The European Water Framework Directive (WFD) (2000/60 EC) requires, for all members, an assessment of the ecological status of aquatic ecosystems. This status has been defined as a result of the structure and the functioning of aquatic systems. Currently, most of the biological methods for lake monitoring are based on their trophic level and not on their functioning. Therefore, the aim of this study is to contribute to a better knowledge of lake trophic functioning i.e. the lake’s ability to transfer the organic matter up to consumers. Two approaches based on the macro-invertebrate communities have been applied on 12 French lakes: The Lake Biotic Index (LBI) (Verneaux & al., 2004), a new lake biological quality assessment method, comprising two indices, each of them giving rise to a peculiar interpretation: the Bl index reflecting a trophic potential and the Df index, interpreted as the trophic functioning result i.e. the efficiency of the organic matter transfer. The carbon isotope ratios (δ13C) of macro-invertebrates sampled according to the LBI method. These ratios have been analysed to test relationships of the LBI interpretations (in term of trophic potential and organic matter transfer) with carbon sources and organic matter recycling. The LBI obtained, from 12 French lakes, showed that systems greatly differed through both their trophic potential and their organic matter transfer. The results, concerning the LBI, showed also that all combinations between high/low trophic potential and efficient/no efficient organic matter transfer exist. The results of carbon isotope ratios, from 7 lakes, revealed great differences between mean δ13C values, standard deviations and the depth related δ13C variations. Based on the macro-invertebrate δ13C results obtained from 7 lakes, preliminary interpretations could be proposed: The littoral δ13C standard deviations would be related to the lake trophic potential; despite none correlation between Bl index and 13C have been obtained, two groups of lakes appeared: lakes with weak trophic potential and standard deviation of δ13C values less than or equal to 1 and lakes with high trophic potential and standard deviation of δ13C values superior to 1. The variations in 13C values between littoral and deep zones, correlated to the Df index, could reflect the trophic functioning. Through the 7 lakes, two types of trophic functioning have been defined: one type based on a high heterotrophic organic matter recycling activity and another based on direct organic matter consumption without recycling.