Origine et flux de carbone dans les réseaux trophiques lacustres : Etude par analyse de la composition en isotopes stables du carbone et de l'azote du zooplancton et des poissons.

摘要

Fish communities occupy the top of food chains. How fish obtain their constitutive carbon depends consequently of all the carbon pathways at the lower levels of the food web. This trophic position confers to fish a potential role of integrators of the underlying processes, at the ecosystem scale. Moreover, stable isotopes composition of an organism depends on all the pathways that transfer carbon up to this organism. Consequently, we wondered in what extent the stable isotopes composition of fish could be a proxy of the trophic functioning of lakes. To address this assumption, whitefish, Coregonus lavaretus, a zooplanktivorous salmonid common in alpine lake, was, in a first time, used as biological model. A preliminary study had revealed the stable isotope composition of whitefish was quite variable among alpine lakes. According to our idea, this variability mirrors differences in the carbon pathways of the food web of these lakes. To understand what was reflected by this variability, we studied initially in what extent it resulted from intrinsic factors (such as lake size or trophic status). This study has then lead us to deepen the role played by the zooplankton compartment in carbon pathways up to Whitefish. In the final stage of this first part, the isotopic composition of whitefish appeared as a proxy for lake trophic functioning. Changes in environmental factors can alter carbon pathways in food webs. If fish are efficient integrators of carbon pathways, impacts of perturbations should be mirrored on their isotopic compositions. Impacts of two anthropogenic perturbations of stable isotope composition of fish were then evaluated : the first one relates to eutrophication-restoration of two alpine lakes, the second one to water levels changes in two Malian reservoirs. Results from these two studies reveal that major changes, ensuing from these perturbation, in carbon origins and fluxes within lake food webs are recorded in the isotopic history of fish communities.