Potential of δ13C and δ15N of cladoceran subfossil exoskeletons for paleo-ecological studies

Stable isotope analyses on cladoceran subfossil exoskeletons retrieved from sediment cores could allow the reconstruction of past changes in lake food webs provided the delta C-13 and delta N-15 values of the exoskeletons reflect those of the organisms' whole body. The relationships between the C and N stable isotope compositions of the exoskeletons and those of the whole body were investigated for two freshwater cladoceran taxa (Bosmina sp. and Daphnia sp.) from modern samples. The C and N stable isotope compositions of the exoskeleton and those of the whole body were strongly correlated. Exoskeleton delta C-13 was similar to the whole body delta C-13 for both taxa. Daphnia exoskeletons were strongly depleted in N-15 (-7.9aEuro degrees) compared to the whole body. Stable isotope analyses were thereafter performed on cladoceran remains from five downcore samples from Lake Annecy, France. Results showed that Bosmina delta N-15 values increased by more than 4aEuro degrees, between the early twentieth and twenty first centuries. Such changes might be the result of changes in nitrogen sources or cycling in the lake and/or of major shifts in Bosmina trophic position within the lake food web. This study sets up the potential of stable isotope analyses performed on cladoceran subfossil remains for paleo-ecological purposes.