Beaver pond biogeochemistry: Acid neutralizing capacity generation in a headwater wetland

WETLANDS, Vol. 13, No. 4. December 1993, pp. 277-292 ~) 1993, The Sociely of Wetland Scientists

BEAVER POND BIOGEOCHEMISTRY: ACID NEUTRALIZING CAPACITY GENERATION IN A HEADWATER WETLAND

Christopher P. Cirmo and Charles T. Driscoll Department of Civil and Environmental Engineering

Syracuse University Syracuse, New York 13244

Abstract: A beaver pond and its associated inlet and outlet waters in the Adirondack Mountains of New York were monitored for major chemical solutes for 26 months in an effort to quantify underlying chemical controls on the production and consumption of acid neutralizing capacity (ANC). The pond was a net annual sink for inlet A1, SO4 ~- , NO~-, and H4SiO4. The pond was a net annual source of dissolved organic carbon (DOC), NH4 +, and Fe z-. Losses of ANC resulting from AI and basic cation retention, as well as organic anion release (RCOO) associated with DOC, were more than offset by SO,-'- and NO3 retention and Fe -'+ and NH4 + release, resulting in a net production of ANC. Rates of ANC generation were 120 meq m--" yr and 310 meq m -2 yr -~ , respectively (based on pond surface area), for the non-summer (October-June) and summer (July-September) periods. Seasonal variations in ANC in the outlet stream were largely associated with Fe 2+ and DOC release, while ANC in the upland inlet stream was associated with AI, NO~ , and basic cations, with much less seasonal variation. Controls on stream chemistry were temporally and longitudinally different for the inlet and outlet streams. The shift to seasonal control of outlet stream ANC by processes associated with organic matter decomposition reactions and anaerobic zone nutrient transformations may be characteristic of headwater wetlands in temperate zones with seasonal temperature extremes. Beaver impoundments and wetlands may also be important in the upstream mobilization or retention of geologically bound solutes like A1, Fe, and HaSiO,. Headwater wetlands, as sinks for solutes associated with acidic deposition and watershed acidification (i.e., SO4 ~--, NO~-, and A1), may play a role in the amelioration of the effects of these solutes on downstream receiving waters and associated biota. Depending on their location in relation to drainage patterns, these ponded systems may influence the nutrient dynamics of receiving waters through nitrogen transformations and organic carbon cycling.

Ke.v Words: Acid deposition, acid neutralizing capacity, Adirondacks, beaver impoundment, biogeochemistry, Castor canadensis, mass balance, pond, watershed, wetland.

I N T R O D U C T I O N

Interest in the ecological importance o f waters impounded by beaver (Castor canadensis Kuhl) in the United States and Canada has a long history. Studies have centered on the role o f beaver impoundments in the alteration of hydrology and as geomorphological agents in landscape ecology (Johnston and Naiman 1987, 1990, Remillard et al. 1987, Naiman et al. 1988). Impacts on hydrology include the creation and main- tenance of riparian wetlands, decreases in stream ve- locity and erosion potential, increases in local water tables, and increased hydraulic residence t ime (Apple 1985, Parker 1986, Woo and Waddington 1990). Bio- geochemical studies have concentrated on the impacts o f these systems on organic carbon dynamics and nutrient cycling (Hodkinson 1975, Naiman et al. 1986). Beaver activity can enhance areal fluxes of methane due to microhabitat changes favoring methanogenic bacteria (Ford and Naiman 1988, Nisbet 1989, Yavit t

277

et al. 1990, 1992, Naiman et al. 1991, Roulet and Ash 1992). Beaver ponds also alter N and P cycling resulting in distinct seasonal trends in N O c , NH4 +, and total-P concentrations (Naiman and Melillo 1984, Francis et al. 1985, Maret et al. 1987, DeVito et al. 1989).

The slowing and settling of particulate organic material results in "highly accretive heterotrophic systems" (Hodkinson 1975) with increased sediment/water con tac t t im e (hydrau l ic res idence t ime) and decreases in the ratio of product ion to respiration (P/ R), resulting in elevated O2 demand. Many shallow water and riparian systems have distinct seasonal anoxic zones, and the rote of anoxic zones in streams, impoundments , and wetlands is impor tant to nutrient cycling and transport (Dahm et al, 1987). With 02 depletion in saturated soils and aquatic sediments, alternate electron acceptors are utilized in the mineral- ization of organic matter. Biotically mediated NO3-,

278 WETLANDS, Volume 13, No. 4, 1993

Fe 3+, and SO, 2- reduction occur, as well as fermen- tation and methanogenesis at highly negative redox potentials. These processes consume H ' and generate acid neutralizing capacity (ANC).

Acid neutralizing capacity is a key parameter in as- sessing the sensitivity of surface waters to inputs of strong acids. Acid neutralizing capacity is defined in a variety of ways depending on the ionic composition of the waters in question. Several authors have noted a discrepancy in measurements of ANC and values calculated by ionic charge balances for softwater systems with high dissolved organic carbon (DOC) concentrations (Perdue 1985, Hemond 1990, Munson and Gher- ini 1993). This discrepancy has been attributed to un- measured organic anions (acids). Recently, Driscoll et al. (1993) calibrated values of solution charge deficit (i.e. organic anions) to a triprotic organic analog model for Adirondack lakes. This approach successfully re- solved the discrepancy between measured and calculated values of ANC. In this study, we represent ANC as

ANC = SBC + [NH4+I + n[Al"+l + 2[Fe 2÷]

- 2[SO42-1 - [NO3-] - [Cl-I - IF-]

- [ R C O 0 ~ - I ( 1 )

where SBC is the sum of basic cation equivalence (2[Ca2*]+2[Mg2-]+[Na+]+[K+]) and [A1 "+] is the equivalence of inorganic monomeric AI (=3 [AP +] + 3 [AI - F] + 3 [AI- SO,] + 3 [AI - Si] + 2 [A1 (OH) 2 + ] + [AI(OH)2+]; A1-F, A1-SO4, and AI-Si are F, SO42-, and Si complexes of A1). The equivalence of strong acid anions is represented as [RCOO, ]. The speciation and equivalence of A1 and organic anions can be estimated from measurements of aqueous A1 fractions, DOC, F , SO42-, dissolved Si (H4SiO4) and pH using a chemical equilibrium model (Schecher and Driscoll 1993). The above ANC expression allows the direct calculation of an ANC budget from analytical measurements and speciation calculations.

as well as in freshwater wetlands (Bayley et al. 1986, Urban et al. 1989), and S transformations have been demonstrated in peats and wetland systems (Wieder et al. 1987). Iron reduction may be important in organic matter decomposition in many surface waters and wetlands (Lovley and Phillips 1986a, Driscoll et al. 1987), and seasonal Fe 3+ and SO42 reduction has been demonstrated in a beaver pond in the Adiron- dacks (Driscoll el al. 1987, Smith et al. 1991).

