The relationships between fish community delta15N and delta13C values and land use in watersheds and in buffer zones of nine French lakes were studied. The results showed inter-lakes variability of mean delta15N and delta13C values for different fish species. The study of correlations between fish delta15N and delta13C values and the proportions from different land use categories in watersheds showed a land use effect on fish delta15N values but not on delta13C values. The results underlined the great impact of manure spreading on grasslands as a nitrate source in our lakes. Nevertheless, the best correlations between fish delta15N values and land use were obtained when the entire fish community (i.e. the whole fish community mean delta15N values in each lake) and all anthropogenic activities (residential lands, camp sites and grasslands for our systems) were considered in either watersheds or in buffer zones. Thus, for our sites, the fish delta15N values are useful to evaluate nitrogen inputs from human and animal wastes at the watershed-scale as well as at the buffer zone-scale. The absence of land use effect on the fish community delta13C values could be at least partly explain by the great importance of heterotrophic activity in the studied lakes.