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

Carbon stable isotope ratios (δ(13)C) are widely used to trace resource transfer pathways, yet δ(13)C 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 We have analyzed data from 42 published studies, supplemented with some unpublished data, to show the determinants of lotic periphyton δ(13)C. At large spatial scales, we observed broad differences in periphyton δ(13)C among biomes and consistent longitudinal variation related to watershed area. Longitudinal increases in δ(13)C indicate the importance of in-stream processes on lotic carbon cycles and autotroph δ(13)C variation. At local spatial scales, periphyton δ(13)C was negatively related to canopy cover and water current velocity and positively related to chlorophyll a density. Autotroph δ(13)C varied among taxonomic groups. Cyanobacteria and red algae had significantly higher and lower δ(13)C than other taxa, respectively. A hierarchical model across spatial scales showed that local controls for periphyton δ(13)C were nested by regional controls, which suggested that productivity and CO(2) availability determine δ(13)C. Overall, our results reveal general patterns of periphyton δ(13)C and provide improved information for study design and the use of δ(13)C in isotopic mixing models in lotic food web studies.