Biogeochemistry 49: 217?239, 2000.
? 2000 Kluwer Academic Publishers. Printed in the Netherlands.
Beaver pond biogeochemical
effects in the Maryland Coastal Plain
DAVID L. CORRELL, THOMAS E. JORDAN & DONALD E. WELLER
Smithsonian Environmental Research Center, PO Box 28, Edgewater, MD 21037, U.S.A.
(author for correspondence)
Received 19 March 1999; accepted 24 September 1999
Key words: ammonium, beavers, landscape, nitrate, phosphate, silicate, total organic-C, total
organic-N, total organic-P, total suspended solids
Abstract. The fluxes and concentrations of materials from two contiguous second-order water-
sheds in the Coastal Plain of Maryland, U.S.A. were measured for six years prior to and six
years subsequent to the formation of a 1.25 ha beaver pond near the bottom of one of the
watersheds. The watersheds have a clay aquiclude and were equipped with V-notch weirs and
continuous volume-integrating water samplers. The beaver pond reduced annual discharge
of water, total-N, total-P, dissolved silicate, TOC, and TSS by 8, 18, 21, 32, 28, and 27%,
respectively. Most of the total-N reduction was due to increased retention of nitrate in the
winter and spring and TON in the winter and summer. Most of the total-P reduction was the
result of retention of both TPi and TOP in the winter and summer. Dissolved silicate retention
peaked in the spring, while TOC and TSS retention peaked in the winter. Prior to the formation
of the beaver pond, concentrations of TON, TPi, TOP, TOC, and TSS had highly significant
correlations with stream discharge, especially in the winter, but subsequent to the pond there
was little or no relationship between these concentrations and stream discharge. However,
concentrations of nitrate in the spring and ammonium in the summer were highly correlated
with stream discharge both before and after the formation of the beaver pond and regres-
sions of discharge versus concentrations of these nutrients explained more of the variation in
concentrations after the formation of the pond.
Beaver (Castor canadensis) were an abundant and important part of the land-
scape in eastern North America at the time of European settlement in the 15th
and 16th centuries, but were extirpated over much of their range for their fur
(Naiman et al. 1988). The Coastal Plain of Maryland was among the many
regions where beaver populations were extirpated. In the 1970?s the U.S.
Fish & Wildlife Service reintroduced beaver to the Coastal Plain of southern
Maryland and beaver populations have subsequently increased rapidly. What
are the biogeochemical effects of this beaver reintroduction on the streams of
the Coastal Plain? While the landscape effects of beaver populations on the
biogeochemistry of the Minnesota region have been explored in considerable
218
detail (e.g. Johnston & Naiman 1987; Johnston et al. 1995; Naiman et al.
1994), the effects of a renewed beaver population in the Coastal Plain may be
somewhat different.
A series of studies of input/output balances for beaver ponds in other
parts of North America have yielded results which allow one to make some
limited generalizations. Beaver ponds in the Adirondacks of New York,
central Ontario, and Wyoming all utilized nitrate (Cirmo & Driscoll 1993,
1996; Devito et al. 1989; Maret et al. 1987). The Adirondacks pond also
utilized silicate (Cirmo & Driscoll 1993, 1996) and, during high flows, the
Wyoming beaver pond retained total-P, TKN, TSS, and alkali-extractable
phosphate (Maret et al. 1987). These results are understandable. Beaver ponds
are shallow, sunny places where periphyton and plankton might be expected
to take up nitrate, silicate, and phosphate. They are also areas of relatively low
water currents where sediments along with their nutrient contents can settle
to the bottom.
The beaver pond in the Adirondacks also exported DOC and ammonium
(Cirmo & Driscoll 1993, 1996; Smith et al. 1991). One of the two ponds
studied in Ontario exported ammonium and both exported TON (Devito et al.
1989). Exports of ammonium and TON could be due to the assimilation of
nitrate and releases of reduced forms of nitrogen and DOC exports could be
the result of primary production within the pond.
No net retention or export of DOC, total-P, or total-N were detected for the
beaver ponds in Ontario (Devito et al. 1989), although they seemed to retain
these nutrients in the summer/fall period and release them in the winter/spring
period. Also, no net retention or export of DOC or POC were found for a
beaver pond in Quebec (Naiman et al. 1986).
These studies may have missed other significant biogeochemical effects of
beaver ponds due to; (a) the lack of continuous volume-integrated sampling,
(b) inadequate hydrologic data, and (c) failure to measure all forms of the
nutrients. In many stream systems the concentrations of nutrients change
rapidly, especially during storm events (e.g. Correll et al. 1999d). Most of the
beaver pond studies cited above relied upon rather infrequent spot sampling.
Most did not continuously measure water discharge. None measured all of
the major forms of carbon, nitrogen, and phosphorus, although Naiman et al.
(1986) measured both DOC and POC.
When beavers constructed a dam in September, 1990, on one of our
long-term Rhode River watershed study sites, it presented an opportunity to
document in detail the biogeochemical effects of the beaver pond. Both this
watershed (#101) and a contiguous watershed (#102) of similar size, slope,
and land use, but with no beavers, have long been equipped with V-notch
weirs and volume-integrating flow-proportional water samplers. Both water-
219
Table 1. Characteristics of Rhode River watersheds.
Watershed Area1 Mean Mean Land Use 1(%)
(ha) Slope2 Discharge3 Forest Row Pasture & Residential Old Fields
(%) (mm/yr) Crops Hay Fields & Roads & Fallow
Beaver Pond 226 7.2 337 49 2 21 14 14
Control 192 6.3 290 52 8 17 13 10
1 The mean of ground truth surveys in 1990 and 1993.
2 Correll and Dixon (1980).
3 Correll et al. (1999b).
sheds have effective aquicludes that allow the sampling of both surface and
groundwater discharges at the weirs. Fluxes of a wide range of nutrients have
been measured continuously for a period of six years prior to and six years
subsequent to the introduction of the beaver pond on watershed 101. This
allows us to compare water discharges, and nutrient and sediment discharges
on watershed 101 before and after the beaver pond, and to make a paired
comparison between watersheds 101 and 102 for the entire 12 years to rule
out effects of interannual weather variation (e.g. Correll et al. 1999 a, b, c).
