Coastal wetlands stabilize shorelines and reduce erosion but the connection between this critical ecosystem service and the composition of coastal plant communities is unclear. Subtropical coastal marshes dominated by low-stature herbaceous species are transitioning to woody mangroves. However, it is unclear how retention and connectivity compare among mangrove- and saltmarsh-dominated coastal habitats during ambient conditions and storms. In 2012, we experimentally removed black mangroves (Avicennia germinans) from 10 coastal plots (24 x 42 m2) to create a 0-100% mangrove cover gradient along the coast of Port Aransas, Texas. We measured differences in autochthonous and allochthonous sediment accretion and soil organic matter dynamics within marsh and mangrove patches (3 x 3 m2). In September 2017, Hurricane Harvey, a Category 4 storm, made direct landfall on our coastal plots. We tested the effects of plot-level percent mangrove cover and patch-level plant identity on soil organic matter, sediment particle characteristics, accretion rates, and erosion before and after the storm as indicators of ecosystem retention and connectivity.
Results/Conclusions
Total root biomass (0-30 cm depth) was higher in all plots with mangroves present, and mangrove patches had 1.7x higher mean and 5x lower variance in root biomass than marsh patches. Sediment particle size was generally coarser in plots with mangroves and finer in the plot without mangroves, suggesting that marshes are more depositional environments and that even at low density mangroves can affect sediment size distribution. Before the storm, short-term patch-level accretion rates decreased with plot-level mangrove cover (reduced marine subsidies), but longer-term accretion rates were positively correlated with plot-level mangrove cover (increased mangrove subsidies). After the storm, marsh patches had higher and more variable sediment accretion rates than mangrove patches. Mangrove patches captured less sediment than marsh patches, and mangrove accretion rates declined with plot-level mangrove cover, similar to measurements of short-term accretion before the storm. Plot-level erosion was up to 3x above ambient conditions in plots with less than 33% mangrove cover. Mangroves enhance retention of autochthonous sediment and organic matter, but can also block allochthonous inputs. The capacity of coastal ecosystems to be resilient to storms may be linked to how these systems balance connectivity to autochthonous and allochthonous sources of sediment and organic matter.