The grazing impact of megaherbivores on sediment accumulation and stabilization functions of seagrass meadows in a subtropical coral reef lagoon

Seagrasses thrive in shallow lagoons between land and fringing coral reefs, serving as a buffer that mitigates the propagation of environmental stressors from land to reefs. However, mass grazing by megaherbivores poses a significant threat to seagrass loss, impacting the sustainability of tropical coastal habitats. Despite the urgency of this issue, few studies have investigated the possible consequences of grazing on the mitigating functions of seagrass beds. To explore the roles of seagrass beds in sediment accumulation and stabilization and assess the effects of herbivory on these functions, we conducted sediment trap experiments in a coral reef lagoon to examine the impact of seagrasses on sediment mobility. Hydrodynamic forces governing sediment mobility were quantified using a customized reef‐scale hydrodynamic model. The results indicated that sediment resuspension primarily driven by bottom shear stress was the primary factor influencing sediment mobility within the lagoon. Resuspension within healthy seagrass beds was generally low and exhibited minimal sensitivity to hydrodynamic forcing. Consequently, river‐borne mineral soil particles predominantly remained confined within the nearshore seagrass zone due to the trapping and stabilizing effects of seagrasses. Since 2019, seagrasses have been severely depleted due to intensive grazing by sea turtles. Simultaneously, sediment resuspension has intensified, and its dependence on bottom shear stress has become evident. Our findings demonstrate that reef seagrass can effectively suppress sediment resuspension by capturing river‐borne mineral soil. Furthermore, the loss of this function through mass grazing could potentially lead to sediment destabilization and erosion.