Hereafter we will refer to watershed 101 as the beaver pond watershed and
watershed 102 as the control watershed or simply the control.
Materials and methods
Research site
The research area (described more fully in Correll et al. 1999a) is located
within the Atlantic Coastal Plain of the U.S.A. in Anne Arundel County,
Maryland (38510 N, 76320 W). Annual precipitation averages 1139 mm
and temperature 13.2 C (160 years; Correll et al. 1999b). The watersheds
have sedimentary fine sandy loam soils. Bedrock is about 1000 m below the
surface, but the Marlboro clay layer forms an effective aquiclude slightly
above sea level throughout the area (Chirlin & Shaffner 1977). Therefore,
each watershed has a perched aquifer, and overland storm flows, interflow,
and groundwater discharges all move to the channel draining each water-
shed. The two watersheds have similar drainage areas, slopes, and land use
(Table 1) and are drained by second order streams.
In the fall of 1990 beavers constructed a dam just above the weir on water-
shed 101, which created a pond that was maintained until the fall of 1996. The
pond was an approximately round shape, with a total area of approximately
220
1.25 ha and a mean depth of about 0.5 m. The surface dimensions of the pond
were determined with an optical range finder.
Sampling
The study period was from September 1984 through August 1996. Discharges
of water from each watershed were measured with sharp-crested 120V-notch
weirs, whose foundations were in contact with the Marlboro clay aquiclude
(Correll 1981). Each weir had an instrument building and a stilling well.
Depths were measured to the nearest 0.3 mm with floats and counterweights
and were recorded every 5 minutes for the beaver pond watershed and every
15 minutes for the control watershed.
Stream water samples were composited and volume-integrated for 7-d
intervals, then collected and returned to the laboratory. A Stevens, model 61R,
flow meter actuated the sampling of an aliquot once every 154 m3 of flow. The
sample water was drawn from the stream channel upstream of the weir and
aliquots were deposited into two plastic containers, one pretreated with 20
ml of 18 N sulfuric acid to prevent biological or enzymatic activity during
storage. Studies of the acid preservation found no detectable conversion of
organic-P, -N, or -C to inorganic forms. After collection, samples were either
analyzed immediately or stored at 4 C.
Sample analysis
Total-P was determined by digestion of acid-preserved samples to ortho-
phosphate with perchloric acid (King 1932). The phosphate in the digestate
and in undigested aliquots was analyzed by reaction with stannous chloride
and ammonium molybdate (APHA 1989). Total phosphate in the undigested,
acid-preserved samples was the sum of dissolved and acid-extractable partic-
ulate phosphate. Total organic-P (TOP) was calculated by subtracting total
phosphate from total-P. Total Kjeldahl N (TKN) was determined by diges-
tion of acid-preserved samples to ammonium with sulfuric acid, Hengar
granules, and hydrogen peroxide (Martin 1972). The ammonium in the
digestate was steam distilled and analyzed by Nesslerization (APHA 1989).
Ammonium was also determined in undigested aliquots by oxidation to nitrite
with alkaline hypochlorite (Strickland & Parsons 1972) and analysis of the
nitrite by reaction with sulfanilamide (APHA 1989). Ammonium was the
sum of dissolved and acid-extractable particulate ammonium. Total organic-
N (TON) was calculated by subtracting ammonium from TKN. The sum of
nitrate and nitrite was measured by reducing nitrate to nitrite with cadmium
amalgam, and analyzing nitrite by reaction with sulfanilamide (APHA 1989).
Unpreserved samples were analyzed for orthosilicate (silicic acid) after filtra-
221
tion through prewashed Millipore 0.45 um filters. Prior to spring of 1991,
samples were analyzed for silicate by reaction with ammonium molyb-
date and colorimetry (Strickland & Parsons 1972). Subsequently, samples
were analyzed for silicate by reaction with ammonium molybdate in a
Technicon Auto-Analyzer (method 696-82W). The concentration of total
suspended solids (TSS) was measured by filtering unpreserved samples
through prewashed, preweighed Millipore 0.45 um filters, drying at 60 C,
and reweighing. Total organic-C (TOC) was analyzed by drying samples at
60 C, followed by reaction with potassium dichromate in 67% sulfuric acid
at 100 C for 3 h (Maciolek 1962). Organic C was calculated from the amount
of unreacted dichromate measured colorimetrically (Mackiolek 1962; Gaudy
& Ramanathan 1964).
Mass balance calculations
We estimated mean annual (Figure 3) and seasonal (Table 5) mass balances
for the beaver pond over the six year period after it was built. Outputs in
stream discharge from the beaver dam were measured directly at the water-
shed weir. Inputs of nitrogen from precipitation were also measured directly
at a site on the same watershed, not far from the pond (Correll et al. 1994;
Jordan et al. 1995). Inputs of materials into the pond from the watershed
were estimated by multiplying control watershed fluxes during the six years
subsequent to the beaver pond by linear regressions of weekly fluxes of mate-
rials from the beaver pond watershed versus control watershed fluxes during
the six years prior to the beaver pond. All regressions were highly significant
(P < 0.00001).
