Integrating megafauna into blue carbon strategies: dugongs could enhance seagrass carbon storage

Coastal seagrass ecosystems occupy a small fraction of the global ocean yet make disproportionately large contributions to carbon capture and storage. In addition, they are increasingly promoted as blue carbon solutions within nationally determined contributions (NDCs). Despite this, most seagrass carbon budgets implicitly assume bottom-up control and very often neglect the functional role of large herbivores. Dugongs (Dugong dugon) are specialist seagrass grazers who may strongly influence seagrass productivity and sediment carbon storage through a combination of biomass removal, sediment disturbance, and rapid nutrient recycling, but their net effect on ecosystem carbon balance remains unknown. We apply a general animal-driven carbon-nutrient cycling model to estimate dugong effects in one of the world’s most important dugong hotspots, the Halodule seagrass beds of Bahrain. We parameterize the sediment-seagrass-dugong model with literature data and validated the model with estimates of net primary production (NPP), net ecosystem carbon balance (NECB) and sediment carbon stocks against literature-based and field measurements comparing scenarios with dugong presence vs. absence. Our results indicate that a realistic dugong aggregation (~700 individuals) can, on average enhance seagrass NPP and NECB by 2.4 times (with a range of uncertainty between 1.1- 4.2 times) and sediment carbon stocks by 2.63 times with a range of uncertainty of 1.1 – 7.6 times) relative to a dugong-absent conditions. This yields substantial additional carbon uptake and storage across the 145 km2 focal conservation area which corresponds to an additional 7.9 x107 kg C y-1 captured and 9.1 x108 kg C stored in sediments. Our findings demonstrate that conserving dugongs and their habitat can significantly increase the climate mitigation value of seagrass ecosystems and that explicitly accounting for animal functional roles is critical to avoid underestimating blue carbon contributions in NDC accounting.