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Restoration of Aquatic Ecosystems: Science, Technology, and Public Policy
On the Savannah River site in South Carolina, characteristic biotic communities have recovered in the 40 years since agricultural usage ceased and natural hydroperiod was reestablished. The persistent seed bank allows rapid recovery of the vegetation of these wetlands if disturbance has not been excessively severe (e.g., soil removal) or long. Dispersal of wetland plant species to recovering bays may be facilitated by waterfowl that use these wetlands when they are inundated. Thus, functional recovery of these systems may occur relatively rapidly after restoration of a more natural hydrologic state.
The Salmon River salt marsh in Oregon (Frenkel and Morlan, 1990) is also an example of a site with intermediate restoration potential— the effects of disturbance are not easily reversible, yet the site exists within a region that has large reserves of natural habitat. In this case, a 21-ha salt marsh was diked in 1961 and used for grazing, as fresh water gradually diluted the salts and allowed pasture vegetation to become established. During the 17-year diked period, the topography subsided 30 to 40 cm due to compaction and loss of soil buoyancy. The U.S. Forest Service breached the dike in 1978 to restore the salt marsh to a functional ecosystem. However, the topographic subsidence was not readily reversible. Ten years after dike breaching, accretion had raised the topography only 2 to 5 cm throughout most of the site. Native salt marsh plants have now reestablished themselves at their appropriate intertidal elevations, but the area of low salt marsh is much larger than that present before diking. From a functional standpoint, aboveground biomass indicates a high level of similarity with productivity in reference wetlands. Thus, the restoration process is under way, but at the current rate of accretion, reestablishment of the predisturbance high salt marsh could take several decades. The case is an example of a site that would fall in the upper left portion of the model shown in Figure 6.3.
In the northern United States and Canada, prairie pothole wetlands were altered by European settlers to facilitate farming (see case study, Appendix A). The poorly drained depression soils were drained by elaborate and extensive tile fields, seasonal inundation was eliminated, and the potholes were converted to tillable land. Approximately one-half of the 20 million acres was drained, with dramatic effects on fish and wildlife habitat. Restoration efforts supported by federal, state, local, and private programs are now under way. Physical measures to restore the natural hydrology, such as breaking the tile fields and filling ditches, followed by natural recovery of wetland plant communities, have restored waterfowl habitat. The chief obstacle is development of a program to persuade individual landowners to take pothole areas out of agricultural production and allow