Abstract
Tidal wetlands provide myriad ecosystem services across local to global scales. With their uncertain vulnerability or resilience to rising sea levels, there is a need for mapping flooding drivers and vulnerability proxies for these ecosystems at a national scale. However, tidal wetlands in the conterminous USA are diverse with differing elevation gradients, and tidal amplitudes, making broad geographic comparisons difficult. To address this, a national-scale map of relative tidal elevation (Z*MHW), a physical metric that normalizes elevation to tidal amplitude at mean high water (MHW), was constructed for the first time at 30 × 30-m resolution spanning the conterminous USA. Contrary to two study hypotheses, watershed-level median Z*MHW and its variability generally increased from north to south as a function of tidal amplitude and relative sea-level rise. These trends were also observed in a reanalysis of ground elevation data from the Pacific Coast by Janousek et al. (Estuaries and Coasts 42 (1): 85–98, 2019). Supporting a third hypothesis, propagated uncertainty in Z*MHW increased from north to south as light detection and ranging (LiDAR) errors had an outsized effect under narrowing tidal amplitudes. The drivers of Z*MHW and its variability are difficult to determine because several potential causal variables are correlated with latitude, but future studies could investigate highest astronomical tide and diurnal high tide inequality as drivers of median Z*MHW and Z*MHW variability, respectively. Watersheds of the Gulf Coast often had propagated Z*MHW uncertainty greater than the tidal amplitude itself emphasizing the diminished practicality of applying Z*MHW as a flooding proxy to microtidal wetlands. Future studies could focus on validating and improving these physical map products and using them for synoptic modeling of tidal wetland carbon dynamics and sea-level rise vulnerability analyses.
Highlights
Tidal wetlands have the capacity to maintain an adaptive resilience to sea-level rise
Z*mean high water (MHW) can provide a physical index of flooding exposure, or “elevation capital,” which could be an early indicator of marsh susceptibility to collapse
We observed latitudinal and regional trends in median Z*MHW and Z*MHW variability that were contrary to our hypothesis, but were supported by a reanalysis of ground-based survey data by Janousek et al (2019)
Summary
Tidal wetlands have the capacity to maintain an adaptive resilience to sea-level rise. Inundation drives elevation change by stimulating belowground biomass input and increasing the availability of sediment which can be trapped and deposited (Morris et al 2002; Kirwan et al 2013; Kirwan et al 2016). Resilience to sea-level rise is not assured or infinite because biological productivity and preservation are limited theoretically by plants’ abilities to fix carbon, and practically by ecological and physical constraints (Morris et al 2016). Local rates of relative sea-level rise (RSLR), which take into account both eustatic and isostatic sea-level change, can vary greatly (Jankowski et al 2017; Horton et al 2018)
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