Methane-derived carbon supports a complex food web in the shallow aquifer

Abstract Although most life on earth is fueled by photoautotrophic food resources, there is increasing evidence for methanogenic methane-derived carbon contributions to freshwater consumers. In one extreme case on the Nyack floodplain in Montana, methane-derived carbon comprised a majority of biomass carbon in stoneflies (Order:Plectopera) living in shallow aquifers underlying river floodplains, helping to explain how large organisms survive in such a carbon-limited system. We studied the food web structure of this system, including five stonefly species, two meiofauna taxa, and one amphipod. We found that trophic positions that varied within and between species were distinguished by δ13C and δ15N signatures indicative of varying, and sometimes preferential, use of methane-derived carbon resources, as well as some combination of trophic level variation within resource classifications. Some of these species likely access food resources at oxic-anoxic interfaces, as suggested by the presence of methanogenic and methanotrophic sequences in gut contents, as well as some evidence for anoxia tolerance. Overall, our work suggests that organisms in shallow aquifer ecosystems have adapted to coexist and use unconventional carbon sources that form in biogeochemically heterogeneous conditions.