Methane metabolism was investigated with respect to depth in intertidal microbial mats of the Great Sippewissett Salt Marsh, Massachusetts. Although sulfate-reducing organisms dominate anaerobic carbon consumption in marine microbial mats, methanogens persist and their activity varies vertically and temporally in the mat system. In the Sippewissett mats, potential methane production for all mat layers was higher in the spring (17.2 +/- 4.5 nmol CH(4) cm(-2) day(-1)) than in the fall (3.0 +/- 1.1 nmol CH(4) cm(-2) day(-1)) and maximal rates were consistently observed in proximity to the chemocline (5-10 mm depth). The methane flux from the mat surface did not vary appreciably over time due to the ability of methanotrophic activity to limit net methane production. Evidence indicates that both aerobic and anaerobic oxidation of methane occurs in this system. The importance of H(2) as a substrate for methanogenesis appeared to be the greatest at the mat surface (0-10 mm), and the proportion of methylotrophic methanogens generally increased with depth. These results suggest that both non-equilibrium H(2) dynamics and the use of non-competitive substrates permit coexistence of methanogens and sulfate-reducing organisms in the mat system.