Marine Wrack Enhances Mangrove Forest Elevation by Reducing Organic Matter Breakdown

Abstract Biogenic mangrove wetlands rely on trapping and burying organic production in soil to pace sea-level rise. However, the fate and role of allochthonous marine wrack in regulating carbon processing and wetland elevation in fringe mangrove forest soils remain unclear. We quantified long-term changes in tidal hydrology and surface soil elevation (2011–2024) across two representative fringe mangrove forest sites (BISC-1, BISC-2) in Biscayne National Park (Florida, USA). To test the influence of marine wrack deposition, we measured the spatiotemporal variation of monthly wrack deposition, litter breakdown rates, soil organic carbon, and stable isotope δ 13 C and δ 15 N content along landward transects (2022–2024). Our results indicate that relative sea-level rise (annual rate ± SE: 8.5 ± 0.4 mm yr − 1 ) was greater than annual rates of surface elevation change at either site (BISC-1 2.1 ± 2.6 mm yr − 1 ; BISC-2 3.9 ± 3.2 mm yr − 1 ). Rhizophora mangle litter breakdown rates (mean ± SE: BISC-1 0.011 ± 0.001 k d − 1 ; BISC-2 0.008 ± 0.001 k d − 1 ) were greater than Thalassia testudinum (BISC-1 0.009 ± 0.001 k d − 1 ; BISC-2 0.005 ± 0.001 k d − 1 ), suggesting that wrack deposition can potentially enhance elevations gains more than mangrove litter. Stable isotope content in soil, mangrove, and seagrass tissue suggested that allochthonous marine wrack contributed between 36.1% ± 0.8% and 53% ± 3.6% to the soil organic carbon content. Our findings emphasize the importance of ecological connectivity in transporting wrack as a marine subsidy that can enhance blue carbon storage and elevation increases by reducing breakdown rates and promoting peat formation in fringe mangrove forests.