Temperate Coastal Wetlands: Morphology, Sediment Processes, and Plant Communities

Abstract

Temperate coastal wetlands include a large variety of environments, from tidal flats and salt marshes to nontidal wetlands at the landward edge, whose hydrology is still influenced by sea level. Salt marsh evolution and accretion depends on several factors such as sediment availability, vegetation cover, marsh platform elevation, and hydrodynamic forces. Different models aim to reproduce the physical and ecological processes driving marsh evolution byusing different levels of simplification. The evolution of a salt marsh from mudflats can be described using a simplified model that considers the differences in elevation between mudflats and salt marshes. Changes in elevation result from the difference between erosion and deposition rates. Models developed to represent vertical salt marsh dynamics use different formulations in terms of both sediment and vegetation dynamics. These models mostly predict that salt marsh accretion rates would increase in more frequently floodedareas and also in the presence of dense vegetation. Finally, the lateral migration of salt marshes due to wind wave erosion is one of the principal causes for salt marsh losses worldwide. By using numerical models and field data, it has been shown that this lateral migration is mainly caused by average weather conditions rather than by extreme storms. Salt marsh plants commonly exhibit clear patterns of zonation, driven by the individual species tolerance to physical stress and biological interactions acting across the elevation gradient. The result is a shore-parallel zonation of plants which is made more complex and spatially variable by the micromorphology of the marsh surface. In low marsh areas, salinity is comparatively low because of regular tidal flushing, but soil salinity levels vary across marsh elevations. In humid warm regions, freshwater input from rain and upland sources moderates salinity at the terrestrial border, but salts can concentrate by evaporation atintermediate marsh elevations. Mid-marshes characterized by higher salinities support a more salt-tolerant flora, and salt accumulations may also lead to the development of bare areas known as salt pans. In humid climates, the upper salt marsh commonly grades into freshwater communities. In arid climates, however, salinities can exceed the limits of even the most tolerant halophytes, and salt flats devoid of vascular vegetation develop near the upland boundary.Tidal marshes throughout the temperate zone would be comparable in terms of ecosystem structure and function. However, substantial differences arise among major geographic regions due to precipitation regimes. Typical patterns of salt marsh plant zonation largely reflect rainfall amount and seasonality, along with the biogeographic distribution of species. In addition, human activities have also exerted profound changes in coastal wetlands, and the processes of wetland loss and degradation have been quite variable in space and time, leading to major regional differences in the extent and ecological integrity of remainingwetland areas. The evolution of salt marshes over time is strongly influenced by climate change and human-induced alterations. Major human impacts are associated with land claims and alterations of fluvial sediment transport. Considering a future climate-enhanced sea level rise, early studies predicted the large-scale loss of coastal wetlands as a consequence of sea level rise exceeding sediment supply. However, salt marsh vulnerability would be largely dependent on biophysical feedback processes that accelerate soil building under a risingsea level. Thus, the future development of coastal wetlands under a rapid sea level rise would be strongly conditioned by human activities that interfere with these feedbacks.