Coastal blue-carbon ecosystems are reshaped by urban development, land reclamation and shoreline engineering. Yet aggregate changes in habitat area or carbon stock reveal the system-level outcome while obscuring the transition pathways and ecological and anthropogenic drivers that produced it. An apparent recovery may therefore arise from fundamentally different processes—from reduced habitat loss to new habitat formation or internal reorganisation—with different implications for its persistence and management. Singapore offers a test because development, conservation and restoration have unfolded concurrently along its compact tropical coastline, allowing contrasting pathways of loss and recovery to be resolved within a single system. We reconstruct Singapore's blue-carbon system from 1986 to 2020 to test for a system-level shift, identify the pathways that reshaped its carbon balance, and resolve the spatial controls on loss and recovery.

We mapped four blue-carbon ecosystems—mangroves, mudflats, sandflats, and seagrass and benthic algae—across eight epochs using a 10-class Landsat framework with 77 spectral, temporal and environmental predictors (overall accuracy: 0.921 ± 0.024). We combined structural-break analysis with site-specific, transition-based carbon accounting across five coastal zones to detect a turning point and quantify pathway-level carbon change. Pixel-scale Random Forest models, interpreted using SHAP values and permutation importance, revealed the ecological, hydro-geomorphic and anthropogenic controls underlying recovery and continued loss.

Singapore's blue-carbon system did not follow a single declining trajectory. A turning point emerged in 2005 (p = 0.011), separating a loss phase from a recovery phase. Blue-carbon area declined by 324 ha during 1986–2005 but increased by 143 ha during 2005–2020, with mangrove dynamics dominating both phases. Land-cover transitions generated a net carbon-stock loss of 170.1 Gg C before 2005 and a net gain of 66.0 Gg C afterwards. Nevertheless, the full-period balance remained negative at −104.1 Gg C: post-2005 gains offset only 39% of the earlier loss. Recovery was not driven by faster formation of new blue-carbon habitat. Instead, it arose from sharply reduced direct blue-carbon losses and a reversal of mangrove–mudflat reorganisation, from mangrove loss before 2005 to mangrove establishment afterwards. Recovery was spatially uneven: mangrove gains were associated with nearby source habitat and suitable tidal-flat substrate, intertidal-flat change with hydro-geomorphic setting, and seagrass change with tidal exposure and urban–maritime disturbance.

Singapore shows that a heavily engineered coastline can reverse long-term blue-carbon decline. Protecting high-carbon habitats, maintaining source connectivity and safeguarding suitable substrates could help convert constrained recovery into sustained carbon gain, offering a practical pathway for rapidly urbanising tropical coasts.