Hydrodynamic habitat thresholds for mangrove vegetation on the shorelines of a microtidal estuarine lagoon

Although understanding the requisite hydrodynamic habitat for mangrove vegetation is vital for successful restoration efforts, few studies have quantitatively assessed the in-situ tolerance of mangroves to relevant physical forcing mechanisms. In this work, hydrodynamic thresholds for the persistence of mangrove vegetation (Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa) are assessed using a hindcast wind-wave model coupled with a high-resolution shoreline survey, with analysis including 383 km of shoreline located in a microtidal estuary along the Atlantic coast of Florida (north Indian River Lagoon, USA). Observed mangrove distribution patterns were most strongly correlated with the modelled wave climate (p < 0.001) and measured intertidal slope (S_o; p < 0.001), and mangrove presence probabilities were maximized for sample sites characterized by low slopes (S_o < 0.5) and weakly energetic waves (80th percentile wave height: H₈₀ < 2.5 cm). Critical wave thresholds (mean ± 95% confidence interval) of H₅₀=4.2 ± 0.4cm and H₈₀ = 8.0 ± 0.5cm were estimated for 50th and 80th percentile wave heights, respectively, representing the wave climate limits above which mangrove presence probabilities fall below 50%. Low intertidal slopes were observed to enhance mangrove wave tolerance (S_o≅0; H₈₀ = 9 cm) while high slopes led to dramatic reductions in threshold wave heights (S_o≅1; H₈₀ = 4 cm). These results have important implications for future restoration efforts, providing the first available quantitative wave thresholds for mangrove habitat suitability.