Turtle Herbivory Suppresses Seagrass Function but Reveals Recovery Potential in a Remote Coral Reef Atoll

Seagrasses are recognised as ecologically and culturally significant habitats across the globe supporting human livelihoods, biodiversity, and carbon storage. In the Cocos (Keeling) Islands, a remote coral reef atoll in the eastern Indian Ocean, seagrasses provide ecosystem services and support culturally, ecologically, and economically important species. However, these meadows have suffered devastating declines, with ongoing green turtle herbivory identified as a key pressure restricting Thalassia hemprichii persistence.To examine how herbivory shapes seagrass traits and function, we compared plots protected from grazing for over two years using exclusion cages against continuously grazed plots across two sites.Seagrass traits at the meadow-scale (shoot density, leaf area and biomass), as well as some functionality (primary production and nutrient storage capacity), were improved with protection from herbivory pressure. Substantial reductions in the belowground biomass in the grazed plots indicate that these seagrasses are operating under severe metabolic constraints. In contrast, some plant-scale traits (photosynthetic rate and growth rate) were not altered despite ongoing herbivory pressure, indicating some capacity for recovery at the individual plant scale.Representing one of the longest turtle exclusion studies in seagrass ecosystems, these results demonstrate that herbivory is driving systemic degradation across multiple biological scales, but these seagrasses currently retain the capacity to recover once grazing pressure is reduced. Synthesis. Tropical seagrass meadows subjected to continuous herbivory pressure can persist in a chronically depleted state. Herbivore exclusion experiments demonstrate that reducing herbivory can unlock the recovery potential of seagrass. These findings highlight the need for nuanced management strategies to safeguard these meadows in a time where seagrasses continue to decline at both localised and global scales, and the global turtle populations are simultaneously on the rise.