Sources of acidity in wetlands have been discussed (Gorham et al. 1984,1985, Kerekes et al. 1986, Urban and Baytey 1986, Bayley et al. 1987, Wood 1989) and include organic acid production (RCOOH), the oxidation of organic or inorganic forms of reduced sulfur and nitrogen, acidic atmospheric inputs (e.g., H2SO4, HNO0, CO2 production from decomposition, and cation exchange processes. The production of large amounts of DOC in wetlands increases the acidity of drainage waters through the dissociation of both strong (RCOO,-) and weak ( R C O Q - ) organic acid functional groups (Driscoll et al. 1989,1993, Hemond 1990). Wetlands and beaver ponds may be a source of biologically available Fe and DOC (Lovley and Phillips 1986b), and the role of DOC in A1 complexation and mobilization, and as carbon input to downstream lakes and streams, is the subject of on-going studies (Tipping et al. 1988, Driscoll et al. 1993).

We studied a stream in a headwater catchment in the Adirondack Mountains of New York to identify and quantify the processes that control the generation and consumption of ANC within a large beaver pond/ wetland complex. We were particularly interested in the longitudinal and seasonal controls on in situ acid neutralizing processes within ponded waters and wetlands. We attempted to quantify the retention/release of chemical species contributing to the overall ANC budget of a headwater stream influenced by a beaver pond/wetland complex within an acid-sensitive catchment.

Acid Neutralizing Capacity and Acidity in Wetlands

The role of headwater wetlands and ponds in neutralizing acidic headwater drainage or exporting acidity to downstream reaches has not been fully explored. Lake sediments and anoxic zones are important sources of ANC that neutralize acidic deposition and acidic upland drainage (Schindler et al. 1980, Kelly et al. 1982, Cook et al. 1986, Schiff and Anderson 1986, Schind le r 1986). Sulfate r educ t ion processes are i m - po r t an t to aquatic ANC generation in hypolimnetic, epilimnetic, and littoral zones in freshwater lakes (Stuiver 1967, Kelly and Rudd 1984, Cook et al. 1986, Schiffand Anderson 1986, Schafran and Driscoll 1987)

METHODS AND MATERIALS

Study Site and Background Information

The study site is Pancake-Hall Creek (43°50 ' N 74°52 ' W) in the Big Moose Lake drainage basin of the south- central Adirondacks (Figure 1). A record of stream chemistry and other watershed characteristics has been established at the site (Goldstein et al. 1987, Driscoll et al. 1987). The morphometry and basin characteristics at P a n c a k e - H a l l Creek are s imi l a r to those o f many beaver-impounded headwater streams in the Adirondacks. The beaver pond associated with this stream has a perennial first and second order inlet, as well as several small, ephemeral inlets along its western

Cirmo & Driscoll, BEAVER POND BIOGEOCHEMISTRY 279

A d i r o n d a c k

be.vo..oo. P 4 / , . z j ~ / ~ ~ tig Moose

Lake I _ . I

1 k m Figure 1. Location map of Pancake-Hall Creek and beaver pond, including sampling sites P4 (inlet), P10 (in-pond), P3 (outlet), and P2 (downstream).

and northern edges. The pond has a volume of 12,500 m ~, an area of 3.0 ha, and a mean depth of 0.4 m (Figure 2). Hydrologic discharge is strongly seasonal, with high-flow periods in early winter (November-Jan- uary) and at spring runoff (March-May). Low flows occur in mid-winter (January-March) and in late summer and early Fall (Figure 2). The pond outlet stream originates at a two-meter high beaver dam and passes a series of smaller secondary dams downstream. Sam- piing locations (Figure 1) include one inlet site (P4), a pelagic profile site (P10)near the deepest point, the outlet immediately below the dam (P3), and a site 150 m downstream (P2). An area of deep till (>3m) is located contiguous and southwest of the ponded area, while the balance of the catchment is underlain by thin till (< 3m) and bedrock.

The catchment is primarily second-growth decidu- ous forest, while floating logs and standing snags dom- inate the pond and wetland. Major tree species include Fagus grandifolia Ehrh., Betula alleghaniensis Britton, Acer saccharum Marsh., Acer rubrum L., and Picea rubens Sarg., with Abies balsamea (L.) Mill. and Tsuga canadensis (L.) Cart. along the shorelines and in the saturated zones around the pond. The shallow area near the entrance of the inlet to the pond is dominated by sedges (Carex spp.) and Sphagnum. Sphagnum species include S. papillosum Lindb. and S. magetlanicum Brid. The southeast shoreline slopes steeply with much less wetland vegetation. Utricutaria species are evident in the pond during the growing season. The sediments are organic and peaty and Sphagnum covered in the shallow zones; some Polytrichum moss species are also present. Sediments with high organic content (of dy or gyttja consistency) are located in the deeper zones near the dam.

/

I p o n d a r e a = 3.0 ha /" /

main OUILR7 DISCHARGE i

DAM Oc~ Jan Apt Jul Oct / - - _ OU'rL~JT p~ ! 9 9 0 1 9 9 1

Figure 2. Bathymetnc map and discharge record of Pan- cake-Hall Creek beaver pond. Contour interval in meters, discharge in millions of liters/day (× 10 6 L/day).

Chemistry and Data Analysis

Water from the four sampling sites was collected at 3-week intervals for 26 months. During this study there were marked variations in water chemistry due to seasonal changes in hydrologic and biological processes. As a result, analysis of water chemistry patterns and mass balances were conducted for the summer period (July through September) and the non-summer period (October through June). Standard chemical methods were used for all analyses (Table 1 ) and suggested hold- ing times were adhered to whenever possible. Quality assurance/quality control (QA/AC) procedures were followed in all analyses (Fordham and Driscoll 1989), including sample and analytical replicates, blanks, and blind audit samples. Fixation of Fe ~ + using the o-phenanthroline method was modified for use in the field, with spectrophotometric readings done within 24 hours of sampling (usually within 3 hours). The bottles were wrapped in aluminum foil and chilled on transport from field to the laboratory. Several timed experiments demonstrated there was no significant change in Fe z+ concentrations in the sample bottles during storage, due to DOC and o-phenanthroline interactions with any available Fe 3+ . Dissolved oxygen (DO) was fixed in the field and titrated within 12 hours using a modified Winkler titration. Temperature was measured in situ, and pH was measured on site or at the laboratory within 12 hours.

The chemical equil ibrium model ALCHEMI (Schecher and Driscoll 1993) was used to calculate the total equivalence ofAl (Al "+ ) based on analytical measurements of total and organic monomeric A1 (A1, and Alo, respectively). Inorganic monomeric A1 (Al3 was calculated as the difference between these two measurements. The ALCHEMI model allows the ther- modynamic calculation of Al complexation with inorganic and organic ligands, as well as the partial pressure of CO2 (Pco2), speciation of inorganic carbon,


Table 1. Chemical analytical methods used in beaver pond study.