Results and discussion
Material concentrations
The beaver pond we studied had many significant effects on the nutrient
dynamics of the stream draining this watershed. Paired comparisons between
the beaver pond and control watersheds show that the water discharge from
the beaver pond watershed was five percent higher than from the control
watershed before the pond and three percent lower subsequently and both
of these differences were highly significant (Table 2). A similar effect was
reported by Burns and McDonnell (1998) for a beaver pond in the Adirondack
Mountains of New York and was attributed to evaporation from the pond. At
our site, average pan evaporation was 11.1, 15.7, and 9.9 mm per season
for spring, summer and fall, respectively (Higman & Correll 1982). Simple
222
Figure 1. Time series of seasonal volume-weighted mean dissolved silicate concentrations
for the beaver pond and control watersheds. An arrow indicates the fall of 1990, when the
beaver pond was constructed. Solid circle data points and solid lines are for the beaver pond
watershed and triangles and dashed lines are for the control watershed.
evaporation from the beaver pond surface would only account for 0.04, 0.6,
and 0.14 percent of average stream flow from this watershed (Correll et al.
1999b). Much of the reduction in flow from the beaver pond may have been
due to high evapotranspiration by the riparian forest fringing the pond. Such
very high evapotranspiration rates were reported for another riparian forest at
a nearby site (Peterjohn & Correll 1986).
Dissolved silicate concentrations from the beaver pond watershed were
not significantly different from the control prior to the pond, but were only
69% of those from the control after the pond (P < 0.0001, Table 2). Volume-
weighted mean seasonal concentrations of dissolved silicate from the beaver
pond watershed prior to the pond were sometimes a little higher or lower
than from the control (Figure 1). After the construction of the pond dissolved
silicate from the beaver pond watershed was lower than from the control
watershed almost every season (Figure 1). Cirmo and Driscoll (1993, 1996)
reported silicate concentrations leaving a beaver pond in the Adirondacks
were only 54% of the concentrations entering the pond. This reduction in
dissolved silicate concentration was probably due to the growth of diatoms in
the periphyton community of the large shallow beaver pond. The pond was
shallow and exposed to full sunlight, an ideal habitat for periphyton growth.
TOC concentrations for the beaver pond watershed were 27% higher (P =
0.02) than those of the control watershed before the pond, but not significantly
different after the pond, indicating a net consumption of TOC by the pond
(Table 2). We presume that much of this reduction in TOC concentrations
was due to the deposition of suspended particulates within the beaver pond
and these particulates contained organic carbon. In contrast, a beaver pond in
Quebec was reported to have no effect on TOC (Naiman et al. 1986), while a
223
Table 2. Comparisons between the beaver pond and control watersheds for six years of weekly data prior to and six years subsequent
to establishment of a beaver pond. Probabilities for comparisons between watersheds prior to and subsequent to the beaver pond are
from paired T-tests. Probabilities for whether the beaver pond watershed changed subsequent to the beaver pond are from general
T-tests of differences between the watersheds before and after establishment of the pond.
Parameter (units) Prior to Beaver Pond Subsequent to Beaver Pond Was the watershed changed
Beaver Control P Beaver Control P significantly due to Beaver? (P)
Dischange (mm/wk) 5.26 5.03 0.007 5.44 5.61 0.03 0.0006
Nitrate (ug N/L) 194 395 < 0.0001 113 249 < 0.0001 0.09
Total NH4 (ug N/L) 191 103 0.0008 406 180 0.04 0.82
TON (ug N/L) 640 563 0.02 601 632 0.92 0.09
TOC (mg C/L) 8.59 6.75 0.02 8.48 8.20 0.30 0.02
Total Pi (ug P/L) 160 169 0.23 201 199 0.74 0.41
TOP (ug P/L) 140 101 0.22 82 101 0.36 0.06
Dis. Silicate (mg Si/L) 10.9 11.1 0.21 7.67 11.1 < 0.0001 < 0.00001
TSS (mg/L) 67.6 47.7 0.03 49.3 56.6 0.25 0.01
224
beaver pond in the Adirondacks exported DOC (Smith et al. 1991; Cirmo &
Driscoll 1996). Two ponds in Ontario retained DOC in the growing season,
but exported the rest of the year (Devito et al. 1989).
TSS concentrations from the beaver pond watershed were 42% higher than
from the control watershed before the pond (P = 0.03), but not significantly
different subsequent to the pond (Table 2), indicating a net retention of TSS
by the pond. This was not surprising since the pond was large enough to
have a long retention time and very little current velocity. A beaver pond in
Wyoming has also been shown to retain TSS, especially during high flows
(Maret et al. 1987; Parker 1986).
Beaver pond watershed nitrate concentrations were lower and ammonium
concentrations were higher than for the control both prior to and subsequent
to the pond (Table 2). Comparisons of mean concentrations from the period
before the beaver pond with the period after the pond illustrate the effects of
variations in weather, even when the time periods are six years (Table 2).
To test directly for changes in water discharge or materials concentrations
due to the beaver pond, we compared differences between weekly values for
the beaver pond and control watersheds prior to and subsequent to the pond
(Table 2). Overall, average water discharges were significantly reduced as a
result of the pond. Nitrate and TON concentrations were lower after the pond,
but the change was only marginal statistically (P = 0.09, Table 2) and there
was no significant change in ammonium. Cirmo and Driscoll (1996) found
a retention of nitrate and release of ammonium. TOC concentrations were
significantly lower after the pond (P = 0.02, Table 2). Phosphate concentra-
tions were not significantly different after the pond, but TOP concentrations
were lower, although the change was only marginal statistically (P = 0.06,
Table 2). Dissolved silicate and TSS concentrations were both significantly
reduced (Table 2). The reductions in concentrations of TSS, TOC, TOP, and
TON were probably due to sedimentation of suspended particulates along
with their carbon, phosphorus, and nitrogen constituents. The small decline
in nitrate was probably due to periphyton uptake and denitrification.
Regressions of beaver pond watershed seasonal weekly concentrations
versus control watershed concentrations were often informative. Relation-
ships that were significant prior to the beaver pond often either changed or
were lost after the pond (e.g. Figure 2). For dissolved silicate there were
highly significant linear regressions in the winter, but with different slopes
and intercepts (Figure 2). However, in the spring, after the pond there was no
significant relationship between silicate concentrations in the two watersheds.
