Fluxes of methane across the sediment-water interface were estimated either by direct measurement with benthic chambers or calculated by applying Fick’s first law.

Benthic fluxes

The methane fluxes across the sediment-water interface were measured using the method of benthic chambers (e. g. Sansone et al. 1998). Fluxes were measured during the summer months (VII, VIII, IX). The plexiglas chamber (2.6 dm3) covered an area 0.0154 m2. The chambers (n = 7) were installed randomly and gently anchored on the substrate without

disturbing the sediment. Samples to determine of initial concentration of CH4 were collected from each chamber before the beginning of incubation. Incubation time was 24 hours. Samples of water were stored in 40 ml glass vials closed by cap with PTFE/silicone septum until analysis.

Diffusive fluxes

Fluxes of methane between the sediment and overlying water were calculated from Fick’s first law as described by Berner (1980):

J = — DS x Ф x (AC / Ax) (1)

where J is the diffusive flux in pg m-2 s-1, Ф is the porosity of the sediment, Ds is the bulk sediment diffusion coefficient in cm-2 s-1, ACtAx is the methane concetrations gradient in pg cm-3 cm-1. Bulk sediment diffusion coefficient (Ds) is based on diffusion coefficient for methane in the water (Do) and tortuosity (0) according to the formula:

Ds = D0P-1 (2)

Tortuosity (0) is possible calculate from porosity according to equation (Boudreau 1996):

в-2 = 1 — Іп(ф2) (3)

Diffusive fluxes of CH4 were determined at all five study sites along the longitudinal profile of the Sitka stream.