Global meta-analysis for controlling factors on carbon stable isotope ratios of lotic periphyton

Carbon stable isotope ratios (delta C-13) are widely used to trace resource transfer pathways, yet delta C-13 variation in freshwater autotrophs is not yet fully understood. We have analyzed data from 42 published studies, supplemented with some unpublished data, to show the determinants of lotic periphyton delta C-13. At large spatial scales, we observed broad differences in periphyton delta C-13 among biomes and consistent longitudinal variation related to watershed area. Longitudinal increases in delta C-13 indicate the importance of in-stream processes on lotic carbon cycles and autotroph delta C-13 variation. At local spatial scales, periphyton delta C-13 was negatively related to canopy cover and water current velocity and positively related to chlorophyll a density. Autotroph delta C-13 varied among taxonomic groups. Cyanobacteria and red algae had significantly higher and lower delta C-13 than other taxa, respectively. A hierarchical model across spatial scales showed that local controls for periphyton delta C-13 were nested by regional controls, which suggested that productivity and CO2 availability determine delta C-13. Overall, our results reveal general patterns of periphyton delta C-13 and provide improved information for study design and the use of delta C-13 in isotopic mixing models in lotic food web studies.