Effects of resource availability and hydrological regime on autochthonous and allochthonous carbon in the food web of a large cross-border river (China).

Resource availability and flooding disturbance restrict the amount of energy available to the upper trophic level consumers and thus determine the trophic structure and energy mobilization in river food webs. In this study, we evaluated the availability of primary and secondary food resources, food web structure (determined by δ13C and δ15N) and relative contributions of autochthonous and allochthonous particulate carbon to aquatic consumers in the Irtysh River, which spans from northwest China to Kazakhstan and suffers from a long frozen period. Despite higher density and biomass, epilithic algae did not make large contributions to aquatic consumers due to the restriction of flow velocity, water depth and turbidity. Aquatic invertebrates specialized in utilization of terrestrial carbon sources, whereas fish varied from aquatic to riparian plants. Different resource use of aquatic consumers across the three reaches in the Irtysh River was ascribed to the spatial distribution of species and resource availability determined by flooding, flood scouring and dam construction. The trophic positions and food chain length at the upper reach were higher than those at the middle and lower reaches. These findings suggest that allochthonous carbon had an advantage over autochthonous carbon in supporting aquatic food webs of the Irtysh River. Higher availability of allochthonous particulate carbon might be relevant to intensive forest cover and high energy flood events in the Irtysh River.