Self-organized sulfide-driven traveling pulses shape seagrass meadows
Ruiz-Reynés, Daniel; Mayol, Elvira; Sintes, Tomàs; Hendriks, Iris E.; Hernández-García, Emilio; Duarte, Carlos M.; Marbà, Núria; Gomila, Damià
Seagrasses provide multiple ecosystem services and act as intense carbon sinks in coastal regions around the globe, but are threatened by multiple anthropogenic pressures, leading to enhanced seagrass mortality that reflects in the spatial self-organization of the meadows. Here we report on the formation of vegetation traveling pulses forming complex spatiotemporal patterns and rings in Mediterranean seagrass meadows. These structures emerge due to an excitable behavior resulting from the coupled dynamics of vegetation and porewater hydrogen sulfide, toxic to seagrass, in the sediment. The resulting spatiotemporal patterns resemble those formed in other physical, chemical, and biological excitable media but on a much larger scale. Based on theory, we derive a model that reproduces the observed seascapes and predict the annihilation of these circular structures as they collide, a distinctive feature of excitable pulses. We show also that the patterns in field images, and the empirically-resolved radial profiles of vegetation density and sediment sulfide concentration across the structures are consistent with predictions from the theoretical model, which shows these structures to have diagnostic value, acting as a harbinger of the terminal state of the seagrass meadows prior to their collapse.