Elevation, drainage, and spatially heterogeneous vulnerability of tidal marshes to sea level rise

Abstract

Tidal marsh stability under Sea Level Rise (SLR) is driven by feedbacks between marsh macrophyte productivity, sediment accretion, and surface elevation gain. Predictive models and empirical studies often assume a spatially uniform tidal regime across the marsh platform, neglecting hydrological variations caused by tidal attenuation, drainage efficiency and terrestrial groundwater influences. This oversimplification can result in misleading estimates of marsh stability, especially in interior marsh of microtidal systems, which are more vulnerable to SLR-induced deterioration. Here we investigated how spatial variation in hydrology alters marsh vulnerability by comparing groundwater and surface water fluctuations between interior marsh zones and tidal creek edges at two microtidal marshes in the Chesapeake Bay. We assessed the deviation between observed and elevation- and tidally-derived estimates of hydroperiod, daily inundation duration, and soil saturation index (SSI). Using growth-hydroperiod response curves of marsh macrophytes, we assessed how these deviations alter marsh vulnerability predictions. We found that both tidal attenuation and weak drainage play dominant roles in shaping interior marsh hydrology. Due to tidal attenuation, water level fluctuations were muted from a 60 cm range at the marsh edge to a 20 [~] 30 cm range in marsh interior 500 m away. Therefore, interior marsh exhibits greater hydrological sensitivity to even minor elevation loss, which presents amplified hydrological stress and accelerated vegetation diebacks. Poor drainage in interior marsh result in a persistently higher water level, and slower recession at low tide, leading to prolonged inundation and soil saturation in interior marshes. As a result, assuming a spatially uniform tidal regime underestimated the hydroperiod (by 22-50%) and soil saturation (by 40-50%), and overestimated plant performance (by 10-25%) in interior marsh zones. Our findings highlight the critical need to incorporate tidal attenuation and drainage heterogeneity into empirical studies and numerical simulations to improve marsh resilience prediction and guide adaptive management strategies.