Analyte Method Reference

pH Potentiometric, glass combination APHA 1985

Cw '+, Mg 2., K +, Na + SO~-' , NO~ , CI F NH44 Acid Neutralizing

Capacity (ANC) Dissolved Inorganic

Carbon (DIC) Dissolved Organic

Carbon (DOC) Dissolved H,SiO~ Total Monomeric

Aluminum (AI,) Organic Monomeric

Aluminum (AIo) Fe 24

Dissolved Oxygen (DO)

electrode, field measurement Atomic absorption (AAS), flame Slavin 1968 Ion chromatography (IC) Small et al. 1975 Ion selective electrode Orion 1976 Phenate colorimet~ Cappo et al. 1987 Strong acid titration, Gran 1952

Gran Plot analysis Acid Purge, infrared (IR) Dohrman 1984

CO2 detection UV enhanced oersulfate Dohrman 1984

oxidation, IR CO, Detection Heteropoly blue colorimetry Cappo et al. 1987 Pyrocatechol violet, Drisco]l 1984

automated analysis Ion exchange, pyrocatechol Driscoll 1984

violet, automated analysis O-phenanthroline, field fixation APHA 1985 Modified Winkler titration, APHA 1985

field fixation

and solid phase saturation indices. The equivalence of organic anions (RCOO-) was estimated as anionic deficit in charge balance diagrams and was calculated with a triprotic organic acid model for use in the ANC budget calculations (Driscoll et al. 1993). Observation of the data from Pancake Hall Creek revealed an increase in the concentration of organic acids (estimated as charge balance discrepancy) with increasing measured DOC (r 2 = 0.26). Organic anions protonate over the pH range of these values, and this process undoubtedly obscured this relationship.

Statistical analyses were done using PC-SAS version 6.03 (SAS Institute 1985). An analysis of variance procedure for unbalanced data (General Linear Model) was used to determine significant differences between sites in annual and seasonal mean solute concentrations. A Duncan's Multiple Range test was used to determine statistically significant differences between individual sites when the analysis of variance indicated a significant difference in solute concentration. Linear regressions were calculated between various chemical parameters and were evaluated using F statistics. Mul- tiple regression of solutes on ANC was also performed (RSQUARE and STEPWISE procedures). A clustering analysis was conducted (CLUSTER procedure) on a correlation matrix of standardized scores of relevant

chemical parameters to evaluate similarity in site chemistry for the inlet, outlet, and in-pond profile sites.

A stream gauging station was activated at a downstream site (Figure 1), and discharge was recorded at fifteen minute intervals from September of 1990 to September of 1991. A staff gauge was installed at the outlet from the pond (P3), and instantaneous discharge measurements were taken on sampling dates to develop a stage-discharge relationship to correlate with the downstream continuous monitoring. This relationship was used to predict mean daily discharge at the pond outlet (P3). Mean daily discharge at the inlet (P4) was estimated by prorating the areal discharge of a non-beaver impounded stream in the nearby (10 km) Woods Lake watershed. Chemical wet deposition flux estimates were obtained from a nearby monitoring site (Nicks Lake) operated by the New York State De- partment of Environmental Conservation (New York State Atmospheric Deposition Monitoring Network 1991). Fluxes of individual solutes for mass balance calculations were computed from the hydrologic budget and the observed solute concentrations from the inlet (P4) and outlet (P3) streams. The solute concentrations of each three-week sampling period were as- sumed to be representative of the concentrations existing for half the period between previous and


20 TEMP ('C) 10

0

DO so SAT (~) 60

6

• INLET (P4) O OUTLET (Ps) n DOWNSTREAM (P2)

1 9 9 0 1 9 9 1

JAN JAN

p H 5

4 3 0 0 DIC (#u c) 200 I 0 0

0 80G

DOC 60D (UM e) 4 0 0

200 0

JAN JAN

1 9 9 0 1 9 9 1

Figure 3. Temporal/longitudinal patterns of temperature (°C), dissolved oxygen saturation (DO~,, %), pH, dissolved inorganic carbon (DIC, #tool C/L) and dissolved organic carbon (DOC, #tool C/L) at pond inlet (P4), outlet (P3), and downstream (P4) sites.

• INLET (P4) O OUTLET (PS) [] DOWNSTREAM (P2) 1990 1991

JAN JAN 3 0 ' " ' . . . . . l . . . . . . . . . . . l . . . . . . . . .

AIi 20 (/,~M) 1 0

A1 ° 6 (~U) 4 ,O

2 0

Cam+ i006080

4 0 - . . . . . . , . . . . . . . . . . . , . . . , . . . .

K ÷ 3 0

• o- .o. .Fbo.o

0 v . . - u " = . , , . . . . . " ~ . ~ M F - . . . , . . . , ' 7 ,'V, , ,

[~M) , oo 50 0

JAN JAN

1990 1 9 9 !

Figure 5. Temporal/longitudinal patterns of inorganic mo- homeric aluminum (AI. umol/L), organic monomeric aluminum (Alo, ~mol/L), Ca ~÷ (~equiv/L), K + (~equiv/L), and HoSiO4 (~mol/L) at pond inlet (P4), outlet (P3), and downstream (P2) sites.

( ~ e q / L )

( / a . eq /L)

H¢SiO ' (

1 0 0

ANC 5 0 ( / ,~eq/L) O

- 5 0

S04z - 150 ( # e q / L ) 1 DO

5 0

N0a- 80 (p . eqTL) 4 0

0 2 0

+

N.H4 15 (#eqTL) I 0

5 50 40

Fe ~+ 50 (b~eq /L) 20

10

• INLET (P4) O OUTLET (Pa) [] DOWNSTREAM (P2) 1990 1991 JAN JAN

1 9 9 0

P

1991

Figure 4. Temporal/longitudinal patterns of acid neutralizing capacity (ANC), SO2-, NOs-, NH4 ÷ and Fe 2+ (all in #equiv/L) at pond inlet (P4), outlet (P3), and downstream (P2) sites.

subsequent sampling dates. The ANC budgets were calculated based on these fluxes and the hydrologic information using equation (1). Huxes were expressed on a pond surface area basis.

RESULTS AND DISCUSSION

Redox Related Trends in Inlet and Outlet Water Chemistry

Time series plots of chemical parameters for the sites are shown in Figures 3-5, and annual and seasonal mean solute concentrations are shown in Table 3. A statistical comparison of the inlet, outlet, and downstream sites is presented in Table 4. Results of mass balance calculations are shown in Table 2, with inputs, outputs, and ne t retention/release of solutes shown. Acid neutralizing capacity (Figure 4) and pH (Figure 3) were significantly higher in outlet waters, particularly in the biologically active, low-flow conditions from July through September (Tables 3 and 4). Increased hydraulic residence time, along with increased rates of O2 consumption and decreased 02 solubility (associated with higher temperature) facilitated the use of alternate terminal electron acceptors by microbes in the decomposition of organic matter. The magnitude of the difference in pH and ANC was lower in the non-


Table 2. Inputs, outputs and retention/release of major solutes from Pancake Hall Creek beaver pond. DOC, DIC, AI. and H~SiQ in moles ha- ' yr- ' . All others in equiv ha ' yr -j. " - " retention, " + " = release.

