A comparison of particulate, dissolved and gaseous carbon in two contrasting upland streams in the UK

Journal of Hydrology 257 (2002) 226±246

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A comparison of particulate, dissolved and gaseous carbon in two contrasting upland streams in the UK J.J.C. Dawson a,*, M.F. Billett a, C. Neal b, S. Hill b a

Department of Plant and Soil Science, University of Aberdeen, Cruikshank Building, Aberdeen AB24 3UU, UK b Centre for Ecology and Hydrology, Wallingford, Oxfordshire OX10 8BB, UK Received 23 April 2001; revised 28 August 2001; accepted 12 October 2001

Abstract Concentrations and ¯uxes of particulate, dissolved and gaseous forms of hydrologic carbon were measured in two contrasting acidic upland streams in NE Scotland and Mid-Wales. Sampling was undertaken at the outlet of each catchment, on a weekly or biweekly basis over 2 years. In addition, spatial variations in concentrations of total organic carbon (TOC), dissolved inorganic carbon (DIC), free CO2 and CH4 are presented for both streams along 14 contiguous sites between the source and the outlet of each catchment. Concentrations of carbon determinants along each stream re¯ected increasing inputs of TOC and gaseous forms of carbon from soil-pore water emanating from deep peat areas; these concentrations subsequently decreased downstream due to changing soil characteristics and in-stream processes. The total annual carbon ¯ux was 191 kg C ha 21 yr 21 from the NE Scotland site compared to 121 kg C ha 21 yr 21 from the Mid-Wales stream, mainly due to signi®cantly higher dissolved organic carbon (DOC) concentrations …p , 0:001† found in the Scottish stream. DOC dominated the carbon ¯ux at both the NE Scotland (88.4%) and Mid-Wales (69.0%) sites, re¯ecting the importance of organic carbon sources in both catchments. Particulate organic carbon (18.5 and 27.4 kg C ha 21 yr 21 for NE 21 Scotland and Mid-Wales, respectively), HCO2 yr 21) and free CO2 ±C (2.62 and 8.75 kg 3 ±C (1.12 and 1.28 kg C ha C ha 21 yr 21) also contributed to the overall carbon ¯ux. The CH4 ±C ¯ux at the outlet of each catchment was less than 0.01 kg C ha 21 yr 21. Climatic differences between the two sites were expressed in terms of signi®cantly higher discharge and temperature, which caused an increase in CO2 and CH4 export from the warmer and wetter Mid-Wales catchment. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Streamwater; Dissolved organic carbon; Carbon dioxide; Methane; Peatland

1. Introduction The biogeochemistry of carbon in aquatic environments and catchment systems is of major importance to many issues. These include carbon dioxide and methane ¯uxes to/from the atmosphere, with its implications for the global climate, and the acid buffering * Corresponding author. Tel.: 144-1224-272693; fax: 144-1224272703. E-mail address: [email protected] (J.J.C. Dawson).

of organic acids in acidic environments, with its implications for `acid rain' research. The hydrobiogeochemistry of carbon is complex as carbon is transported in streamwater in particulate, dissolved and gaseous forms. Particulate organic carbon (POC) is the fraction retained on a ®lter with pore sizes between 0.45 and 1.0 mm. Dissolved organic carbon (DOC) is associated with the ®ltrate, although colloidal material will be included if ®lters greater than 0.7 mm are used (Thurman, 1985). Dissolved inorganic carbon (DIC) is also contained in the ®ltrate

0022-1694/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0022-169 4(01)00545-5

J.J.C. Dawson et al. / Journal of Hydrology 257 (2002) 226±246

HCO32

CO22 3

and in streams comprises and ions. These anions are associated with gaseous carbon (free CO2) via the carbonate equilibria (Stumm and Morgan, 1981). Acidic upland catchments in the UK associated with peaty soils are frequently water saturated and thus important in maintaining high concentrations of CO2 within the soil pores (Skiba and Cresser, 1991), as well as providing anoxic areas for methane generation (Jones and Mulholland, 1998). Carbon compounds particularly prevalent in upland peats, such as humic and fulvic acids, play a central role in the regulation of surface water pH (Driscoll et al., 1989), as well as being complexing agents for other ions, nutrients and heavy metals (Hope et al., 1994). In upland peat-rich systems, DOC and POC originate largely from terrestrial sources and are an important component of streamwaters particularly those draining upland catchments with peaty soils (Fiebig et al., 1990; Grieve, 1991; Hope et al., 1997a). The strongest relationships between streamwater DOC and different catchment variables such as soil carbon pools and percentage peat cover occur in small catchments (Hope et al., 1997b; Aitkenhead et al., 1999). In these low pH waters, DIC tends to contribute a smaller proportion of the total carbon ¯ux (Neal and Hill, 1994) and only occurs as dissolved free CO2 and HCO2 3 ions (Stumm and Morgan, 1981). Generally, although DIC concentrations tend to be high at higher pH values, across the whole pH range a factor of only 4 times the concentration was found (Neal and Hill, 1994). In upland streams, CO2 concentrations are in excess of CO2 concentrations in equilibrium with the atmosphere (Rebsdorf et al., 1991; PinÄol and Avila, 1992; Neal and Hill, 1994; Dawson et al., 1995). For these streams, concentrations and ¯uxes progressively decrease downstream as CO2 inputs are reduced and outgassing to the atmosphere occurs (Skiba and Cresser, 1991; Dawson et al., 1995). Methane, in general, is also supersaturated in streams, ranging from 1 to 598 times the atmospheric equilibrium concentration (Jones and Mulholland, 1998). The main control on small-scale spatial changes in streamwater DOC concentrations in upland headwaters is the difference between soil water inputs from different parts of the catchment as the stream ¯ows through different soil types (Grieve, 1990).

