- Email: [email protected]
Groundwater flow and dissolved carbon movement in a boreal peatland
Journal
ELSEVIER
Journal of Hydrology
191 (1997) 122-138
Groundwater flow and dissolved carbon movement in a boreal peatland J.M. Waddingtonlwa, N.T. Rouletb ‘Department OfGeography, York University, 4700 Keele street, North York, Ont. M3J IPZ, Canada bDepartmentof Geography. McGill University and the Centre for Climate and Global Change Research, 805 Sherbrooke Street W.. MontrpOl, Que. H3A 2K6, Canada Received 4 February
1995; revised 28 December
1995; accepted
9 March 1996
Measurement of the groundwater flux and the consequent advection of dissolved carbon (DOC (dissolved organic carbon), CH4 and CO*) were made in a boreal peatland in northern Sweden in summer 1993. The early summer gradients in hydraulic head indicated a downward flux of water in the peatland, but after a persistent mid-summer dry period the gradients changed to produce an outward radial flow from the centre to the margins of the peatland. This shift in gradients corresponded to the removal of some of CH., and CO* dissolved in groundwater. The concentrations of dissolved CH, and COr were spatially variable. with the highest observed in the centre of the peatland. In contrast, DOC concentrations were relatively constant near the centre of the peatland, and switched in the margin from being the highest during dry periods to lowest during wet periods. The changes in concentration of CH4, COr, and DOC are a function of the seasonal patterns of production and consumption, but groundwater flow is significant in the redistribution of dissolved carbon and the spatial patterns of concentration. The calculated mass flux of dissolved carbon was significant: it was approximately 20% of annual CO* fixation. 8 1997 Elsevier Science B.V.
1. Introduction Many wetlands in the high latitudes are peatlands because the complete decomposition of organic material. Peatlands lo6 ha (Sjiks, 1980) and are most common in the boreal and peatlands store an estimated 455 Gt of carbon or one-third
’Resent address: 4K1, Canada. 0022-1694/97/$17.00
BWrtment of Geography,
McMaster
Unversity,
1280 Main Street W., Hamilton,
8 1997- Elsevier Science B.V. All rights mx%ved
PIf SOO22-1694(96)03075-2
their wet conditions inhibit cover an estimated 500 x subarctic regions. Northern of the world’s soil carbon
ht.
L8S
J.M. Waddington, N.T. Roulet/Joumal of Hydrology 191 (1997) 122-138
123
(Gorham, 1991). Although they are believed to represent a net sink of carbon, they exchange a large amount of carbon to the atmosphere (CO*, CH,), and via water flow (DOC (dissolved organic carbon), CH4, COZ) to adjacent ecosystems. With drier conditions or drainage, the stored carbon in peatlands can become mineralized and lost to the atmosphere through oxidation (Armentano and Menges, 1986), whereas under wetter conditions, CH4 emission (Moore et al., 1990) and leaching of DOC (Moore, 1987; Koprivnjak and Moore, 1992) are major losses of carbon. The fluxes of dissolved carbon are directly and indirectly controlled by the hydrology of the peatland. This paper examines the hydrology of a boreal peatland and investigates the role of hydrology in the movement of dissolved carbon (DOC, CHJ, COZ) in the peatland. This water-borne flux of dissolved carbon is important in the determination of the peatland carbon budget. DOC export from peatlands is a function of DOC production and the hydrologic pathways by which water flows in the peatland and adjacent upland regions (Koprivnjak and Moore, 1992). AMU~ DOC export from peatlands is between 2 and 40 g mm2(Mulholland and Kuenzler, 1979; Moore, 1987; Moore and Jackson, 1989; Urban et al., 1989; Dalva and Moore, 1991). Koprivnjak and Moore (1992) found that DOC in streams draining subarctic catchments in the summer was positively correlated with the ratio of the area of the peatland to the area of the catchment. Many northern peatlands, however, lack welldefined streams, so flow is usually very diffuse (Price and Woo, 1988). The spatial and temporal variability of DOC concentrations within peatlands is large (Moore, 1987). Fringe areas of peatlands that are assumed to be influenced by groundwater and/or upland runoff may have higher nutrient levels which can enhance carbon remineralization and increase pore-water DOC levels. Pore-water concentrations of DOC generally increase in the summer because of higher evapotranspiration rates and plant tissue decomposition (Moore, 1987), but pore-water dilution can occur during large summer and autumn rainfall events. ‘Ihe spatial and temporal variability of dissolved CH4 and CO2 is also large. Numerous studies (e.g. Moore et al., 1990; Buttler et al., 1991; Nilsson and Bohlin, 1993) have indicated high concentrations of dissolved CO2 and CHJ at depth in peatlands. In general, dissolved concentrations increase with depth, reflecting differences in production and consumption, the importance of gas transport and peat characteristics (Nilsson and Bohlin, 1993), and the possible effects of surface dilution from lateral flow and precipitation. However, decreasing gas concentrations near the bottom of some profiles (Williams and Crawford, 1984; Brown et al., 1990; Nilsson and Bohlin, 1993) and younger radiocarbon dates of dissolved CO2 in deeper peat than shallow peat (Charman et al., 1992, 1994; Aravena et al., 1993) indicate an additional groundwater influence in the redistribution of gases. Changes in the lateral and vertical movement of water during the season should, therefore, have a major influence on the redistribution of dissolved CH, and CO2 in a peatland, as is the case for DOC. Lateral water movement in peatlands is controlled by the position of the water table (Bay, 1969), the slope of the peatland (Ivanov, 1981), and the hydraulic properties of the peat (Ivanov, 1981). The flux of water is greatest during periods of high water table because the upper layers of the peat profile have the greatest hydraulic conductivities (Ivanov, 1981; Verry et al., 1988). Lateral flow in peatlands having surface microtopographic (hummocks and hollows) and mesotopographic (ridges, pools, lawns) features
124
J.M. Waddington, N.T. RouletlJoumal
ofHydrology191(1997) 122-138
is impeded by lower hummock and ridge hydraulic conductivities (Swanson and Grigal,
1988; Quinton, 1991). Although it has generally been assumed, based on low hydraulic conductivity (Ingram, 1983) and pore-water chemistry (Gorham and Hofstetter, 1971). that the dominant direction of water flow in peatlands is lateral, there is increasing evidence (hydraulic heads and isotope geochemistry) suggesting that vertical groundwater flow can be significant (Siegel and Glaser, 1987; Charman et al., 1992; Aravena et al., 1993). There have been no studies that have systematically examined the redistribution of dissolved CO* or CH4 via groundwater flow in peatlands. In this study, a boreal patterned peatland was instrumented to examine the role of groundwater hydrology in the redistribution of dissolved carbon in northern peatlands. The objectives of this study were to: (1) determine the change in the lateral and vertical flux of water during the summer season; (2) determine the spatial and temporal variability of dissolved carbon storage and its movement; (3) examine the role of hydrologic pathways in the transport and redistribution of dissolved carbon.
2. Study area The study peatland, Stor-Amyran, is located 15 km south of UmeH, Vbterbotten, Sweden (63”44’N, 20’06’E) near the coast of the Gulf of Bothnia. Star-Amyran is a concentric raised bog, typical of peatlands in central and northern Sweden (Mikkell et al., 1992). central and northern Finland (Ruuhij&vi,01960) and the boreal and subarctic regions of North America (Zoltai et al., 1988). Star-Amyran is a Sphagnum-dominated, raised, ombrogenic peatland that developed around a creek that flowed into the Gulf of Bothnia (Nilsson, 1992). Stratigraphy indicates that Star-Amyran was originally a soligenie peatland (Nilsson, 1992), but today only a small soligenic zone exists where water flows into the wetland from uplands. The peatland is dominated by a 12 ha ombrogenic feature in the centre (Fig. 1) that rises 1.4 m above the surrounding peatland. Tbe ombrogenie feature comprises a series of ridges, open water pools, and vegetated lawns and has no surface inflow or outflow. The ridges are dominated by Sphagnum fuscum, Calluna vulgaris, Andromeda polifolia, Rubus chamaemorus, and Ledurn palustre, whereas the lawns are dominated by S. majus or S. balticum and Eriophorum vaginatum (Mikkelii et al., 1992). Pools vary in area and depth but are generally deeper than 1 m in the western area and are 25 cm deep at the eastern margin. The ombrogenic feature of the peatland can be divided into four areas based on topography: a floating mat, the hinge (identified as the boundary between the concave and convex region of the raised ombrogenic area), a plateau, and finally the eastern peatland margin (Fig. 1). Peat depths range from 2.5 m at the eastern margin to over 4 m at the plateau. The peat is underlain by a silty clay.
3. Methods The research was conducted between 2 1 May and 13 September 1993 and was designed to exam$e the role hydrologic pathways have in the transport of DOC, CO*, and CH4 in the Stor-Amyran raised ombrogenic area. Four locations on a 300 m transect through the
J.M. Waddington, N.T. Rouler/Joumal
of Hydrology 191 (1997) 122-138
125
Fig. 1. Sampling locations along the 300 m transect(A-A’) in the Star-&ran peatland.The inset illustratesthe location of the instrumentsat each sampling location. Thin lines indicate the mesotopographicridges.
