Old methane fuels modern river food web

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 enter 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 macro–seeps where underground deposits connect to the surface
via fissures and fractures. We explored the contribution of methane from
natural gas macro–seeps 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 macro–seeps had
significantly depleted δ13C values compared to animals from reference
reaches. Within natural gas macro–seep 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 macro–seep 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
zooplankton, with up to 33% 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.