Non-Summer (t0/1/90 to 6/30/91) Summer (7/1/91 to 9/30/91

Constituent in Out Net In Out Net

DO(? 38000 44000 +6000 23000 26000 +3000 RCOO 3200 3900 +700 2000 2700 +700 C i 940 900 - 4 0 330 330 0 NO3- 8500 6200 -2300 1300 0 -1300 F 590 520 - 7 0 170 130 - 4 0 SO2 15000 13000 -2000 7200 1600 -5600 NH~ + 330 880 +550 40 370 +330 Na ÷ 4000 3400 -600 2300 1000 -1300 K ~ 1600 1700 +100 420 460 +40 Ca 2+ 10800 10100 -700 3700 2100 -1600 Mg "~ 2700 2800 ~ 100 1200 620 -580 A1, 3100 2400 -700 290 130 -160 AI ~* 8400 6300 -2100 700 42 -660 Fe 2* 0 670 +670 170 1500 +1300 H.SiO~ 13000 10500 -2500 7800 460 -7300

s u m m e r period. Seasonal patterns of tempera ture and DO reflect the increased hydraulic residence t ime cre- ated by the pond and wetland during low flow (Figure 3). Outlet s t ream tempera ture was 4 to 5 °C higher than

Table 3. Mean solute concentrations at the beaver pond inlet (P4) and outlet (P3) streams, from June 1989 to Sep- tember 1991. All values in ueq/L cxcept DOC, DIC, AI~, AIo, and H,SiO, in (umol/L), Tcmperature in °C, and DO in % saturation, ann = annual, sum = summer, n o n s u m - nonsummer.

Outlet (P3) Para- Inlet (P4) Non- meter Ann Sum Nonsum Ann Sum sum

Temp 7.5 13 4.3 9.8 18 5.2 DO~, 86 85 86 70 63 74 pH 4.8 4.9 4.8 5.2 5.6 5.0 ANC -11 -4 .9 14 14 37 0.2 DOC 300 340 270 440 640 330 DIC 52 64 47 150 170 130 SO42÷ 120 12D 130 100 72 120 NO~" 48 15 65 32 4.0 48 F 3.6 2.5 4.1 3.8 3.3 4.0 C1- 8.4 7.5 8.9 9.0 7.7 9.8 NH4 ~ 3.5 4.2 3.2 9.8 11.8 8.7 Na ~ 39 50 32 32 35 30 K ÷ 10 8.2 12 15 17 14 Ca ~÷ 77 72 80 75 65 81 Mg 2÷ 21 21 21 22 20 23 SBC 150 150 150 140 140 150 AI~ 13 4,4 18 7.8 1.0 12 AI,, 4.0 3.9 4.0 3.4 3.3 3.4 Fe ~+ 1.9 3.0 1.3 15 27 7.5 H4SiO, 120 150 100 65 34 84

in the inlet in mid - summer . Decreases in DO saturation (DOs~t) at the pond outlet (P3) were evident, although reaeration of the s t ream occurred before the downs t ream site (P2). Dissolved oxygen in outlet waters was depleted by microbial decomposi t ion o f organic material within the pond, as reflected by increases in outlet concentrat ions o f DIC and DOC. Mean hydraulic residence t ime in the low-flow s u m m e r period was about 50 days, decreasing to 35 days during high flows, and frequently to less than one day during large hydrologic events. Product ion o f D I C was evident under the ice (Dec--March) and in low-flow summer periods, coinciding with a bui ldup of decomposition related CO: (up to 170% of equil ibrium Pco2). Elevated DIC concentrations were not evident down- s t ream f rom the pond (site P2), mos t likely due to re- equilibration o f the flowing outlet water with a tmo- spheric CO2 (Pcoz = 10-3~ arm) and 02. The lowest DO~,t and highest D I C values occurred upon immediate outflow f rom the pond (P3) in all seasons. The pond was a source of D O C throughout the year, with approximate ly 15,800 moles o f C as D O C exported per year f rom the pond.

A loss of SO~ 2- was evident in the pond and coincided with DOC, DIC, and A N C release, implicat ing microbial reduction processes. Inlet SO, 2 concentrations showed little seasonal trend, al though a statistically insignificant decline appeared after June o f 1989, perhaps reflecting a decline in a lmospher ic SO, 2- deposition noted in the Adirondack region (Driscoll and van Dreason 1993). The pond was a net sink for SO42-, with m u c h of the retention (44%) occurring during the s u m m e r period (Table 2), The release of SO42 associated with drying and oxidation of exposed peat or

Cirmo & Driscoll, BEAVER P O N D B I O G E O C H E M I S T R Y 283

Table 4. Comparison of mcans of stream chemistry- for beaver pond inlet (P4), outlet (P3), and one downstream site (P2) using Duncan's Multiplc Range Test. Paramcter units and descriptions as defined in Table 3. (ns = not significant, * = p < 0.05, • * = p < t3.01).

Annual Summer Non -summer

Temp * P2 > P4 * P3 > P2, P4 ns DO~,~ ** P4 > P2 > P3 ** P4 > P2 > P3 ** P4 > P2, P3 pH **P2, P3 > P4 **P2, P3 > P4 * P2, P3 > P4 ANC **P2, P3 > P4 **P2, P 3 > P4 * P2, P3 > P4 DOC **P2, P3 > P4 **P2, P 3 > P4 * P 2 > P 4 DIC * P3 > P 2 > P4 **P3 > P 2 > P 4 * P3, P 2 > P4 SO]- ** P4 > P3, P2 ** P4 > P3, P2 ns NO~ * P4 > P3, P2 * P 4 > P3 ns F ns ns ns C1 ns ns ns NH~- **P3, P2 > P4 **P2, P 3 > P 4 * P 4 > P 2 Na t ns * P 4 > P 2 , P3 ns K + ** P3 > P4 ** P2, P3 > P4 ns Ca -~÷ ns ** P2 > P3 ns Mg 2 , ns ns ns SBC ns ** P2 > P3 ns AI, ** P4 > P2, P3 ** P4 > P3, P2 ** P4 > P3, P2 AI,, * P4 > P3, P2 ns ns Fe '-~ * P3, P 2 > P4 **P3 > P2 > P4 ns H~SiO, * * P 4 > P 2 , P3 * * P 4 > P2, P3 * P 4 > P 3 , P2

sediment was not observed in this study, but its potential release during drought years could have implications for periodic acidification o f downs t ream waters (Driscoll et al. 1987, LaZerte 1993).

The beaver pond was an impor tan t sink for inputs of NO3- (Table 2, Figure 4). The largest NO~- fluxes occurred in the n o n - s u m m e r period, and NO3- input exceeded output in all seasons (Table 2). Watershed retention o f NO3- during the active s u m m e r growing season was evident f rom inlet seasonal variations, but NO3- concentrat ions were always above detection limits in inlet waters. In the outlet s tream, NO3 concentrations were at or below detection l imits f rom July through September. Wetlands and headwater ponds could serve as net sinks for excess NO3 runoff f rom the catchment , especially during the growing season. More important ly , the potential increases in s t ream and lake loadings of NO3 due to hypothesized watershed nitrogen saturation (Aber et al, 1989, Stoddard and Murdoch 1991, Driscoll and Van Dreason 1993) may be mit igated by NO3 retention or r emova l in wetlands. Nitrogen would be expected to be retained in pond biomass , while denitrification and NO3 reduction mos t likely occur in sediments and microsi tes throughout the pond ('Nairnan and Melillo 1984, Fran- cis et al. 1985, DeVito et al. 1989).

