Methanogens aren't what they eat: littoral sediment methanogenesis is influenced by lake characteristics rather than terrestrial organic matter inputs

Freshwater systems are responsible for large amounts of CH4 efflux to the atmosphere. In particular, shallow lake sediments harbour methanogen communities with several described mechanisms for CH4 to evade in situ oxidation. Shallow lake sediments are also connected to the surrounding terrestrial ecosystem, and are subject to frequent influx of terrestrial organic matter—much of which settles in the sediments. Despite this connection, little work has examined how sediment methanogens are affected by the flux of fresh terrestrial OM, and the factors that shape their communities. Here, we experimentally created lake sediments with different ratios and concentrations of coniferous and deciduous tree litter. These sediments were placed in mesocosms, and installed in three boreal lakes near Sudbury, Canada, that varied in both water clarity and trophic status. We found that lower water clarity corresponded with higher rates of methanogenesis, but increasing litter inputs above a certain threshold did not. The different lakes also had rates of processing for high molecular weight C, which resulted in different levels of inhibition between the three lakes. Differences between lakes also explained some variation in methanogen community structure, whereas leaf litter ratio did not, and the only differences with litter concentration was between sediments with litter versus the inorganic only controls. Therefore, we conclude that water clarity, and trophic status could be an important factor in modulating methanogenesis in lake sediments.