centre of the ombrogenic feature were instrumented for measurements of water table position, peat surface level, and hydraulic head (Fig. 1). Precipitation was monitored continuously using a tipping bucket rain gauge at the floating mat site. Measurement of the properties of the peat were made at three mesotopographic features (ridge, pool, and lawn landfotms) at each of the four locations. At each site, samples of DOC and dissolved CH4 and CO* were obtained to determine the spatial variability and flux of dissolved carbon throughout the study period. 3.1. Groundwater jZu.x Groundwater flow nets were constructed using hydraulic head data obtained from a series of groundwater wells and piezometers installed along the transect. Piezometers, made from 1.5 cm ID PVC pipe screened over a 10 cm interval, were installed to depths of 0.25,0.50,0.75, 1.O, 2.0, and 3.0 m. Water levels within the piezometers were monitored weekly, and water table and peat surface level were monitored using potentiometric water level recorders. All water level sensors and a tipping bucket rain gauge were read every
126
J.M. Waddingron, N.T. Roulet/Joumalof Hydrology191 (1997) 122-138
minute and averaged every half-hour by a datalogger. Hydraulic conductivity was determined for every piezometer by the Hvorslev water level recovery method (Freeze and Cherry, 1979). The groundwater flux across each sampling location was calculated from the weekly flow nets using Darcy’s law. Groundwater fluxes were converted to an equivalent depth of runoff by dividing the volumetric flux by the area at each location. Areas, determined from air photographs and a survey of the peatland, were 59 800 m*, 25 500 m*, 21660 m*, and 59000 m* for the floating mat, hinge, plateau, and peatland margin locations, respectively. 3.2. Dissolved carbon sampling and analysis Profiles of dissolved CH4 and CO2 were measured every 7-10 days. Water samples were taken every 2.5 cm in the top 15-25 cm of the peat and then at various intervals to a depth of 3 m. Water samples were withdrawn using a stainless steel tube fitted with a series of stopcocks and connected to a 10 ml plastic syringe. After the water sample was withdrawn an equal amount of air was drawn into the syringe and the water was degassed into the head space by shaking vigorously for 2 mm. The CH4 concentration in the head space was determined on a Perkin-Elmer (Norwalk, CT, USA) gas chromatograph equipped with a Porapaq Q (Millipore Water Chromographic, Milford, MA, USA) column and a flame ionization detector (FJD). Calibration gases of nominally 12, 103,1000,5000, and 10000 ppmv were used depending on the concentration range of the samples being analysed. The CO2 head space concentration was measured by a Perk&Elmer gas chromatograph equipped with a Porapaq T column and a thermal conductivity detector (TCD). Calibration gases of 500,1000,10 000, and 20 000 ppmv were used. Sampling for DGC was conducted at all sites once every month. Water was withdrawn from the groundwater wells and piezometers using a hand pump. Samples were filtered (0.45 pm), stored at 4’C and sent to McGill University (Montreal, Quebec) at the end of the field season, where they were analysed on a Shimadzu (Kyoto, Japan) TGC 5050 analyser. 3.3. Calculations of the mass flux of dissolved carbon To determine the significance of groundwater transport of dissolved carbon it is necessary to compute the mass flux of carbon from one portion of the peatland to another. The peatland has been divided into four zones (see above) based on position and topography. The mass flux of groundwater from one zone to another was computed assuming the boundaries between zones extended perpendicularly from the surface to the base of the peatland. The width of the flow zone was the width ofthe peatland at this point. This discharge was converted into a unit area depth of subsurface runoff by dividing by the area of the zone that the groundwater is from. Conversely, this same discharge can be expressed as a depth of subsurface runoff received if the same discharge is divided by the area of the zone that receives the groundwater. The mass flux of dissolved carbon is computed by multiplying the zonal discharge by the sum of the integrated concentrations of DGC, and dissolved CO2 and CH.+ The mass flux of carbon, either loss or gain, is converted into a unit area by dividing the mass flux by the area of the zone that contributes or receives the flux. This is done so the water-borne flux of carbon can be compared with the exchange of carbon across.
J.M. Waddington,N.T. Roulet/Joumalof HydrologyI91 (1997) 122-138
127
Table I Hydraulicconductivity (cm s-‘) at floating mat, binge, plateau, and margin sites Site within a location
Pleating mat
Hinge
Lawn (25 cm) Lawn (50 cm) Lawn (100 cm) Lawn (200 cm) Ridge (25 cm) Ridge (50 cm) Ridge (100 cm) Ridge (200 cm) Pool (25 cm) Pool (50 cm) Pool (100 cm) Pool (200 cm) Pool (300 cm)
1.1 x 10-Z 6.4 x 10” 6.2 x lOa 2.4 x 10” 7.5 x lo4 2.8 x 10” 5.7 x 10” 3.7 x 104 4.5 x 10-3 5.6 x 1O-3 3.1 x 104 2.0 x 104 1.5 x 10”
8.1 x 1.5 x 1.4 x 6.3 x 1.7 x 2.4 x 1.1 x 3.3 x
lo-’ 10-l 10-3 1OA 10-2 10” lo+ 104
Plateau
Margin
n.a. 1.3 x 1.2 x 6.8 x 5.2 x 3.3 x 1.9 x 2.1 x
1.5 x 8.6 x 8.3 x 1.5 x 1.0 x 3.6 x 7.5 x 4.9 x 6.0 x 7.9 x 4.2 x 2.7 x 1.9 x
10-I 10” 10” 1O-3 10” 104 10-4
10-2 10” lo+ IO” 10-3 10” lo4 104 10” 10” lo4 lo4 IO”
n.a., Not applicable.
4. Results 4. I. General hydrology The peat hydraulic conductivity in Stor-finyran ranged from 2 x lo-’ to 8 x 10” m s-’ (Table 1). The hydraulic conductivity of the mesotopographic ridges was approximately l-2 orders of magnitude lower than the hydraulic conductivity of adjacent lawn peat at similar depths. The 30 year average precipitation $or the length of the study period at the UmeH airport, approximately 13 km north of Stor-Amyran, was 219 2 51 mm, whereas total precipitation during the 1993 study period was 257 mm. Precipitation events were not evenly distributed during the study period (Fig. 2(a)): much of June and July experienced no or little precipitation, whereas late May and August were characterized by higher precipitation. The temporal variation in precipitation is reflected in the position of the water table (Fig. 2(b)). The periods representing dry and wet moisture conditions are arbitrarily defined by the mean water table position at the margin lawn site: dry conditions (water table less than -1 cm depth at the margin lawn) and wet conditions (water table -1 cm or greater at the margin lawn). During wet periods, the water table rose to a maximum water table height of 4 cm above the peat (Fig. 2(b)), and overland flow was observed in individual lawn segments, but the water table remained below the lowest points of the adjacent ridges, causing water to pond on the upslope edge of the ridges. Consequently, the differences in hydraulic conductivity between ridges-hummocks and lawns-hollows and the prevalence of the me~otopographic ridge features eliminated surface flow during the summer months in Stor-Amyran. Furthermore, the ridges represent recharge zones that cause flow to diverge outwards, further reducing the possibility of surface flow. Accordingly, pundwater flow dominated the lateral flow in the peatland during summer months at Stor-Amyran.
128
J.M. Waddington. N.T. Roulet/Joumal of Hydrology 191 (1997) 122-138
(JWW)
UO!W@!=Jd
J.M. Wddington, N.T. Roulet/loumal of Hydrology 191 (1997) 122-138
0
100 ~
(ml
129
200
Fig. 3. Groundwater flow net for the cross-section A-A’ during (a) wet conditions (water table -1 cm or greater) and (b) dry conditions (water table less than -1 cm).
4.2. Groundwater jhx
The hydraulic gradients and flow direction in the raised ombrogenic feature changed between dry and wet periods (Fig. 3). Groundwater flow during wet periods (61% of the summer period) was from a recharge area in the plateau towards the margin and floating mat locations. A greater hydraulic gradient occurred adjacent to the pools at the margin of the raised peatland (Fig. 3(a)). A stagnation point formed under the top of the plateau during the period of sustained water table drawdown (Fig. 3(b)), creating an outward radial flow centred around a depth of 2 m. Combining the flow nets with the hydraulic conductivities, the plateau site lost 3250 m3 of groundwater during the study period (143 mm towards the hinge and 157 mm towards the margin), whereas the hinge and floating mat gained 3800 m3 and 4280 m3 (149 mm and 72 mm), respectively (Table 2). The groundwater flux was greatest at the peatland
130
J.M. Waddington,N.T. Roulet/Joumal of Hydrology191 (1997) 122-138
Fig. 4. Cross-sectionalong the A-A’ transectillustratingthe mean concentrationdistributionof DOC during (a) wet and (b) dry conditions. (Concentrationsare in mg 1-l; isopleths are 2 mg I-’ increments.)
margin because of higher hydraulic conductivity and hydraulic gradient. Here, the seasonal groundwater flux was 9440 m3 (160 mm) and the subsurface runoff varied from 1.0 to 1.7 mm day-’ over the period of study. Similarly, the daily subsurface runoff into the floating mat and hinge sites varied over the study period from 0.5 to 0.8 mm day-’ and from 1.1 to 1.8 mm day-‘, respectively. 4.3. Concentrations of DOC, dissolved CH,, and dissolved CO2
DOC concentrations were slightly higher at two sites and slightly lower at the other hvo sites (Fig. 4) between dry and wet conditions. The pattern of concentrations of dissolved CO2 and dissolved CH4 showed distinct differences between dry and wet conditions: dissolved COz (Fig. 5) and CH4 (Fig. 6) concentrations were both higher under wet conditions than dry conditions. DOC concentrations ranged from 21.0 to 29.0 mg 1-l in the peatland, with the lowest mean seasonal concentrations at the plateau site (22.2 mg 1-t). The highest mean seasonal
J.M. Waddington, N.T. RouletLJownal of Hydrology 191 (1997) 122-138
6
100
mmnc0 (ml
26s
666
Fig. 5. Cross-section along the A-A’ transect illustrating the mean concentration distribution of dissolved CO2 during (a) wet and (b) dry conditions. (Concentrations are in mg 1-l; isopleths are 5 mg 1-l increments.)
concentrations were at the floating mat site (27.3 mg 1-l). The concentration of DOC with depth changed during the season (Fig. 4). During a dry period in late July, DOC concentrations in the surface peat were 6-8 mg 1-l higher than in the peat at a depth greater than 2 m (Fig. 4(a)), whereas during a wet period surface concentrations were 6-8 mg 1-t lower than DOC at depth (Fig. 4(b)). This indicates that the concentrations are highest at the surface during dry periods. The DOC concentration in the surface layer at the plateau site did not change significantly between wet and dry periods. Dissolved CO* concentrations also showed different patterns between the dry and wet periods (Fig. 5). During the dry period, concentrations were greatest below the hinge and plateau sites (maximum 30 mg 1-l. 682 PM), whereas during the wet periods maximum concentrations were approximately 50 mg 1-l (1136 PM). With the exception of these locations, concentrations generally increased from 5-8 mg 1-l (100-200 PM) near the surface to over 12 mg 1-l (273 PM) at depth (Fig. 5). The cross-sectional profiles of dissolved CHI concentrations (Fig. 6) indicate higher concentrations during wet periods. During this time maximum dissolved CH4
132
J.M. Woddngton, N.T. RouletLJoumal
of Hydrology
191 (1997)
122-138
Table 2 Concentrationsand mass flux of HOC, dissolved CH,, and CO*, based on sampling from 21 May to 13 September 1993 Mean seasonal concentrations
Groundwater Plux (m’)
Runoff (mm)
Mat
4280
72
Hinge
3800
149
Plateau (west) - 1540
- 143
Plateau (east)
- 1710
- 157
!WO
160
Margin
DOC (mg 1-l) 27.3 (29.0,25.7) 24.3 (24.4, 24.2) 22.2 (21.0, 23.4) 22.2 (21.0, 23.4) 23.7 (23.4, 24.1)
CH4
02
(mg 1-Q
(mg 1-V
3.0 (1.2.3.9)
10.4 (4.5, 15.0) 14.8 (11.0,22.1) 12.3 (6.0, 17.5) 12.3 (6.0, 17.5) 10.5 (9.1, 11.9)
:678, 3.0) 1.6 (0.9, 2.7) 1.6 (0.9, 2.7) 2.9 (1.5.3.7)
Mass flux (mg C m-* day-‘) 18.4 40.1 - 27.9 - 30.5 41.8
Numbersin parenthesesreferto mean values during dry and wet conditions. Individual component fluxes are in the mass of the compound, whereas the mass flux is calculated in grams of carbon from weekly volumeweighted concentrationsand weekly groundwaterfluxes.