227

However, in-stream processes modify these DOC inputs, thus providing a secondary control on the spatial variability of DOC concentrations and ¯uxes. Similar processes also control the concentrations and ¯uxes of HCO2 3 and free CO2, although in-stream processes (including outgassing of CO2) have signi®cantly more in¯uence on spatial changes occurring in these two determinants than on DOC (Dawson et al., 2001a). Although there is a large amount of published work on individual carbon determinants or occasionally, a combination of two components (Neal and Hill, 1994; Dawson et al., 1995; Hope et al., 1997a,b; Jones and Mulholland, 1998), few studies have estimated the combined particulate, dissolved and gaseous carbon ¯uxes in upland streams. An integrated approach is needed to gain a proper understanding of the hydrobiogeochemical functioning of carbon within upland riverine environments. This shortfall is addressed here, based on a 2-year study (01/09/96±31/08/98) of small-scale downstream changes in carbon concentration in two acid upland peat-rich catchments from different climatic regions of the UK; a Scottish catchment near the headwaters of the River Dee (Brocky Burn) and a mid-Wales catchment at the headwaters of the River Severn (Hafren). They represent distinct endmembers in that: 1. The Brocky Burn site represents a colder and drier peat-dominated system characteristic of lower rainfall, acidic regions of Scotland with signi®cant snow cover and sub-zero air temperatures during the winter period. 2. The Hafren site is representative of warmer and wetter peat-dominated areas in Wales with high rainfall and milder meteorological conditions than the Scottish site (air temperatures are typically above zero with little snow during the winter months). The aims of this study were, speci®cally, to quantify total carbon (DOC, POC, HCO2 3 ; CO2 and CH4) ¯uxes from each catchment. Areas within each catchment that generate contrasting patterns of downstream variation in concentration are identi®ed and the different contributions of each carbon determinant to the overall carbon ¯ux from both streams quanti®ed.

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Table 1 (a) Characteristics of the Brocky Burn and Upper Hafren catchments. (b) Mean (range) solute chemistry of Brocky Burn and Upper Hafren streamwaters (All determinants are in mg l 21 except for Total Fe and Al (both mg l 21), Gran-alkalinity (mequiv. l 21) and pH; nd ˆ not determined) Brocky Burn a

Upper Hafren b Mid-Wales (SN828892) 0.93 South-east facing 535±635 1.4 Acid moorland vegetated with acidic grassland (Nardus, Festuca) and peaty mires Peats (.0.40 m depth)

Management

NE Scotland (NO615832) 1.30 South facing 270±549 2.7 Heather moorland (Calluna, Erica) and rushes (Juncus) and grasses in riparian and ¯ush sites Peats (.1.5 m depth) with humic podzols, regosols and ¯uvisols Granite and granite derived colluvial and ¯uvio-glacial material Heather burning for grouse

Sheep grazing

Climate/Hydrology01/09/96-31/08/98 Rainfall (mm yr 21) Mean discharge (m 3 s 21) Evapotranspiration (%) Mean air temperature (8C) Mean water temperature (8C) Stream gradient c (%)

1164 0.036 25.6 7.5 7.5 (0.7±16.7) 10.3

2726 0.062 22.9 7.3 7.7 (1.0±14.5) 7.1

(b) Mean (range) solute chemistry pH Total OC Total IC Ca 21 Mg 21 Na 1 K1 NH1 4 Cl 2 F2 SO22 4 NO2 3 PO32 4 Total Fe Total Al Gran-Alkalinity SiO2

5.17 (4.24±6.87) 12.2 (5.60±31.1) 1.18 (0.00±2.52) 2.03 (1.04±3.42) 0.70 (0.44±1.00) 5.22 (2.71±6.32) 0.31 (0.18±0.52) 0.05 (0.01±0.13) 7.43 (4.99±8.72) 0.00 (0.00±0.02) 3.64 (2.30±5.10) 0.05 (0.00±0.21) 0.00 (0.00± 0.00) nd nd 55.77 (267.7±164) 7.82 (2.99±11.9)

5.33 (4.35±7.16) 1.30 (0.00±6.70) nd 0.56 (0.17±1.32) 0.63 (0.03±1.00) 3.50 (1.86±6.09) 0.14 (0.05±0.61) 0.01 (0.00±0.12) 5.73 (3.50±13.5) 0.03 (0.00±0.40) 2.43 (1.28±4.94) 1.02 (0.00±4.55) 0.01 (0.00±0.06) 87.8 (15.0±806) 84.1 (13.0±307) 9.67 (258.8±60.5) 1.68 (0.30±20.5)

(a) Characteristics Location Area (km 2) Aspect Altitude range (m) Mainstem length (km) Vegetation Soils Geology

a b c

Greywackes, mudstones and shales

Brocky Burn data from 06/96 to 06/97(Smart, unpublished data). Upper Hafren data from 17/07/90 to 08/12/98 (Hill and Neal, unpublished data). Stream Gradient ˆ altitude range (km)/stream length (km) £ 100.

2. Methods 2.1. Study areas Table 1 gives catchment characteristics, climate

and solute chemistry. Brocky Burn is a small (130 ha) headwater stream in the Glen Dye catchment (NE Scotland) and is a tributary of the River Dee. Brocky Burn is 2.7 km in length and drains in a southerly direction from the Cairn of Edendocher (549 m).

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229

Fig. 1. Maps of (a) Brocky Burn (NE Scotland) and the (b) Upper Hafren (Mid-Wales) showing the location of the 14 downstream sampling sites; the discharge and ¯ux data are determined at the lowermost site of each stream.

The catchment consists largely of heather moorland with rushes and grasses present in the riparian zone and ¯ush sites and is managed for grouse by periodic burning. Soils in the catchment are derived from granite and acidic colluvial and ¯uvio-glacial material, comprising 59% histosol, 22% humic podzol, 19% regosol and ,1% ¯uvisol. Peat depths of .5.0 m have been recorded in the upper section of the catchment. Mean annual precipitation (September 1996± September 1998) collected from two rain collectors situated in the upper (1196 mm) and lower (1131 mm) sections of the catchment was 1164 mm. Discharge was continuously monitored at a ¯ume using a datalogger obtaining a reading every 5 min and average hourly discharge for the study period was 0.036 (0.002±0.989) m 3 s 21. Average annual evapotranspiration loss (discharge minus precipitation) was 25.6%. The Upper Hafren drains from a plateau above the tree line of the Plynlimon Forest in Mid-Wales in a

southeasterly direction for 1.4 km into the Afon Hafren, a major tributary of the River Severn. The Upper Hafren is a small (93 ha) acidic semi-natural moorland catchment used mainly for sheep grazing and ramblers en route to the Source of the River Severn. The geology of the Upper Hafren consists of quartz-rich greywackes and grits, which are overlain by fractured blue±grey mudstones and shales. Soils are mainly peat with depths .40 cm; on the plateau where drainage is impeded, 1±2 m deep peats occur. Mean annual precipitation is approximately 2518 mm with average annual evapotranspiration loss 23.8% (Hill and Neal, 1997). Discharge was measured at a ¯ume intensively monitored by The Centre for Ecology and Hydrology (Plynlimon) on the Afon Hafren further downstream. Discharge on 15 occasions for a wide range of ¯ow regimes at the ¯ume were related to discharge, measured by dilution gauging, at the outlet of the Upper Hafren. A strong discharge relationship between the ¯ume and the

