Lithology constrains the origination of autochthonous carbon in river ecosystems

Dissolved inorganic carbon (DIC) provides a substrate for primary production in the lotic ecosystems, yet carbon's biogeochemical origination in the lotic food webs is still poorly constrained. Here, we assembled a global dataset of isotopic composition (i.e., ¹³C/¹²C or δ¹³C) of DIC and periphyton (algae being the primary producers) in river waters, and carried out a field study in two catchments respectively with carbonate and silicate dominated lithologies on the Tibetan Plateau. A two-endmember mixing model based on the datasets indicated that δ¹³C and concentrations of DIC in the river waters were largely determined by the catchment-scale chemical weathering of different lithologies. Meanwhile, a significant correlation was obtained between δ¹³C-DIC and δ¹³C-periphyton in the datasets, strongly implying that the origination of periphyton carbon was largely regulated by the catchment lithologies. The δ¹³C-periphyton compositions are also affected by isotopic fractionations during algal primary production, which, in turn, were closely related to the relationships between primary productivity and DIC availability in the rivers. The study advances our understanding of the origination and transfer of carbon biogeochemically bridging the geosphere and biosphere in the lotic ecosystems.