concentrations were about three times greater than maximum concentrations during dry periods (Table 2). Furthermore, dissolved CH4 concentrations during this period were between 15 and 30% of dissolved COr, which represents ahnost the same amount of carbon. Highest dissolved concentrations are located below the two pools and near the stagnation points below the hinge and plateau (Fig. 6). Like dissolved CO*, dissolved CH4 concentrations generally increased two- to three-fold from the surface to the bottom of the peat. 4.4. Mass flux of dissolved carbon and change in storage The product of the weekly groundwater flux and volume-weighted DOC, dissolved CH4, and dissolved CO2 concentrations equals the water-borne mass flux of carbon in the peatland. The floating mat, hinge and margin sites received an input of carbon from the plateau region, whereas the plateau site lost carbon during the study period (Table 2). Over 27 mg C rn-’ day-’ was lost from the plateau site, whereas inputs of carbon to the hinge and margin sites were over 40 mg C rn-’ day-‘. Total input of dissolved carbon to the floating mat site was slightly less than half the input to the hinge site. DOC accounted for between 82 and 84% of the mass flux of water-borne carbon at the sites. The proportion of dissolved CO* in the mass flux of carbon at the hinge and plateau sites (13.2% and 12.4%, respectively) was slightly higher than the proportion at the floating mat and margin sites (8.7% and 9.996, respectively). The opposite trend held for dissolved CH4, where dissolved CH4 accounted for approximately 4% and 7% of the mass flux for the middle (hinge and plateau) and edge (floating mat and margin) sites, respectively. The mean peatland storage of dissolved carbon during wet and dry periods was
133
J.M. Waddington, N.T. Rovlet/Joumal of Hydrology 191 (1997) 122-138
0
100
Dbtaneo (m)
200
3m
Fig. 6. Cross-section along the A-A’ transect illustrating the mean concentration of dissolved CH, during (a) wet and (b) dry conditions. (Concentrations are in mg I-‘; isopleths are 0.5 mg I-’ increments.)
calculated using the mean volume-weighted dissolved concentrations and assuming a mean peat porosity of 0.8 (Table 3). Total dissolved storage was 13% lower during dry conditions than wet conditions. DOC storage was 1% higher during dry conditions than wet conditions, whereas dissolved CH4 and CO2 storage were 68% and 56% lower during dry periods than during wet periods, respectively. 5. Discussion
Like many patterned peatlands, Stor-Amyran lacks a well-defined drainage network and consequently overland flow is diffuse. The presence of micro- and mesotopography impedes overland flow because the hydraulic conductivity of the hummocks and ridges is much lower than that of the hollows and lawns. Furtbettnore, persistent water table mounds in the ridges further impede overland flow and have been shown to minimize poolto-pool water transfer in a Labrador bog (Price and Maloney, 19%). During wet periods, this results in the ponding of water on the upslope edge of the ridge, which maintains a water table at the peat surface and in some areas sustains pools. Some of this ponded water is lost to evaporation, but much of the water also moves as groundwater flow to the
134
J.M. Waddington, N.T. Roulet/Joumal of Hydrology 191(1997)
122-138
Table 3 Mean peatland storage of DOC, dissolved CH,, and dissolved CO*. during wet and dry conditions from 2 1 May to 13 September 1993
Dot ‘334 co2
Total
Dry (g C m-*)
Wet (g C m-*)
284.8 9.2 23.8 317.8
282.8 28.6 54.2 365.6
AS (g C m-*) 2.0 - 19.4 - 30.4 - 47.8
AS indicates the difference in mean storage between wet and dry conditions. Negative numbers refer to a loss in dissolved carbon storage under dry conditions.
perimeter of the peatland. Consequently, the traditional view of overland flow as the dominant hydrologic flow path in peatlands (Ingram, 1983) is unsuitable for assessing the overall flux of water in this common type of northern peatland. Rather, determination of the groundwater flux is needed to calculate the flux of water in a peatland of this type. The total subsurface runoff at the plateau site was 143 mm towards the hinge and 157 mm towards the margin, approximately equalling the flux inputs to the hinge (149 mm) and margin (160 mm) sites. This indicates that the increase in groundwater flux at the hinge and margin sites from the plateau can be attributed to the increase in the source area from the central region of the peatland to the perimeter. The drop in the depth of groundwater flow between the hinge and floating mat sites is probably in response to the change in the hydrologic flow paths during dry conditions (Fig. 3(b)). During dry conditions, water discharges part way between the hinge and floating mats in a region marked by numerous small pools. Additional water recharges in this zone at the floating mat ridge and flows towards the floating mat region. The depth of flow at the floating mat site is only 50% of that at the hinge site, but total groundwater flow is slightly higher, suggesting that the discharge and recharge between the two sites are roughly offset. The total rainfall during the study period was 257 mm, whereas the depth of groundwater flow entering the hinge and margin sites was 149 mm and 160 mm, respectively. Consequently, an amount equivalent to approximately 60% of the depth of rainfall incident upon the ombrogenic feature leaves the margins of the ombrogenic feature during the summer months as groundwater flow. An amount equivalent to 40% of rainfall is lost as evaporation. Because the initial and final water table positions during the study are fairly similar, the net storage change over the study period is small. This translates into a mean daily evaporation rate of approximately 0.9 mm, which is in the range of that observed for other northern bogs (Virta, 1966). 5.1. Groundwaterflow
reversal
A reversal in the direction of the groundwater flow occurred below the plateau during dry conditions. Reversals of hydraulic head and consequent losses of CH4 have been documented during drought conditions in the Glacial Lake Agassiz Peatlands in northern Minnesota (Romanowicz et al., 1993). Unlike those reversals, which were caused by changes in the proportion of the regional and local discharge systems, however, the
J.M. Waddington, N.T. RouletLJoumal
of Hydrobgy
191 (1997) 122-138
135
reversal in this study was driven by internal mechanisms. During dry periods, evaporation and drainage to the margins cause the water table at the plateau lawn to drop several centimetres below the surface. The drop in the water table alters the thickness of the peatland aquifer and results in a greater influence of local flow systems (T&h, 1963). Undet these conditions, the surface topography has less control on the groundwater flow in Stor-Amyran. The upward flow of water in the peatland, during dry conditions, is also important for the redistribution of dissolved carbon in the peatland, as it augments the diffusive transport of gases in the peatland (Romanowicz et al., 1993) and also releases the normal downward pressure that inhibits the release of CO2 and CH4 in bubbles (e.g. Moore et al., 1990; Windsor et al., 1992). Evidence for these mechanisms is substantiated by the existence of an episodic release of CH4 during the reversal in groundwater flow direction. The episodic release of CH4 resulted in a large negative change in stored CH4 and CO2 (Table 3). CH4 flux from the plateau lawn, measured using static enclosures, increased from 2.5 mg ms day-’ the week before the reversal to over 58 mg mm2day-’ during the reversal (Waddington and Roulet, 1996). The flux of CH4 then returned to 2.0 mg mm2day-’ the following week, even though the flow reversal persisted. Large CH4 losses also occurred during groundwater flow reversals in the Glacial Lake Agassiz Peatlands (Romanowicz et al., 1993). The release of large CH4 storage during drought conditions in peatlands indicates that groundwater flow reversals, although important in the net transport of dissolved carbon in peatlands, are also an important mechanism for understanding the carbon balance of peatlands and the air-water exchange of carbon in peatlands.