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sampling site enabled an hourly average discharge of 0.062 (0.007±1.580) m 3 s 21 for the study period to be calculated for the sampling site on the Upper Hafren. 2.2. Sampling and analysis Intensive spatial sampling, undertaken to measure small-scale changes in the concentrations of carbon determinants along the main stems of both streams, was carried out on three separate occasions: Brocky Burn, 14/07/97, 11/06/98, 07/07/98; Upper Hafren, 17/10/96, 02/10/97, 19/08/98. Samples were collected from 14 sites along the length of both streams (Fig. 1a and b). The distance between successive sample sites on the main stems varied from 50 to 300 m. For the determination of annual carbon ¯uxes the streams were sampled on a weekly or biweekly basis for 2 years from 01/09/96 to 31/08/98 at the lowermost site of each catchment. However, data for POC and CO2 at the Upper Hafren site were only collected from 19/11/96 onwards (21 months) and missing concentrations from earlier weeks were calculated from subsequent concentration-discharge and concentration-water temperature relationships, respectively. A 550 ml glass bottle was used to collect samples for DOC and POC and a 125 ml plastic bottle for Gran-alkalinity for the routine study. However, for downstream sampling, DOC and POC samples were collected in 25 ml glass vials for ease of sampling. Streamwater CO2 and CH4 concentrations were determined by headspace analysis (Kling et al., 1991, 1992; Hope et al., 1995). This involves the collection of a streamwater sample in a 60 ml luer-lock syringe that is sealed underwater to avoid outgassing. A headspace was created in the syringe and equilibrated with the streamwater sample by vigorously shaking at stream temperature for 1 min. A 10±15 ml sample of the headspace gas was then transferred into a 20 ml nylon gas-tight syringe for subsequent analysis (Kling et al., 1991). Stream temperature and pH were also recorded. Samples were transported back to the laboratory and stored at 4 8C prior to analysis. Samples from the 550 ml glass bottles were ®ltered through pre-ashed and weighed Whatman GF/F glass micro-®bre ®lters (Wetzel and Likens, 1991). The ®ltrate (containing components ,0.7 mm) was used for the determination of DOC concentrations by ultra-violet oxidation and infrared gas analysis

(LABTOC, Pollution and Process Monitoring). The remaining material on the ®lter was used to determine particulate organic matter (POM) by loss-on-ignition at 375 8C for 16 h; POC was subsequently calculated by a regression equation used for non-calcareous soils (Ball, 1964). The downstream samples, used for more detailed spatial analysis, remained un®ltered and total organic carbon (TOC) (DOC 1 POC) and DIC were analysed using the LABTOC. Bicarbonate±C was measured by Gran-titration with 0.005 M H2SO4 to two end-points (pH 4.5 and 4.0) using a GK2401C combined glass/reference electrode (Radiometer, Copenhagen). In the calculation of Gran alkalinity and HCO2 3 concentration (Neal et al., 1998) allowances were made for the effects of ionic strength on the relationships between H 1 concentration and activity (g ) using the Davis equation. Gran alkalinity in these acidic streams is mainly associated with HCO2 3 ; although some Al, much of it organically bound, will contribute to the alkalinity. A study of a number of Plynlimon catchments estimated that alkalinity derived from Al sources was mainly less than 15 mequiv. l 21 (Neal, 1988). Accurate pH measurements were made using an enclosed pH system preventing outgassing of CO2 prior to analysis, which is important for the calculation of HCO2 3 (Neal et al., 1998). Quality control checks were undertaken regularly using a standard 0.5 £ 10 24 M sulphuric acid standard (pH 4.01 ^ 0.04). Gas samples were analysed within 24 h for CO2 and CH4 concentration using a gas chromatograph (Chrompack 9001) with attached methanizer and FID. 2.3. Flux calculations and statistical analysis Interpolation methods, used to determine ¯uxes of both suspended material and dissolved organic carbon (Walling and Webb, 1985; Hope et al., 1997a) are used when concentration values are obtained from non-continuous sampling. Walling and Webb (1985) set out ®ve commonly used estimation procedures to calculate river loads, two of which have been recently used to calculate ¯uxes of organic carbon in NE Scotland (Hope et al., 1997a). Method 5 (Eq. (1)) is the preferred method when continuous discharge measurements are available and is recommended by the Paris Commission for the measurement of river ¯uxes

J.J.C. Dawson et al. / Journal of Hydrology 257 (2002) 226±246 Table 2 Mean and range of water temperature and discharge measured at the lowermost sites on Brocky Burn and the Upper Hafren

Brocky Burn 14/07/97 11/06/98 07/07/98 Upper Hafren 17/10/96 02/10/97 19/08/98

Q (m 3 s 21)

Mean temperature (Range) (8C)

0.0082 0.0183 0.0040

12.4 (7.2±16.3) 8.2 (6.1±10.1) 11.3 (7.9±14.8)

0.0816 0.0197 0.0253

7.9 (6.9±9.1) 10.0 (9.4±11.0) 12.0 (10.3±17.9)

(Littlewood, 1992). The equation used to calculate the ¯ux of each carbon determinant from spot sampling was: Total Flux ˆ

K

P …C Q † P i i Qr Qi

…1†

231

where k is the conversion factor to take account of time period under investigation, Ci is instantaneous concentration at time of sampling (g m 23), Qi is instantaneous discharge at time of sampling (m 3 s 21) and Qr is the mean discharge (m 3 s 21) from continual discharge measurements. Fluxes (kg C ha 21 yr 21) were calculated for each carbon determinant using the earlier method at each study site. Standard errors (95% con®dence intervals) for each ¯ux were calculated using the method described by Hope et al. (1997a). A Pearson Correlation matrix was produced for each stream between carbon determinants and physical characteristics from the 2-year data set. t-Tests were used to compare the two sites for all chemical and physical data determinants. The percentage contribution that each carbon component made to the overall carbon ¯ux from both streams was also calculated.

