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

PAULA PRATOLONGO; ANDREW PLATTER; JASON KIRBY; MARK BRINSON

Coastal Wetlands: an ecosystem integrated approach

Elsevier

Año: 2008;

Temperate coastal wetlands include a large variety of environments, ranging from tidal flats to fen woodland communities or barren salt flats. While tidal flats typically occur at the seaward end, there is a large variation in plant communities at the landward margin, depending on climatic and groundwater influences. In humid climates, sea level controls the groundwater position providing the waterlogged conditions necessary for fen wetlands development.  Hageman (1969) termed ‘perimarine zone’ the area where freshwater wetlands persist under sea level control. In arid climates where freshwater inputs are scarce, salt flats occur in the perimarine zone, where the elevated levels of soil salinity eliminate vegetation. In the intertidal zone, the primary abiotic control on wetland structure and function is the hydroperiod. In the ‘ramp’ model of salt marsh accretion the hydroperiod and the availability of sediments determine the amount of particles settling on the salt marsh surface. Under the ramp model, sites located lower in the tidal frame experience more sediment deposition on each tide and, as elevation increases, hydroperiod and net sedimentation are reduced. In the ‘creek’ model of salt marsh accretion, tidal channels capture and funnel the incoming tide, explaining the observed increase in sediment grain size with proximity to the creek margin. Levees generally develop on creek banks where more of the coarser sediment load is deposited. While the ramp model accounts for widespread and gradual trends in vertical accretion, the creek model is focused more on the local scale and potentially rapid development of three-dimensional sedimentary features. The result of the interaction between hydrodynamics, elevation and vegetation is a shore-parallel zonation of plants which is made more complex and spatially variable by the micromorphology of the marsh surface. The lower marsh, regularly inundated by salt water usually consists of hardy pioneer genera, and at higher levels a greater diversity of plants can colonize. In humid climates there would be a gradational transition from the upper salt marsh limit into a freshwater reed swamp which creates a clear zonation of plant species at different elevations. 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 through the temperate zone would be comparable in terms of ecosystem structure and function. However, substantial differences arise among major geographic regions due firstly to precipitation and secondly to 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 remaining wetland areas. The evolution of salt marshes over time is strongly influenced by climate change and human populations.  Human alterations are associated with land claims and engineering works for navigation and erosion/flood protection, as well as environmental pollution. Indirect human alterations include the delivery of pollution from far-field sources and allied changes to terrestrial sediment delivery. 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, or the influence of sea-level rise on marsh productivity.  However, there is some evidence to suggest that, at some locations, the geomorphic response of salt marshes would not be sediment limited.