flow
5.2. Net transport Analysis of the net transport of dissolved carbon in Stor-Amyran allows for a comparison between the controls of groundwater flow and production of the movement of carbon in the peatland. The mean daily dissolved carbon mass flux from the plateau towards the hinge and margin was -27.9 mg C mm2day-’ and -30.5 mg C mm2day-’ (Table 2), which represents 70% and 73% of the mean daily mass flux entering the hinge and margin sites, respectively. There is an increase in the transport of dissolved carbon from the plateau to the margins of the peatland. The net increase in carbon to the hinge and margin sites is 12.2 mg C rnw2day-’ and 11.3 mg C me2 day-‘, respectively. This increase in carbon flux can be attributed to both an increase in the rate of production of DOC, CH4, and CO2 and the groundwater flow along the hydrologic pathway between the recharge and discharge sites. Because the depth of flow increases only slightly from the plateau to the hinge and margin sites (Table 1). however, much of the increase in mass flux must be attributed to an increase in the in situ production. Conversely, the decrease in mass flux between the hinge and floating mat sites is attributed to a dominant change in the groundwater flow. The mean daily mass flux of carbon at the floating mat site is only 45% of the mass flux at the hinge location, whereas relative to the total flux, only a 7% increase exists at the floating mat site compared with the hinge location. Because mean DOC, dissolved C02, and dissolved CH4 concentrations are generally higher at the floating mat site relative to the hinge location, the decrease in net transport is attributed to the decrease in the depth of
136
J.M. Waddington.N.T. Rouler/Joumal
ofHydrology191(1997)
122-138
groundwater flow between the two sites (Table 2). This decrease is due to the seasonal formation of the small discharge zone between the two sites. 5.3. Redistribution of dissolved carbon Concentrations of DOC, dissolved COZ, and dissolved CH4 change between dry and wet conditions within the peatland. Increased dissolved CO2 and dissolved CH4 below the hinge and plateau appear to be related to the existence of a groundwater flow stagnation point and the result of the groundwater flow reversal. The change in DOC concentrations probably occurs in response to changes both in evaporation and precipitation (Koprivnjak and Moore, 1992) and changes in the hydrologic pathway. Under dry conditions high surface concentrations are likely to be accentuated by evaporation, whereas under wet conditions dilution from precipitation decreases the surface concentration. Changes in the concentration of DOC at depth at the margin sites, however, is probably controlled by the changes in the flow pattern source of the groundwater flow at the centre of the peatland. 5.4. Implications for carbon cycling in peatlands The existence of a dominant local groundwater flow system and groundwater flow reversals in the peatland contradicts the idea that raised ombrogenic peatlands are recharge areas with negligible groundwater flow (Ingram, 1983). Using T&h’s conceptual model of groundwater flow as an analogy (T&h, 1963), it can be generalized that recharge areas of topographic highs (plateau and ridges) represent regions of carbon loss, whereas the topographic lows represent regions of carbon gains. Although it has been shown that changes in the net transport along this hydrologic pathway are likely to occur, the translocation of dissolved carbon has many implications regarding carbon cycling in peatlands. The translocation of carbon via groundwater is important for determining the carbon balance of a peatland. This study has shown that the change in dissolved carbon storage in a peatland is dynamic and subject to changes in the groundwater flux. Furthermore, this study has shown that the movement of dissolved carbon in the peatland can represent both a major loss (at the plateau location) and a major gain of carbon (floating mat and margin locations) to the carbon balance of different zones of the peatland. Comparing the mass flux of carbon via groundwater with the atmospheric fluxes of COZ and CH4 at the same sites (Waddington and Roulet, 1996) provides evidence of the importance of this component. At the plateau site the carbon loss via groundwater was ten times greater than the flux of CH4 to the atmosphere, whereas at the hinge and peatland margin sites the dissolved carbon input was approximately 1.l- 1.5 times greater than the atmospheric loss of CHb. Furthermote, the dissolved component input to the floating mat region was approximately 20% of the total carbon uptake by plants during the same time period. Consequently, groundwater flow and dissolved carbon movement are important in the exchange of carbon in boreal peatlands. The substrate available for methanogenesis has often been considered to be limited to the peat itself, which is often of poor quality (Nilsson and Bohlin, 1993). With the movement of younger substrate into different zones in a peatland (e.g. Aravena et al., 1993), however, there is the possibility of in situ methanogenesis and the further increase in the
J.M. Waddington. N.T. Roulet/Joumal of Hydrology 191 (1997) 122-138
137
concentration of methane. Although many studies of carbon fluxes have concentrated on characterizing the substrate quality for methanogenesis (Nilsson, 1!392), the transport of dissolved carbon (CO*, CH4, and DOC) represents another potential source of carbon that has not been generally considered in the cycling of carbon in peatlands.
Acknowledgements
We would like to thank C. Mikkell, G. Granberg, M. Nilsson, R. Swanson and I. Bergman for their assistance in the field and laboratory. We also thank J. Price, K. Devito, A. Hill and two anonymous reviewers for providing comments on an early draft of this manuscript. J.M.W. was supported by a National Sciences and Engineering Research Council of Canada (NSERC) postgraduate scholarship and the Martin Gelber International Studies Award, York University. The research was funded by an NSERC research grant.to N.T.R. Logistical assistance was provided by the Department of Forest Ecology, Swedish University of Agricultural Sciences, and the Department of Physical Geography, University of Umei.
References Aravena, R., Warner, B.G., Charman. D.I., Belyea, L.R., Mathur, S.P. and Dinel. H., 1993. Carbon isotopic composition of deep carbon gases in an ombrogenous peatland, northwestern Ontario, Canada. Radiocarbon, 35: 27 l-276. Armentano, T.V. and Menges, C.V., 1986. Patterns of change in the carbon balance of organic-soil wetlands of the temperate zone. 1. Ecol., 74: 755-774. Bay, R.R., I!%%.Runoff from small peatland watersheds. J. Hydrol., 9: 90- 102. Brown, A., Marthur, S.P. and Kushner, D.J., 1990. An ombrotrophic bog as a methane reservoir. Global Biogeochem. Cycles, 3: 205-213. Buttle~, A.J., Dinel, H., Levesque. M. and Mathur, S.P., 1991. The relation between movement of surface water and gaseous methane in a basin bog with a novel instrument. Can. J. Soil Sci.. 71: 427-438. Charman, D.J., Aravena, R. and Warner, B.G., 1992. Isotope geochemistry of gas and water samples from deep peats in boreal Canada. SUO, 43: 199-201. Chm. D.J., Warner. B.G. and Aravena, R., 1994. Carbon dynamics in a forested peatland in north-eastern Ontario. Canada. J. Ecol., 82: 55-62. Dalva. M. and Moore, T.R.. 1991. Sources and sinks of dissolved organic carbon in a forested swamp catchment. Biogeochemistry, 15: 1-19. Freeze, R.A. and Cheny, J.A., 1979. Groundwater. Prentice-Hall, Englewood Cliffs, NJ, 604 pp. Gorhar% E., 1991. Northern peatlands: role in the carbon cycle and probable responses to climate warming. Bcol. Appl., I: 182-195. Gorham, E. and Hofstetter, R.H., 1971. Penetration of bog peats and lake sediments by tritium from atmospheric fallout. Ecology, 52: 898-902. Ingram, H.A.P., 1983. Hydrology. In: A.J.P. Gore (Editor), Ecosystems of the World, Vol. 4A, Mires: Swamp, Bog, Fen, and Moor. Elsevier, Amsterdam, pp. 67-158. Ivanov, K.E., 1981. Water Movement of Mirelands (translated from Russian by A. Thompson and H.A.P. Ingram). Academic Press, London. Koprivnjak, J.-F. and Moore, T.R., 1992. Sources, sinks, and fluxes of dissolved organic carbon in subarctic fen catchments.
Arct. Alp. R~.s., 24: 204-210.
138
J.M. Waddington, NT. RouletLJoumal of Hydrology 191(1997)
122-138
Mikkehi. C., Sundh. I., Eilertsson, J., Svensson, B.H. and Nilsson, M., 1992. Methane emissions from a Swedish peatland area: temporal and spatial variation. In: Proc. 9th Int. Peat Congr., Vol. 3. International Peat Society, Uppsala Sweden, pp. 152-165. Moore, T.R., 1987. Patterns of dissolved organic matter in subarctic peatlands. Earth Surf. Processes Landforms, 12: 387-397. Moore, T.R. and Jackson, R.J., 1989. Dynamics of dissolved organic carbon in forested and disturbed catchments, Westland, New Zealand. 2. Larry River. Water Resour. Res., 25: 1331-1339. Moore, T.R., Roulet, N.T. and Knowles, R., 1990. Spatial and temporal variability and environmental controls of CH4 flux from subarctic fens. Global Biogeochem. Cycles, 4: 29-46. Mulholhutd. P.J. and Kuenzler. E.J., 1979. Organic carbon export from upland and forested-wetland watersheds. Limnol. Gceanogr., 24: 960-966. Nilsson, M., 1992. Fungi and bacteria in peat and peat forming plant communities, PhD Dissertation, Swedish University of Agricultural Sciences, Urn&. Nilsson, M. and Bohlin, E., 1993. Methane and carbon dioxide concentrations in bogs and fens-with special reference to the effects of the botanical composition of the peat. J. Ecol., 81: 615-625. Price, J.S. and Maloney, D.A., 1996. Hydrology of a patterned bog-fen complex in southeastern Labrador, Canada. Nord. Hydrol., in press. Price, J.S. and Woo, M.K.. 1988. Studies of a subarctic coastal marsh, I. Hydrology. J. Hydrol., 103: 275-292. Quinton. W.L., 1991. The hydrology of a subarctic patterned wetland. M.Sc. Thesis, Department of Geography, York University, North York, Ont.. 140 pp. Romanowicz, E.A., Siegel, D.I. and Glaser. P.H., 1993. Hydraulic reversals and episodic methane emissions during drought cycles in mires. Geology, 21: 231-234. Ruuhijtirvi, R., 1960. Faber die regionale Einteilung der nordfinnische Moore. Ann. Sot. Zool. Bot. Fenn. Vanamo, 31: l-360. Siegel, D.I. and Glaser, P.H., 1987. Groundwater flow in a bog-fen complex, Lost River peatland, nordiern Minnesota. J. Ecol., 75: 743-754. Sjors, H., 1980. Peat on earthz multiple use or conservation? Ambio, 9: 303-308. Swanson, D.K. and Grigal, D.F., 1988. A simulation model of mire patterning. Gikos, 53: 309-314. T6th, J., 1%3. A theoretical analysis of ground-water flow in small drainage basins. J. Geophys. Res., 68: 47954812. Urban, N.R., Bayley. S.E. and Eisenreich, S.J., 1989. Export of dissolved organic carbon and acidity from peatlands. Water Resour. Res., 25: 1619-1628. Verry. E.S., Brooks, K.N. and Batten, P.K., 1988. Streamflow response from an ombrotrophic mire. In: Proc. Int. Symp. Hydrology of Wetlands in Temperate and Cold Regions, Vol. 1. Academy of Finland, Helsinki, pp. 5259. Virta, J., 1966. Measurement of evapotranspiration and computation of water budget in treeless peatlands in the natural state. Phys. Math. Fenn.. 32: l-69. Waddington, J.M. and Roulet, N.T., 1996. Atmosphere-wetland carbon exchanges: scale dependency of CO2 and CH, exchange on the developmental topography of a peatland. Global Biogeochem. Cycles, 10: 233-245. Williams, R.T. and Crawford, R.L., 1984. Methane production in Minnesota peatlands. Appl. Environ. Microbiol., 47: 1266-1271. Windsor, J., Mooore, T.R. and Roulet, N.T., 1992. Episodic fluxes of methane from subarctic fens. Cart. J. Soil Sci., 72: 441-452. Zohai, S.C., Taylor, S., Jeglum, J.K., Mills, G.F. and Johnson, J.D., 1988. Wetlands of boreal Canada. In: Wetlands of Canada. Ecological Classification Series, No. 24. Sustainable Development Branch, Environment Canada and Polyscience Publications, Montmal, Que.
