Highest rates of methanogenesis and sulfate reduction in the surfacesediments, integrated over 0–25 cm b.s.f., were observed on the shelf(70–253 m, 0.06–0.1 and 0.5-4.7 mmol m−2 d−1, respectively),while lowest rates were discovered at the deepest site (1024 m, 0.03 and0.2 mmol m−2 d−1, respectively). The addition of methanolresulted in significantly higher surface methanogenesis activity, suggestingthat the process was mostly based on non-competitive substrates – i.e.,substrates not used by sulfate reducers. In the deeper sediment horizons,where competition was probably relieved due to the decrease of sulfate, theusage of competitive substrates was confirmed by the detection ofhydrogenotrophic activity in the sulfate-depleted zone at the shallow shelfstation (70 m).
Surface methanogenesis appeared to be correlated to the availability oflabile organic matter (C ∕ N ratio) and organic carbon degradation (DICproduction), both of which support the supply of methanogenic substrates. Anegative correlation between methanogenesis rates and dissolved oxygen in thebottom-near water was not obvious; however, anoxic conditions within the OMZmight be advantageous for methanogenic organisms at the sediment-waterinterface.
Our results revealed a high relevance of surface methanogenesis on theshelf, where the ratio between surface to deep (below sulfate penetration)methanogenic activity ranged between 0.13 and 105. In addition, methaneconcentration profiles indicated a partial release of surface methane intothe water column as well as consumption of methane by anaerobic methaneoxidation (AOM) in the surface sediment. The present study suggests thatsurface methanogenesis might play a greater role in benthic methanebudgeting than previously thought, especially for fueling AOM above thesulfate–methane transition zone.
