Tracking sedimentation process under progressive mangrove propagation in low-energy bays

Mangroves play a critical role in wave attenuation, carbon storage and tidal flat accretion. However, the mechanisms linking mangrove sedimentation to tidal flat deposition environment were not well documented. This study reconstructed the historical sedimentation process of the Mai Po Mangrove Reserve, a low-energy system with limited human disturbance, using in situ observations, sediment core samples and remote sensing images. The results show that sediments at the seaward mangrove fringe experienced two coarsening phases during 1990–2024, with mean grain size decreasing from 6.58 ± 0.77 φ before 2006 to 5.80 ± 0.66 φ after 2006 and further to 5.20 ± 0.65 φ after 2017. Within the mangrove interior, mean grain size increased from 3.35 ± 0.38 φ to 5.14 ± 0.95 φ after 1996, and subsequently decreased to 3.96 ± 0.39 φ after 2017, remaining consistently coarser than the fringe. Meanwhile, the mass accumulation rate (MAR) declined markedly from 1.40 g cm⁻² yr⁻¹ in 1999 to 0.94 g cm⁻² yr⁻¹ in 2016 and then sharply rebounded to 1.31 ± 0.07 g cm⁻² yr⁻¹, whereas MAR within the interior steadily decreased, with a mean value of 0.54 ± 0.12 g cm⁻² yr⁻¹ over the study period, consistently lower than the fringe. The seaward expansion of northern mangroves intercepted tidal and fluvial sediments before they reached the sampling sites, driving sediment coarsening and the initial MAR decline. In contrast, Shenzhen River regulation enhanced fine sediment supply to the mangrove interior, leading to early-stage fining. Weaker hydrodynamic forcing and shorter hydroperiods maintained lower sediment accumulation in the interior. Additionally, occasional typhoon events triggered sediment coarsening in both zones, while sea-level rise exerted only minor influence. These findings provide a theoretical basis for resilience evaluation of mangrove wetland systems and the protection and restoration policy setting.