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] Mean transit time (MTT) is being increasingly used as a metric of hydrological function in intercatchment comparisons. Estimating MTT usually involves relating the temporally varying input concentration of a conservative tracer to the signal in the stream using various transfer functions as transit time distributions (TTDs). Most studies have been confined to data collection periods of 1-2 years at single sites, often limiting the transferability of the findings as such short periods usually only capture a narrow range of climatic variability within a spatially restricted area. In this study, we use longer-term (up to 17 years) weekly input-output relationships of Cl À to estimate MTTs using a range of TTD models in 20 headwater catchments (ranging from <1 to 35 km 2 ) in seven geomorphologically and climatically distinct parts of the Scottish Highlands. The MTTs obtained from a Gamma distribution model were the best identified and ranged from about 50 to 1700 days for individual catchments. The MTTs, in conjunction with GIS analysis of landscape characteristics and climatic indices, allowed the development of a robust multiple-regression model to establish the relative importance of different landscape and climate controls on MTTs. The best model combines the prediction variables percent responsive soil cover, drainage density, precipitation intensity, and topographic wetness index and yields R 2 adj = 0.88. Cross validation shows small absolute error, suggesting that the model can be used to estimate MTTs in ungauged headwater catchments throughout the Scottish Highlands and potentially in similar regions where comparable information is available.

No abstract

No abstract

Variation in dissolved organic carbon (DOC) concentrations of surface waters is a consequence of process changes in the surrounding terrestrial environment, both within annual cycles and over the longer term. Long-term records (1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) of DOC concentrations at six catchments (0.44-10.0 km 2 ) across a climatic transect in Scotland were investigated for intra-annual relationships to evaluate potential long-term seasonal patterns. The intra-annual mode of DOC export contrasted markedly between catchments and appeared dependent on their hydrological characteristics. Catchments in wetter Central Scotland with high rainfall-runoV ratios, short transit times and well-connected responsive soils show a distinct annual periodicity in DOC concentrations throughout the long-term datasets. Increased DOC concentrations occurred between June and November with correspondingly lower DOC concentrations from December to May. This appears unrelated to discharge, and is dependent mainly on higher temperatures driving biological activity, increasing decomposition of available organic matter and solubility of DOC. The drier eastern catchments have lower rainfall-runoV ratios, longer transit times and annual drying-wetting regimes linked to changing connectivity of soils. These are characterised by seasonal DOC concentration-discharge relationships with an autumnal Xush of DOC. Temperature inXuences the availability of organic matter for DOC transport producing a high DOC concentration-discharge relationship in summer/autumn and low DOC concentration-discharge relationship in winter/spring. These two distinct modes of seasonal DOC transport have important implications for understanding changes in DOC concentrations and export brought about by climate change (temperature and precipitation) and modelling of aquatic carbon losses from soil-types under diVerent hydrological regimes.