A knowledge of the stable carbon isotopic ratio of methane S13C-CH4 in natural systems can be useful in studies of the mechanisms and pathways of CH4 cycling (Sansone et al. 1997). Values of carbon isotope signature of methane (S13C-CH4) indicate biogenic nature of the methane, being usually in the range -27 % up to -100 % (Conrad 2004; Michener & Lajtha 2007). Whiticar et al. (1986) demonstrated that methane in freshwater sediments is isotopically distinguished by being relatively enriched in 13C (S13C = -65 to -50%) in contrast to marine sediments (-110 to -60%). Accordingly, the two precursors of methane, namely acetate and CO2/H2, yield methane with markedly different S13C values; methane from acetate is relatively enriched in 13C. Average minimum in the carbon isotopic composition of CH4 (-61.4 %) occurred deeper in sediments (60 cm) while average maximum in S13C-CH4 occured in the lower sediment depth of 30 cm. Enrichment of 13C in CH4 probably reflects aerobic CH4 oxidation because oxidation would result in residual CH4 with S13C-CH4 values less negative than the source CH4 (Barker & Fritz 1981; Chanton et al. 2004). However, this effect has been observed only at the study site IV.