Carbon incorporated into diatom frustule walls is protected from degradation enabling 5 analysis for carbon isotope composition (δ13Cdiatom). This presents potential for tracing carbon 6 cycles via a single photosynthetic host with well-constrained ecophysiology. Improved 7 understanding of environmental processes controlling carbon delivery and assimilation is 8 essential to interpret changes in freshwater δ13Cdiatom. Here relationships between water 9 chemistry and δ13Cdiatom from contemporary regional data sets are investigated. Modern 10 diatom and water samples were collected from river catchments within England and lake 11 sediments from across Europe. The data suggest dissolved, biogenically produced carbon 12 supplied proportionately to catchment productivity, was critical in the rivers and soft water 13 lakes. However, dissolved carbon from calcareous geology overwhelmed the carbon 14 signature in hard water catchments. Both results demonstrate carbon source characteristics 15 were the most important control on δ13Cdiatom, with a greater impact than productivity. 16 Application of these principles was made to a sediment record from Lake Tanganyika. 17 δ13Cdiatom co-varied with δ13Cbulk through the last glacial and Holocene. This suggests carbon 18 supply was again dominant and exceeded authigenic demand. This first systematic evaluation 19 of contemporary δ13Cdiatom controls demonstrates that diatoms have the potential to supply a 20 record of carbon cycling through lake catchments from sediment records over millennial 21 timescales.