) low biomass croplandscropland dominated by crops with lower biomass, designated using cover type (e.g. grains) or climate (semiarid regions); (3) cropland/woodlandsmixed landscape in which cropland predominated over forest; (4) cropland/othersmixed landscape in which cropland predominated over other cover types, i.e., forest, shrubland, built areas. Compared to cropland/woodland, this cover category had lower green biomass. Total proportion of land in agriculture was estimated by adding categories (1) and (2). Soil drainage categories for each site were also obtained (14). There were seven possible categories: very poor, poor, imperfect, moderately well, well, rapid, and excessive. Welldrained areas were categorized as those where water was removed from the soil readily but not rapidly; excess water flowed downward into underlying pervious material or laterally as subsurface flow. These soils commonly retained optimum amounts of moisture for plant growth after rains or addition of irrigation water (14).
Results Individual mallard juveniles from western Canada had feather δ15N values that ranged from +6.1 to +23.7‰. Arithmetic mean δ15N values at individual sites ranged from +7.1 to +17.4‰ (Table 1). Significant intersite differences in mean feather isotope values were observed (ANOVA, p < 0.0001). Feather isotope data were log10-transformed prior to this analysis to meet normality assumptions. Consistently lower δ15N values were observed in feather samples collected from northern Canada. The land surrounding these northerly sites was not used for agriculture (Table 1). Highest feather δ15N values were observed at more southerly locations and corresponded with areas that contained a large proportion of agricultural land (Table 1). Nonparametric correlations were used to examine the relationship between isotope values and land use activities. Mean feather δ15N values correlated positively with the proportion of land in high biomass crops (rs ) 0.65, p < 0.01) (Figure 3a) and with the total proportion of land in agriculture (rs ) 0.64, p < 0.01) (Figure 3b). Feather δ15N values were not significantly correlated with the proportion of low biomass cropland, cropland/forest, or 3484
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FIGURE 3. Relationship between mean feather δ15N values and the amount of land in agriculture as defined by (A) % cover by high biomass crop and (B) % total land in agriculture (see text for details). Each point represents one sampling location; numbers beside each point refer to the sites shown in Figure 2. cropland/other (p > 0.1). Most of the sites encompassed well-drained soils (Table 1). Statistically significant intersite differences were also observed in mean feather δ15C values (Kruskal-Wallis ANOVA, p < 0.03; Table 1). However, mean feather δ15C values were not correlated with any measure of land-use (p > 0.1), and so they are not discussed further. The δ15N results suggest that agricultural land use was important in controlling feather δ15N values. To investigate the possible influence of specific nitrate sources on feather δ15N values, the isotope values were adjusted to account for trophic isotopic fractionation of 15N. This lead to δ15N
estimates for nitrate entering the base of the foodweb that facilitated comparisons with numerous δ15N values previously published for different natural and anthropogenic nitrate sources. The procedure that was used to estimate baseline nitrate δ15N values using feather measurements was comprised of three steps: (1) define the proportion of animal and plant foods in the mallard duckling diet, (2) account for the trophic fractionation of 15N assuming that δ15N values increased by about 3.4‰ per trophic level (15), and (3) estimate baseline nitrate δ15N values by subtracting the trophic fractionation factors associated with the two food types from the δ15N value of the feather. The following details describe how each of these steps was accomplished. During the first 2 weeks after hatch, the diet of ducklings of many species consists almost entirely of aquatic invertebrates (16, 17), and invertebrate availability is an important factor regulating duckling growth and survival (18). As development continues, mallard ducklings begin to consume progressively greater amounts of plant foods (16, 19). Most of the ducklings in this study were about 4 weeks in age. Therefore, their diet was characterized as having consisted of 