Seagrasses are marine flowering plants forming underwater meadows that provide multiple key ecosystem services. These ecosystems are one of the most important carbon sinks in the biosphere. Unfortunately, seagrass ecosystems are currently experiencing major anthropogenic pressures. However, given the numerous ecological feedback mechanisms at stake, seagrass degradation may result in potential irreversible shifts (tipping points). My thesis project aims to model the coupled ecological and biogeochemical processes that determine the functioning of carbon dynamics in seagrass ecosystems. It also aims to evaluate the effect of global change stressors on carbon fluxes, and to identify potential tipping points at which these ecosystems may abruptly switch from carbon sinks to sources due to global change stressors.