Abrupt Loss of Soil Organic Carbon Following Disturbance in Seagrass Ecosystems

Abstract

Seagrasses are key carbon sinks in the biosphere and, when intentionally conserved or restored, constitute a promising natural solution 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 acknowledged, 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 stoichiometric model that couples ecological and biogeochemical functioning to assess the effects of three major stressors—mechanical damage, eutrophication, and warming—on the carbon storage capacity of seagrass ecosystems. After parameterizing our model for the Mediterranean seagrass Posidonia oceanica (L.) Delile, we explored these stress cases to identify the processes and feedbacks that can cause ecological tipping points leading to changes in biogeochemical dynamics. The model shows that when ecological tipping points occur, they cascade into biogeochemistry and precipitate abrupt losses of carbon storage. Importantly, even without a tipping point, carbon storage still declined abruptly rather than gradually along stressor gradients. Yet, the dynamics of carbon losses depended on the type of stressor, indicating the need to further test the relative contribution of biotic and abiotic drivers in shifting seagrasses from carbon sinks to carbon sources.