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publications > paper > groundwater control of mangrove surface elevation: shrink and swell varies with soil depth > introduction


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Soil surface elevation is an important response variable in wetland environments (Childers et al. 1993). Soil elevation affects hydroperiod, inundation frequency, and soil oxidation-reduction state. The hydrological conditions of a site are known to substantially affect soil processes including sedimentation, erosion, and the shrink and swell of soil materials. Soil elevation and surface flooding have been identified as important factors in wetland species colonization, recruitment, and survival (McMillan 1971; Rabinowitz 1978a,b; Ellison and Fransworth 1993; Cornu and Sadro 2002). Changes in soil surface elevation can be an important indicator of soil processes that are linked to hydrology, as well as those attributed to bioturbation (Ford and Grace 1998), decomposition (Cahoon et al. 2003), and subsidence (Cahoon et al. 1995). Soil surface elevation change is an integration of several processes occurring within the soil profile; yet most methods used to measure surface elevation changes do not distinguish among processes within the profile (Kaye and Barghoorn 1964; Childers et al. 1993; Cahoon et al. 1995). The elevation loss from subsidence and the elevation gain from accretion are incorporated into the absolute change in soil elevation. It is possible to partition the change in soil elevation into its component processes of surface accretion and subsurface expansion or compaction using the surface elevation table-marker horizon approach (Cahoon et al. 1995).

In a 3-yr study of a coastal mangrove forest along Shark River, Everglades National Park, Florida, soil surface elevation was found to vary linearly (R2 = 0.38) with surface water stage 15-30 d prior to sampling (Smith and Cahoon 2003). The investigation was limited in that the benchmarks used to measure soil elevation extended just 4 m into the soil and stopped approximately 2 m above the limestone bedrock. Processes occurring below the 4-m deep benchmark were not included in the elevation readings. The influence of processes within the active root zone (e.g., root growth and decomposition or shrink and swell) on soil elevation could not be determined because the benchmarks integrated processes over the entire 4-m soil column. Because of these limitations we added sampling devices that allowed us to measure the shallow active root zone (0-0.35 m) and the deeper soil zone (4-6 m).

We present here a study of soil elevation dynamics in the lower Shark River drainage basin that includes the entire soil profile and distinguishes between three depths within the soil profile; 0-0.35, 0-4, and 0-6 m. Our main objective was to investigate the relationship among changes in soil surface elevation and changes in the hydrological parameters of river stage and groundwater piezometric head pressure at the site over the three depths. We wanted to determine the relative contribution to soil elevation by each of the four components of the soil profile: surface (i.e., accretion), shallow zone (active root zone, 0-0.35 m), middle zone (0.35-4 m), and bottom zone (4-6 m).

A comprehensive understanding of the influences of hydrology on the soil profile at this site is of considerable importance. The site is located in the Shark River estuary downstream of the Shark River Slough, receives freshwater inputs from the Greater Everglades drainage, and is under the influence of upstream water management practices of the Greater Everglades. The Everglades drainage is currently undergoing an ecosystem restoration concentrating on modifying water deliveries to mimic predrainage flows. In addition to the changing freshwater flows linked to restoration, this mangrove forest is affected by sea level rise. Determining how hydrology influences the specific soil zones and surface elevation will allow managers to make more informed decisions regarding these two opposing hydrological processes.

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