Abstract

Mangroves are widely promoted as coastal bio-shields, yet their capacity to stabilize shorelines in sediment-deprived, high-energy deltas remains poorly quantified at fine spatial scales. This study addresses whether sustained mangrove cover actively mitigates long-term shoreline erosion, or whether these ecosystems instead function as passive indicators of underlying geomorphic change, in the Muthupet wetland (1995–2025). Using Landsat imagery, the Digital Shoreline Analysis System (DSAS), and the Combined Mangrove Recognition Index (CMRI), shoreline movement was coupled with localized biological cover change through Thiessen polygon interpolation. Over three decades, the coastline was highly unstable and retreating, with over 76% of transects actively eroding under sediment deprivation. Contrary to the bio-shield assumption, segments supporting denser mangrove cover in 1995 underwent the greatest landward retreat, whereas sparsely vegetated transects remained comparatively stable, and sustained cover did not significantly reduce shoreline fluctuation. The strong synchronization between physical shoreline retreat and habitat contraction (rs = 0.611, p < 0.0001) is consistent with the Muthupet mangroves functioning as passive indicators of coastal instability rather than active stabilizers. Consistent with observations from other sediment-deprived deltas such as Mekong, these findings necessitate a shift in regional coastal management toward a geomorphology-first framework that prioritizes sediment–budget restoration over vegetation planting alone.

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