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Reduction of High Nitrate Concentrations in a Central New York State Stream Impounded by Beaver
Richard Lawrence Klotz

Northeastern Naturalist, Volume 17, Issue 3 (2010): 349–356

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2010 NORTHEASTERN NATURALIST 17(3):349–356 Reduction of High Nitrate Concentrations in a Central New York State Stream Impounded by Beaver Richard Lawrence Klotz* Abstract - The influence of Castor canadensis (North American Beaver) impoundments on the loss of nitrate (NO3 -) from stream water was monitored for 14 months at four sites in Homer Gulf Creek in central New York State. A relatively high NO3 - concentration of 1.56 (0.14) mg N/L (mean (SEM)) at the upstream site was reduced by a daily average of 35.5% as the water passed through the beaver ponds. There was a significant difference (P < 0.001) in the mean values among the four sites, and significant differences were measured between the upstream site and each of the three downstream sites. The reduction in NO3 - within the upstream beaver pond was significantly (P = 0.005) greater during the warmer months (April–September) of the study vs. the October–March period, suggesting biological processes were responsible. Stream water NO3 - may have been transformed through biological processes, including the microbial process of denitrification, enhanced in the beaver ponds as a result of anoxic sediments, sufficient supplies of labile organic matter, and increased water residence time. Introduction Historical records indicate that Castor canadensis Kuhl (North American Beaver or just Beaver) was ubiquitous in North America prior to human exploitation, likely inhabiting many small streams (Naiman et al. 1988). The flow in these streams was probably impeded by a series of beaver dams, resulting in ponds separated by the stream channels. By the end of the nineteenth century, the North American Beaver was locally extinct in large parts of its original range, and its numbers greatly reduced in much of the rest as a result of overexploitation (Outwater 1996). As a result of restoration efforts, estimates put the number of Beaver in the United States near the end of the twentieth century at between 7–12 million, most in the Great Lakes region and along the Mississippi River (Outwater 1996). The distinctive dam building of the Beaver slows stream velocity and results in the collection of large amounts of sediment and organic matter in the impounded area (Naiman et al. 1986). The saturation of the soil (Naiman et al. 1994) and increased respiratory activity (Songster-Alpin and Klotz 1995) associated with the organic matter create anaerobic zones in the pond sediments along the length of the stream, thereby influencing the nitrogen (N) cycle and promoting denitrification, the conversion of nitrate (NO3 -) to gaseous forms of N (Dahm et al. 1987). Previous studies (Cirmo and Driscoll 1993, Correll et al. 2000, Devito and Dillon 1993, Maret et al. 1987, Margolis *Department of Biological Sciences, State University of New York College at Cortland, Cortland, NY 13045; 350 Northeastern Naturalist Vol. 17, No. 3 et al. 2001) found that beaver impoundments reduced stream water levels of NO3 -, an important nutrient. However, most of the beaver ponds studied were in relatively undisturbed watersheds with upstream NO3 - concentrations less than the mean of 0.31 mg N/L found in a study of over 300 streams in small forested watersheds in the United States (Binkley et al. 2004). Many streams in the United States have high N levels from anthropogenic sources (Smith et al. 2003). Two hypotheses were tested in this study. Because beaver ponds provide enhanced conditions for NO3 - removal, I hypothesized that NO3 - loss along this relatively high-N stream would be similar to that in low-N, Beaver- impounded streams previously studied. I also hypothesized that if the NO3 - loss is due to biological activity, then one would expect differences in NO3 - loss between winter and summer. Field Site Description The beaver impoundments were located on a first-order segment of Homer Gulf Creek in a rural area of the township of Homer in Cortland County, NY at 042°40'12.62"N, 076°15'25.23"W (Fig. 1). The study area