Peatland streams potentially represent important conduits for the exchange of gaseous carbon between the terrestrial ecosystem and the atmosphere. We investigated how gaseous evasion of carbon from the stream surface compared with downstream carbon transport at three locations on a Scottish headwater stream. Carbon dioxide was consistently above atmospheric saturation in the stream, with mean concentrations of 159.1, 81.8, and 29.5 mol L Ϫ1 at the lower, middle, and upper sites, respectively (i.e., 7.6, 3.9, and 1.2 times in excess of atmospheric equilibrium concentrations). Methane concentrations in stream water were much lower but showed a similar pattern. Rates of gaseous evasion from the stream surface to the atmosphere, determined experimentally using direct measurement of dissolved gas concentrations in conjunction with coinjection of conservative solute and volatile gas tracers, also declined downstream. Combined stream losses of all forms of carbon from the entire catchment (i.e., degassing from the stream surface and exports downstream) totaled 54,140 kg C yr Ϫ1. Evasion of carbon dioxide from the stream surface accounted for 34% of this total, compared to 57% lost as dissolved organic carbon via export downstream. When expressed per unit area of watershed, the gaseous C evasion from the stream represents a loss of 14.1 g C m Ϫ2 yr Ϫ1, which equals 28-70% of the estimated net carbon accumulation rate for such peatlands. This study shows that gaseous carbon loss from the surface of temperate headwater streams can be both spatially variable and significant in terms of rates of net annual land surface-atmosphere exchange at the catchment scale.Although there is an extensive literature on dissolved organic carbon (DOC) in streams, much less attention has been given to gaseous forms of carbon (e.g., carbon dioxide and methane) in stream water (Hope et al. 1994). Typically, both carbon dioxide and methane are supersaturated in surface waters, with both streams and lakes frequently exhibiting gaseous partial pressures many times in excess of atmospheric equilibrium (Kling et al. 1991;Cole et al. 1994;Hamilton et al. 1994;Dawson et al. 1995; Jones and Mulholland 1998a,b). By evasion of this excess gaseous carbon, surface waters can act as conduits for a significant flux of carbon from terrestrial pools to the atmosphere (Kling et al. 1991;Cole et al. 1994;Hamilton et al. 1994). Most of the published evasion flux estimates are for lakes (e.g., Cole et al. 1994) and large rivers (e.g., DeAngelis and Scranton 1993;Raymond et al. 1997). However, small streams are where much groundwater initially enters surface drainage systems carrying a large load of dissolved gases, yet there are few published studies where gaseous carbon efflux rates have actually been measured for such streams (Jones and Mulholland 1998a,b).Using measurements of carbon dioxide and methane content of stream water in conjunction with tracer experiments involving the coinjection of a conservation solute and a volatile gas tracer, we quan...
.[1] Any change in the ability of northern peatlands to act as a sink for atmospheric CO 2 will play a crucial part in the response of the Earth system to global warming. We argue that a true assessment of the sink-source relationships of peatland ecosystems requires that losses of C in drainage waters be included when determining annual net C uptake, thus connecting measurements of stream C fluxes with those made at the land surfaceatmosphere interface. This was done by combining estimates of net ecosystem exchange (NEE) with stream water measurements of TOC, DIC, and gaseous C loss, in a 335-ha lowland temperate peatland catchment (55°48.80 0 N, 03°14.40 0 W) in central Scotland over a 2-year period (1996)(1997)(1998). Mean annual downstream C flux was 304 (±62) kg C ha À1 yr À1 , of which total organic carbon (TOC) contributed 93%, the remainder being dissolved inorganic carbon (DIC) and free CO 2 . At the catchment outlet evasion loss of CO 2 from the stream surface was estimated to be an additional 46 kg C ha À1 yr À1 . Over the study period, NEE of CO 2 -C resulted in a flux from the atmosphere to the land surface of 278 (±25) kg C ha À1 yr À1 . Net C loss in drainage water, including both the downstream flux and CO 2 evasion from the stream surface to the atmosphere, was therefore greater or equal to the net annual C uptake as a result of photosynthesis/respiration at the land surface. By combining these and other flux terms, the overall C mass balance suggests that this system was either acting as a terrestrial C source or was C neutral.
Peatland streams have repeatedly been shown to be highly supersaturated in both CO 2 and CH 4 with respect to the atmosphere, and in combination with dissolved (DOC) and particulate organic carbon (POC) represent a potentially important pathway for catchment greenhouse gas (GHG) and carbon (C) losses. The aim of this study was to create a complete C and GHG (CO 2 , CH 4 , N 2 O) budget for Auchencorth Moss, an ombrotrophic peatland in southern Scotland, by combining flux tower, static chamber and aquatic flux measurements from 2 consecutive years. The sink/source strength of the catchment in terms of both C and GHGs was compared to assess the relative importance of the aquatic pathway. During the study period (2007)(2008) the catchment functioned as a net sink for GHGs (352 g CO 2 -Eq m À2 yr À1 ) and C (69.5 g C m À2 yr À1 ). The greatest flux in both the GHG and C budget was net ecosystem exchange (NEE). Terrestrial emissions of CH 4 and N 2 O combined returned only 4% of CO 2 equivalents captured by NEE to the atmosphere, whereas evasion of GHGs from the stream surface returned 12%. DOC represented a loss of 24% of NEE C uptake, which if processed and evaded downstream, outside of the catchment, may lead to a significant underestimation of the actual catchment-derived GHG losses. The budgets clearly show the importance of aquatic fluxes at Auchencorth Moss and highlight the need to consider both the C and GHG budgets simultaneously.
[1] Boreal streams represent potentially important conduits for the exchange of carbon dioxide (CO 2 ) between terrestrial ecosystems and the atmosphere. The gas transfer coefficient of CO 2 (K CO2 ) is a key variable in estimating this source strength, but the scarcity of measured values in lotic systems creates a risk of incorrect flux estimates even when stream gas concentrations are well known. This study used 114 independent measurements of K CO2 from 14 stream reaches in a boreal headwater system to determine and predict spatiotemporal variability in K CO2 . The K CO2 values ranged from 0.001 to 0.207 min −1 across the 14 sites. Median K CO2 for a specific site was positively correlated with the slope of the stream reach, with higher gas transfer coefficients occurring in steeper stream sections. Combining slope with a width/depth index of the stream reach explained 83% of the spatial variability in K CO2 . Temporal variability was more difficult to predict and was strongly site specific. Variation in K CO2 , rather than pCO 2 , was the main determinant of stream CO 2 evasion. Applying published generalized gas transfer velocities produced an error of up to 100% in median instantaneous evasion rates compared to the use of actual measured K CO2 values from our field study. Using the significant relationship to local slope, the median K CO2 was predicted for 300,000 km of watercourses (ranging in stream order 1-4) in the forested landscape of boreal/nemoral Sweden. The range in modeled stream order specific median K CO2 was 0.017-0.028 min −1 and there was a clear gradient of increasing K CO2 with lower stream order. We conclude that accurate regional scale estimates of CO 2 evasion fluxes from running waters are possible, but require a good understanding of gas exchange at the water surface.
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