Antoine Le Vilain, Oscar Serrano, Elisa Thébault, Eugenia T. Apostolaki, Vasilis Dakos 

Seagrasses are key carbon sinks in the biosphere and, hence, promising nature-based solutions for climate change mitigation. Unfortunately, they are also experiencing major anthropogenic and climatic pressures that can lead to seagrass degradation or even result in difficult-to-reverse abrupt shifts (i.e., tipping point responses) to complete loss. Although the possibility of tipping point responses in seagrass ecological dynamics has been addressed, the potential cascading effect of tipping points on biogeochemical dynamics, shifting seagrass ecosystems from carbon sinks to carbon sources, remains largely unexplored. In this context, we developed a mechanistic model coupling ecological and biogeochemical dynamics to assess the effects of global change stressors on the carbon storage capacity of seagrass ecosystems. After parameterising our model for the Mediterranean seagrass (Posidonia oceanica), we explored different stress scenarios -namely “mechanical damage”, “eutrophication”, and “warming”- to identify the processes, feedbacks, and the most critical parameters that can cause ecological tipping points leading to changes in biogeochemical dynamics. The model showed that, even in the absence of a tipping point, carbon storage was still lost abruptly along stress gradients rather than gradually driven by a cascade of ecological to biogeochemical dynamics. Yet, the dynamics of carbon losses depended on the type of stress, indicating the need to further test the relative relevance of biotic and abiotic drivers in shifting seagrasses from carbon sinks to sources.