Radiocarbon reveals depleted carbon in attached benthic assemblages of persistent pools in a tropical non-perennial river

Abstract Radiocarbon-depleted ("old") carbon can be bioavailable in aquatic ecosystems, but its role in persistent-pool food webs of non-perennial rivers is largely unknown. We measured hydrochemistry, environmental tracers and carbon isotopes in water and biota from four main-channel pools and one floodplain spring along the Martuwarra Fitzroy River, Western Australia, over two dry seasons. Dissolved inorganic (DIC) and organic (DOC) carbon remained near modern in all pools (97.6–103 and 95.1–101.0 percent Modern Carbon, pMC, respectively), yet attached benthic assemblages (periphyton and biofilm) were substantially more depleted, with periphyton as low as 57.1 pMC (~ 4,500 apparent radiocarbon years). Depletion did not track groundwater connectivity: the most depleted periphyton occurred where groundwater indicators were weakest. This biota–water mismatch suggests localised old-carbon sources of unresolved origin. Plausible sources include groundwater-derived DIC/DOC at connected sites, respired aged sedimentary organic carbon, methane-derived and rock-derived (carbonate or petrogenic) carbon. Pool consumers were near modern (~ 97.5–100.5 pMC). At the floodplain spring, strongly depleted DIC (30.4 pMC) co-occurred with depleted charophytes, aquatic insects and fish, although all biota were more modern than DIC. DIC and DOC radiocarbon may therefore fail to represent the 14 C composition of carbon incorporated into aquatic biomass, in either direction.