Post-depositional alteration of stable isotope signals by preferential degradation of algae-derived organic matter in reservoir sediments

Abstract Post-depositional degradation of organic matter (OM) in freshwater sediments is crucial for driving the biogeochemical dynamics and influencing the consequential carbon burial. This process also often causes diagenetic alterations on paleoenvironmental proxies. Yet, mechanisms behind the degradation of sedimentary OM and the depth-related variations in stable isotope ratios can only be explained in part. Degradation of sedimentary OM in two drinking water reservoirs with contrasting trophic states (eutrophic versus mesotrophic) and catchment land-use (agriculture versus forestry) were studied with a combination of typical chemical treatments to group sedimentary organic matter into four different biochemical fractions, i.e., carbohydrates, proteins, lignins and lipids. Our results show that the degradation of sedimentary OM was mainly driven by preferential degradation of aquatic proteins and carbohydrates. In a subsequent step, lipids seemed to be degraded. This process entails more pronounced reduction of sedimentary OM and associated depth variations in δ13C and δ15N in the eutrophic reservoir of agricultural catchment, when compared to the mesotrophic reservoir with a forest dominated catchment. Moreover, changes in the relative proportions of biochemical components in sedimentary OM have more pronounced impact on δ15N isotope values relative to the ones of δ13C. Our findings also suggest that algae-derived OM is primarily responsible for the post-depositional alterations of elemental and isotopic compositions of sedimentary OM. This may be of importance for estimates of carbon sinks in lakes and reservoirs.