Seasonal variability in the δ13C and δ15N values of the zooplankton taxa in two alpine lakes

δ13C and δ15N values in crustacean taxa were measured once a month in two alpine lakes over a 2-year period in order to address seasonal changes in the isotope composition of the zooplankton. Daphnia sp. was used as the ‘pelagic baseline’ (i.e. primary sources fueling the pelagic food web), as it exhibited the lowest δ13C and δ15N values among the crustacean taxa. Seasonal changes resulting from modifications in the trophic position of other crustacean taxa were assessed by comparing their seasonal δ13C and δ15N values to those of Daphnia sp. Baseline modifications were addressed by relating them to physical, chemical and biological changes in the epilimnion. In Lake Geneva, all the crustacean taxa rely on the same carbon source and occupy relative trophic positions that display no major changes over time. In contrast, in Lake Annecy, the crustacean food web was found to be much more complex. The crustacean taxa apparently obtained their carbon from differing primary sources and their relative trophic positions changed during the year. This study raises the question of the ambiguity of the pelagic baseline concept when applied to zooplankton trophic studies.