Spatial Dynamics and Vegetation Densification of Mangroves in Ambon Bay Derived from NDVI–NDMI Analysis

Abstract

Mangrove ecosystems perform critical ecological functions in tropical coastal zones. Globally, these ecosystems have experienced significant areal decline, underscoring the urgency of systematic spatiotemporal monitoring. This study quantified changes in mangrove coverage across Teluk Ambon, Maluku Province, over a nine-year period (2017–2026) using multi-temporal Sentinel-2 L2A imagery processed through a dual-index classification approach combining NDVI and NDMI. Classification was performed in ArcMap 10.8, and accuracy was assessed against 324 validation points derived from Google Earth, yielding an overall accuracy of 91% and a Kappa coefficient of 0.86. Total mangrove coverage increased from 51.98 ha in 2017 to 59.70 ha in 2026, a net gain of 14.8%. The dominant ecological trajectory was not uniform spatial expansion but progressive canopy densification: the dense mangrove class expanded from 7.37 ha to 21.75 ha. Change detection analysis revealed that the sparse-to-dense transition was the principal driver of this structural development, contributing 9.38 ha in 2017–2021 and accelerating to 14.06 ha in 2021–2026, while canopy degradation declined markedly from 8.18 ha to 1.18 ha between periods. Concurrently, colonization of non-vegetated surfaces by sparse mangrove increased from 4.46 ha to 9.15 ha, indicating that active establishment complemented successional densification in the later period. These results indicate that Teluk Ambon’s mangrove ecosystem is undergoing simultaneous structural consolidation and new-area establishment, with implications for blue carbon accounting, coastal management, and the prioritization of conservation interventions.