Fig. 2. Downstream spatial variability in pH along the main stem of Brocky Burn and the Upper Hafren.

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Fig. 3. Downstream spatial variability in (a) TOC (b) DIC (c) free CO2 ±C and (d) CH4 ±C concentrations along the main stem of Brocky Burn and the Upper Hafren.

3. Results 3.1. Downstream spatial variability Changes in discharge and water temperature for each downstream sampling run are shown in Table 2. With the exception of the ®rst sample run on the Upper Hafren (17/10/96), all sampling was carried out at or near base ¯ow. The weather on all the days was dry and discharge remained steady throughout sampling. Mean streamwater temperature during the downstream sampling runs was higher at Brocky Burn than the Upper Hafren (Table 2), although throughout the sampling period, the Upper Hafren had a mean water temperature ,1 8C above that of Brocky Burn. Downstream variations in pH (Fig. 2) were related to discharge, the highest discharge days produced the lowest overall pH values for the 3 sampling runs at each site. At Brocky Burn, pH initially decreased from

the top of the catchment as the water entered the peatrich areas, 0.85 km downstream. From this point onwards, pH steadily increased from between 4.48± 5.55 and 5.01±6.71 respectively, at the lowermost site. On the Upper Hafren, however, there was a continuous increase in pH from between 4.20 and 4.46 at the source of the stream to 4.97±5.93 at the lowest site. Downstream changes in the concentrations of TOC (POC 1 DOC), DIC, free CO2 and CH4 on the three sampling dates are shown in Fig. 3. At Brocky Burn, TOC concentrations (Fig. 3a) at the stream source ranged from 1.19 to 6.06 mg l 21 and increased to a maximum of 10.0±25.3 mg l 21 as the stream passed through the deep peats in the central area of the catchment. TOC concentrations then declined to between 7.90 and 19.0 mg l 21 at the outlet. TOC concentrations at the source of the Upper Hafren were initially much higher than at Brocky Burn (8.56±22.0 mg l 21),

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233

Fig. 3. (continued)

then decreased rapidly in the upper reaches of the catchment to 1.68±3.78 mg l 21 at the lowermost site. Concentrations of DIC in both streams (Fig. 3b) were more variable than TOC and also relatively low. Concentrations were lowest (,1.5 mg l 21) when discharge was at its highest. At Brocky Burn, DIC concentrations were low near the stream source and increased to between 1.21 and 3.21 mg l 21 close to the peat-rich central area. Further downstream, concentrations of DIC on all three sampling dates then started to fall and continued to decrease to between 0.1 and 1.65 mg l 21 at the lowermost site. Although initially on the Upper Hafren, there was a slight increase in DIC concentrations to between 1.32 and 2.06 mg l 21, at the catchment outlet DIC was ,0.80 mg l 21. All three sets of downstream samples, for each catchment, showed consistent and reproducible trends in CO2 ±C concentrations (Fig. 3c). Free CO2 ±C concentrations in the main stem of Brocky

Burn ranged from ,0.40 mg l 21 in streamwater derived from the poorly developed regosols at the stream source, to 3.28 mg l 21 as the stream passed through the peaty areas of the catchment. The lowest concentrations of CO2 ±C in this area occurred on the day associated with highest discharge. Two clear peaks in free CO2 ±C concentration were observed in this part of the catchment, before it decreased to ,0.28 mg l 21 at the outlet. Free CO2 ±C concentrations in the Upper Hafren were also initially low (,1.11 mg l 21) and increased rapidly in the areas of deepest peat to between 1.35 and 5.63 mg l 21; lowest concentrations occurring on the day with the highest discharge. From 0.30 km downstream, there was a steady decrease in free CO2 ±C concentration to ,0.43 mg l 21 at the outlet, although a slight increase occurred between 0.63 and 0.87 km downstream from the source. At both sites, CH4 ±C concentrations (Fig. 3d) followed a

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Fig. 3. (continued)

similar spatial pattern to CO2 ±C concentrations, although concentrations were considerably lower. 3.1. Long-term monitoring The mean concentration and range of various physical determinants and carbon concentrations at the outlet of Brocky Burn and the Upper Hafren for the 2-year period are shown in Table 3. Mean discharge at Brocky Burn was signi®cantly lower …p , 0:001† than the discharge for the corresponding period at the Upper Hafren. Although mean water temperature was slightly higher in the Upper Hafren, the difference was not statistically signi®cant. In addition, mean pH between the two sites was not signi®cantly different; both streams remained acidic throughout with a mean pH of ,5.5. Mean DOC concentrations were at least 4 times higher …p , 0:001† at Brocky Burn compared to the Upper Hafren. With respect to POC, mean concentrations between

the sites were similar, although during storm events POC increased to a greater extent in the Upper Hafren. Mean HCO2 3 ±C concentrations were signi®cantly higher …p , 0:001† at Brocky Burn, although concentrations varied considerably over the study period. At both sites, the range in HCO2 3 ±C concentrations varied from undetectable levels at pH values ,4.5 to 2.19 mg l 21 at Brocky Burn and 0.74 mg l 21 at the Upper Hafren. Mean free CO2 ±C concentrations were signi®cantly lower at Brocky Burn …p , 0:001† compared to the Upper Hafren and were equivalent to epCO2 values of 1.3 and 1.5, respectively. Methane±C was undetectable (,0.01 mg l 21) at this point in both streams. Pearson correlation matrices for the routine 2-year period monitoring are shown in Table 4. Most correlations showed stronger relationships (higher r 2values) between determinants at the Upper Hafren than Brocky Burn, in particular for pH with CO2 ±C (Fig. 4a), temperature with CO2 ±C (Fig. 4b) and DOC

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235

Fig. 3. (continued)

with temperature (Fig. 4c). The relationship between discharge and DOC can be strengthened at both sites if the data is split seasonally (Fig. 5). Two DOC relationships were obtained for the time period studied; a `high DOC' and `low DOC' period. High DOC conditions prevailed through the summer until the ®rst storm in the autumn. Low DOC conditions

then persisted throughout the remainder of the autumn, winter and spring following each autumnal storm when the DOC concentration-discharge relationship reverted back to their respective summer/ autumn curves. However, the low DOC period continued for longer at Brocky Burn compared with the Upper Hafren site.

