My Ph.D. research explores the application of carbon and nitrogen stable isotopes (SI) to describe the carbon cycle and food dynamic of northern lakes and reservoirs.

In order to accuratly understand consumers SI composition, the baseline signature of the system must be determined. I compared the various methods used in the litterature to assess the carbon signature of algae in the pelagic zone of lakes. This study indicates that the signature of a primary consumer (ie Daphnia) or the signature of particulate organic matter corrected for algal biomass are robust algal signatures. On the other hand, we have found that the signature of dissolved inorganic carbon produces a significantly lighter algal signature because of the difficulty to assess carbon fractionation in freshwaters.

Based on carbon SI, I have looked at the sources of carbon for zooplankton in oligtrophic systems dominated by allochthonous carbon as part of their particulate and dissolved organic bulk. Autochthonous carbon remain the main source of carbon in oligotrophic ecosystems, suggesting a strong decoupling between the microbial loop and the classical food-web.

In aquatic ecosystems, the nitrogen isotopic composition (d15N) of primary consumers may exhibit wide spatial and temporal variations arising from a number of different sources, which has important consequences when comparing multiple ecosystems food webs. The aim of this study was to assess the sources of variation in the d15N signature of various zooplankton taxa from a series of oligotrophic lakes and large reservoirs situated in the boreal ecoregion of Northern Canada. Seasonality and taxonomy were the most important sources of variance (35 and 51%, respectively) for zooplankton d15N. An ANCOVA based on temperature and taxonomy was constructed to predict d15N values of zooplankton and we verified its validity by using several data sets from the literature. Our results suggest that the model takes into account both variations due to differences in nutrient loads occurring between spring and summer and nitrogen transformation processes.

For more informations: http://www.science.uwaterloo.ca/~jmarty/jmarty.htm