Influence of watershed’s anthropogenic activities on fish nitrogen and carbon stable isotope ratios in nine French lakes

The relationships between fish community δ15N and δ13C values and land use in watersheds and in buffer zones of nine French lakes were studied. The results showed inter-lakes variability of mean δ15N and δ13C values for different fish species. The study of correlations between fish δ15N and δ13C values and the proportions from different land use categories in watersheds showed a land use effect on fish δ15N values but not on δ13C 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 δ15N values and land use were obtained when the entire fish community (i.e. the whole fish community mean δ15N 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 δ15N 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 δ13C values could be at least partly explained by the small size of these lakes in which it has been shown that consumers, namely macroinvertebrates, have δ13C values largely influenced by organic matter recycling activity