Table 3 Mean and range of various determinants in both Brocky Burn and the Upper Hafren (all carbon determinants are expressed in mg l 21 ***shows signi®cant difference …p # 0:001†)

Q (m 3 s 21) Water temperature ( oC) pH DOC POC HCO3 ±C CO2 ±C epCO2

Brocky Burn

Upper Hafren

***0.036 (0.002±0.989) 7.5 (0.7±16.7) 5.11 (4.11±7.11) ***14.5 (5.78±41.0) 0.49 (0.06±4.54) ***0.69 (,0.01±2.19) ***0.32 (0.22±0.54) ***1.3 (0.9±2.4)

0.062 (0.007±1.580) 7.7 (1.0±14.5) 5.31 (4.46±6.87) 2.88 (1.40±5.88) 0.43 (,0.01±7.22) 0.18 (,0.01±0.74) 0.39 (0.09±0.77) 1.5 (1.0±2.3)

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Table 4 Pearson correlation values at Brocky Burn and the Upper Hafren for pH, discharge, water temperature and carbon determinants (®gure in bold refer to Brocky Burn …n ˆ 73†; ®gure in italics refer to the Upper Hafren …n ˆ 101†; ***ˆ p # 0:001; **ˆ p # 0:01; *ˆ p # 0:05)

POC HCO2 3 ±C CO2 ±C epCO2 Log Q Temperature pH

DOC

POC

HCO2 3 ±C

CO2 ±C

epCO2

Log Q

Temperature

***0.526 ***0.500 ***20.493 ***20.421 20.125 20.144 0.078 0.026 ***0.515 ***0.694 0.176 **0.300 ***20.613 ***20.610

20.178 20.164 20.099 20.014 0.053 0.099 ***0.483 ***0.415 0.077 0.031 **20.330 **20.306

0.120 **20.288 ***0.398 ***20.314 ***20.825 ***20.781 **0.351 ***0.390 ***0.896 ***0.829

***0.537 ***0.850 20.085 0.167 ***20.484 ***20.839 0.043 **20.296

**20.317 *0.240 **0.333 ***20.636 *0.254 ***20.346

**20.310 *20.209 ***20.924 ***20.931

*0.271 ***0.319

Fig. 4. Correlations between (a) pH and CO2 ±C (b) water temperature and CO2 ±C and (c) water temperature and DOC.

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237

Fig. 4. (continued)

3.2. Fluxes The ¯uxes (1/295% con®dence intervals) of each carbon determinant are shown in Table 5. Annual DOC ¯uxes were higher at Brocky Burn (169 kg C ha 21 yr 21) than the Upper Hafren (83.5 kg C ha 21 yr 21), whereas POC, HCO2 3 ±C and CO2 ±C ¯uxes were lower at the outlet of Brocky Burn. The total carbon ¯uxes from Brocky Burn and the Upper Hafren were 191 and 121 kg C ha 21 yr 21, respectively. The percentage contributions of each carbon determinant to the overall carbon ¯ux (Fig. 6) showed that the total ¯ux of carbon from both sites was dominated by DOC, which was more important at Brocky Burn (88.4%) compared to the Upper Hafren (69.0%). This was due to the relative importance of POC to the overall carbon ¯ux at the Upper Hafren (22.7%) compared to Brocky Burn (9.7%). Although, free CO2 contributed 1.5% of the total carbon ¯ux at Brocky Burn, it was a more important component of the total carbon ¯ux on the Upper Hafren (7.2%). The

combined ¯uxes of HCO32 ±C and CH4 ±C were ,1% of the total carbon ¯ux at both sites.

4. Discussions 4.1. Downstream spatial variability Spatial patterns in DOC along headwater streams are closely related to soil changes within the catchment (Rees et al., 1989). At Brocky Burn, increased concentrations of TOC (DOC and POC) as the stream passed from regosols to peat were more marked at higher ¯ows than at lower ¯ow. There appears to be a similar process occurring at the Upper Hafren. The observed decrease in TOC concentrations down the Upper Hafren from near the source to the lowermost site is presumably related to a decrease in peat depth with altitude and in-stream processing (Hill and Neal, 1997; Dawson et al., 2001a). Although at the lower sites highest TOC concentrations were associated

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Fig. 4. (continued)

with the highest discharge, at the stream source there appeared to be a dilution effect with discharge, as TOC concentrations in streamwater emanating from pools at the top of the catchment were higher at lower discharge. This has also been observed in other permanently ¯ooded areas such as wetlands where deep groundwater ¯owpaths are less important (Schiff et al., 1998; Hinton et al., 1998). In these areas leaching and ¯ushing will cause a decrease in DOC concentrations with increased discharge. DIC concentrations were variable along both headwater catchments and mostly ,3.0 mg l 21, re¯ecting the acidic nature of the streams and low groundwater inputs (Reid et al., 1981). When discharge was high, the pH was ,5.0 in the main section of the streams, shifting the carbonate equilibria in favour of free CO2; this was subsequently lost to the atmosphere downstream by outgassing, causing lower HCO2 3 and hence DIC concentrations. Although there are inputs of DIC from tributaries (Dawson et al., 2001a) that maintain

the free CO2 and HCO2 3 concentration in the upper and middle sections of Brocky Burn, DIC concentrations subsequently decreased further downstream. The streamwater chemistry of individual tributaries may therefore show a greater groundwater in¯uence, because of lower water volumes associated with fracture patterns in the underlying granite at Brocky Burn. A downstream decrease in DIC also occurred at the Upper Hafren with occasional DIC ¯uctuations along the stream length. The Upper Hafren catchment has been shown to be part of a highly heterogeneous system of ¯ow pathways and chemical weathering on scales of less than 100 m (Neal et al., 1997). Free CO2 concentrations had steadily decreased downstream over the upper part of the catchment, which suggests that in the lower section of the catchment diffuse areas and tributaries are the most likely source of DIC (in the form of HCO2 3 ) and account for smallscale changes in the DIC spatial concentrations. Although DIC represents both free CO2 ±C and

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239

Fig. 5. Concentration-discharge relationships for two distinct sets of data (High and Low DOC) at Brocky Burn and the Upper Hafren.

