Impacts of coastal dynamics on blue carbon ecosystems: Assessing long-term carbon balance in mangroves of Amazon coast

Coastal erosion is a significant driver of global mangrove loss, yet its net effect on carbon stocks remains poorly understood. This uncertainty arises from sampling limitations and the potential for inland migration and accretion to offset losses. In this study, we estimated the total ecosystem carbon stocks (TECS) of mangroves undergoing erosion and accretion on a tropical island along the Amazon coast. The carbon balance was quantified over a 38-year period by combining field sampling with remote sensing. We found pronounced spatial variability in mangrove TECS (250–1373 Mg C ha⁻¹), closely linked to forest structure, soil properties, and coastal hydrodynamics. Results revealed an accentuated net decline of 55–77 % in TECS between 1985 and 2023, with an estimated loss of approximately 2500 Mg C. These losses are driven by an asymmetry in the carbon balance, highlighting that newly mangrove stands cannot fully compensate for the carbon lost from mature forests characterized by high biomass and organic-rich soils. Ultimately, our results reinforce the importance of local studies to improve large-scale estimates of carbon stocks and other ecosystem services provided by mangrove forests. • Coastal dynamics drive major shifts in Amazon mangroves carbon stocks. • Total Ecosystem Carbon Stocks varied widely (250–1373 Mg C ha⁻¹) across sites. • High carbon stocks were linked to peat layers in Rhizophora -dominated mangroves. • Changes on mangrove extant revealed a high decline in TECS, ∼ 2400 Mg C loss. • Results highlight the importance of localized processes in carbon assessments.