Concentrat ions o f NH4 ~ in the outlet s t ream were consistently higher than the inlet (Figure 4, Table 3) and are likely due to organic-N mineral izat ion. Con-

centrat ions of NH4 + in the outlet s t ream were erratic, perhaps reflecting nitrification as water was reaerated or warmed during the summer . A m m o n i u m concentrations downs t ream f rom the beaver pond (P2) were greater during the n o n - s u m m e r period, indicating a longer residence t ime for NH4 ÷ t ransported f rom the pond in the colder months possibly due to lower nitrification rates. On a mass basis, the pond was a net upland source of NH4 ~ to downs t ream waters. This was only a transient source o f ANC since this NH4 + was eventually oxidized downst ream.

Significant export o f Fe 2+ occurred throughout the year (Figure 4, Table 2), with large pulses f rom July to September and again during the under-ice period. Some of this Fe 2+ was lost to oxidat ion due to reaeration immedia te ly downstream. During low-flow periods, Fe oxyhydroxide precipitates were noted on the s t ream- bed just below the dam (Smith et al. 1991). Like NH4*, product ion of Fe z+ was not pe rmanen t ANC production, as regards the whole s t ream reach, due to this oxidation. Any released Fe 2+ would be expected to reoxidize slowly under acidic condit ions (S tumm and Morgan 1981) or to react with sulfides produced f rom SO4 -~- reduction within the pond, with subsequent precipitation of Fe sulfide minerals. Photo-reduct ion of Fe 3+ to Fe 2÷ may also occur during the summer , which could also contr ibute to the observed release (Madsen et al. 1986, McKnight et al. 1988). However , since there is strong evidence of Fe 2" release f rom anoxic


Table 5. Significance levels, slopes and r -~ values of linear regressions of major solutes on ANC for the downstream site (P2), outlet (P3) and inlet (P4) of the beaver pond. (ns = not significant, * = p < 0.05, ** = p < 0.01).

Downstream (P2) Outlet (P3) Inlet (P4)

DOC ** 0.09 .28 ** 0.09 .52 ns 8 0 4 2 ** -- 1.0 .61 ** -0.55 .53 ns NO3 ** -0.87 .38 ** -0.44 .45 * -0.13 .22 F ns ns * -5 .0 .20 CI ns ns * -0.72 .12 N H 4- ** 4.5 .45 ** 3.1 .38 ns Na + ** 2.9 .74 ** 2.1 .28 ** 0.45 .29 K + ** 3.5 .3[ ** 2.4 .26 ** 0.37 .25 Ca-" ns ** -1.7 .23 ns Mg 2 + ns ns ns A1, ** -3.6 .46 ** -2 .0 .56 ** -0.55 .24 AI" ** - t.2 .46 ** 0.67 .53 * -0.15 .17 Fe ~+ ** 2.7 .62 ** 1.5 .73 ns

zones in the summer, as well as under ice in winter from the same zones, we feel confident that most Fe 2+ release is likely due to microbial Fe 3+ reduction processes.

Basic Cation Comparisons in Inlet and Outlet Waters

Basic cation (Ca 2+, Mg 2+ , Na + , K + ) concentrations were not statistically different between inlet and outlet, except during the summer period (Table 4). Mass flux calculations (Table 2) indicated a summer retention o f Na +, Ca-'-, and Mg 2--. On a seasonal basis, Ca 2~ was somewhat lower in both inlet and outlet waters during the summer period, presumably due to enhanced biological uptake and the exchange o f H ÷ for basic cations by Sphagnum, peat, and sediments in the pond and associated wetlands (Clymo 1963, Craigie and Maass 1966, Andrus 1986). A potential retention role o f beaver ponds for basic cations was revealed in a study of the outlet stream chemistry at another beaver pond in the Adirondacks (Gubala and Driscoll 1990). The collapse o f the beaver dam at that site resulted in a temporary release of basic cations, revealing that pond sediments and vegetation had been cation exchange sinks.

Linear regressions of Ca 2 + on ANC (Table 5) showed a negative correlation at the immediate pond outlet (P3) suggesting a relationship between Ca z+ release and ANC generation by other processes within the pond. The exchange of Ca 2+ on organic acid functional groups would be enhanced during the higher pH/lower A1 n u m m c r pc r iod , a n d enchangeab le Ct~ 2+ w o u l d l ike ly be displaced by elevated inputs o f H - and A1 during the winter and spring. The positive correlation of K + and Na + with ANC at all three sites (Table 5) suggests an underlying influence o f mineral weathering and sur-

ficial geology. The mineralogy o f the soils is largely quartz, plagioclase, and K feldspar (April and Newton 1985). These minerals could have a continuous influence on water chemistry as the water moves through the watershed. The net export o f K + (Table 2) may be attributed to decomposit ion o f leaf litter inputs, which have been shown to be a large source o f K + (Vogt et al. 1987). Sodium was retained in the pond, especially in summer months, indicating active exchange during the growing season.

Aluminum and Silica Comparisons in Inlet and Outlet Waters

Transformations involving AI speciation warrant discussion (Figure 5, Table 3) since inorganic monomeric A1 species (Ali) have been implicated in potential toxicity to aquatic biota in acidic systems (Driscoll et al. 1980, Sehofield and Trojnar 1980, Hall et al. 1985). A negative relationship of ANC with AI, was evident (Table 5), indicating mobili ty of A1 during acidic conditions and retention during high ANC conditions. The trend o f declining Alt concentrations from inlet to outlet was particularly evident during high-flow periods with high AI, concentrations in the inlet stream. This retention was largely associated with declines in the AI~ fraction, since concentrat ions o f Alp were relatively unchanged. Hydrolysis of inlet A1 and precipitation within the pond are the probable mechanisms o f retention. Chemical equil ibrium calculations indicated a sharp reduction in AL as water passed through the pond, with the ratio of Al~/Alt nearly halved throughout the s u m m e r per iod . U p to 90o/0 o f AI, e x p o r t e d f rom the pond in the summer occurred as Alp. During the non-summer period, Alp comprised only 20% of outlet AI, and AP + concentrations were much higher. Mass balance calculations indicated that 620 mol Alt ha -t


SUMMER (July-Sep) NON-SUMMER (Oct-June) 250 250

~, ,~ , . tu iv/L t~ t ; ou - 200

I 5 0

100

50

0

Figure 6.

_ _ F e f # +F n+ .-- , A1 ------

• . . .- HC03-- • H+/~ ~ " ~ NO a - N H 4 " ~

S B C S B C S B C

"-~..... S 0 4 z -

S B C

I N L E T O U T L E T I N L E T O U T L E T

2 0 0

150

100

50

0

Distribution of ionic solutes for the beaver pond inlet (P4) and outlet (P3) for the summer and non-summer periods.

yr t (on an annual basis) were retained in the pond, or approximately 24% of inlet Alt. Of this retained AI, 82% was AI,, the labile or potentially toxic form. Alu- minum retention in this system may benefit aquatic biota downstream. It should be noted that the pond served as a source of A1 to the stream during one col- lection date in January 1990 (mostly Alo), and during spring snowmelt in 1991 (mostly AI,). This A1 release was most likely due to the dissolution and flushing of AI previously precipitated in the pond.