Table 5 Fluxes and 95% con®dence intervals of carbon determinants at Brocky Burn and the Upper Hafren (all carbon ¯uxes are in kg C ha 21 yr 21) Brocky Burn

Upper Hafren

DOC POC HCO32 ±C CO2 ±C CH4 ±C

169 (119) 18.5 (17.9) 1.12 (2.07) 2.62 (1.75) , 0.01

83.5 (37.7) 27.4 (19.0) 1.28 (1.17) 8.75(3.80) , 0.01

Total C-¯ux

191

121

HCO2 3 ±C; much of the excess CO2 will have degassed prior to analysis by the aqueous carbon analyser. Thus, in some sections of the stream, higher CO2 ±C than DIC concentration was recorded. The headspace analysis technique for free CO2 enables the measurement of all excess carbon in the stream without any loss from the sample. Free CO2 concentrations in these catchments were within a range of 1±15 times atmospheric pressure and this range is similar to that reported elsewhere for headwater streams in NE Scotland and MidWales (Neal and Hill, 1994; Dawson et al., 1995). However, in streamwater ¯owing through the deep

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Fig. 6. Percentage ¯ux of each carbon determinant at Brocky Burn and the Upper Hafren for the period 01/09/96-31/08/98.

peat areas of Brocky Burn and the Upper Hafren, free CO2 concentrations corresponding to epCO2 values of 14.1 and 27.8, respectively, were observed. Spatial variations in streamwater CO2 concentrations are in¯uenced by a number of terrestrial factors including temperature dependent soil and root respiration rates, soil moisture, O2 and nutrient availability and soil organic matter content (Howard and Howard, 1993; Jones and Mulholland, 1998). The two study catchments undergo cycles of wetting and drying and thus experience periods of both aerobic and anaerobic conditions (Pulliam, 1993). There is evidence that signi®cant production of CO2 and CH4 occurs alternately in the anoxic transition zone (Clymo and Pearce, 1995). In addition, CO2 can be produced by non-methanogenic anaerobic respiration pathways but is signi®cantly less than CO2 production under aerobic conditions (Yavitt et al., 1987; Bridgham and Richardson, 1992; Magnusson, 1993; Pulliam, 1993). Although anaerobic CO2 production in freshwater North

Carolina peatlands has been shown to be several orders of magnitude more than methanogenesis (Bridgham and Richardson, 1992). The rate of CO2 escape from soils to the atmosphere affects the CO2 concentrations reaching the stream and is dependent on the water table depth, soil porosity and water holding capacity (Moore and Knowles, 1989; Skiba and Cresser, 1991; Dawson et al., 2001a). Stream turbulence, velocity, depth and gradient as well as biotic processes will in¯uence CO2 concentrations once it is present in the stream system (Rebsdorf et al., 1991). In Brocky Burn, initial low concentrations of free CO2 were most likely due to (i) low soil respiration rates in the poorly developed mineral soils at the top of the catchment and (ii) ease of escape of CO2 to the atmosphere through thin, coarse textured soils. A marked increase in CO2 concentration in the stream associated with the peat may be due to a decrease in the rate of CO2 escape from peaty soils to the atmosphere (i.e. increased CO2 retention) due to their low

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porosity, in addition to increased CO2 production compared to the mineral soils (Magnusson, 1993; Hope, unpublished data). This is further enhanced by continual inputs of CO2-rich water from tributaries ¯owing through the peats (Dawson et al., 2001a). A rapid decrease in CO2 concentrations occurred at 1.74 km downstream from the source of Brocky Burn as inputs of CO2 from tributaries ceased, outgassing continued and soils within the catchment changed from peats to humic podzols, which were found to have signi®cantly lower concentrations of CO2 (Hope, unpublished data). In the Upper Hafren, highest free CO2 concentrations were found in streamwater draining the deep peats at the head of the catchment. There was a rapid decrease in concentration downstream from this point as CO2 inputs to the stream from less saturated soils were reduced and outgassing continued. In Scottish blanket peats, emission of CO2 from soils to the atmosphere was 40% greater in dry areas than wet areas (Chapman and Thurlow, 1996). Moore and Knowles (1989) also found that CO2 evolution from peats to the atmosphere increased linearly with lowering of the water table. These studies suggest that production of CO2 still continues further downstream in the Upper Hafren catchment, although much of the soil-respired CO2 does not reach the stream. Free CO2 concentrations eventually decreased in both catchments to values close to atmospheric equilibrium, although in Brocky Burn concentrations were closer to equilibrium with the atmosphere, presumably because the stream outlet was a greater distance from the high soil CO2 sources and the steep stream gradient in the lower section of the catchment. Methane concentrations followed a similar pattern to that of free CO2 in both catchments with highest concentrations observed in areas of the stream draining deep peats. At the lowermost site in both catchments, CH4 was undetectable. The more rapid decline in CH4 concentrations over the lower section of the streams, compared to CO2, is probably due to outgassing and oxidation losses as well as the lack of further inputs from more aerated soils (Pulliam, 1993; Jones and Mulholland, 1998). Concentrations of CH4 in the peat areas were higher at the Upper Hafren than in the peat areas at Brocky Burn possibly due to the more highly saturated state of the soils in the former, promoting anaerobic conditions for the bioconversion