The summer and fall silicate concentration patterns were similar to those of
the spring. Nitrate concentrations from the beaver pond watershed also had
highly significant but different linear regressions versus the control watershed
225
Figure 2. Dissolved silicate weekly concentrations from the beaver pond and control water-
sheds. Solid circle data points and the solid regression line are from the time prior to the
pond and open square data points and dashed regression line are from the time after the pond.
Winter regressions for the upper panel: [SiO4 ] before beavers = 0.425 [control SiO4 ] + 6.68;
R2 = 0.27, P < 0.0001. [SiO4 ] after beavers = 0.582 [control SiO4] + 2.92; R2 = 0.26, P <
0.0001. Spring regressions for the lower panel: [SiO4] before beavers = 0.525 [control SiO4]
+ 5.14; R2 = 0.37, P < 0.0001. [SiO4] after beavers = -0.161 [control SiO4] + 9.45; R2 =
0.03, P = 0.13.
before and after the pond was built in the winter, spring, and fall. However, in
the summer there was a high correlation for nitrate concentrations before the
pond, but no significant relationship after the pond. This loss of relationship
in concentrations between the beaver pond watershed and the control was
also observed in the summer for TON, TOP, and TSS and in the fall for TSS.
We believe that changes reflect the fact that the dynamics of the beaver pond
are those of a small shallow lake, rather than the dynamics of a second order
stream.
Material fluxes
Fluxes of nutrients and TSS had a different pattern for the beaver pond water-
shed than for the control watershed (Table 3). During the six years subsequent
to the beaver pond, average annual flux of nitrate from the control watershed
was about the same as for the prior time period, but the fluxes of ammonium,
dissolved silicate, and TOC were higher, and the fluxes of TON, phosphate,
226
Table 3. Comparisons of mean annual fluxes between the beaver pond and control
watersheds for six years prior to and subsequent to establishment of a beaver pond.
Parameter (units) Prior to Beaver Pond Subsequent to Beaver Pond
Beaver Control Beaver Control
Discharge (mm/yr) 272 260 282 291
Nitrate (g N/ha yr) 642 917 540 933
TKN (g N/ha yr) 2270 2120 1730 1850
TON (g N/ha yr) 1980 1870 1430 1580
Total NH4 (g N/ha yr) 285 252 308 285
Total-P (g P/ha yr) 975 1020 565 702
Total-Pi (g P/ha yr) 508 588 377 452
TOP (g P/ha yr) 465 435 188 250
Dis. Silicate (kg Si/ha yr) 28.2 23.1 25.9 27.3
TOC (kg C/ha yr) 26.3 20.8 31.8 36.7
TSS (kg/ha yr) 300 188 111 114
TOP, and TSS were lower (Table 3). However, subsequent to the beaver pond
the fluxes of nitrate and dissolved silicate from the beaver pond watershed
were reduced, much as found by Cirmo and Driscoll (1993, 1996). While the
other material fluxes changed in the same direction as the control watershed,
the declines in TOP and TSS fluxes were larger than in the control (Table 3),
indicating the net removal of these materials in the beaver pond, similarly to
the findings of Maret et al. (1987).
The biogeochemical effects of the beaver pond were seasonal. For
example, the beaver pond effect on increased ammonium flux was only signi-
ficant in the fall (Table 4). Before the pond, fall ammonium fluxes were
not significantly different from the control, but after the pond ammonium
flux from the beaver pond watershed was 2.7 times higher than the control
(P = 0.02, Table 4), however, our test for whether the beaver pond caused this
change was marginal (P = 0.09). One should keep in mind that fall fluxes are
low to begin with, due to the low water discharge in the fall. Increased releases
of ammonium in the fall were probably due to lower redox potentials in the
bottom sediments of the pond. The pond effects on decreased nitrate fluxes
were only significant in the winter and spring, but this is when most of the
annual nitrate flux occurred. Both nitrate concentrations and water discharge
were low in the summer and fall. Beaver pond effects on decreased flux of
dissolved silicate were significant in all seasons except the fall (Table 4). In
all seasons except spring, nitrate flux was reduced by the pond and in the
227
spring, although nitrate flux increased, it increased much less than for the
control watershed 102 (Table 4). One might expect to see a decreased flux
of nitrate due, in part, to the opportunity for denitrification within the beaver
pond.
The beaver pond retained or volatilized about 30 kg nitrate-N/ha yr, 0.8
kg ammonium-N/ha yr, and 57 kg TON/ha yr, or about 18% of its average
total-N inputs (Figure 3). Looked at seasonally, winter was the period of most
total-N retention (59 kg/ha, Table 5), with some retention in the spring and
summer and no retention in the fall. Nitrogen retention in the winter and
summer was dominated by TON (41 and 25 kg N/ha, respectively; Figure 5).
There was an equal amount of nitrate retention in the winter and spring (17
kg N/ha; Figure 5). While atmospheric deposition was included in these mass
balances, the contribution to the overall budget was small (Figure 3), due to
the fact that the surface area of the pond was only 0.5% that of the water-
shed. However, atmospheric deposition to the watershed was important and
contributed to the watershed nitrogen discharges (Correll et al. 1994; Jordan
et al. 1995). Our results can be compared to those of Cirmo and Driscoll
(1996) who found that a 1.26 ha beaver pond retained 8.33 kg nitrate-N/ha
(mostly during the nongrowing season), but released 0.44 kg ammonium-
N/ha. Devito et al. (1989) found that two beaver ponds retained 4.6 and 6.1
kg nitrate-N/ha, but exported 3.2 and 7.2 kg TON/ha. One of these ponds
retained a trace of ammonium, but the other released 3.2 kg ammonium-N/ha
(Devito et al. 1989). A major difference was the fact that our watersheds are
more disturbed and discharge much larger amounts of nitrogen.