Trends in H4SiO4 concentrations were also distinctly seasonal (Figure 5), with retention by pond waters from May through November. Mass balance calculations indicated H4SiO4 retention throughout the annual cycle, with 45% of this retention during the summer period (Table 2). Diatom proliferation is extensive in the open water pelagic zone, on sediment as epilithic growth, and as attached periphyton on Sphagnum and other macrophytes (personal observation). Although H4SiO4 is not directly involved in ANC contributing reactions, enhanced SiO2 solubility at elevated pH and DOC has recently been demonstrated in peat pore- waters and ground water (Bennet et al. 1991, Hiebert and Bennet 1992). Silica dissolution may have been involved in the elevated H4SiO4 concentrations found at depth in the pond (discussed later). This H4SiO4 retention may have implications for downstream diatom growth.

Ionic Interactions in Inlet and Outlet Chemistry

Ionic equivalency diagrams for the pond inlet (P4) and outlet (P3) during the summer and non-summer periods (Figure 6) revealed underlying controls of seasonal and longitudinal chemistry. The ionic strength

of both inlet and outlet water was higher during the non-summer period, with less biological uptake. Sul- fate concentration was lower in the outlet in all seasons, with correspondingly higher organic anion (RCOO-) and HCO3- concentrations. Outlet waters were dominated by SO, 2-, basic cations, H C O 3 , RCOO , and Fe 2+, whi[e inlet chemistry was controlled by basic cations, A1 "+, H + , SO42-, and NO~ .

Correlations between ANC and major chemical constituents differed at the inlet, pond outlet, and downstream site, indicating changes in control on seasonal variations in ANC as water moved through the wetland (Table 5). Inlet waters (P4) were generally of lower ionic strength and showed positive correlations of ANC with the basic cations K + and Na ' and negative correlations with AI "+ and NO3-. The ANC of waters exiting the pond (site P3) were correlated with parameters associated with organic decomposition reactions in the pond: NH4 +, Fe 2+, SO42 , NO3-, and DOC. During times of short interaction of water with pond sediments and reducing zones (early winter, spring runoff, etc.), measured ANC and AI, were transported more conservatively.

A positive correlation between H4SiO4 and ANC suggests that weathering reactions are important in the supply of ANC in the upland system. Correlations of AI and NO3- with ANC in upland soil water of this catchment in another study (Driscoll el al. 1987) indicated strong relationships of AI~ with NO3- and with ANC in O~ horizon soils and forest floor leachate. This pattern was reflected in the relationship of AP + with NO3- in the inlet tributary stream draining these soils (Al,+=19+0.55*NO3 -, in ~eq/L, r2=0.59, p<0.01). At all stream sites, this relationship was significant only during the winter months, indicating that processes that would otherwise serve to immobilize AI or se-


O Dissolved Oxygen (umol /L)

-

JUL JAN JUL .rAN IUL

Figure 7. Temporal/spatial patterns (isopleths) of dissolved oxygen (~mo]/L) for the in-pond site (PI 0) from June 1989 to September 1991. Blocked out areas represent ice-cover.

quester NOj were inactive at this time. There was a significant relationship between Na + and H4SiO4 in the inlet stream (Na+=7.3+0.26,H~SiO4, in ;,mol/L, r2=0.59, p<0.01) in all seasons and in the downstream outlet (P2) during the summer only (Na+=29+0.21, H4SiOn, in ~mol/L, r2=0.51, p<0.01). These results indicate that soil and mineral weathering influence on ANC is likely in the headwater inlet, while late summer period chemistry at the downstream site (P2) may be influenced by ground-water baseflow originating in the deeper till to the west of the stream.

Seasonal variations in outlet stream ANC (P3 and P2) were controlled by the release of Fe z~ , DOC, and NH4 + , and by the retention of SO42-, NO3-, and A1 °~- . The negative relationship of ANC with Ca 2+ in the outlet reflected the potential for exchange of Ca 2+ on sediments or biologically active exchange sites (e.g., Sphagnum). Differences between the pond outlet (P3) and the downstream site (P2) in processes associated with ANC generation reflected the reoxidation of the reduced species Fe 2÷ and NH4 +. The continued influence of pond processes on stream chemistry, even at downstream site (P2), was indicative of the importance of the proximity and placement of upstream and headwater wetlands in affecting receiving water chemical and nutrient loads.

A m m o n i u m

~ ,

2 " \ /

~ JUL J~ JUL JA~ J~L Figure 8. Temporal/spatial patterns (isopleths) of NHc 0~equiv/L) for the in-pond site (P10) from June 1989 to September 1991. Blocked out areas represent ice-cover.

F e r r o u s i r o n , Fe(II) ( u e q / L ) . _ . 0

v

P" 1 4 ,x

JUL JAN JUL JAN JUL

Figure 9. Temporal/spatial patterns (isopleths) of Fe 2+ (~e- quiv/L) for the in-pond site (Pl0) from June 1989 to Sep- tember 1991. Blocked out areas represent ice-cover.

The results of multiple regression analysis of solutes on the dependent variable ANC to the three sites, supported the hypothesis of variable chemical control on ANC as the stream passed through the wetland system. A six-variable model was chosen using two different fitting algorithms (RSQUARE and STEPWISE) in PC- SAS. The inlet water (P4) was fit to a regression in the order H4SiO4, Al °+, Cl-, Na +, NOj-, and K+; outlet water (P3) was fit in the order A1 "+ , Fe 2+, SO, 2-, NH4 ÷, DOC, and DOlt. The downstream site (P2) was fit to a multiple regression on ANC in the order Na + , SO, 2- , NH4 +, DO~,, H4SiO4, AI "+ , indicating a return of some of the influence of baseflow in this downstream reach.

Spatial and Temporal Chemical Trends Within the Ponded Waters

An examination of chemical depth profiles (at the pelagic site, P10) indicated temporal and spatial variation in ANC generation (Figures 7-10), A weak mid- summer and winter thermal stratification developed between spring and fall mixing periods. Temperature profiles indicated spring and fall periods of mixing of the water column coinciding with ice-out and fall cool-

Acid N e u t r a ] i z i n g C a p a c i t y ( u e q / L )

JUL JAN JUL JAN JUL-

Figure 10. Temporal/spatial patterns (isopleths) of measured acid neutralizing capacity (ANC, ~cquiv/L) for the in- pond site (P I0) from June 1989 to September 199 I. Blocked out areas represent ice-cover.


Table 6. Significance levels, slopes and r e values of linear regressions of major solutes on ANC for the pond surface, 1 meter, and 2 meter sites of the beaver pond. (ns = not significant, * = p < 0.05, ** = p < 0.01).