incorporated two impoundments, an upstream one with a length of 87 m and area of 4000 m2, and a downstream pond with a length of 160 m and area of 3300 m2. Breaks were made in the upstream dam sometime between 30 March and 20 April 2006, which lowered the water level and decreased the water residence time in that impoundment, but the pond remained. Sites along Homer Gulf Creek were located as follows: Site 1 was located in a free-flowing section of stream 170 m above the upstream end of the first beaver pond, Site 2 was located just downstream of the first dam, Site 3 was located just downstream of the second dam, and Site 4 was located in a stream channel 153 m Figure 1. Map of the study area. 2010 R.L. Klotz 351 downstream of Site 3. The stream channel between Sites 3 and 4 provided a segment for comparison with the beaver-impounded reaches upstream. The total distance along the first-order stream reach from Site 1 to Site 4 was 570 m. Within this study area, the stream gradient was 0.75%. The two ponds were shallow, the first impounded by a 48-m-long dam 1 m in height, and the second by a 30-m-long dam 1 m in height. State Route 41A, a lightly traveled two-lane road ran the length of the study area; the guard rail of the road was situated from 12–27 m from the edge of the ponds. The 2.02-km2 watershed was comprised of deciduous forest land (76%), cropland and pasture (22%), and shrub and brush rangeland (2%). The beaver dams were constructed over a period of about two years beginning approximately four years prior to the start of this study. The original stream had a cobble bottom, similar to the reaches at sites 1 and 4. Methods Water samples were collected at about biweekly–monthly intervals from June 2005 thru July 2006. Duplicate samples were taken from free-flowing locations at the sites in acid-cleaned, 60-ml polyethylene bottles and stored on ice. Nitrate N was measured the day of collection on a Dionex ion chromatograph (GP 50 gradient pump and ED40 electrochemical/conductivity detector) with linear conductivity response determined using NO3 - standards of 0, 1.0, and 3.0 mg N/L using AS-11 guard and analytical columns. Pond areas were determined by making measurements at 10-m intervals at 90o from a predetermined compass line along one side of the study area. Watershed area and boundaries were determined using Terrain Navigator V. 5.01 by Maptech, Inc., Andover, MA. The percentages for various land-cover types in the watershed were determined with a polar planimeter on an image from GoogleEarth. Statistical analyses were carried out using SigmaStat 3.5 by Systat Software, Inc., San Jose, CA. A one-way ANOVA was used to test for differences in NO3 - among the four sites, and the Holm-Sidak method for multiple pair-wise comparisons was used to determine which pairs of sites differed in their means. The Pearson product moment correlation coefficient was calculated to determine the strength of the linear dependence between % N loss and mm precipitation. The Mann-Whitney rank sum test was used to compare the means of two groups of non-ranked samples where the populations had non-normal distributions. The % reduction in NO3 - in the first beaver pond was calculated for each date as: 100 (1 - Site 2 N/Site 1 N), where Site 1 N and Site 2 N are the NO3 - -N concentrations for Sites 1 and 2, respectively. Daily precipitation data were obtained from the State University of New York College at Cortland weather station located 10.2 km southeast of the study area. Results Nitrate concentrations over the 14 months of the study for Homer Gulf Creek show daily upstream Site 1 values to be typically higher than 352 Northeastern Naturalist Vol. 17, No. 3 downstream Site 4 values (Fig. 2). Nitrate (mean [SEM]) was 1.56 (0.14) mg N/L at Site 1, 0.92 (0.10) at Site 2, 0.81 (0.13) at Site 3, and 0.71 (0.11) at the downstream Site 4 (Fig. 3). One-way ANOVA with the Holm-Sidak method for multiple pair-wise comparisons found a very high significant difference (P < 0.001) in the mean values among the sites, and significant differences between Sites 1 vs. 4, 1 vs. 3, and 1 vs. 2. There were no statistically signifi- cant differences between the other pairs of sites. Over the 14 months of the study, the stream water NO3 - N