Our beaver pond retained a little of its phosphate, especially in the winter,
and 19 kg/ha yr of its TOP inputs, or about 21% of its average total-P inputs
(Figure 3, 5). The seasons of greatest P-retention were the winter and summer
and some TPi may have been released in the fall (Table 5; Figure 5). It should
be stressed that these mass balances were independently estimated for each
season and for the year. This becomes evident when the net seasonal mass
balances for total-N and total-P are summed and compared with the annual
mass balances, estimated separately. The seasonal total-N retentions sum to
135 kg N/ha yr versus the annual retention of 109 kg N/ha yr. The seasonal
total-P retentions sum to 34 kg P/ha yr versus the annual retention of 35 kg
P/ha yr. Devito et al. (1989) found that one of their ponds retained 0.1 kg
total-P/ha while the other exported 0.3 kg total-P/ha.
Our beaver pond retained 1,680 kg of dissolved silicate-Si/ha yr
(Figure 3), or about 32% of inputs, and the highest seasonal retention was 480
kg Si/ha in the spring (Table 5). This corresponds to a retention of 57.8 kg
Si/ha yr in a beaver pond in the Adirondacks of New York (Cirmo & Driscoll
1996). Our pond also retained about 2,200 kg TOC/ha yr (Figure 3) or 28%
228
Table 4. Comparisons among wataershed discharges (mm/season) and seasonal fluxes for the
beaver pond and control watersheds for six years before and six years after the construction of a
beaver dam. NS = probability > 0.10.
Parameter Flux or Nutrient Concentration Probability
Beaver Watershed Control Watershed Beaver vs Control Watershed
Before After Before After Before After Changed by
Beaver? (P )
Winter
Discharge 91.9 95.4 89.3 103 NS 0.002 0.002
Nitrate (g N/ha) 284 225 355 395 0.0001 < 0.00001 0.03
NH4C(g N/ha) 65.1 73.5 52.1 75.1 NS NS NS
TON (g N/ha) 480 363 346 444 NS NS 0.10
TOC (kg C/ha) 7.98 4.91 5.01 6.70 NS 0.006 NS
Total-Pi (g P/ha) 96.3 73.9 76.1 90.3 NS NS NS
TOP (g P/ha) 87.1 40.3 67.2 51.4 NS NS NS
H4Si04 (kg Si/ha) 9.43 7.80 8.35 8.39 < 0.00001 0.0002 < 0.00001
TSS (kg/ha) 87.1 40.3 67.2 51.4 NS NS NS
Spring
Discharge 104 143 97.9 144 0.05 0.05 NS
Nitrate (g N/ha) 233 256 266 401 NS < 0.0001 0.01
NH4C(g N/ha) 82.9 123 90.6 120 NS NS NS
TON (g N/ha) 540 765 512 771 NS NS NS
TOC (kg C/ha) 7.05 9.31 6.17 8.89 NS NS NS
Total-Pi (g P/ha) 129 192 125 213 NS NS NS
TOP (g P/ha) 111 99.5 94.3 119 NS NS NS
H4Si04 (kg Si/ha) 10.8 9.53 9.53 11.1 0.0003 0.001 < 0.00001
TSS (kg/ha) 59.6 61.8 33.1 58.9 NS NS NS
Summer
Discharge 48.4 23.2 45.4 23.9 0.08 NS 0.10
Nitrate (g N/ha) 84.4 18.1 197 83.0 0.003 0.0002 0.10
NH4C(g N/ha) 102 63.2 76.0 72.0 0.005 NS NS
TON (g N/ha) 723 179 762 262 NS 0.09 NS
TOC (kg C/ha) 7.96 2.37 6.55 2.92 NS NS NS
Total-Pi (g P/ha) 187 63.6 294 117 0.04 NS NS
TOP (g P/ha) 216 27.3 213 67.2 NS 0.03 NS
H4Si04 (kg Si/ha) 5.46 1.52 5.38 2.02 NS 0.001 0.01
TSS (kg/ha) 144 15.2 114 31.8 NS 0.08 NS
229
Table 4. Continued.
Parameter Flux or Nutrient Concentration Probability
Beaver Watershed Control Watershed Beaver vs Control Watershed
Before After Before After Before After Changed by
Beaver? (P )
Fall
Discharge 27.9 19.7 27.8 19.7 NS NS NS
Nitrate (g N/ha) 40.0 21.9 95.8 114 0.001 0.003 NS
NH4C(g N/ha) 34.9 48.1 32.2 18.0 NS 0.02 0.09
TON (g N/ha) 242 126 244 96.2 NS NS NS
TOC (kg C/ha) 3.40 1.67 3.16 1.41 NS NS NS
Total-Pi (g P/ha) 95.9 48.0 93.1 31.1 NS 0.002 NS
TOP (g P/ha) 51.0 21.0 59.9 12.3 NS NS NS
H4Si04 (kg Si/ha) 3.18 2.18 3.47 4.04 NS 0.01 NS
TSS (kg/ha) 43.4 12.6 23.6 5.58 0.07 0.02 NS
 Paired T -Tests of weekly fluxes or volume-intergrated mean concentrations were used
when comparing watersheds for the same time periods, and general T -Tests of weekly data
were used to compare differences between watearsheds before and after the establishment
of the beaver pond.
of the inputs from the stream to the pond, with the season of greatest retention
in the winter, and little or no retention in the spring and fall (Table 5). This
corresponds to a net release of 83.8 kg DOC/ha yr from pond in the Adiron-
dacks (Cirmo & Driscoll 1996). Thus, our beaver pond had a net retention
of allochthonous TOC inputs from the watershed and autochthonous TOC
inputs and was an overall heterotrophic system, as reported for a beaver pond
in Alberta (Hodkinson 1975). In contrast, a beaver pond in the Adirondacks
released 85 kg DOC/ha yr (Cirmo & Driscoll 1993, 1996), while of two ponds
in Ontario, one retained 22 kg DOC/ha yr and one released 70 kg DOC/ha yr
(Devito et al. 1989). Our beaver pond also retained about 10 t of TSS/ha yr,
or about 27% of inputs, per year and most of this retention was in the winter.
