While an implicit part of the national goal of no-net-loss involves mitigation for unavoidable impacts to wetlands,an explicit part of the goal is the restoration of wetlands where possible to recover the historical quality of the remaining acreage base (Conservation Foundation, 1987). Restoration may be required as part of a permitting process, but restoration efforts may also be prompted by environmental resource management goals for habitat or water quality improvement in keeping with the net- recovery clause of the national no-net-loss goal. In either case, degraded wetlands present restoration opportunities for improvements to water quality, habitat, water storage and other functions, and these opportunities can be particularly useful for watershed-scale environmental planning. The goal of restoration is typically to reestablish wetland ecosystems to levels that existed prior to human influence. Wetland creation can include regulatory mitigation or commercial and private creation efforts outside of regulatory requirements. A useful volume was recently released by the National Academy of Sciences addressing restoration of wetlands and other aquatic ecosystems from a management standpoint (National Research Council, 1992).
Degraded, prior converted wetlands may offer opportunities for innovative management approaches (which may require permitting). In the southeastern coastal plain, a mixed- use, aquaculture-silviculture (crayfish-timber) enterprise can be quite successful (Mitsch and Gosselink, 1993). The hydrologic cycle of a bottomland hardwood forest can be simulated by winter impoundment of a prior converted or degraded swamp or area planted in flood-tolerant tree species. The crayfish are harvested in the spring and summer. Such a system can restore bottomland hardwood community structure and provide water quality benefits of nutrient removal. Inflow of toxic compounds must be monitored closely, however, because crayfish accumulate them. Since timber rotations are long, generally 20 - 50 years, this system can provide wildlife habitat as well, particularly if it is not intensively managed.
Constructed wetlands can provide many of the water quality improvement functions of natural wetlands with the advantage of control over location, design, and management to optimize those water quality functions. Constructed wetlands are not typically intended to replace all of the functions of natural wetlands, but emphasize certain features to maximize pollutant removal efficiency and to minimize point source and nonpoint source pollution prior to its entry into streams, natural wetlands, and other receiving waters. Wetlands created for habitat, water quantity, aesthetic and other functions as well as water quality functions typically call for different design considerations than those used solely for water quality improvement.
For example, since a bar has peak demand during Saturday night and a church has peak demand during Sunday morning, they can efficiently share parking if located near to each other (usually within a block or so).
The U.S. Forest Service has published guidance on reforesting previously cleared riparian areas and renovating degraded riparian areas for the protection of receiving water quality (Welsch, 1991). The guidance is directed toward agricultural and silvicultural land uses and emphasizes that riparian buffers are meant to be used as part of a sound land management system including upland best management practices, and can be damaged and functionally impaired otherwise.
This tailored design approach to constructed systems generally makes them less suitable as wildlife habitat than natural wetlands. Nevertheless, constructed wetlands are often designed with ancillary wildlife values in mind, for example, incorporating open water for waterfowl usage. While species diversity of vegetation and microflora and fauna are lower in treatment wetlands, bird usage can be higher than that in adjacent natural wetlands because of the more eutrophic, and hence more productive, aquatic conditions in the loaded systems (McAllister, 1993, in Kadlec, 1995). A major concern with the use of constructed wetlands for wildlife habitat is the potential for concentrating accumulated pollutants up the food chain, with deleterious effects to birds and other consumers. While wildlife impacts have been observed in several instances with wetlands created for habitat (see the Wetlands Loss and Degradation section), these appear related to agricultural irrigation return flows in the West or hazardous waste site releases (Knight, 1993). So far, no similar problems are documented for constructed treatment wetlands (Kadlec, 1995; Knight, 1993), but the potential for harm exists with some metals and other compounds (Knight, 1993), and the issue requires continued evaluation.
Hammer (1992) envisions a holistic watershed wetland management approach involving a hierarchical arrangement of restored or created wetlands within a watershed landscape. Following conventional on-farm BMP systems, first-order control involves constructed wetlands designed specifically for animal wastewater, processing facility wastewater, or septic tank effluent treatment. Second-order control also occurs at the individual farm level, and consists of constructed wetland/upland systems, such as the nutrient/sediment control system described above, for treating cropland runoff or discharge from animal wastewater treatment systems, and providing some ancillary benefits as well. Third-order control requires a larger, watershed picture, and involves nutrient/sediment control systems, constructed wetland/pond complexes, and restored or created wetlands and riparian areas along many small streams higher in the watershed, providing water quality, hydrologic buffering, life support, and other values. Finally, fourth-order control uses large wetlands low in the watershed primarily for hydrologic buffering and habitat support values in addition to limited water quality benefits. First- and second-order systems are located within the bounds of individual farms and require active operation to maintain optimum treatment performance, while third- and fourth-order elements provide water quality benefits to runoff from numerous farms or entire watersheds, and function without intervention.
For example, an office complex canefficiently share parking facilities with a restaurant or theaters, sinceoffices require maximum parking during weekdays, while restaurants and theatersrequire maximum parking during evenings and weekends.
Mitsch (1993) observed in a comparison of experimental systems using phosphorus as an example that retention as a function of nutrient loading will generally be less efficient in downstream wetlands than in smaller upstream wetlands. Wetlands (floodplains) along higher-order streams influence water quality to a much smaller degree, since the upland runoff that passes through them and joins the stream is a much smaller fraction of the total stream flow than it is for headwater wetlands. Wetlands along large streams do, however, provide water quality benefits during flood events, a function that headwater wetlands do not provide. Mitsch (1993) cautioned that the downstream wetlands could retain more mass of nutrients than upstream systems, and that a placement tradeoff might be optimum. From a management standpoint, creating many smaller wetlands around a watershed would mean dealing with more landowners, but taking less land out of production on any one farm than creating a few large wetlands, and is more fair in terms of not asking any landowner to contribute more than what is needed to treat the runoff from their land (van der Valk and Jolly, 1993).
While management of restored or created wetlands should as a rule emulate the functions of undisturbed marshes, there may be times when single- or priority-objective management is appropriate. For a given wetland site, a restoration or creation management strategy must involve determination of the most important values to be obtained, and of whether a single, exclusive value outweighs the suite of values to be obtained from historic restoration. If a single-purpose wildlife use is sought, such as certain fish utilization, management may result in manipulation of marsh hydrology at the expense of other species and wetland functions. For example, game fish species require consistently deep water, yet shallow, emergent-plant-depth water levels provide the highest plant species diversity and greatest overall wildlife use of marshes (Mitsch and Gosselink 1993; Kent 1994b). At the same time, waterfowl require different structural conditions depending on species needs for feeding (divers versus dabblers), nesting, or staging (Weller 1981; Kent 1994b). In general, a ratio of no more than 1:1 open water to emergent vegetation maximizes waterfowl use (Weller 1981). Thus, tradeoffs are inevitable when structural components of a wetland, such as water level, are artificially manipulated. Any management strategy beyond reestablishment of historical functions must weigh these tradeoffs in light of management goals.
For this reason, the subject of wildlife management is located here under wetland restoration and creation. Some additional wildlife management discussion occurs under the Management of Exempt Wetlands subsection for the same reason, since exempt systems are typically degraded and offer the possibility of improvement through active management.