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

Carbon stable isotope ratios (δ¹³C) are widely used to trace resource transfer pathways, yet δ¹³C 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 We have analyzed data from 42 published studies, supplemented with some unpublished data, to show the determinants of lotic periphyton δ¹³C. At large spatial scales, we observed broad differences in periphyton δ¹³C among biomes and consistent longitudinal variation related to watershed area. Longitudinal increases in δ¹³C indicate the importance of in-stream processes on lotic carbon cycles and autotroph δ¹³C variation. At local spatial scales, periphyton δ¹³C was negatively related to canopy cover and water current velocity and positively related to chlorophyll a density. Autotroph δ¹³C varied among taxonomic groups. Cyanobacteria and red algae had significantly higher and lower δ¹³C than other taxa, respectively. A hierarchical model across spatial scales showed that local controls for periphyton δ¹³C were nested by regional controls, which suggested that productivity and CO₂ availability determine δ¹³C. Overall, our results reveal general patterns of periphyton δ¹³C and provide improved information for study design and the use of δ¹³C in isotopic mixing models in lotic food web studies.