Analyzing the role of land use and shoreline changes on carbon sequestration in mangrove forests of Nijhum Dwip, Bangladesh

Mangrove islands experience combined pressures from changes to their shores, conversion of land for agriculture, and declining blue carbon storage. Still fewer researchers look at these processes combined. For this land cover classification, the study area Nijhum Dwip, Bangladesh was examined using Landsat 5 TM (1990) and Landsat 8 OLI (2020) at 30 m resolutions. The study mapped out seven land-use and land-cover classes (LULC), extracted the shorelines using NDWI and MNDWI measurement and assessed shoreline changes using Digital Shoreline Analysis System (DSAS) with a Net Shoreline Movement (NSM), End Point Rate (EPR) and Long-term Rate of Change (LRR) and assessment of carbon stocks using InVEST carbon model. Between 1990 and 2010, the mangrove area diminished by over 8.34 km². Built-up land area occupied only 0.10 km² in 1990 but expanded to 0.51 km² in 2020. Besides, mudflat area increased by about 12.30 km² during the 1990–2010 period but then fell below 10 km² by 2020. The southern shoreline experienced the strongest retreat, which exceeded 500 m in some sections. Overall, the long-term average shoreline erosion rate was about 22 m yr⁻¹ . The northern segments acquired land via sedimentation. Carbon stores in mangroves have fallen from about 4093.19 Mg(C) in 1990–2640.09 Mg(C) in 2010, before recovering to about 3327.24 Mg(C) in 2020. The study adds a new spatial framework linking multi-decadal land-use change, shoreline dynamics, and modeled carbon variation on a small deltaic island. The results indicate that newly gained land will not always lead to a net increase in blue carbon, as it takes time for new mudflats to stabilize and become sufficiently protected prior to the establishment of mangroves. The existing mangroves must be protected and their conversion restricted. Corridors for their migration must also be conserved. The restoration of mangroves must be prioritized on stable mudflats and accreting margins for long-term climate resilience.