concentration was reduced by a daily average of 35.5% from the upstream Site 1 to downstream Site 4. The % NO3 - loss was significantly correlated (Pearson product moment correlation coefficient = -0.658; P < 0.001) with mm of precipitation on the day prior to sampling. The only three dates (14 June 2005, 14 October 2005, and 2 December 2005) when NO3 - concentrations were higher at the downstream vs. upstream station followed heavy rain; rainfall totals for the day prior to sampling were the highest for the 14 June and 14 October 2005 collections (23.6 and 22.6 mm, respectively), and a total of 53.1 mm of precipitation fell on the three days prior to the 2 December 2005 collection date. The largest downstream reductions in the concentration of NO3 - occurred during the warmer months of the study. The reduction in NO3 - in the first beaver pond was determined by the Mann-Whitney rank sum test to be significantly (P = 0.005) greater during the April–September months of the study vs. the October–March period (Table 1). Figure 2. Nitrate concentrations at the four stations at the Homer Gulf site during the 15-month study. Tick marks on x-axis represent the beginning of the month listed. Error bars represent ± 1 SD. 2010 R.L. Klotz 353 Discussion The significant reduction in streamwater NO3 - along Homer Gulf Creek (Fig. 3) extends the findings of others to show that relatively high levels of the nutrient are reduced along beaver-impounded streams. The only previous study on beaver-impounded streams with comparable NO3 - levels was conducted in a different environment, an acidic (pH: 4.5–5; ANC: -50 to 0 μeq/L) stream in the Adirondacks of New York state (Cirmo and Driscoll 1993), where NO3 - was significantly higher above (mean of 2.98 mg N/L) vs. below (1.98 mg N/L) the beaver-impounded region. Other studies were conducted on streams with lower NO3 -. Devito and Dillon (1993) found that NO3 - inputs exceeded outputs by 35% for a headwater Beaver pond in Figure 3. Mean nitrate concentrations at Homer Gulf Creek at the four sites during the study. Error bars represent ± 1 SEM. One way ANOVA with the Holm-Sidak method for multiple pair-wise comparisons found a significant difference (P < 0.001) in the mean values among the sites, and significant differences (unadjusted P < 0.001 for each) between Sites 1 vs. 4, 1 vs. 3, and 1 vs. 2. Table 1. Per cent decrease in daily nitrate nitrogen concentration between Sites 1 and 2 at Homer Gulf Creek. The Mann-Whitney Rank Sum Test found the April–September vs. the October– March periods to be significantly different (P = 0.005). % reduction Time period n Mean SE April–September 13 45.5 9.7 October–March 8 12.4 3.3 All dates 21 32.2 6.9 354 Northeastern Naturalist Vol. 17, No. 3 central Ontario; NO3 - concentrations generally ranged from about 0.01 to 0.2 mg N/L. Along a stream in Wyoming, NO3 - concentrations were reduced seasonally by 21–64% as the stream flowed through the beaver-impounded area; seasonal averages ranged from 0.14–0.60 mg N/L upstream of the impoundment (Maret et al. 1987). On the coastal plain of Maryland, stream water NO3 - concentrations averaged 0.194 mg N/L prior to construction of a Beaver dam, and 0.113 mg N/L after construction (Correll et al. 2000). Margolis et al. (2001) found that beaver impoundments acted as sinks for NO3 - on two Appalachian Mountain streams in Pennsylvania and Maryland, with discharge-weighted NO3 - concentrations of 0.037 and 0.190 mg N/L. The factors potentially responsible for the significant decrease in NO3 - concentration as the stream water passed through the upstream beaver dam include geological as well as biological processes. Input of low-N groundwater along the study reach may have contributed to the decrease. Biological factors were likely involved. The significantly higher % reduction in NO3 - concentration between sites 1 and 2 during the warm months as compared to during the cold months (Table 1) suggests biological processes within the upstream beaver pond were involved in the significant reduction in NO3 - concentration. Also, the lack of a significant change in NO3 - as the water flowed in the stream channel between sites 3 and 4 (Fig. 3) suggests that the significant reduction in high NO3 - levels entering the first pond (Site 1 vs. 2) was due to processes in the beaver pond and not in the stream channel. Studies on the removal of N from streams not impounded by Beaver reveal a biological cause (Birgand et al. 2007, Mulholland et al. 2008), and the findings provide insights into possible processes by which beaver ponds may effect N removal. These biological processes include gaseous loss as a result of microbial transformations (e.g., denitrification) and storage as a result of assimilation (Arango et al. 2008, Pribyl et al. 2005). The extensive reviews by Seitzinger et al. (2006) and Birgand et al. (2007) found that denitrification is the only factor leading to ultimate N removal in most systems, with assimilation resulting in only short-term storage of N that may eventually lead to permanent loss through denitrification. They concluded that denitrifi- cation mainly occurs in the sediments, where anaerobic conditions combine with sufficient supplies of labile organic matter and NO3 - for the denitrifying microorganisms. The construction of dams and the impoundment of water by Beaver enhance the conditions known to promote denitrification in streams. The sediments that accumulate within the beaver ponds are finer grained than those in free-flowing stream reaches (Songster-Alpin and Klotz 1995), and NO3 - loss from water over fine vs. coarse sediments is much greater (Wyer and Kay 1989). Sediments from four beaver ponds (all that were measured) near Homer Gulf Creek had higher per cent organic matter (Klotz 1998) that was more labile, as evidenced by 13- to 35-fold increases in areal microbial respiratory activities (Songster-Alpin and Klotz 1995), than sediments from free-flowing upstream reaches. Respiration of the labile organic matter leads to the anoxic conditions necessary for denitrifying microorganisms (Hill and 2010 R.L. Klotz 355 Sanmugadas 1985). The increased water residence time in impoundments (Birgand et al. 2007, Seitzinger et al. 2006) allows for NO3 - to be removed from the water by the denitrifiers in the sediments. The significant negative correlation between % NO3 - loss and mm precipitation the day prior to sampling found in this study suggests that increased discharge and shorter water residence time as a result of precipitation limits NO3 - removal. Beaver impoundments provide for a greatly increased sediment surface area per length of stream; beaver ponds in Cortland County had 21–111 times greater surface area than the original streams (Songster-Alpin and Klotz 1995). Beaver dams also raise the water level of the stream, which hydrologically connects the stream to its banks, an important factor found to promote denitrification and lead to NO3 - reduction in urban streams (Kaushal et al. 2008). These impoundment characteristics which promote denitrification may have played an important role in the significant reduction of high NO3 - concentrations in the upstream beaver pond of this study (Site 1 vs. 2; Fig. 3). This study extends the findings of others to show that high levels of NO3 - are reduced along a Beaver-impounded stream type common to the northeastern United States. Many streams in the conterminous United States have similarly high NO3 - concentrations (Smith et al. 2003). Coastal eutrophication is a growing problem linked to high levels of NO3 - from anthropogenic sources (Howarth and Marino 2006). The removal of NO3 - along beaver-impounded streams is important for reducing the concentration of this nutrient prior to its reaching marine waters. Acknowledgments I thank C. Muirhead, M. McCormick, and J. Tibbles for assisting with the laboratory and field analyses, and D. Barclay, P. Jeffers, and L.K. Klotz for assisting with technical matters. Literature Cited Arango, C.P., J.L. Tank, L.T. Johnson, and S.K. Hamilton. 2008. Assimilatory uptake rather than nitrification and denitrification determines nitrogen-removal patterns in streams of varying land use. Limnology and Oceanography 53:2558–2572. Binkley, D., G.G. Ice, J. Kaye, and C.A. Williams. 2004. Nitrogen and phosphorus concentrations in forest streams of the United States. Journal of the American Water Resources Association 40:1277–1291. Birgand, F., R.W. Skaggs, G.M. Chescheir, and J.W. 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