50% animal and 50% plant foods. Assuming that, baseline δ15N values were estimated by subtracting the isotope fractionation factors associated with the two food types from the δ15N values of the feathers. Since there is little isotope fractionation of 15N in plants in most natural nitrogenlimited systems (20, 21), terrestrial plant foods would have had a δ15N value similar to the nitrogen entering the base of the foodweb. Whether this assumption applies to aquatic plants in nitrogen-rich environments remains to be determined, but our assumption here is that it does. Animal foods, i.e., aquatic invertebrate primary consumers, would have had δ15N values about 3.4‰ greater than the δ15N values in the plants they consumed. Feather δ15N values would have reflected a further 3.4‰ trophic level fractionation. Therefore, if ducklings were eating a mixed diet of invertebrates and plants, baseline nitrogen δ15N values were estimated by subtracting the expected isotopic fractionations associated with those foods (50% animal foods × 6.8% + 50% plant foods × 3.4‰ ) 3.4‰ + 1.7‰ ) 5.1‰) from the feather values. This led to mean estimated baseline δ15N values ranging from +2.0 to +12.3‰ (see Table 1). The baseline estimate is the result of several assumptions: (A) that δ15N, on average, is enriched by ∼3.4‰ per trophic level, (B) feather values exhibit a 3.4‰ diet-tissue trophic fractionation, similar to other tissues, (C) the duckling diet is 50% invertebrates and 50% plant, and (D) diet-tissue trophic fractionation factors are the same in young birds as those measured in adults. To address (A) and (B), diet-tissue fractionation factors have been previously measured (22). A carnivorous bird species (Larus delawarensis) exhibited a mean diet-feather δ15N fractionation factor of +3.0 ( 0.2‰, similar to the +3.4‰ value used in this study. Birds that had a greater proportion of plant material in their diets had lower diet-feather δ15N fractionation factors, i.e., chicken (Gallus gallus) +1.1 ( 0.1‰, quail (Coturnix japonica) +1.6 ( 0.1‰. A +3.4‰ diet-tissue fractionation factor is probably a better representation of the general trophic isotopic fractionation of 15N in aquatic foodwebs (15). It may have been a high estimate of the dietfeather trophic fractionation if plant foods were much more important than animal foods in the diet of mallard ducklings. However, this is unlikely given what is known from previous dietary studies (16, 19). If a lower diet-tissue fractionation factor were used, then the baseline δ15N estimates would have been higher because a smaller dietary fractionation factor would have been applied to the feather δ15N values. With respect to assumption (C), if the duckling diet had consisted of 100% animal material, then feather values would have been subtracted by a higher dietary isotopic fraction-
ation factor (+6.8‰) resulting in lower baseline estimates. Conversely, if the diet had been comprised of 100% plant material, then feather values would have been subtracted by a lower fractionation factor (+3.4‰), and baseline δ15N values would have been higher. However, it is unlikely that either of these two extremes reflect the true diet of mallard ducklings based upon the findings from mallard dietary studies (16, 19). We are confident, however, that our estimates assuming equal proportions of animal and plant foods in the duckling diet reflected a realistic situation. Finally, with respect to assumption (D), Hobson and Clark (22) found that age was not a factor affecting diet-tissue fractionation factors. Therefore, the diet-feather fractionation factors reported by Hobson and Clark (22) likely apply to the duckling feathers examined here. Estimation of baseline δ15N values allowed us to compare those values with the δ15N values measured in a variety of natural and anthropogenic nitrate sources. Baseline estimates (+2 to +4‰) stemming from the analysis of feathers from nonagricultural areas (sites 1-4) were consistent with baseline nitrate δ15N values found in natural soils (ranging from +2 to +6‰) (Figure 1). Most of the baseline δ15N estimates originating from the analysis of feathers from agricultural sites were intermediate between the δ15N values observed in synthetic fertilizers/fertilized soils and those measured in animal waste (Figure 1). Only feathers from site 10 (Frank Lake) displayed δ15N values in the middle of the range exhibited by manure nitrate δ15N (Table 1, Figure 1).