Surface One Meter Two Meters D O C ** 0 . 0 4 .29 ** 0 .03 .14 ** 0.11 .47 SOa 2 ns ns ** - l . 8 7 .55 NO,- ** -0.16 .19 ** -0.24 .18 ** - l .0 .32 F ns n s n s C I - ns ns n s N H ~ ' ns ns ** 1.6 .48 Na + ns ns ns K + ns ns ** 4.5 .34 Ca 2 ' ns ns ns Mg -~* ns ns ns AL * 0.70 .18 ** 1.1 .36 ** -3.1 .25 AI "~ ** -0.20 .16 ** -0.38 .36 * -1.2 .27 Fe 2+ ns ns ** 0.60 .54

ing, respectively. The mixing periods were reflected in uniform profiles for all chemical species. A clinograde 02 profile developed at the deepest site in the pond concurrently with temperature stratification (Figure 7). Increasing concentrations of NH4 ~- (Figure 8), Fe e+ (Figure 9), and DOC were found with depth during these periods. These increases coincided with an ac- cumulation of measured ANC during thermally strat- ified, low-flow periods (Figure 10). Corresponding concentrations of SO42- and NO~ were near detection limits at these times. The O2 depletion and ANC buildup reflected the importance of ANC generation through anaerobic decomposition processes in the near-sediment zone. Unstable weather conditions, including storms and occasional high winds, seemed to cause a temporary breakdown of this thermal and O2 stratification. This condition was also noted in the NH4 + and DIC profiles, with periodic buildup and decline of concentrations throughout the yearly cycle. Development of anoxic conditions under ice and the release of ac- cumulated reduced species with spring thaw and ice- out could constitute a major proportmn of the annual chemical export budget from this pond (DeVito et al. 1989). Both AI~ and Alp concentrations declined with depth in the pond, indicating the role of the near- sediment zone in the hydrolysis and deposition of A1 species. This is a potentially important annual ANC sink in the pond. There was also a seasonal accumu- lation of H, SiO4 at depth with the O~ depletion. Ac- cumulation of H4SiO4 may be due to the settling and dissolution of planktonic and periphytic diatoms or dissolution of diatom frustules that have collected in sediments from epilithic algae.

Linear correlations of solute concentrations with ANC for the in-pond sites at 0-, 1-/and 2-meter depths (Table 6) revealed significant relationships of redox- related chemical parameters with the deepest site in

the pond. The positive relationships of ANC with NH4 +, Fe 2+ , and DOC reflect the importance of their release to ANC generation with depth, as do the negative correlations with SO42-, NO~-, and AI with their retention. Fewer (and weaker) correlations were evident at the surface site, Potassium was significantly correlated with ANC production in the deepest loca- "fion only, which seemed to reflect the cerrelation of K ~ release with litter decomposition (Vogt et al. 1987).

Chemical Similarity of Inlet, Outlet, and In-Pond Sites

In an effort to determine the chemical similarity of sampling sites with season, a clustering analysis of the inlet, outlet, and depth profile sites was performed. Results for the non-summer period (Figure 1 l a) revealed that the inlet, outlet, and pond surface (0-meter) sites were most similar, while the 1- and 2-meter depth sites were fairly isolated. This pattern supports the time-series observations, where in-pond chemical processes (those controlled by sediment/water contact) were not as effective in controlling stream chemistry in the non-summer period. Solutes in inlet waters seemed to move conservatively in a uniform layer under the ice in winter. Large late-winter storms and spring ice-out could induce turbulent flow under the ice or on the pond surface, resulting in a late winter flush of materials. Aulenbach (1992), using an end member mixing analysis (EMMA) at another beaver- impounded stream in the Adirondacks, determined that the chemistry of a downstream site showed an increasing contribution from deeper beaver-pond water with the progression of a large late-winter hydrologic event. Turbulent mixing of otherwise isolated deeper pond waters would seem to be implicated.

A similar clustering of sites for the summer period


A. Non-Summer

Om 2m Outlet Inlet

1 I I Im

0.5

0.8

1.00 ] 1.10 I J

B. S u m m e r

0m lm 2m I 0. ]

0 . 9

Outlet Inlet

0.7

I 1.34

Figure 11. Cluster diagram of sampling sites by chemical similarity for the a) summer (July-Sep) and b) non-summer (Oct-June) periods. Numbers are relative distances. In- let=P4, Outlet=P3, 0m = pond surface site, l m = 1 meter depth site, 2m = 2 meter depth site.

(Figure 11 b) indicated a distinctly different situation, with greater chemical similarity between the outlet and the deepest pond depth site. The inlet stream did not cluster with any of the other sampling sites. In the mid- and late-summer months, base flow at the outlet site was maintained by water exiting through and below the dam, or as "under tow" and "throughflow" in the terminology of Woo and Waddington (1990). This observation supports the hypothesis that summer- and early-fall outlet flows reflected the chemistry of the near-sediment zone in the pond. These periods of very low inlet flow, along with maximum evapotranspira- tion and biological activity, should provide the wetland with its greatest opportunity to directly influence stream chemistry.

Acid Neutralizing Capacity Budget

An ANC budget based on pond surface area was developed from outlet and inlet fluxes of relevant chemical constituents (Table 2, Figure 12), Overall A N C p r o d u c t i o n b a s e d o n s o u r c e a n d s i n k d i f f e r e n c e g revealed a net annual ANC production rate of 160 meq m 2 yr- 1 Non-summer ANC production was 120 meq m -2 yr - ' , while summer ANC production was 310 meq m -2 yr -1. The components of these budgets sup-

A N C B U D G E T S 1,0O0

SUMMER ~o0 NON-SUMMER net ANC prod = al0 800 net ANC prod = 120

/CI.F ret ' / 8oo j j ,o0 ~ Nil4 rel/-!_N_Q_3_.r_et

Fe rel - - - ~ ~

2oo [ - - _.__ SO4 rat loo I AI ret SO4 reti' L ~ I I rNret 0 I

SINKS SOURCES SINKS SOURCES

Figure 12. Acid neutralizing capacity (ANC) budgets (me- quiv m -z yr ' of pond surface area) for Pancake-Hall Creek beaver pond for the period September 1990 to September 1991, based on mass balances on individual ions. Non-summer (October-June) and summer (July-September) periods. Al=aluminum equivalency, SBC=sum of basic cations, Org=organic acids, SO4=sulfate, NO3=nitrate, NH4=ammonium, Fe=ferrous iron, Cl=chloride, F-fluoride, tel=release, ret=retention.

port the time-series and regression analyses in showing that summer ANC production was dominated by SO42- retention and Fe 2+ release (65% and 17% of ANC production, respectively), while non-summer ANC production was much lower and controlled by NO~ retention (34%), lower amounts of SO42 retention (33%) a n d NH4 + release (13%). Inorganic A1 retention was a major sink for ANC during the non-summer period (550/0 of ANC loss). Organic acid production and basic cation retention were important ANC sinks in the yearly cycle, with basic cation retention becoming much more important in the active biological summer season. The magnitude of the NO~ and A1 n' retention was skewed to the non-summer months since much of the entire annual influx of these substances entered the pond during high-flow periods in early spring, and were included in budget calculations for the non-summer period.