ELSEVIER
Journal of Hydrology
191 (1997) 122-138
Groundwater flow and dissolved carbon movement in a boreal peatland J.M. Waddingtonlwa, N.T. Rouletb ‘Department OfGeography, York University, 4700 Keele street, North York, Ont. M3J IPZ, Canada bDepartmentof Geography. McGill University and the Centre for Climate and Global Change Research, 805 Sherbrooke Street W.. MontrpOl, Que. H3A 2K6, Canada Received 4 February
1995; revised 28 December
1995; accepted
9 March 1996
Measurement of the groundwater flux and the consequent advection of dissolved carbon (DOC (dissolved organic carbon), CH4 and CO*) were made in a boreal peatland in northern Sweden in summer 1993. The early summer gradients in hydraulic head indicated a downward flux of water in the peatland, but after a persistent mid-summer dry period the gradients changed to produce an outward radial flow from the centre to the margins of the peatland. This shift in gradients corresponded to the removal of some of CH., and CO* dissolved in groundwater. The concentrations of dissolved CH, and COr were spatially variable. with the highest observed in the centre of the peatland. In contrast, DOC concentrations were relatively constant near the centre of the peatland, and switched in the margin from being the highest during dry periods to lowest during wet periods. The changes in concentration of CH4, COr, and DOC are a function of the seasonal patterns of production and consumption, but groundwater flow is significant in the redistribution of dissolved carbon and the spatial patterns of concentration. The calculated mass flux of dissolved carbon was significant: it was approximately 20% of annual CO* fixation. 8 1997 Elsevier Science B.V.
1. Introduction Many wetlands in the high latitudes are peatlands because the complete decomposition of organic material. Peatlands lo6 ha (Sjiks, 1980) and are most common in the boreal and peatlands store an estimated 455 Gt of carbon or one-third
’Resent address: 4K1, Canada. 0022-1694/97/$17.00
BWrtment of Geography,
McMaster
Unversity,
1280 Main Street W., Hamilton,
8 1997- Elsevier Science B.V. All rights mx%ved
PIf SOO22-1694(96)03075-2
their wet conditions inhibit cover an estimated 500 x subarctic regions. Northern of the world’s soil carbon
ht.
L8S
J.M. Waddington, N.T. Roulet/Joumal of Hydrology 191 (1997) 122-138
123
(Gorham, 1991). Although they are believed to represent a net sink of carbon, they exchange a large amount of carbon to the atmosphere (CO*, CH,), and via water flow (DOC (dissolved organic carbon), CH4, COZ) to adjacent ecosystems. With drier conditions or drainage, the stored carbon in peatlands can become mineralized and lost to the atmosphere through oxidation (Armentano and Menges, 1986), whereas under wetter conditions, CH4 emission (Moore et al., 1990) and leaching of DOC (Moore, 1987; Koprivnjak and Moore, 1992) are major losses of carbon. The fluxes of dissolved carbon are directly and indirectly controlled by the hydrology of the peatland. This paper examines the hydrology of a boreal peatland and investigates the role of hydrology in the movement of dissolved carbon (DOC, CHJ, COZ) in the peatland. This water-borne flux of dissolved carbon is important in the determination of the peatland carbon budget. DOC export from peatlands is a function of DOC production and the hydrologic pathways by which water flows in the peatland and adjacent upland regions (Koprivnjak and Moore, 1992). AMU~ DOC export from peatlands is between 2 and 40 g mm2(Mulholland and Kuenzler, 1979; Moore, 1987; Moore and Jackson, 1989; Urban et al., 1989; Dalva and Moore, 1991). Koprivnjak and Moore (1992) found that DOC in streams draining subarctic catchments in the summer was positively correlated with the ratio of the area of the peatland to the area of the catchment. Many northern peatlands, however, lack welldefined streams, so flow is usually very diffuse (Price and Woo, 1988). The spatial and temporal variability of DOC concentrations within peatlands is large (Moore, 1987). Fringe areas of peatlands that are assumed to be influenced by groundwater and/or upland runoff may have higher nutrient levels which can enhance carbon remineralization and increase pore-water DOC levels. Pore-water concentrations of DOC generally increase in the summer because of higher evapotranspiration rates and plant tissue decomposition (Moore, 1987), but pore-water dilution can occur during large summer and autumn rainfall events. ‘Ihe spatial and temporal variability of dissolved CH4 and CO2 is also large. Numerous studies (e.g. Moore et al., 1990; Buttler et al., 1991; Nilsson and Bohlin, 1993) have indicated high concentrations of dissolved CO2 and CHJ at depth in peatlands. In general, dissolved concentrations increase with depth, reflecting differences in production and consumption, the importance of gas transport and peat characteristics (Nilsson and Bohlin, 1993), and the possible effects of surface dilution from lateral flow and precipitation. However, decreasing gas concentrations near the bottom of some profiles (Williams and Crawford, 1984; Brown et al., 1990; Nilsson and Bohlin, 1993) and younger radiocarbon dates of dissolved CO2 in deeper peat than shallow peat (Charman et al., 1992, 1994; Aravena et al., 1993) indicate an additional groundwater influence in the redistribution of gases. Changes in the lateral and vertical movement of water during the season should, therefore, have a major influence on the redistribution of dissolved CH, and CO2 in a peatland, as is the case for DOC. Lateral water movement in peatlands is controlled by the position of the water table (Bay, 1969), the slope of the peatland (Ivanov, 1981), and the hydraulic properties of the peat (Ivanov, 1981). The flux of water is greatest during periods of high water table because the upper layers of the peat profile have the greatest hydraulic conductivities (Ivanov, 1981; Verry et al., 1988). Lateral flow in peatlands having surface microtopographic (hummocks and hollows) and mesotopographic (ridges, pools, lawns) features
124
J.M. Waddington, N.T. RouletlJoumal
ofHydrology191(1997) 122-138
is impeded by lower hummock and ridge hydraulic conductivities (Swanson and Grigal,
1988; Quinton, 1991). Although it has generally been assumed, based on low hydraulic conductivity (Ingram, 1983) and pore-water chemistry (Gorham and Hofstetter, 1971). that the dominant direction of water flow in peatlands is lateral, there is increasing evidence (hydraulic heads and isotope geochemistry) suggesting that vertical groundwater flow can be significant (Siegel and Glaser, 1987; Charman et al., 1992; Aravena et al., 1993). There have been no studies that have systematically examined the redistribution of dissolved CO* or CH4 via groundwater flow in peatlands. In this study, a boreal patterned peatland was instrumented to examine the role of groundwater hydrology in the redistribution of dissolved carbon in northern peatlands. The objectives of this study were to: (1) determine the change in the lateral and vertical flux of water during the summer season; (2) determine the spatial and temporal variability of dissolved carbon storage and its movement; (3) examine the role of hydrologic pathways in the transport and redistribution of dissolved carbon.
2. Study area The study peatland, Stor-Amyran, is located 15 km south of UmeH, Vbterbotten, Sweden (63”44’N, 20’06’E) near the coast of the Gulf of Bothnia. Star-Amyran is a concentric raised bog, typical of peatlands in central and northern Sweden (Mikkell et al., 1992). central and northern Finland (Ruuhij&vi,01960) and the boreal and subarctic regions of North America (Zoltai et al., 1988). Star-Amyran is a Sphagnum-dominated, raised, ombrogenic peatland that developed around a creek that flowed into the Gulf of Bothnia (Nilsson, 1992). Stratigraphy indicates that Star-Amyran was originally a soligenie peatland (Nilsson, 1992), but today only a small soligenic zone exists where water flows into the wetland from uplands. The peatland is dominated by a 12 ha ombrogenic feature in the centre (Fig. 1) that rises 1.4 m above the surrounding peatland. Tbe ombrogenie feature comprises a series of ridges, open water pools, and vegetated lawns and has no surface inflow or outflow. The ridges are dominated by Sphagnum fuscum, Calluna vulgaris, Andromeda polifolia, Rubus chamaemorus, and Ledurn palustre, whereas the lawns are dominated by S. majus or S. balticum and Eriophorum vaginatum (Mikkelii et al., 1992). Pools vary in area and depth but are generally deeper than 1 m in the western area and are 25 cm deep at the eastern margin. The ombrogenic feature of the peatland can be divided into four areas based on topography: a floating mat, the hinge (identified as the boundary between the concave and convex region of the raised ombrogenic area), a plateau, and finally the eastern peatland margin (Fig. 1). Peat depths range from 2.5 m at the eastern margin to over 4 m at the plateau. The peat is underlain by a silty clay.
3. Methods The research was conducted between 2 1 May and 13 September 1993 and was designed to exam$e the role hydrologic pathways have in the transport of DOC, CO*, and CH4 in the Stor-Amyran raised ombrogenic area. Four locations on a 300 m transect through the
J.M. Waddington, N.T. Rouler/Joumal
of Hydrology 191 (1997) 122-138
125
Fig. 1. Sampling locations along the 300 m transect(A-A’) in the Star-&ran peatland.The inset illustratesthe location of the instrumentsat each sampling location. Thin lines indicate the mesotopographicridges.