Discussion Studies that measured nitrogen isotopes in wildlife usually interpret δ15N as providing an indication of the trophic position of the organism (23). However, other studies comparing δ15N values among sites have emphasized the importance of considering potential differences in nitrogen isotope values at the base of the food web (24-27). Obviously, in this case, ducklings do not exhibit the wide variety of feeding behaviors that would be required to account for the large range in feather δ15N values (range ) 17.6‰) observed here. Previous work has shown that different sources of anthropogenic nitrogen have distinct δ15N values (7, 8). Thus, it is possible that the intersite differences in feather isotope values observed here mainly reflected the different isotopic values of nitrogen entering the base of the duckling foodweb. Using the feather values, we estimated the δ15N values in nitrate entering the base of the foodweb. Feathers collected from the four sites (1-4) where there was no agricultural activity had feather isotope values (+2 to +4‰) that were consistent with baseline δ15N values found in noncultivated soils (ranging from +2 to +6‰, Figure 1). However, most of the δ15N baseline estimates originating from the analysis of samples from agricultural areas were intermediate between synthetic fertilizer/fertilized soil values and the values measured in animal waste (Table 1, Figure 1). δ15N values in feathers from one of the sites (Frank Lake, AB) resulted in baseline nitrate δ15N estimates similar to those observed in nitrate from animal manure. The extent to which animal manure is applied in this region merits further detailed examination. These results may provide an indication of the cumulative impact of animal waste and fertilizer nitrate on food web δ15N values. Nevertheless, we recognize that there are other factors that may have contributed to the elevated δ15N values observed here (see ref 8). Soil and watershed denitrification, in particular, could be an important process contributing to the higher baseline δ15N values (8). Unfortunately, there have been no such systematic surveys on the Canadian Prairies documenting the extent to which soil and wetland denitrification takes place (28). However, because denitrification is an anaerobic process, it may be more important in areas of poor soil drainage (8, 29). Examination VOL. 35, NO. 17, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of the soil drainage categories indicated that most of the sites had well-drained soils (Table 1). Therefore, there is indirect evidence that denitrification may not have been responsible for the differences observed in feather δ15N values. However, it is possible that denitrification may occur under anaerobic conditions that can be found in wetlands that waterfowl use. Furthermore, landscape topography may be important with elevated δ15N values being found in soils from low-lying areas (29). Obviously, denitrification cannot be ruled out as an important process regulating intersite differences in feather δ15N values, but it remains to be seen how denitrification could explain the observed relationship between percent land in agriculture and feather δ15N values. Additional complications arise from the fact that biological samples originating from different ecosystems may have slightly different “natural” δ15N values. For example, forest organisms have slightly lower δ15N values than those observed in organisms from savanna or saline ecosystems (30-32). However, the magnitude of these natural ecosystem differences is quite small and so cannot account for the large intersite differences we observed in feather δ15N values. Similarly, the role of climate, i.e., precipitation amount, is known to affect soil and plant δ15N values over large spatial scales (33). However, the annual amounts of precipitation received in the vicinity of the 17 sampling sites were very similar (data compiled by ecodistrict from 1961 to 1990, mean for the 17 sites ) 367 ( 60 mm) (13). Therefore, climatological influences on feather δ15N values were probably not important over this geographic region. It is also possible that the aquatic plants consumed by waterfowl varied significantly in their δ15N values, and these differences could have contributed to the intersite differences in feather δ15N values measured here (G. Cabana, personal communication). Clearly, much more research is required to elucidate the relative importance of these processes in regulating δ15N values in wildlife tissues. However, we believe that nitrogen additions stemming from long-term land-use practices may be the single most important factor in regulating feather δ15N values at smaller landscape scales. Whether this is related to the addition of animal manure and/or alterations in nitrogen cycling as a result of cultivation remains to be determined. Previous studies have documented the effect of nutrient additions on δ15N values in a wide variety of ecosystems and suggested that δ15N values may be useful in tracing nutrient sources related to human activities (6, 26, 34-37). Few studies, however, have documented the relationship between land use activities on a broad scale and isotopic values in environmental samples. In a study in Vermont, Harrington