It should be noted that summed seasonal mass balances were not close to
annual mass balances for TOC or TSS and these values should be viewed
with caution.
Effects of variation in stream discharge
The concentrations and fluxes of materials in the stream draining the beaver
pond were often modified from what would have been expected based on the
behavior of this watershed prior to the beaver pond and the behavior of the
230
Figure 3. Mean annual mass balances of inputs and outputs from a beaver pond on the Rhode
River watershed in the Maryland Coastal Plain for the six years subsequent to the pond
formation. Wet deposition fluxes of nitrogen and all discharge fluxes were measured directly.
Watershed input fluxes were estimated from control watershed outputs and linear regressions
of beaver pond watershed outputs versus control watershed outputs during the six years prior
to the beaver pond. All regressions were significant at the P < 0.00001 level.
231
Table 5. Seasonal comparisons of estimated six year mean inputs to a Beaver
pond and directly measured outputs as stream discharge. Inputs are the sum
of measured atmospheric wet depositon (N only) and mean stream inputs
estimated from the control watersheds and linear regressions of fluxes from
the beaver pond watershed versus the control during the six years prior to the
construction of the Beaver pond.
Parameter Inputs Outputs
A. Winter
Nitrate (kg N/winter) 72 51
Ammonium (kg N/winter) 19 17
TON (kg N/winter) 133 82
Total-N (kg/winter) 224 150
Total Pi (kg P/winter) 28 17
TOP (kg P/winter) 18 9.1
Total-P (kg/winter) 46 26
Dis. Silicic Acid (t Si/winter) 2.2 1.8
TOC (t C/winter) 2.9 1.1
TSS (t/winter) 50 9.1
B. Spring
Nitrate (kg N/spring) 79 58
Ammonium (kg N/spring) 22 28
TON (kg P/spring) 173 170
Total-N (kg/spring) 270 259
Total Pi (kg P/spring) 42 43
TOP (kg P/spring) 29 22
Total-P (kg/spring) 71 66
Dis. Silicic Acid (t Si/spring) 2.8 2.2
TOC (t C/spring) 2.0 2.1
TSS (t /spring) 22 14
232
Table 5. Continued.
Parameter Inputs Outputs
C. Summer
Nitrate (kg N/summer) 11 4.1
Ammonium (kg N/summer) 23 14
TON (kg N/summer) 71 40
Total-N (kg/summer) 101 59
Total Pi (kg P/summer) 21 14
TOP (kg P/summer) 17 6.2
Total-P (kg/summer) 38 21
Dis. Silicic Acid (t Si/summer) 0.49 0.34
TOC (t C/summer) 0.99 0.54
TSS (T/summer) 13 3.4
D. Fall
Nitrate (kg N/fall) 13 4.9
Ammonium (kg N/fall) 6.4 11
TON (kg N/fall) 25 28
Total-N (kg/fall) 42 44
Total pi (kg P/fall) 7.3 11
TOP (kg P/fall) 1.0 4.7
Total-P (kg/fall) 8.3 16
Silicic Acid (t Si/fall) 0.82 0.49
TOC (t C/fall) 0.41 0.38
TSS (t/fall) 1.2 2.8
control watershed for the entire time period. These modifications were also
different seasonally. Within a season how did variations in stream discharge
affect material retention or export by the pond? In many cases the creation
of the beaver pond seemed to uncouple the stream chemistry from short-
term variations in discharge. For example, the concentration of TON in the
233
Figure 4. Seasonal mean fluxes of nitrate and TON into and out of the beaver pond. Fluxes
into the pond are the sum of direct atmospheric wet deposition and watershed discharges.
winter had a highly significant relationship to stream discharge prior to the
pond, but little or no relationship to discharge after the pond (Table 6). This
change in winter relationship was similar for concentrations of TOC, TPi,
TOP, and TSS (Table 6). A similar change was noted for TOP and TSS in the
summer and for TPi in the fall (Table 6). These uncouplings were probably
due to the high rates of deposition of particulates along with the nutrients
234
Figure 5. Seasonal mean fluxes of TPi and TOP into and out of the beaver pond.
235
they contained on the bottom of the pond. The concentrations of suspended
particulates increased rapidly as stream discharge increased, but the beaver
pond trapped much of the increased load of suspended particulates.
For another group of materials the beaver pond either had no effect
or increased the relationship between concentrations and stream discharge.
For example, nitrate concentrations had a highly significant correlation with
discharge in the spring prior to the pond, but this correlation increased
(explained more of the variation in nitrate concentration) subsequent to the
formation of the pond (Table 6). Similarly, ammonium concentrations in
the summer had a significant negative correlation with discharge before the
pond, but a higher correlation with discharge after the pond and this rela-
tionship was extended into the fall (Table 6). The higher nitrate and lower
ammonium concentrations during periods of high flow may have reflected
internal nitrogen processing in the beaver pond. During periods of high
flow the pond was more likely to be aerobic and nitrification may have
been favored, while during periods of low flow the pond may have been
more hypoxic and produced more ammonium. Some of the retained TON
(Figure 7) may have been mineralized during the warmer time periods.
Implications and research needs
Landscape managers should pay attention to the impacts of the resurgence
of beaver populations both in the Maryland Coastal Plain and World-wide.
Beaver have been reintroduced to many habitats in North America and Europe
and their populations are increasing rapidly. Their natural predators have been
extirpated from much of their natural range, leaving humans as the primary
control on their ultimate population. As their populations increase they
have both short-term localized effects on stream ecosystems and long-term
landscape-level effects on the biogeochemistry of whole regions.
At the short-term, local level beaver remove riparian forest and change
the character of streams. In many developed landscapes, such as the Mary-
land Coastal Plain, most of the remaining forest is riparian and the overall
wisdom of allowing beaver to remove much of this forest at a given site is
controversial. The ponding and exposure to the sun result in warmer water
temperatures and many changes in water quality. Still at the short-term, local
level, when the pond is abandoned the resulting beaver meadow undergoes
plant succession and the sediments and nutrients that had been trapped in the
beaver pond begin to change. The soils of the meadow may become more
aerobic and undergo mineralization of organic matter, releasing some of their
mineral nutrients.