SIGNIFICANCE AND CONCLUSIONS

The role of headwater wetlands in the neutralization of acidic drainage waters has received little attention in comparison with in-lake and terrestrial ANC generation. Wetlands play an important role in altering the biogeochemistry of stream waters; headwater wetland a r e a s , as a n i n t e r f a c e b e t w e e n t e r r e s t r i a l a n d aquatic systems, have the first opportunity to alter pri- mary drainage water chemistry. With abundant organic sediment and anoxic zones as sites for ANC generation, beaver ponds offer a unique "aquatic island"


in a terrestrial "sea" where substantial alterations in water chemistry are possible. Beaver ponds contain deeper waters than some typical marshes and bogs, providing longer contact time with sediments and the development of anoxic zones.

Terrestrial watershed ANC generation rates of 120 meq m -2 yr -t have been reported at a watershed near the present study (Schafran and Driscoll t987), and Schindler (1986) has reported terrestrial production rates of from - 2 0 to 193 meq m 2 yr ~, with most rates being under 90 meq m- 2 yr- '. Estimates of aquatic ANC production for lakes on the Canadian Shield range from 118 to 208 meq m -2 yr -~. Comparison of these rates with the annual rate calculated in this study reveals the importance of these beaver-impounded waters as sources of ANC to inlet streams. With a combination of the reduction and/or assimilation of SOn 2 and NO~-, and as sinks for AI, beaver impoundments serve as both sources and sinks of ANC and as buffer/ transformation zones in the transport of these substances. Recent trends seem to show a leveling off or decline in SO42- deposition inputs in the eastern Unit- ed States (Driscoll and van Dreason 1993). Headwater wetlands could be important in moderating these declines, as studies using stable sulfur isotope profiles have demonstrated that SO, 2- reduction rates respond directly to input SO4 z- (Nriagu and Soon 1985, Fry 1986).

Nitrate concentrations in drainage water seem to be increasing in certain areas of the eastern United States (Stoddard and Murdoch 1991, Driscoll and van Drea- son 1993). The maturing of eastern forests and a trend in increased NO3- concentrations in stream water (Abet et al. 1989, Driseoll and Van Dreason 1993) has prompted hypotheses regarding the "'nitrogen saturation" phenomenon. The role of wetlands as headwater sinks/transformers for this potential increased NO3- runoff should be explored. The importance of headwater wetlands as modifiers of NO3- inputs to receiving waters is supported since the beaver pond in this study acts as a sink for NO3- in all seasons. The shift from a system dominated by NO3 in the headwaters to NH4 + and possibly organic N in the downstream reaches has been demonstrated. This change may have consequences for the nutrient and acidification status of the receiving lake, particularly on a seasonal basis.

Although DOC production is demonstrated as being a significant component of the ANC budget throughout the year, it is represented as an ANC sink since the organic acid equivalency (RCOO-) calculated using a triprotic model has been shown to be representative of strong acid groups based on low calculated pK~ values in the Adirondack region (Driseoll et al. 1993). In the Adirondacks, DOC is generally negatively correlated with ANC (Munson et al. 1990a, 1990b). The

positive correlation of DOC with ANC in this study stream indicates that DOC production is coincidental with those processes associated with ANC production from organic decomposition in the pond. Nevertheless, the release of organic solutes serves to depress pH and ANC in drainage waters and offset the net production of ANC by wetland processes (e.g., SO42- and NO3 reduction).

The role of wetlands in the immobilization of AI has not been fully investigated. The beaver pond at Pan- cake Hall Creek seems to be acting as a sink for AI that has been mobilized in the watershed by acidic drainage waters. Both total AI transport and its speciation arc changed, with the less toxic organic forms more important in downstream reaches. As sinks for terrestrial H4SiO4, especially during the growing season, these ponded waters may serve to limit H4SiO4 transport to downstream lakes. It is doubtful that the diatom communities in the receiving waters would be H4SiO 4 limited, but the potential role of these upstream wetlands in the cycling of HaSiO4 has not been fully explored. There is evidence from surveys of Adiron- dack lakes that beaver activity at small lake outlets may inhibit ground-water H4SiO4 in-seepage due to excessive siltation and an increase in hydraulic head of ponded waters (Schofield 1993).

The mobilization of Fe has implications for the immediate downstream sediment benthos (Smith et al. 1991) and may play a role in the mobilization of terrestrial Fe to downstream reaches and lake sediments. In this wetland, there is evidence that the released Fe 2+ is reoxidized downstream so that the ANC generated by this process cannot be considered "permanent." At the same time, our estimate of the contribution of Fe 2+ to net ANC generation is conservative in that it is likely that some Fe 2+ released by reduction processes is immediately precipitated as ferrous sulfide (in the pres- ence of any sulfide formed during SO42- reduction). This Fe 2÷ would not be observed during analysis or be counted in an ANC budget calculated by mass balance. Wetlands located closer to lake inlets or as part of the littoral zone around lakes may be sites for the production of permanent ANC from Fe reduction (as iron sulfide precipitates) since there would be less reoxidation than in flowing streamwater.

Seasonal production and release of DOC and NH4 ÷ has implications for receiving water trophic characteristics and water quality. If ANC can be generated within beaver ponds in sufficient amounts to appre- ciably affect the acidity of drainage waters, the role of wetlands in biogeochemical cycling takes on new light in these acidified systems. Along with the retention of AI and basic cations and the potential transformation of nitrogen, beaver-impounded waters could serve as future sources for retained substances in the event of


catastrophic or gradual dam collapse. Collapse of beaver dams is a cyclical phenomenon, and the potential for massive single event releases of sequestered chemical species needs to be taken into account in models of stream and lotic ecosystem behavior. Beaver impoundments existing on side tributaries to major drainages could be potential sources of acidity due to the addition of organic acids to stream water, while impoundments that develop in depressions that directly intersect a main drainage channel should serve as sites of increased hydraulic residence time and sediment contact. The net annual impact of wetland areas on acidic drainage may be more subtle, since the acidity transported during major runoffevents (spring snow melt, spring rains, etc.) may constitute a major per- centage of the annual budget. During these periods, the wetland system in this study is somewhat short-cir- cuited, and the wetland has less influence on stream water quality. This is probably common in eastern temperate systems.

In addition to the role of beaver ponds in creating landscape heterogeneity at the aquatic-terrestrial interface (Johnston and Naiman 1987, 1990, Naiman et al. 1988), these ponds may have a biogeochemical role in the production of ANC in acid-sensitive catchments. Current models of watershed acidification do not ad- equately consider the role of wetlands or the biogeochemical transformation processes occurring within them. Calculations of ANC budgets using different models result in a variety of processes controlling ANC, depending on the model formulations and processes accounted for (Cook et al. 1992). Future modeling ef- forts should include a consideration of the effects and importance of wetland areas within acidic-lake catchments in both the production and consumption of ANC.

ACKNOWLEDGMENTS

Support for this research was provided by the Em- pire State Electric Energy Research Corporation, The Electric Power Research Institute, the U.S. Fish and Wildlife Service, and Living Lakes, Inc. We also appreciate the help of Dr. Robert Newton with the watershed hydrology and Brent Aulenbach and Kimberly Bowes for their help in the field. We appreciate the cooperation of Dr. and Mrs. Steinhausen for use of the research site.

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Manuscript received 29 April 1993; accepted 20 August 1993.

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Beaver pond biogeochemistry: Acid neutralizing capacity generation in a headwater wetland