centre of the ombrogenic feature were instrumented for measurements of water table position, peat surface level, and hydraulic head (Fig. 1). Precipitation was monitored continuously using a tipping bucket rain gauge at the floating mat site. Measurement of the properties of the peat were made at three mesotopographic features (ridge, pool, and lawn landfotms) at each of the four locations. At each site, samples of DOC and dissolved CH4 and CO* were obtained to determine the spatial variability and flux of dissolved carbon throughout the study period. 3.1. Groundwater jZu.x Groundwater flow nets were constructed using hydraulic head data obtained from a series of groundwater wells and piezometers installed along the transect. Piezometers, made from 1.5 cm ID PVC pipe screened over a 10 cm interval, were installed to depths of 0.25,0.50,0.75, 1.O, 2.0, and 3.0 m. Water levels within the piezometers were monitored weekly, and water table and peat surface level were monitored using potentiometric water level recorders. All water level sensors and a tipping bucket rain gauge were read every
126
J.M. Waddingron, N.T. Roulet/Joumalof Hydrology191 (1997) 122-138
minute and averaged every half-hour by a datalogger. Hydraulic conductivity was determined for every piezometer by the Hvorslev water level recovery method (Freeze and Cherry, 1979). The groundwater flux across each sampling location was calculated from the weekly flow nets using Darcy’s law. Groundwater fluxes were converted to an equivalent depth of runoff by dividing the volumetric flux by the area at each location. Areas, determined from air photographs and a survey of the peatland, were 59 800 m*, 25 500 m*, 21660 m*, and 59000 m* for the floating mat, hinge, plateau, and peatland margin locations, respectively. 3.2. Dissolved carbon sampling and analysis Profiles of dissolved CH4 and CO2 were measured every 7-10 days. Water samples were taken every 2.5 cm in the top 15-25 cm of the peat and then at various intervals to a depth of 3 m. Water samples were withdrawn using a stainless steel tube fitted with a series of stopcocks and connected to a 10 ml plastic syringe. After the water sample was withdrawn an equal amount of air was drawn into the syringe and the water was degassed into the head space by shaking vigorously for 2 mm. The CH4 concentration in the head space was determined on a Perkin-Elmer (Norwalk, CT, USA) gas chromatograph equipped with a Porapaq Q (Millipore Water Chromographic, Milford, MA, USA) column and a flame ionization detector (FJD). Calibration gases of nominally 12, 103,1000,5000, and 10000 ppmv were used depending on the concentration range of the samples being analysed. The CO2 head space concentration was measured by a Perk&Elmer gas chromatograph equipped with a Porapaq T column and a thermal conductivity detector (TCD). Calibration gases of 500,1000,10 000, and 20 000 ppmv were used. Sampling for DGC was conducted at all sites once every month. Water was withdrawn from the groundwater wells and piezometers using a hand pump. Samples were filtered (0.45 pm), stored at 4’C and sent to McGill University (Montreal, Quebec) at the end of the field season, where they were analysed on a Shimadzu (Kyoto, Japan) TGC 5050 analyser. 3.3. Calculations of the mass flux of dissolved carbon To determine the significance of groundwater transport of dissolved carbon it is necessary to compute the mass flux of carbon from one portion of the peatland to another. The peatland has been divided into four zones (see above) based on position and topography. The mass flux of groundwater from one zone to another was computed assuming the boundaries between zones extended perpendicularly from the surface to the base of the peatland. The width of the flow zone was the width ofthe peatland at this point. This discharge was converted into a unit area depth of subsurface runoff by dividing by the area of the zone that the groundwater is from. Conversely, this same discharge can be expressed as a depth of subsurface runoff received if the same discharge is divided by the area of the zone that receives the groundwater. The mass flux of dissolved carbon is computed by multiplying the zonal discharge by the sum of the integrated concentrations of DGC, and dissolved CO2 and CH.+ The mass flux of carbon, either loss or gain, is converted into a unit area by dividing the mass flux by the area of the zone that contributes or receives the flux. This is done so the water-borne flux of carbon can be compared with the exchange of carbon across.
J.M. Waddington,N.T. Roulet/Joumalof HydrologyI91 (1997) 122-138
127
Table I Hydraulicconductivity (cm s-‘) at floating mat, binge, plateau, and margin sites Site within a location
Pleating mat
Hinge
Lawn (25 cm) Lawn (50 cm) Lawn (100 cm) Lawn (200 cm) Ridge (25 cm) Ridge (50 cm) Ridge (100 cm) Ridge (200 cm) Pool (25 cm) Pool (50 cm) Pool (100 cm) Pool (200 cm) Pool (300 cm)
1.1 x 10-Z 6.4 x 10” 6.2 x lOa 2.4 x 10” 7.5 x lo4 2.8 x 10” 5.7 x 10” 3.7 x 104 4.5 x 10-3 5.6 x 1O-3 3.1 x 104 2.0 x 104 1.5 x 10”
8.1 x 1.5 x 1.4 x 6.3 x 1.7 x 2.4 x 1.1 x 3.3 x
lo-’ 10-l 10-3 1OA 10-2 10” lo+ 104
Plateau
Margin
n.a. 1.3 x 1.2 x 6.8 x 5.2 x 3.3 x 1.9 x 2.1 x
1.5 x 8.6 x 8.3 x 1.5 x 1.0 x 3.6 x 7.5 x 4.9 x 6.0 x 7.9 x 4.2 x 2.7 x 1.9 x
10-I 10” 10” 1O-3 10” 104 10-4
10-2 10” lo+ IO” 10-3 10” lo4 104 10” 10” lo4 lo4 IO”
n.a., Not applicable.
4. Results 4. I. General hydrology The peat hydraulic conductivity in Stor-finyran ranged from 2 x lo-’ to 8 x 10” m s-’ (Table 1). The hydraulic conductivity of the mesotopographic ridges was approximately l-2 orders of magnitude lower than the hydraulic conductivity of adjacent lawn peat at similar depths. The 30 year average precipitation $or the length of the study period at the UmeH airport, approximately 13 km north of Stor-Amyran, was 219 2 51 mm, whereas total precipitation during the 1993 study period was 257 mm. Precipitation events were not evenly distributed during the study period (Fig. 2(a)): much of June and July experienced no or little precipitation, whereas late May and August were characterized by higher precipitation. The temporal variation in precipitation is reflected in the position of the water table (Fig. 2(b)). The periods representing dry and wet moisture conditions are arbitrarily defined by the mean water table position at the margin lawn site: dry conditions (water table less than -1 cm depth at the margin lawn) and wet conditions (water table -1 cm or greater at the margin lawn). During wet periods, the water table rose to a maximum water table height of 4 cm above the peat (Fig. 2(b)), and overland flow was observed in individual lawn segments, but the water table remained below the lowest points of the adjacent ridges, causing water to pond on the upslope edge of the ridges. Consequently, the differences in hydraulic conductivity between ridges-hummocks and lawns-hollows and the prevalence of the me~otopographic ridge features eliminated surface flow during the summer months in Stor-Amyran. Furthermore, the ridges represent recharge zones that cause flow to diverge outwards, further reducing the possibility of surface flow. Accordingly, pundwater flow dominated the lateral flow in the peatland during summer months at Stor-Amyran.
128
J.M. Waddington. N.T. Roulet/Joumal of Hydrology 191 (1997) 122-138
(JWW)
UO!W@!=Jd
J.M. Wddington, N.T. Roulet/loumal of Hydrology 191 (1997) 122-138
0
100 ~
(ml
129
200
Fig. 3. Groundwater flow net for the cross-section A-A’ during (a) wet conditions (water table -1 cm or greater) and (b) dry conditions (water table less than -1 cm).
4.2. Groundwater jhx
The hydraulic gradients and flow direction in the raised ombrogenic feature changed between dry and wet periods (Fig. 3). Groundwater flow during wet periods (61% of the summer period) was from a recharge area in the plateau towards the margin and floating mat locations. A greater hydraulic gradient occurred adjacent to the pools at the margin of the raised peatland (Fig. 3(a)). A stagnation point formed under the top of the plateau during the period of sustained water table drawdown (Fig. 3(b)), creating an outward radial flow centred around a depth of 2 m. Combining the flow nets with the hydraulic conductivities, the plateau site lost 3250 m3 of groundwater during the study period (143 mm towards the hinge and 157 mm towards the margin), whereas the hinge and floating mat gained 3800 m3 and 4280 m3 (149 mm and 72 mm), respectively (Table 2). The groundwater flux was greatest at the peatland
130
J.M. Waddington,N.T. Roulet/Joumal of Hydrology191 (1997) 122-138
Fig. 4. Cross-sectionalong the A-A’ transectillustratingthe mean concentrationdistributionof DOC during (a) wet and (b) dry conditions. (Concentrationsare in mg 1-l; isopleths are 2 mg I-’ increments.)
margin because of higher hydraulic conductivity and hydraulic gradient. Here, the seasonal groundwater flux was 9440 m3 (160 mm) and the subsurface runoff varied from 1.0 to 1.7 mm day-’ over the period of study. Similarly, the daily subsurface runoff into the floating mat and hinge sites varied over the study period from 0.5 to 0.8 mm day-’ and from 1.1 to 1.8 mm day-‘, respectively. 4.3. Concentrations of DOC, dissolved CH,, and dissolved CO2
DOC concentrations were slightly higher at two sites and slightly lower at the other hvo sites (Fig. 4) between dry and wet conditions. The pattern of concentrations of dissolved CO2 and dissolved CH4 showed distinct differences between dry and wet conditions: dissolved COz (Fig. 5) and CH4 (Fig. 6) concentrations were both higher under wet conditions than dry conditions. DOC concentrations ranged from 21.0 to 29.0 mg 1-l in the peatland, with the lowest mean seasonal concentrations at the plateau site (22.2 mg 1-t). The highest mean seasonal
J.M. Waddington, N.T. RouletLJownal of Hydrology 191 (1997) 122-138
6
100
mmnc0 (ml
26s
666
Fig. 5. Cross-section along the A-A’ transect illustrating the mean concentration distribution of dissolved CO2 during (a) wet and (b) dry conditions. (Concentrations are in mg 1-l; isopleths are 5 mg 1-l increments.)
concentrations were at the floating mat site (27.3 mg 1-l). The concentration of DOC with depth changed during the season (Fig. 4). During a dry period in late July, DOC concentrations in the surface peat were 6-8 mg 1-l higher than in the peat at a depth greater than 2 m (Fig. 4(a)), whereas during a wet period surface concentrations were 6-8 mg 1-t lower than DOC at depth (Fig. 4(b)). This indicates that the concentrations are highest at the surface during dry periods. The DOC concentration in the surface layer at the plateau site did not change significantly between wet and dry periods. Dissolved CO* concentrations also showed different patterns between the dry and wet periods (Fig. 5). During the dry period, concentrations were greatest below the hinge and plateau sites (maximum 30 mg 1-l. 682 PM), whereas during the wet periods maximum concentrations were approximately 50 mg 1-l (1136 PM). With the exception of these locations, concentrations generally increased from 5-8 mg 1-l (100-200 PM) near the surface to over 12 mg 1-l (273 PM) at depth (Fig. 5). The cross-sectional profiles of dissolved CHI concentrations (Fig. 6) indicate higher concentrations during wet periods. During this time maximum dissolved CH4
132
J.M. Woddngton, N.T. RouletLJoumal
of Hydrology
191 (1997)
122-138
Table 2 Concentrationsand mass flux of HOC, dissolved CH,, and CO*, based on sampling from 21 May to 13 September 1993 Mean seasonal concentrations
Groundwater Plux (m’)
Runoff (mm)
Mat
4280
72
Hinge
3800
149
Plateau (west) - 1540
- 143
Plateau (east)
- 1710
- 157
!WO
160
Margin
DOC (mg 1-l) 27.3 (29.0,25.7) 24.3 (24.4, 24.2) 22.2 (21.0, 23.4) 22.2 (21.0, 23.4) 23.7 (23.4, 24.1)
CH4
02
(mg 1-Q
(mg 1-V
3.0 (1.2.3.9)
10.4 (4.5, 15.0) 14.8 (11.0,22.1) 12.3 (6.0, 17.5) 12.3 (6.0, 17.5) 10.5 (9.1, 11.9)
:678, 3.0) 1.6 (0.9, 2.7) 1.6 (0.9, 2.7) 2.9 (1.5.3.7)
Mass flux (mg C m-* day-‘) 18.4 40.1 - 27.9 - 30.5 41.8
Numbersin parenthesesreferto mean values during dry and wet conditions. Individual component fluxes are in the mass of the compound, whereas the mass flux is calculated in grams of carbon from weekly volumeweighted concentrationsand weekly groundwaterfluxes.
