Sediment distribution in shallow estuaries at fine scale: in situ evidence of the effects of three-dimensional structural complexity of mangrove pneumatophores

One of the main services offered by mangroves is their capacity for trapping sediment. We investigated how spatial complexity of pneumatophores of Avicennia marina may influence fine-scale sediment particle size distribution. Using realistic three-dimensional models captured from pneumatophore patches, indices of complexity (the area/volume ratio, the Getis-Ord Gi* statistic) were calculated to quantify mangrove root structural complexity in five 1 × 1 m2 plots. The complexity of pneumatophores in 16 0.25 × 0.25 m2 subplots in each of the 5 plots was measured and its relationship with the relative abundance of fine sediment particles (clay and silt, <63 µm) was assessed. Results showed the complexity of the neighbouring subplots in the direction of incoming water was a major factor driving the trapping of suspended silt and clay, thus underpinning the function of mangrove aboveground structures in the distribution of fine particles. This simple low-cost technique to measure the complexity of mangroves demonstrates how further investigations may quantify the relationship between this complexity and their capacity to trap sediment with data derived from actual real-world models rather than based on simplistic, simulated structures. This information will be valuable in guiding future efforts in mangrove rehabilitation and restoration.