Old methane fuels modern river food web

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Abstract Dominant ecological paradigms provide theoretical frameworks for two basal sources of carbon to riverine food webs: carbon fixed by algae and submerged macrophytes (autochthonous) and carbon fixed by terrestrial vascular plants that enters aquatic ecosystems as litter (allochthonous). An additional, less‐studied, pathway for carbon to enter rivers is as methane that is produced in small quantities via methanogenesis by anaerobic methanogenic archaea. Because rivers are generally well‐oxygenated, methane is not often recognised as a substantial contributor to riverine carbon budgets. However, methane may also enter rivers via natural gas macroseeps where underground deposits connect to the surface via fissures and fractures. We explored the contribution of methane from natural gas macroseeps to food webs by comparing δ13C values of animals and their foods from reaches rich in dissolved methane with reaches where methane concentrations were at background levels. Animals collected from river reaches with natural gas macroseeps had significantly depleted δ13C values compared to animals from reference reaches. Within natural gas macroseep reaches, methane served as the principal basal carbon source for mayfly primary consumers (Tasmanocoenis spp.), contributing up to 55% of their total carbon budget. Methane‐derived carbon persisted across multiple trophic levels of the natural gas macroseep food web, subsidising up to 19% of the carbon in Decapoda and 28% in carnivorous fish. Among Decapoda, carbon of terrestrial origin dominated biomass contributions, supplying up to 55% of carbon to Macrobrachium australiense and 56% to Parataya australiensis. Autochthonous carbon contributions were highest among mayflies, with up to 30% of their carbon fixed by algae. Our findings establish the prevalence of a natural gas‐derived methane–methanotrophic bacteria–primary consumer pathway. We emphasise its significance as a potentially dominating energy conduit in rivers with high concentrations of dissolved methane, demonstrating feeding preference redundancy at low trophic levels. Additionally, we provide useful information for the incorporation of different sources of carbon in riverine food webs. Read the free Plain Language Summary for this article on the Journal blog.