concentrations were about three times greater than maximum concentrations during dry periods (Table 2). Furthermore, dissolved CH4 concentrations during this period were between 15 and 30% of dissolved COr, which represents ahnost the same amount of carbon. Highest dissolved concentrations are located below the two pools and near the stagnation points below the hinge and plateau (Fig. 6). Like dissolved CO*, dissolved CH4 concentrations generally increased two- to three-fold from the surface to the bottom of the peat. 4.4. Mass flux of dissolved carbon and change in storage The product of the weekly groundwater flux and volume-weighted DOC, dissolved CH4, and dissolved CO2 concentrations equals the water-borne mass flux of carbon in the peatland. The floating mat, hinge and margin sites received an input of carbon from the plateau region, whereas the plateau site lost carbon during the study period (Table 2). Over 27 mg C rn-’ day-’ was lost from the plateau site, whereas inputs of carbon to the hinge and margin sites were over 40 mg C rn-’ day-‘. Total input of dissolved carbon to the floating mat site was slightly less than half the input to the hinge site. DOC accounted for between 82 and 84% of the mass flux of water-borne carbon at the sites. The proportion of dissolved CO* in the mass flux of carbon at the hinge and plateau sites (13.2% and 12.4%, respectively) was slightly higher than the proportion at the floating mat and margin sites (8.7% and 9.996, respectively). The opposite trend held for dissolved CH4, where dissolved CH4 accounted for approximately 4% and 7% of the mass flux for the middle (hinge and plateau) and edge (floating mat and margin) sites, respectively. The mean peatland storage of dissolved carbon during wet and dry periods was
133
J.M. Waddington, N.T. Rovlet/Joumal of Hydrology 191 (1997) 122-138
0
100
Dbtaneo (m)
200
3m
Fig. 6. Cross-section along the A-A’ transect illustrating the mean concentration of dissolved CH, during (a) wet and (b) dry conditions. (Concentrations are in mg I-‘; isopleths are 0.5 mg I-’ increments.)
calculated using the mean volume-weighted dissolved concentrations and assuming a mean peat porosity of 0.8 (Table 3). Total dissolved storage was 13% lower during dry conditions than wet conditions. DOC storage was 1% higher during dry conditions than wet conditions, whereas dissolved CH4 and CO2 storage were 68% and 56% lower during dry periods than during wet periods, respectively. 5. Discussion
Like many patterned peatlands, Stor-Amyran lacks a well-defined drainage network and consequently overland flow is diffuse. The presence of micro- and mesotopography impedes overland flow because the hydraulic conductivity of the hummocks and ridges is much lower than that of the hollows and lawns. Furtbettnore, persistent water table mounds in the ridges further impede overland flow and have been shown to minimize poolto-pool water transfer in a Labrador bog (Price and Maloney, 19%). During wet periods, this results in the ponding of water on the upslope edge of the ridge, which maintains a water table at the peat surface and in some areas sustains pools. Some of this ponded water is lost to evaporation, but much of the water also moves as groundwater flow to the
134
J.M. Waddington, N.T. Roulet/Joumal of Hydrology 191(1997)
122-138
Table 3 Mean peatland storage of DOC, dissolved CH,, and dissolved CO*. during wet and dry conditions from 2 1 May to 13 September 1993
Dot ‘334 co2
Total
Dry (g C m-*)
Wet (g C m-*)
284.8 9.2 23.8 317.8
282.8 28.6 54.2 365.6
AS (g C m-*) 2.0 - 19.4 - 30.4 - 47.8
AS indicates the difference in mean storage between wet and dry conditions. Negative numbers refer to a loss in dissolved carbon storage under dry conditions.
perimeter of the peatland. Consequently, the traditional view of overland flow as the dominant hydrologic flow path in peatlands (Ingram, 1983) is unsuitable for assessing the overall flux of water in this common type of northern peatland. Rather, determination of the groundwater flux is needed to calculate the flux of water in a peatland of this type. The total subsurface runoff at the plateau site was 143 mm towards the hinge and 157 mm towards the margin, approximately equalling the flux inputs to the hinge (149 mm) and margin (160 mm) sites. This indicates that the increase in groundwater flux at the hinge and margin sites from the plateau can be attributed to the increase in the source area from the central region of the peatland to the perimeter. The drop in the depth of groundwater flow between the hinge and floating mat sites is probably in response to the change in the hydrologic flow paths during dry conditions (Fig. 3(b)). During dry conditions, water discharges part way between the hinge and floating mats in a region marked by numerous small pools. Additional water recharges in this zone at the floating mat ridge and flows towards the floating mat region. The depth of flow at the floating mat site is only 50% of that at the hinge site, but total groundwater flow is slightly higher, suggesting that the discharge and recharge between the two sites are roughly offset. The total rainfall during the study period was 257 mm, whereas the depth of groundwater flow entering the hinge and margin sites was 149 mm and 160 mm, respectively. Consequently, an amount equivalent to approximately 60% of the depth of rainfall incident upon the ombrogenic feature leaves the margins of the ombrogenic feature during the summer months as groundwater flow. An amount equivalent to 40% of rainfall is lost as evaporation. Because the initial and final water table positions during the study are fairly similar, the net storage change over the study period is small. This translates into a mean daily evaporation rate of approximately 0.9 mm, which is in the range of that observed for other northern bogs (Virta, 1966). 5.1. Groundwaterflow
reversal
A reversal in the direction of the groundwater flow occurred below the plateau during dry conditions. Reversals of hydraulic head and consequent losses of CH4 have been documented during drought conditions in the Glacial Lake Agassiz Peatlands in northern Minnesota (Romanowicz et al., 1993). Unlike those reversals, which were caused by changes in the proportion of the regional and local discharge systems, however, the
J.M. Waddington, N.T. RouletLJoumal
of Hydrobgy
191 (1997) 122-138
135
reversal in this study was driven by internal mechanisms. During dry periods, evaporation and drainage to the margins cause the water table at the plateau lawn to drop several centimetres below the surface. The drop in the water table alters the thickness of the peatland aquifer and results in a greater influence of local flow systems (T&h, 1963). Undet these conditions, the surface topography has less control on the groundwater flow in Stor-Amyran. The upward flow of water in the peatland, during dry conditions, is also important for the redistribution of dissolved carbon in the peatland, as it augments the diffusive transport of gases in the peatland (Romanowicz et al., 1993) and also releases the normal downward pressure that inhibits the release of CO2 and CH4 in bubbles (e.g. Moore et al., 1990; Windsor et al., 1992). Evidence for these mechanisms is substantiated by the existence of an episodic release of CH4 during the reversal in groundwater flow direction. The episodic release of CH4 resulted in a large negative change in stored CH4 and CO2 (Table 3). CH4 flux from the plateau lawn, measured using static enclosures, increased from 2.5 mg ms day-’ the week before the reversal to over 58 mg mm2day-’ during the reversal (Waddington and Roulet, 1996). The flux of CH4 then returned to 2.0 mg mm2day-’ the following week, even though the flow reversal persisted. Large CH4 losses also occurred during groundwater flow reversals in the Glacial Lake Agassiz Peatlands (Romanowicz et al., 1993). The release of large CH4 storage during drought conditions in peatlands indicates that groundwater flow reversals, although important in the net transport of dissolved carbon in peatlands, are also an important mechanism for understanding the carbon balance of peatlands and the air-water exchange of carbon in peatlands.