241

of organic matter to CH4 (Wolin and Miller, 1987; Jones and Mulholland, 1998). The porosity of the soils and the height of the water table with its associated aerobic and anaerobic zones (Moore and Knowles, 1989) will change downstream affecting the proportion of soil CO2 and CH4 that reaches the stream. The more freely draining and hence less saturated soils occur further downstream in both catchments with subsequent decreases in the streamwater concentrations of both gases. 4.2. Long-term monitoring Temporal changes in DOC concentrations were positively correlated with discharge …p , 0:001† at both sites. This is consistent with previous work in a range of British upland catchments (Reid et al., 1981; Grieve, 1984, 1990) in which the change in DOC concentration re¯ects a change in hydrological pathways (Grieve, 1990) as water ¯ows through more organic-rich soil horizons following rainfall events. However, the mean concentration of DOC at the lowermost site in Brocky Burn was …p , 0:001† signi®cantly higher than at the Upper Hafren lowermost site. Although both the Brocky Burn and Upper Hafren catchments are dominated by peats, peat depth and hence the amount of organic matter per hectare are very different. Peat depths of .5.0 m have been observed in the middle shallow section of the Brocky Burn catchment, whereas Hill and Neal (1997) reported peat depth of 1±2 m at the source of the Upper Hafren. Lower concentrations of DOC in the Upper Hafren are due to differences in the organic matter pool and possibly to a dilution effect caused by substantially greater annual precipitation in similar sized catchments. Moreover, the contrasting land-use between the two catchments will have an in¯uence on DOC production. Grazing, as occurs at the Upper Hafren, will not only remove above ground carbon, but also switch the balance of above and below ground carbon allocation. Heather burning removes above ground and changes species composition resulting in carbon inputs to soil and DOC that maybe quite different from how the soils at the site developed (Brooks, pers. comm.) The slightly warmer climatic conditions in Wales, however, may result in greater organic matter decomposition and hence an increase in DOC production.

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Relationships between DOC concentrations and discharge (Fig. 5) in both catchments showed a similar biannual pattern with both `high' and `low' periods of DOC production and subsequent transport. Each year, following the ®rst major high ¯ow in autumn, the catchment appears to enter a dormant period associated with a reduction in heterotrophic processing of soil carbon. This lowers the mobile DOC pool available to be ¯ushed from the catchment (Brooks et al., 1999) and hence lowers DOC concentrations in the stream for a given discharge. However, in both catchments, when higher temperatures lead to increased respiration, decomposition and exudation of organic matter, higher DOC concentrations occur in the stream. The switch from low mobile DOC conditions back to high mobile DOC conditions was less obvious than the changeover (high to low) following the autumnal high ¯ows and maybe due to differences in hydrology, soil temperature and/or moisture content, which trigger an increase in soil carbon processing. During spring at Brocky Burn, although higher temperatures might increase organic matter decomposition within the catchment, DOC concentrations in the streamwater do not respond. It is possible that the DOC produced at this time of year is utilised within the catchment in a `closed' system of carbon cycling. However, the Mid-Wales catchment had an appreciably shorter low mobile DOC period than the NE Scotland catchment. At Brocky Burn, periods of low DOC production ended on 22/7/97 and 2/6/98 each year, whereas at the Upper Hafren, the low mobile DOC period ended by 29/4/97 and 12/5/98, respectively. This switch may be due to longer growing seasons in the warmer and wetter Mid-Wales catchment. At Brocky Burn, the heather moorland has a shorter growing season compared to the acidic grassland at the Upper Hafren. Differences in soil± plant system terrestrial processes between the two catchments, which includes variations in litter, the rhizophere, soil microbial processes, soil carbon formation and DOC production, may also delay the onset of the high mobile DOC period in the Brocky Burn catchment. Particulate organic carbon concentrations were not signi®cantly different between the two streams and both tended to increase with increasing discharge …p , 0:001†: However, it was only during high ¯ows when differences were apparent between the two sites.

Maximum POC concentrations were higher at the Upper Hafren compared to Brocky Burn because the former has increased stream discharges and energy enhancing erosion. The Upper Hafren also has a higher potential for erosion due to increased disturbance by humans and sheep. Compared to Brocky Burn, more highly eroded areas occurred, particularly at the top of the catchment within the peaty mires, where bare soil and peat hags were prominent. Although HCO2 3 ±C concentrations were low due to a lack of inorganic sources within the catchments, Brocky Burn had signi®cantly higher mean HCO2 3 ±C concentrations …p , 0:001† at its lowermost site than the Upper Hafren. This may be due to either a DIC-rich tributary in Brocky Burn (Dawson et al., 2001a) that enters the main stem close to the outlet, or higher precipitation at the Upper Hafren, leading to 2 a dilution of soil-derived HCO2 3 : The HCO3 ±C concentrations at each site were highly correlated with pH (positively) and discharge (negatively) …p , 0:001†: The strong relationship with discharge (Table 4) is explained by changes in hydrological pathways as the proportion of groundwater contribution to the overall ¯ow decreases with increasing ¯ow. At a certain discharge when pH is ,4.5, the amount of HCO2 3 ±C is undetectable as streamwater is dominated by soil water and possibly overland ¯ow with little or no inorganic carbon buffering capacity. Mean free CO2 concentrations were signi®cantly higher in the Upper Hafren …p , 0:001† than Brocky Burn. This could be caused by the smaller distance and shallower gradient of the Upper Hafren catchment (Table 1a) from its source to the lowermost site, which restricts the opportunity for outgassing of soil-derived CO2. In addition, increased soil moisture in the Upper Hafren will reduce losses of soil CO2 to the atmosphere. There was no correlation between CO2 ±C and discharge at either site, although a strong negative relationship …p , 0:001† with temperature did exist (Table 4). Isotopic data in Brocky Burn shows that soil-derived CO2 does not equilibrate immediately with the atmosphere but does so progressively downstream (Palmer et al., 2001). Atmospheric derived CO2 becomes more signi®cant downstream as increased mixing between the stream and the atmosphere occurs. Thus, any correlation between discharge and CO2 becomes progressively weaker at sites further downstream as soil-derived CO2 inputs

