Warming increases the proportion of primary production emitted as methane from freshwater mesocosms

Abstract Methane (CH 4 ) and carbon dioxide (CO 2 ) are the dominant gaseous end products of the remineralization of organic carbon and also the two largest contributors to the anthropogenic greenhouse effect. We investigated whether warming altered the balance of CH 4 efflux relative to gross primary production (GPP) and ecosystem respiration (ER) in a freshwater mesocosm experiment. Whole ecosystem CH 4 efflux was strongly related to temperature with an apparent activation energy of 0.85 eV. Furthermore, CH 4 efflux increased faster than ER or GPP with temperature, with all three processes having sequentially lower activation energies. Warming of 4 °C increased the fraction of GPP effluxing as CH 4 by 20% and the fraction of ER as CH 4 by 9%, in line with the offset in their respective activation energies. Because CH 4 is 21 times more potent as a greenhouse gas, relative to CO 2 , these results suggest freshwater ecosystems could drive a previously unknown positive feedback between warming and the carbon cycle.