Transfer of C from naturally depleted peat to freshwater benthic invertebrates in a controlled laboratory system.

Radiocarbon (C) is a key radionuclide in radioactive waste management due to its long half-life and potential integration into the global carbon cycle. However, its incorporation into freshwater organisms remains poorly understood. This study investigated the transfer of C into two benthic invertebrates, Lumbriculus variegatus and Chironomus riparius, in a controlled experimental system. Naturally C depleted peat was used as the primary carbon source and substrate for the organisms, enabling clear source tracing without artificial labeling but represented a simplified scenario compared to natural sediments with higher nutritional quality. An isotope mixing model based on the natural C abundance partitioned C sources between peat and dietary inputs (fish food and unbleached strips of paper towel). Results showed significantly higher peat-derived C contribution in C. riparius (40 %) than in L. variegatus (<3 %), likely due to more active substrate-feeding of recalcitrant peat during early instars. In contrast, L. variegatus with slower C turnover likely relied on previous dietary C (fish food and unbleached strips of paper towel) or assimilation from fish food in the feeding group rather than feeding on recalcitrant peat with larger particles. Survival rates differed, with lower chironomid survival possibly influenced by a combination of peat characteristics (recalcitrant and acidic), food distribution, and sensitivity of early instars. While these findings provide baseline data for biosphere models of C transfer, the use of peat and simplified conditions limits direct extrapolation to natural systems. Further studies with representative sediments and ecological complexity are needed to improve risk assessments.