Allochthonous versus autochthonous carbon subsidies in small river food webs depend on seasonality and riparian tree species

The river continuum concept (RCC), which proposes that terrestrial allochthonous inputs, namely, leaf litter, is the main source of organic carbon in trophic webs of forested headwater streams, was based on gut content analyses of benthic invertebrates. Modern methods, such as stable isotope analysis (SIA), fatty acid (FA) biomarker analysis, and elemental analysis (C:N stoichiometry), indicate autochthonous sources (periphytic microalgae) to be the main basal source of carbon in streams. However, these modern methods were applied in a comparatively limited number of cases. We hypothesize that: (1) proportions of incorporation of organic carbon of leaf litter by benthic consumers can be higher in spring than in autumn due to conditioning of the leaves during winter, and (2) carbon of leaf litter of certain tree species can be incorporated in river food webs to a higher degree than carbon of other tree species. Using SIA, FA, and C:N, allochthonous and autochthonous sources of organic matter and their quality in food webs of a small forest river in the Russian Far East were studied. It was found that some benthic animals obtained organic carbon mainly or exclusively from the allochthonous basal source, leaf litter. The other animals relied heavily or exclusively on the autochthonous basal source of organic carbon, periphytic microalgae, but some of them shifted to the conditioned leaf litter as the principal basal source in spring. Leaves of one of the tree species, Manchurian alder, appeared to be significantly more valuable food for primary consumers than leaves of the other studied species. Thus, the RCC should not be rejected but rather specified in terms of the seasonality and species composition of riparian vegetation.