et al. (6) found a positive relationship between the percentage of land in agriculture and the δ15N values in streamwater nitrate. They also found higher δ15N values in detritus, algae, invertebrates, and fish from watersheds containing more agricultural land. They concluded that the entry of anthropogenic nitrogen into these watersheds was an important factor regulating differences in δ15N values. However, they emphasized the difficulty of attributing the increases in δ15N values to any one particular source of nitrogen because of the confounding effects associated with the isotopic fractionations that can occur in both soils and groundwater. They did, suggest, however, that the higher δ15N values observed in agricultural watersheds were consistent with those expected were animal waste an important source of nitrogen. Kendall et al. (38) also reported higher δ15N values in nitrate draining Vermont agricultural lands and hypothesized that the higher δ15N values may have reflected the influence of nitrogen originating from animal waste. Cabana et al. (39) developed an empirical model predicting nitrogen isotope values in aquatic invertebrates from land-use information. Their work suggested that aquatic invertebrates 3486
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from rivers dominated by urban and agricultural lands would have higher δ15N values (up to +20‰). These studies provide additional evidence to suggest that nitrogen sources associated with different land use practices may be responsible for long-term alteration of δ15N values in environmental samples. The results presented here provide further evidence that nitrogen from nonpoint source agricultural sources was entering nontarget aquatic systems and that the δ15N values associated with these sources were eventually passed up the foodweb. On the Canadian Prairies, the intensification of livestock operations during the 1990s has been well documented (40). The number of livestock farms has decreased, while the numbers of livestock has significantly increased. Concentrating greater numbers of animals within smaller areas poses a problem for the disposal of animal waste (3). From 1976 to 1991, the total amount of nitrogen associated with manure production on the Canadian Prairies remained fairly constant (272 000 to 292 000 tonnes/yr); however, in 1996 this amount increased to 366 000 tonnes (40). In 1991 and 1996, approximately the same proportion (∼65%) of the total production was applied to agricultural lands resulting in total manure nitrogen application estimates for the Prairies of 194 553 tonnes in 1991 and 234 742 tonnes in 1996. If these values are further apportioned according to province, the changes between 1991 and 1996 resulted in an increase in the application of manure nitrogen in Saskatchewan by 19% and in Alberta by 54%. The larger increase in Alberta is interesting because the apparent relationship between feather δ15N values and percent land in agriculture is most obvious for the Alberta sites (Figure 3). These relatively recent increases in manure-nitrogen loadings may be cause for concern and reflect the need for enhanced monitoring of landscape nutrient levels and cumulative impacts of landscape nitrogen loadings. Because Canada lacks a systematic program to monitor water-quality nationwide, opportunistic monitoring may play an important role in assessing the impact of anthropogenic activities on environmental quality. The results presented here represent an example of how existing programs focused on waterfowl population management may be adapted to supply insights into other environmental issues. A corollary of this study points to the potential utility of stable nitrogen isotopes as tracers of the origins of migratory birds. Nitrogen isotope measurements may provide another means to assess the degree to which avian species are being produced in agricultural landscapes and may provide new insights into the degree to which agricultural lands act as sources of bird populations. As agricultural intensification and landscape-scale nitrogen additions continue, periodic measurement of stable nitrogen isotope values in waterfowl feathers may play a useful role in assessing the efficacy of management practices designed to limit the impacts of agriculturally derived nutrients on Prairie and other landscapes.
Acknowledgments This study would not have been possible without the suggestions and cooperation of the many biologists who provided feather samples. These included the following: John Bidwell, Elizabeth Buelna, M. Drat, Dave Duncan, Carl Ferguson, E. M. Martin, Dave Mossop, Richard Popko, Paul Pryor, Kent Sinnott, John Solberg, Phil Thorpe, James Voelzer, and the other CWS and USFWS personnel who contributed to this work. The authors also wish to thank Ian Marshall for providing the land cover data and Jason Duffe for his valuable GIS expertise. Geoff Koehler assisted with stable isotopic measurements. Insightful comments provided by K. Keenleyside and two journal reviewers greatly improved the manuscript.
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Received for review December 13, 2000. Revised manuscript received May 2, 2001. Accepted June 5, 2001. ES001970P
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