At the large-scale, long-term level, the landscape becomes a mosaic of
current and former beaver sites along stream networks. As a result stream
236
Table 6. Seasonal regressions of weekly mean material concentrations versus beaver
pond watershed water discharge (Q). Only regressions with a P < 0.10 are given.
Separate regressions are given for the period of six years before the beaver pond was
formed and the period of six years after the beaver pond.
Before Beavers
Winter Nitrate (ug N/L) = 16.0Q + 135; R2 = 0.28, P < 0.00001
Spring Nitrate (ug N/L) = 10.3Q + 90.6; R2 = 0.21, P = 0.00002
Spring Ammonium (ug N/L) = 188Q?0:519; R2 = 0.29, P = 0.08
Summer Ammonium (ug N/L) = 289Q ?0:198; R2 = 0.25, P = 0.02
Winter TON (ug N/L) = 36.4Q + 104; R2 = 0.12, P = 0.002
Summer TON (ug N/L) = 6.65Q + 780; R2 = 0.05, P = 0.09
Winter TOC (mg C/L) = 8.64Q ? 14.5; R2 = 0.30, P < 0.00001
Winter TPi (ug P/L) = 9.72Q ? 5.62; R2 = 0.27, P < 0.00001
Summer TPi (ug P/L0 = 7.14Q + 229; R2 = 0.04, P = 0.08
Fall TPi (ug P/L) = 17.9Q + 151; R2 = 0.08, P = 0.02
Winter TOP (ug P/L) = 10.8Q ? 29.2; R2 = 0.33, P < 0.00001
Spring TOP (ug P/L) = 5.81Q + 40.2; R2 = 0.16, P = 0.0003
Summer TOP (ug P/L) = 13.6Q + 147; R2 = 0.13, P = 0.006
Fall TOP (ug P/L) = 10.6Q + 70.8; R2 = 0.11, P = 0.02
Winter TSS (mg/L) = 7.88Q ? 32.8; R2 = 0.32, P < 0.00001
Spring TSS (mg/L) = 4.25Q + 7.74; R2 = 0.21, P = 0.00003
Summer TSS (mg/L) = 8.90Q + 102; R2 = 0.07, P = 0.05
Fall TSS (mg/L) = 9.37Q + 51.1; R2 = 0.12, P = 0.02
After Beavers
Winter Nitrate (ug N/L) = 11.3Q + 96.5; R2 = 0.28, P < 0.00001
Spring Nitrate (ug N/L) = 7.78Q + 20.1; R2 = 0.39, P < 0.00001
Summer Nitrate (ug N/L) = 5.48Q + 31.9; R2 = 0.06, P = 0.03
Winter Ammonium (ug N/L) = 112Q?0:251; R2 = 0.07, P = 0.08
Spring Ammonium (ug N/L) = 238Q?0:516; R2 = 0.22, P = 0.05
Summer Ammonium (ug N/L) = 409Q?0:368; R2 = 0.63, P = 0.0005
Fall Ammonium (ug N/L) = 228Q?0:290; R2 = 0.48, P = 0.008
Summer TOC (mg C/L) = 104Q ?0:0819; R2 = 0.18, P = 0.04
Winter TOP (ug P/L) = 1.22Q + 33.0; R2 = 0.06, P = 0.03
Spring TOP (ug P/L) = ?1.53Q + 88.8; R2 = 0.05, P = 0.06
Fall TOP (ug P/L) = 9.35Q + 56.9; R2 = 0.08, P = 0.02
Spring TSS (mg/L) = ?1.35Q + 63.3; R2 = 0.04, P = 0.09
Fall TSS (mg/L) = 5.63Q + 37.8; R2 = 0.07, P = 0.02
Spring Dis. Silicate (mg Si/L) = ?0.0350Q + 7.34; R2 = 0.04, P = 0.09
Summer Dis Silicate (mg Si/L) = 0.141Q + 5.55; R2 = 0.05, P = 0.04
237
corridors are changed from rather narrow, well defined zones in the landscape
to much wider, physically more complex, and biologically much more diverse
and productive landscape regions. This effect should be most pronounced in
very flat terrain, such as many parts of the Coastal Plain. Over time a given
region, such as the Maryland Coastal Plain will develop populations of beaver
meadows of various ages and sizes. Each of these beaver meadows will be
undergoing both plant and soil succession. It will probably take on the order
of a century for the stream networks of this landscape with their populations
of beaver meadows to approach a new biogeochemical quasi-equilibrium.
Assuming that humans allow beaver populations to remain at moderate
to high levels for a long enough time, what will be the implications for
the management and health of aquatic ecosystems from headwaters stream
corridors to coastal waters? Although the implications will be to some extent
regionally specific, some general effects of a resurgent beaver population
are clear. The hydrology will be altered by increased evapotranspiration and
reduced storm surges. In most settings stream temperatures will be increased
during the growing season. A large fraction of the suspended sediments and
their nutrient contents will be converted to beaver meadows and much of these
sediments and nutrients will remain trapped for the long-term. Riparian veget-
ation will be extended spatially and greatly increased in diversity. Mineral
nutrient fluxes to down-stream systems such as large lakes, estuaries, and
coastal waters will be decreased. These implications have thus far been
largely overlooked by land managers even though these changes have been
under way for decades. There are now thousands of beaver ponds on the
Maryland Coastal Plain where there were none a few decades ago. Similar
changes are occurring in many parts of North America and Europe.