flow
5.2. Net transport Analysis of the net transport of dissolved carbon in Stor-Amyran allows for a comparison between the controls of groundwater flow and production of the movement of carbon in the peatland. The mean daily dissolved carbon mass flux from the plateau towards the hinge and margin was -27.9 mg C mm2day-’ and -30.5 mg C mm2day-’ (Table 2), which represents 70% and 73% of the mean daily mass flux entering the hinge and margin sites, respectively. There is an increase in the transport of dissolved carbon from the plateau to the margins of the peatland. The net increase in carbon to the hinge and margin sites is 12.2 mg C rnw2day-’ and 11.3 mg C me2 day-‘, respectively. This increase in carbon flux can be attributed to both an increase in the rate of production of DOC, CH4, and CO2 and the groundwater flow along the hydrologic pathway between the recharge and discharge sites. Because the depth of flow increases only slightly from the plateau to the hinge and margin sites (Table 1). however, much of the increase in mass flux must be attributed to an increase in the in situ production. Conversely, the decrease in mass flux between the hinge and floating mat sites is attributed to a dominant change in the groundwater flow. The mean daily mass flux of carbon at the floating mat site is only 45% of the mass flux at the hinge location, whereas relative to the total flux, only a 7% increase exists at the floating mat site compared with the hinge location. Because mean DOC, dissolved C02, and dissolved CH4 concentrations are generally higher at the floating mat site relative to the hinge location, the decrease in net transport is attributed to the decrease in the depth of
136
J.M. Waddington.N.T. Rouler/Joumal
ofHydrology191(1997)
122-138
groundwater flow between the two sites (Table 2). This decrease is due to the seasonal formation of the small discharge zone between the two sites. 5.3. Redistribution of dissolved carbon Concentrations of DOC, dissolved COZ, and dissolved CH4 change between dry and wet conditions within the peatland. Increased dissolved CO2 and dissolved CH4 below the hinge and plateau appear to be related to the existence of a groundwater flow stagnation point and the result of the groundwater flow reversal. The change in DOC concentrations probably occurs in response to changes both in evaporation and precipitation (Koprivnjak and Moore, 1992) and changes in the hydrologic pathway. Under dry conditions high surface concentrations are likely to be accentuated by evaporation, whereas under wet conditions dilution from precipitation decreases the surface concentration. Changes in the concentration of DOC at depth at the margin sites, however, is probably controlled by the changes in the flow pattern source of the groundwater flow at the centre of the peatland. 5.4. Implications for carbon cycling in peatlands The existence of a dominant local groundwater flow system and groundwater flow reversals in the peatland contradicts the idea that raised ombrogenic peatlands are recharge areas with negligible groundwater flow (Ingram, 1983). Using T&h’s conceptual model of groundwater flow as an analogy (T&h, 1963), it can be generalized that recharge areas of topographic highs (plateau and ridges) represent regions of carbon loss, whereas the topographic lows represent regions of carbon gains. Although it has been shown that changes in the net transport along this hydrologic pathway are likely to occur, the translocation of dissolved carbon has many implications regarding carbon cycling in peatlands. The translocation of carbon via groundwater is important for determining the carbon balance of a peatland. This study has shown that the change in dissolved carbon storage in a peatland is dynamic and subject to changes in the groundwater flux. Furthermore, this study has shown that the movement of dissolved carbon in the peatland can represent both a major loss (at the plateau location) and a major gain of carbon (floating mat and margin locations) to the carbon balance of different zones of the peatland. Comparing the mass flux of carbon via groundwater with the atmospheric fluxes of COZ and CH4 at the same sites (Waddington and Roulet, 1996) provides evidence of the importance of this component. At the plateau site the carbon loss via groundwater was ten times greater than the flux of CH4 to the atmosphere, whereas at the hinge and peatland margin sites the dissolved carbon input was approximately 1.l- 1.5 times greater than the atmospheric loss of CHb. Furthermote, the dissolved component input to the floating mat region was approximately 20% of the total carbon uptake by plants during the same time period. Consequently, groundwater flow and dissolved carbon movement are important in the exchange of carbon in boreal peatlands. The substrate available for methanogenesis has often been considered to be limited to the peat itself, which is often of poor quality (Nilsson and Bohlin, 1993). With the movement of younger substrate into different zones in a peatland (e.g. Aravena et al., 1993), however, there is the possibility of in situ methanogenesis and the further increase in the
J.M. Waddington. N.T. Roulet/Joumal of Hydrology 191 (1997) 122-138
137
concentration of methane. Although many studies of carbon fluxes have concentrated on characterizing the substrate quality for methanogenesis (Nilsson, 1!392), the transport of dissolved carbon (CO*, CH4, and DOC) represents another potential source of carbon that has not been generally considered in the cycling of carbon in peatlands.
Acknowledgements
We would like to thank C. Mikkell, G. Granberg, M. Nilsson, R. Swanson and I. Bergman for their assistance in the field and laboratory. We also thank J. Price, K. Devito, A. Hill and two anonymous reviewers for providing comments on an early draft of this manuscript. J.M.W. was supported by a National Sciences and Engineering Research Council of Canada (NSERC) postgraduate scholarship and the Martin Gelber International Studies Award, York University. The research was funded by an NSERC research grant.to N.T.R. Logistical assistance was provided by the Department of Forest Ecology, Swedish University of Agricultural Sciences, and the Department of Physical Geography, University of Umei.
References Aravena, R., Warner, B.G., Charman. D.I., Belyea, L.R., Mathur, S.P. and Dinel. H., 1993. Carbon isotopic composition of deep carbon gases in an ombrogenous peatland, northwestern Ontario, Canada. Radiocarbon, 35: 27 l-276. Armentano, T.V. and Menges, C.V., 1986. Patterns of change in the carbon balance of organic-soil wetlands of the temperate zone. 1. Ecol., 74: 755-774. Bay, R.R., I!%%.Runoff from small peatland watersheds. J. Hydrol., 9: 90- 102. Brown, A., Marthur, S.P. and Kushner, D.J., 1990. An ombrotrophic bog as a methane reservoir. Global Biogeochem. Cycles, 3: 205-213. Buttle~, A.J., Dinel, H., Levesque. M. and Mathur, S.P., 1991. The relation between movement of surface water and gaseous methane in a basin bog with a novel instrument. Can. J. Soil Sci.. 71: 427-438. Charman, D.J., Aravena, R. and Warner, B.G., 1992. Isotope geochemistry of gas and water samples from deep peats in boreal Canada. SUO, 43: 199-201. Chm. D.J., Warner. B.G. and Aravena, R., 1994. Carbon dynamics in a forested peatland in north-eastern Ontario. Canada. J. Ecol., 82: 55-62. Dalva. M. and Moore, T.R.. 1991. Sources and sinks of dissolved organic carbon in a forested swamp catchment. Biogeochemistry, 15: 1-19. Freeze, R.A. and Cheny, J.A., 1979. Groundwater. Prentice-Hall, Englewood Cliffs, NJ, 604 pp. Gorhar% E., 1991. Northern peatlands: role in the carbon cycle and probable responses to climate warming. Bcol. Appl., I: 182-195. Gorham, E. and Hofstetter, R.H., 1971. Penetration of bog peats and lake sediments by tritium from atmospheric fallout. Ecology, 52: 898-902. Ingram, H.A.P., 1983. Hydrology. In: A.J.P. Gore (Editor), Ecosystems of the World, Vol. 4A, Mires: Swamp, Bog, Fen, and Moor. Elsevier, Amsterdam, pp. 67-158. Ivanov, K.E., 1981. Water Movement of Mirelands (translated from Russian by A. Thompson and H.A.P. Ingram). Academic Press, London. Koprivnjak, J.-F. and Moore, T.R., 1992. Sources, sinks, and fluxes of dissolved organic carbon in subarctic fen catchments.
Arct. Alp. R~.s., 24: 204-210.
138
J.M. Waddington, NT. RouletLJoumal of Hydrology 191(1997)
122-138
Mikkehi. C., Sundh. I., Eilertsson, J., Svensson, B.H. and Nilsson, M., 1992. Methane emissions from a Swedish peatland area: temporal and spatial variation. In: Proc. 9th Int. Peat Congr., Vol. 3. International Peat Society, Uppsala Sweden, pp. 152-165. Moore, T.R., 1987. Patterns of dissolved organic matter in subarctic peatlands. Earth Surf. Processes Landforms, 12: 387-397. Moore, T.R. and Jackson, R.J., 1989. Dynamics of dissolved organic carbon in forested and disturbed catchments, Westland, New Zealand. 2. Larry River. Water Resour. Res., 25: 1331-1339. Moore, T.R., Roulet, N.T. and Knowles, R., 1990. Spatial and temporal variability and environmental controls of CH4 flux from subarctic fens. Global Biogeochem. Cycles, 4: 29-46. Mulholhutd. P.J. and Kuenzler. E.J., 1979. Organic carbon export from upland and forested-wetland watersheds. Limnol. Gceanogr., 24: 960-966. Nilsson, M., 1992. Fungi and bacteria in peat and peat forming plant communities, PhD Dissertation, Swedish University of Agricultural Sciences, Urn&. Nilsson, M. and Bohlin, E., 1993. Methane and carbon dioxide concentrations in bogs and fens-with special reference to the effects of the botanical composition of the peat. J. Ecol., 81: 615-625. Price, J.S. and Maloney, D.A., 1996. Hydrology of a patterned bog-fen complex in southeastern Labrador, Canada. Nord. Hydrol., in press. Price, J.S. and Woo, M.K.. 1988. Studies of a subarctic coastal marsh, I. Hydrology. J. Hydrol., 103: 275-292. Quinton. W.L., 1991. The hydrology of a subarctic patterned wetland. M.Sc. Thesis, Department of Geography, York University, North York, Ont.. 140 pp. Romanowicz, E.A., Siegel, D.I. and Glaser. P.H., 1993. Hydraulic reversals and episodic methane emissions during drought cycles in mires. Geology, 21: 231-234. Ruuhijtirvi, R., 1960. Faber die regionale Einteilung der nordfinnische Moore. Ann. Sot. Zool. Bot. Fenn. Vanamo, 31: l-360. Siegel, D.I. and Glaser, P.H., 1987. Groundwater flow in a bog-fen complex, Lost River peatland, nordiern Minnesota. J. Ecol., 75: 743-754. Sjors, H., 1980. Peat on earthz multiple use or conservation? Ambio, 9: 303-308. Swanson, D.K. and Grigal, D.F., 1988. A simulation model of mire patterning. Gikos, 53: 309-314. T6th, J., 1%3. A theoretical analysis of ground-water flow in small drainage basins. J. Geophys. Res., 68: 47954812. Urban, N.R., Bayley. S.E. and Eisenreich, S.J., 1989. Export of dissolved organic carbon and acidity from peatlands. Water Resour. Res., 25: 1619-1628. Verry. E.S., Brooks, K.N. and Batten, P.K., 1988. Streamflow response from an ombrotrophic mire. In: Proc. Int. Symp. Hydrology of Wetlands in Temperate and Cold Regions, Vol. 1. Academy of Finland, Helsinki, pp. 5259. Virta, J., 1966. Measurement of evapotranspiration and computation of water budget in treeless peatlands in the natural state. Phys. Math. Fenn.. 32: l-69. Waddington, J.M. and Roulet, N.T., 1996. Atmosphere-wetland carbon exchanges: scale dependency of CO2 and CH, exchange on the developmental topography of a peatland. Global Biogeochem. Cycles, 10: 233-245. Williams, R.T. and Crawford, R.L., 1984. Methane production in Minnesota peatlands. Appl. Environ. Microbiol., 47: 1266-1271. Windsor, J., Mooore, T.R. and Roulet, N.T., 1992. Episodic fluxes of methane from subarctic fens. Cart. J. Soil Sci., 72: 441-452. Zohai, S.C., Taylor, S., Jeglum, J.K., Mills, G.F. and Johnson, J.D., 1988. Wetlands of boreal Canada. In: Wetlands of Canada. Ecological Classification Series, No. 24. Sustainable Development Branch, Environment Canada and Polyscience Publications, Montmal, Que.