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.
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
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; email@example.com.
350 Northeastern Naturalist Vol. 17, No. 3
et al. 2001) found that beaver impoundments reduced stream water levels of
-, 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
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
- loss is due to biological activity, then one would expect differences
- 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.
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
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
- 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.
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
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)
- 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
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
- 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).
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
- 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
- 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
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
- 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
- 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
beaver-impounded streams is important for reducing the concentration of
this nutrient prior to its reaching marine waters.
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
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