Organic carbon (Corg) and bulk nitrogen (Nbulk) isotope compositions (respectively d13Corg and d15Nbulk) of sedimentary rocks deposited in the Archean and Proterozoic eons are commonly used as proxies for the biomass d13Corg and d15Nbulk values in paleo-environment and paleo-ecosystem studies. Many sedimentary rocks from these eons were deposited under ferruginous conditions, but it remains unknown whether C and N isotope signatures are preserved during diagenetic degradation of organic matter under this chemical state. Lake Pavin, Massif Central, France, is a permanently stratified freshwater lake with anoxic and ferruginous deep waters and represents a unique natural site for evaluating the impact of organic matter decomposition predominantly through iron redox cycling and methanogenesis. Here we present C and N isotope compositions of sediment collected from traps placed at different depths in the water column and from cores recovered in the anoxic zone of the lake. The d13Corg and d15Ntot values in the living biomass of Lake Pavin are estimated from the shallowest sediment traps. The biomass d13Corg values show a seasonally-controlled bimodal distribution, around -23.3 ± 1.7 ‰ (2SD) in September and November (organic matter produced in summer and autumn) and -28.0 ± 2.6 ‰ (2SD) in April and June (organic matter produced in winter and spring). These seasonal variations are attributed to variable d13C values in dissolved inorganic carbon, inherited from fluctuation in the relative rates of photosynthesis versus respiration. The d15Ntot values are near -1 ‰ for most seasons, indicating that the dominant N source to photosynthetic biomass is atmospheric N2. Along the water column profile, organic matter mineralization released about 20 % of C and 30 % of N, with only moderate effects on d13Corg and d15Ntot (decreased by 1.3 and 0.4 ‰, respectively). Although the water column data show significant seasonal variations, global annual fluxes, estimated from regular trap collection over 15 months, are remarkably consistent with the top sediment composition. In the sedimentary column, microbial degradation through ferric iron reduction and methanogenesis induces a loss of 40-50% Corg and 50-60% Ntot, with negligible modification of d13Corg and d15Ntot. We conclude that although the C/N ratio increases significantly as organic matter is degraded in the water column and in the first ~15 cm of the sedimentary column, d13Corg and d15Ntot are well preserved, supporting the idea that C and N isotope signatures can be reliably interpreted as tracers of primary biomass in sedimentary rocks deposited in anoxic and ferruginous environments.