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become less signi®cant (Dawson, unpublished data). The relationship between discharge and CO2 ±C (Table 4) in the Upper Hafren compared to Brocky Burn is stronger (higher r 2 values) because of the shorter distance from the main soil CO2 source (peats) to the catchment outlet. The results also suggest that the majority of free CO2 has degassed before reaching the outlet as concentrations become more dependent on the solubility of free CO2 (Butler, 1982; Hope et al., 1995) at different temperatures (Fig. 4b). However, at smaller temporal (diurnal) scales, further variation will occur due to biotic processes such as respiration, photosynthesis and decomposition (Hoffer-French and Herman, 1989; Rebsdorf et al., 1991; Dawson et al., 2001b). 4.3. Fluxes The differences between ¯uxes at the two sites were in part related to discharge and clearly linked to climate (Hinton et al., 1997). However, the DOC ¯ux at Brocky Burn (169 kg C ha 21 yr 21) was higher than at the Upper Hafren (83.5 kg C ha 21 yr 21) because of differences in the amount of organic matter (per unit area) contained within the respective catchments. This is common in other peat dominated headwater catchments in the UK and indicates that soil carbon is the major source of organic carbon to the stream (Aitkenhead et al., 1999; Dawson et al., 2001a). There was not as great a difference in DOC ¯uxes as there was in DOC concentrations between the two catchments, as the lower concentrations in the Upper Hafren are partly balanced by its higher discharge for a similar sized catchment. The Upper Hafren ¯ux values are within the normal range (10± 100 kg C ha 21 yr 21) for most UK river systems, although the Brocky Burn ¯ux values are higher than the typical range (Hope et al., 1994). POC ¯uxes ranged from 18.5 to 27.4 kg C ha 21 yr 21, which is on the upper end of the range (2.8±15.8 kg C ha 21 yr 21) found in other upland catchments in Britain (Reynolds, 1986; Hope et al., 1997a). The POC:DOC ratio was substantially higher at the Upper Hafren (32.8%) compared to Brocky Burn (10.9%). The ratio of DOC to POC can vary substantially and POC generally becomes more important with increasing stream order and size (Thurman, 1985). However, POC is more important

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in the smaller Upper Hafren catchment because precipitation is higher and the catchment appears to be more eroded, enhancing particulate sources to the stream. Fluxes of HCO2 3 ±C were low (1.12 and 1.28 kg C ha 21 yr 21) compared to many upland stream systems (Hope et al., 1994). The main reason for this was that both headwater catchments contain acid parent material and mean pH remains low …pH , 6:3†: The majority of DIC is therefore present as free CO2 (CO2 (aq) and H2CO3) rather than HCO2 3 (Komada et al., 1998). One of the reasons that free CO2 ±C ¯uxes from the Upper Hafren (8.75 kg C ha 21 yr 21) were higher than from the Brocky Burn catchment (2.62 kg C ha 21 yr 21) was because discharge was higher for the Hafren. Estimated ¯uxes for free CO2 in upland rivers are rare in the literature, but previous work suggests that ¯uxes do tend to be higher in MidWales than in NE Scotland. Indirect measurements from Wales suggest ¯uxes of 12.4 kg ha 21 yr 21 DIC of which 87% was undissociated free CO2 (Neal and Hill, 1994). Free CO2 ±C ¯uxes of 2.6±4.4 kg C ha 21 yr 21 from streams in NE Scotland has been estimated using direct methods (Dawson et al., 1995; Dawson, unpublished data). The estimates from the Upper Hafren and Brocky Burn are therefore in line with previous values, although higher free CO2 ¯uxes will occur in highly heterotrophic (polluted) systems, where respiration exceeds removal of CO2 by photosynthesis at all times (Odum, 1956). Even in the study streams, ¯uxes will probably be underestimates as stream CO2 concentration increases during darkness due to continual respiration with no photosynthetic removal of CO2. A 2.5 times increase in CO2 concentration during the night has been recorded in streams from NE Scotland (Dawson et al., 2001b). Although DOC dominated the total carbon ¯ux at both sites, it was more important at Brocky Burn (88.4%) than the Upper Hafren (69.0%). This was in part because POC contributed more to the overall carbon ¯ux at the Upper Hafren (22.7%) than at Brocky Burn (9.7%). Fluxes of HCO2 3 ±C as a percentage of the total carbon ¯ux were relatively insignificant for both catchments. Free CO2 contributed 1.5% of the total carbon ¯ux at Brocky Burn although it was a signi®cantly more important component in the

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Upper Hafren (7.2%). One of the main reasons why gaseous carbon ¯uxes are low at the outlets of catchments is that outgassing of CO2 and CH4 occurs upstream. Recent work in Brocky Burn suggests that the CO2 and CH4 ¯uxes to the atmosphere from streams (140±284 kg C ha 21 yr 21) draining peatland systems are comparable to the total downstream carbon ¯uxes in the catchment (Hope et al., 2001).

The results presented from these two typical peatland sites have signi®cance beyond the UK uplands, particularly in terms of climatically and seasonally affected losses of carbon from the terrestrial environment as DOC, and losses of CO2 and CH4 from the peatland areas. In addition, they clearly demonstrate the role that differences within catchment soil carbon pools have in controlling streamwater chemistry and the potential impact of disturbance on this important terrestrial carbon store.

5. Conclusions Carbon ¯uxes from these contrasting acidic peatland systems are dominated by DOC and POC (.90%). Inorganic carbon is a smaller part of the total carbon ¯ux and is mainly exported as free CO2. Methane was undetectable at the outlet of the catchments, as outgassing of peat-derived CH4 occurred further upstream. Variation in climate, in particular precipitation and temperature, is a major controlling factor on the ¯uxes of the different forms of carbon exported from the two contrasting study areas. The initial depth of peat is in part caused by climate, with lower temperatures in NE Scotland leading to greater peat accumulation. Increased depth of organic matter and lower annual precipitation are the major reasons why DOC concentrations in Brocky Burn are higher. The water table at the Upper Hafren site is closer to the surface and standing surface water is common at the top of the catchment. This leads to higher concentrations of CO2 and CH4 in the upper part of the Upper Hafren, partly because drainage is so slow. Increased temperatures will also enhance degradation of DOC to CO2 and CH4 in the Upper Hafren compared to Brocky Burn leading to an increase in concentrations in the stream. Changes in CO2 and CH4 concentrations downstream at both sites are related to spatial variations in soil type and condition. Both catchments exhibited higher DOC concentrations in the streams for a given discharge in the summer and early autumn compared to the winter/spring period. The period of low mobile DOC conditions was much shorter at the Upper Hafren compared to Brocky Burn. Differences in soil±plant system terrestrial processes, as well as warmer temperatures earlier in the spring, will contribute to the beginning and end of the high/low mobile DOC periods in both streams.

Acknowledgements The authors would like to thank Fasque Estates for allowing access to the Brocky Burn site and the Centre for Ecology and Hydrology (Plynlimon) for their co-operation and assistance during ®eldwork at the Upper Hafren site and the reviewers for their helpful comments. J.J.C. Dawson also wishes to thank the Natural Environment Research Council for providing ®nancial support.

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