Effects of algal and terrestrial carbon on methane production rates and methanogen community structure in a temperate lake sediment

Summary 1. Sources of atmospheric CH 4 are both naturally occurring and anthropogenic. In fact, some anthropogenic activities may influence the production of CH 4 from natural sources, such as lakes. 2. Ongoing changes in the catchment of lakes, including eutrophication and increased terrestrial organic carbon export, may affect CH 4 production rates as well as shape methanogen abundance and community structure. Therefore, inputs from catchments to lakes should be examined for their effects on CH 4 production. 3. We added algal and terrestrial carbon separately to lake sediment cores and measured CH 4 production. We also used quantitative polymerase chain reaction and terminal restriction fragment length polymorphism to determine the effects of these carbon additions on methanogen abundance and community composition. 4. Our results indicate that CH 4 production rates were significantly elevated following the addition of algal biomass. Terrestrial carbon addition also appeared to increase methanogenesis rates; however, the observed increase was not statistically significant. 5. Interestingly, increased CH 4 production rates resulted from increases in per‐cell activity rather than an increase in methanogen abundance or community compositional shifts, as indicated by our molecular analyses. 6. Overall, anthropogenic impacts on aquatic ecosystems can influence methanogenesis rates and should be considered in models of global methane cycling and climate.