Lake characteristics influence how methanogens in littoral sediments respond to terrestrial litter inputs

Shallow lake sediments harbor methanogen communities that are responsible for large amounts of CH₄ flux to the atmosphere. These communities play a major role in degrading in-fluxed terrestrial organic matter (t-OM)-much of which settles in shallow near-shore sediments. Little work has examined how sediment methanogens are affected by the quantity and quality of t-OM, and the physicochemical factors that shape their community. Here, we filled mesocosms with artificial lake sediments amended with different ratios and concentrations of coniferous and deciduous tree litter. We installed them in three boreal lakes near Sudbury, Canada that varied in trophic status and water clarity. We found that higher endogenous nutrient concentrations led to greater CH₄ production when sediment solar irradiance was similar, but high irradiance of sediments also led to higher CH₄ concentrations regardless of nutrient concentrations, possibly due to photooxidation of t-OM. Sediments with t-OM had overall higher CH₄ concentrations than controls that had no t-OM, but there were no significant differences in CH₄ concentrations with different t-OM compositions or increasing concentrations over 25%. Differences among lakes also explained variation in methanogen community structure, whereas t-OM treatments did not. Therefore, lake characteristics are important modulators of methanogen communities fueled by t-OM.