We believe that our study points to the need for more research of two
types on the biogeochemical effects of beavers. More mass balance studies of
both active beaver ponds and beaver meadows of various ages are needed
to test the generality of our results and the results of others in both the
Coastal Plain and in other physiographic regions. More mechanistic studies
are also needed to better understand nutrient dynamics within these inter-
esting habitats. Whether the results from a few dozen studies of beaver
ponds in more northern latitudes of North America can be transferred to the
Maryland Coastal Plain or many other regions of the World is still an open
question.
238
Acknowledgments
This research was supported by a series of grants from the Smithsonian
Environmental Science Program, the National Science Foundation, and the
National Oceanic and Atmospheric Agency?s Coastal Oceans Program.
References
APHA (American Public Health Association) (1989) Standard Methods for the Examination
of Water and Wastewater (17th Edn). APHA, Washington DC
Burns DA & McDonnell JJ (1998) Effects of a beaver pond on runoff processes: comparison
of two headwaters catchments. J. Hydrol. 205: 248?264
Chirlin GR & Schaffner RW (1977) Observations on the water balance for seven sub-basins
of Rhode River, Maryland. In: Correll DL (Ed) Watershed Research in Eastern North
America (pp 277?306). Smithsonian Press, Washington DC
Cirmo CP & Driscoll CT (1993) Beaver pond biogeochemistry: acid neutralizing capacity
generation in a headwater wetland. Wetlands 13: 277?292
Cirmo CP & Driscoll CT (1996) The impacts of a watershed CaCO3 treatment on stream and
wetland biogeochemistry in the Adirondack Mountains. Biogeochem. 32: 265?297
Correll DL & Dixon D (1980) Relationship of nitrogen discharge to land use on Rhode River
watersheds. Agro-Ecosyst. 6: 147?159
Correll DL (1981) Nutrient mass balances for the watershed, headwaters intertidal zone, and
basin of the Rhode River estuary. Limnol. Oceanogr. 26: 1142?1149
Correll DL, Jordan TE & Weller DE (1994) Long-term nitrogen deposition on the Rhode River
watershed. In: Hill P & Nelson S (Eds) Toward a Sustainable Watershed: The Chesapeake
Experiment (pp 508?518). Chesapeake Res. Consort. Publ. 149, Edgewater, MD
Correll DL, Jordan TE & Weller DE (1999a) Effects of precipitation and air temperature on
phosphorus fluxes from Rhode River watersheds. J. Environ. Qual. 28: 144?154
Correll DL, Jordan TE & Weller DE (1999b) Effects of interannual variation of precipitation
on stream discharge from Rhode River subwatersheds. J. Am. Water Resour. Assoc. 35:
1?10
Correll DL, Jordan TE & Weller DE (1999c) Effects of precipitation and air temperature on
nitrogen discharges from Rhode River watersheds. Water Air Soil Pollut. 115: 547?575
Correll DL, Jordan TE & Weller DE (1999d) Transport of nitrogen and phosphorus from
Rhode River watersheds during storm events. Water Resour. Res. 35: 2513?2521
Devito KJ, Dillon PJ & Lazerte BD (1989) Phosphorus and nitrogen retention by five
precambrian shield wetlands. Biogeochemistry 8: 185?204
Gaudy AF & Ramanathan M (1964) A colorimetric method for determining chemical oxygen
demand. J. Water Pollut. Control Fed. 36: 1479?1487
Higman D & Correll DL (1982) Seasonal and yearly variation in meteorological parameters at
the Chesapeake Bay Center for Environmental Studies. In: Correll DL (Ed) Environmental
Data Summary for the Rhode River Ecosystem (pp 1?159). Smithsonian Environmental
Research Center, Edgewater, MD
Hodkinson JD (1975) Energy flow and organic matter decomposition in an abandoned beaver
pond ecosystem. Oecologia 21: 131?139
Johnston CA & Naiman RJ (1987) Boundary dynamics at the aquatic-terrestrial interface: The
influence of beaver and geomorphology. Landscape Ecol. 1: 47?57
239
Johnston CA, Pinay G, Arens, C & Naiman RJ (1995) Influence of soil properties on the
biogeochemistry of a beaver meadow hydrosequence. Soil Sci. Soc. Am. J. 59: 1789?1799
Jordan TE, Correll DL, Weller DE & Goff NM (1995) Temporal variation in precipitation
chemistry on the shore of Chesapeake Bay. Water Air Soil Pollut. 83: 263?284
King EJ (1932) The colorimetric determination of phosphorus. Biochem. J. 26: 292?297
Maciolek JA (1962) Limnological Organic Analyses by Quantitative Dichromate Oxidation.
US Fish Wildlife Serv. Publ.
Maret TJ, Parker, M & Fannin TE (1987) The effect of beaver ponds on the nonpoint source
water quality of a stream in southwestern Wyoming. Wat. Res. 21: 263?268
Martin DF (1972) Marine Chemistry, Vol. 1. Dekker, New York
Naiman RJ, Melillo JM & Hobbie JE (1986) Ecosystem alteration of boreal forest streams by
beaver (Castor Canadensis). Ecology 67: 1254?1269
Naiman RJ, Johnston CA & Kelley JC (1988) Alteration of North American streams by beaver.
Bioscience 38: 753?762
Naiman RJ, Pinay G, Johnston CA & Pastor J (1994) Beaver influences on the long-term
biogeochemical characteristics of boreal forest drainage networks. Ecology 75: 905?921
Peterjohn WT & Correll DL (1986) The effect of riparian forest on the volume and chem-
ical composition of baseflow in an agricultural watershed. In: Correll DL (Ed) Watershed
Research Perspectives (pp 244?262). Smithsonian Press, Washington, DC
Smith ME, Driscoll CT, Wyskowski BJ, Brooks CM & Cosentini CC (1991) Modification of
stream ecosystem structure and function by beaver (Castor canadensis) in the Adirondack
Mountains, New York. Can. J. Zool. 69: 55?61
Strickland JDH & Parsons TR (1972) A Practical Handbook of Seawater Analysis (2nd Edn).
Bull. Fish. Res. Bd. Can. 165