A mechanistic-based framework to understand how dissolved organic carbon is processed in a large fluvial lake

Dissolved organic carbon (DOC) plays a fundamental role in the ecology of aquatic systems. Our current understanding of the DOC dynamic in fluvial systems, however, is mostly based on empirical observations, often derived from controlled experiments. Although such experiments provide valuable insights on how DOC is processed, they do not inform us of the underlying mechanisms involved, which limits our understanding of the biogeochemical cycling of DOC in natural fluvial systems. We developed and validated a mechanistic-based model that provides a framework to understand how DOC is processed in a large fluvial lake ecosystem. Our approach relies on a simple multiple-reactor, kinetic transport system that integrates the principal processes involved in the kinetics of labile (DOC_L) and semilabile (DOC_SL) DOC pools. Our model reveals that during their transition along the longitudinal axis (~24 h, ~50 km), both DOC pools were utilized by bacteria and transformed differentially according to their sources. A large fraction of DOC_L was preferentially used for biomass production, whereas DOC_SL completed bacterial carbon demand. Based on the simulations, our results further suggest that the processing of DOC_L drives the ecosystem metabolism through a priming effect on DOC_SL, emphasizing the critical role of residence time on DOC biogeochemical cycling in fluvial lakes. This framework provides a theoretical basis for further development that could be adapted to take into account not only other aspects of DOC cycling but also the prevailing hydrodynamic conditions.