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A coupled geomorphic and ecological model of tidal marsh evolution.

ABSTRACT: The evolution of tidal marsh platforms and interwoven channel networks cannot be addressed without treating the two-way interactions that link biological and physical processes. We have developed a 3D model of tidal marsh accretion and channel network development that couples physical sediment transport processes with vegetation biomass productivity. Tidal flow tends to cause erosion, whereas vegetation biomass, a function of bed surface depth below high tide, influences the rate of sediment deposition and slope-driven transport processes such as creek bank slumping. With a steady, moderate rise in sea level, the model builds a marsh platform and channel network with accretion rates everywhere equal to the rate of sea-level rise, meaning water depths and biological productivity remain temporally constant. An increase in the rate of sea-level rise, or a reduction in sediment supply, causes marsh-surface depths, biomass productivity, and deposition rates to increase while simultaneously causing the channel network to expand. Vegetation on the marsh platform can promote a metastable equilibrium where the platform maintains elevation relative to a rapidly rising sea level, although disturbance to vegetation could cause irreversible loss of marsh habitat.

SUBMITTER: Kirwan ML

PROVIDER: S-EPMC1851060 | biostudies-literature | 2007 Apr

REPOSITORIES: biostudies-literature

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A coupled geomorphic and ecological model of tidal marsh evolution.

Kirwan Matthew L ML Murray A Brad AB

Proceedings of the National Academy of Sciences of the United States of America 20070326 15

The evolution of tidal marsh platforms and interwoven channel networks cannot be addressed without treating the two-way interactions that link biological and physical processes. We have developed a 3D model of tidal marsh accretion and channel network development that couples physical sediment transport processes with vegetation biomass productivity. Tidal flow tends to cause erosion, whereas vegetation biomass, a function of bed surface depth below high tide, influences the rate of sediment dep ...[more]

PMID: 17389384

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Project description:Over 17 years, species richness and seed bank densities, initially very high, decreased, and Phragmites australis, an invasive, came to dominate the seed bank and vegetation. Despite a persistent and varied seed bank, Phragmites will likely preclude contributions from the seed bank or newly dispersed propagules to vegetation development. Background and aims A created tidal freshwater wetland, with no extant seed bank, was monitored to evaluate the colonization potential of a tidal river, vegetation and seed bank changes, and also species persistence. Methodology The 32.3-ha mitigation, adjacent to the Delaware River, was completed in 1994. Three sites, with differing completion dates, and three locations (channel edge, midpoint, upland edge) were sampled. From 1995 to 1999, surveys permitted a view of initial dynamics involving soil seed bank density or field vegetation cover, as well as species composition. Vegetation was monitored through 2011, and in 2011 a seed bank survey was also undertaken. Principal results Between 1995 and 1999, 177 species emerged from soil samples, with a maximum of 32.3 ± 1.8 (SE)/sample. Eight species were added in 2011, indicating continuing dispersal and input. Comparing species richness across three sites for midpoint locations, 2011 totals and species/sample were lower (61–90 % and 53–72 %, respectively) than 1998 values. In vegetation plots during the first 5 years, 72 cover species were present. In 2011, there were 44; 14 were new and the majority (13) were woody and part of an overhanging edge canopy. Dispersal was primarily via water. Overall, the trend was towards lower seed bank densities, except for Mikania scandens and Phragmites australis which had higher densities, as well as cover values. Species richness also decreased. Species varied in persistence in the seed bank and vegetation, and among sites and with location. Initial site colonization differences were still apparent in 2011 seed bank samples. While present, Lythrum salicaria did not reduce species richness; Phragmites, in contrast, had substantial impact. Conclusions The single most important change was an increase in Phragmites in the vegetation. Vegetation dynamics are now determined by this species, and the persistent seed bank and continuing dispersal input have little obvious future in vegetation development.

Project description:If individuals can perceive and manage risks, they may alter their behaviors based on prior experience. This expectation may apply to nest site selection of breeding birds, for which adaptive behavioral responses may enhance fitness. Birds that nest in tidal marshes have adapted to the challenges posed primarily by periodic, monthly tidal flooding and secondarily by predation. We investigated adaptive responses in nesting behavior of the saltmarsh sparrow (Ammospiza caudacutus), an obligate tidal-marsh-breeding bird, using 536 nests monitored across 5 years. Using linear mixed effects models, we tested whether nest characteristics differed among nests that were successful, depredated, or flooded, and we investigated whether females made changes in nest structure and placement according to outcome of their previous nesting attempt. Nest characteristics differed among females with different nest fates. Fledged and depredated nests were built higher in the vegetation and in higher elevation areas of the marsh than those that flooded. Successful nests had more canopy cover and were comprised of a lower proportion of high marsh vegetation (Spartina patens) than those that were flooded or depredated. Females with nests that failed due to flooding constructed subsequent nests higher in the vegetation and at higher elevation than those that were successful in their prior attempt, consistent with a response to previous experience. Eighty-five percent of females renested within the average home range core area distance (77 m), indicating a high degree of nest placement fidelity. Females for which nests were depredated in their prior nesting attempt renested at a greater distance than females for which the previous nesting attempts were successful. Our findings suggest saltmarsh sparrows exhibit plasticity in nesting behavior, which may be important for balancing selective pressures in a dynamic environment. This plasticity, however, is insufficient to enable them to adapt to the increased flooding predicted with sea-level rise.

Project description: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.Supplementary informationThe online version contains supplementary material available at 10.1007/s12237-021-01027-9.

Dataset Information

A coupled geomorphic and ecological model of tidal marsh evolution.

Publications

A coupled geomorphic and ecological model of tidal marsh evolution.

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