Carbon dioxide and methane evasion from a temperate peatland stream

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 headwate r stream. Carbon dioxide was consistently above atmospheric saturation in t he stream, with mean concentrations of 159.1, 81.8, and 29.5 mu mol L-l at the lower, middle, and upper sites, respectively (i.e., 7.6, 3.9, and 1.2 t imes in excess of atmospheric equilibrium concentrations). Methane concentr ations in stream water were much lower but showed a similar pattern. Rates of gaseous evasion from the stream surface to the atmosphere, determined ex perimentally using direct measurement of dissolved gas concentrations in co njunction with coinjection of conservative solute and volatile gas tracers, also declined downstream. Combined stream losses of all forms of carbon fr om the entire catchment (i.e., degassing from the stream surface and export s downstream) totaled 54,140 kg C yr(-1). Evasion of carbon dioxide from th e stream surface accounted for 34% of this total, compared to 57% lost as d issolved organic carbon via export downstream. When expressed per unit area of watershed, the gaseous C evasion from the stream represents a loss of 1 4.1 g C m(-2) yr(-1), which equals 28-70% of the estimated net carbon accum ulation rate fur such peatlands. This study shows that gaseous carbon loss from the surface of temperate headwater streams can be both spatially varia ble and significant in terms of rates of net annual land surface-atmosphere exchange at the catchment scale.