Novel functional drainage density for Blue Carbon Ecotones

Species distribution depends on the topology of ecosystems jointly with bioclimatic factors. Hydrological networks, as terraformers, play a pivotal role in shaping Blue Carbon Ecotones (BCEs) by influencing ecosystem functions such as ecological dispersal and distribution, water biogeochemistry, and tide-runoff balance, which impact ecohydrological feedback and climate. Yet, mapping hydrological networks is vital for developing effective restoration and conservation strategies that are enhancing ecosystemic fitness and services. Here, a comprehensive Functional Drainage Density (FDD) was developed by integrating a four-dimensional flow connectivity (CSI) as a multiplicative factor of the traditional drainage density (DD), at a pixel resolution of ∼ 100 km 2 globally. We showed that the overlap of BCEs’ ecohydrological niches through FDD is a prerequisite for their co-occurrence. However, the co-occurrence requires trade-offs to reach a Pareto-optimal balanced state; hence, the rarity of the BCEs overlap, i.e., only 23.4% and 4.0% for coastal areas with two and three co-existing BCE topologies. FDD is a critical predictor of BCE shifts, where high FDD supports the one single BCE within their climate niche versus enhancing diversity. Positive functional feedback for co-occurring BCEs suggest that their co-occurrence is highly beneficial for co-dependent habitats such as oyster reefs, and ecosystem-scale restoration projects. The ocean-land ecotone, as much as riparian areas, is home to the vast majority of people and is the most prone to critical risks and opportunities. Our results emphasize that the optimal BCE portfolio should consider FDD for enhancing BCEs function and services, as well as other ecotones at the basin-scale, via conservation and restoration strategies. • Functional drainage density predicts the ecosystemic structure and function of BCEs. • Hydrological networks and flows sculpt the spatial patterns of BCEs. • Low and high FDD encourages the expansion of one single BCE. • Intermediate FDD enhances the cooperative Pareto-optimal coexistence of BCEs. • Pareto optimal BCEs is associated to the highest habitat diversity and health.