For their size, freshwater ecosystems contain a disproportionately large number of the world's species. More than 40% of the world's fish species and roughly 12% of the animal species reside in freshwater habitats which themselves cover only about 1% of the earth's surface (World Resources Institute et al., 1998). With respect to wetlands, in recent years, numerous social benefits (or "ecosystem services") of wetlands have been identified. In a table adapted from Kusler (1983), the National Research Council (1992) presents fifteen separate services provided by wetlands (reproduced as Table 3). Among them are flood conveyance and storage, sediment control, and recreation. On a local level, smaller towns (~ 10,000 population) can rely on wetlands for tertiary wastewater treatment at half the cost or even less compared to technology-based advanced treatment methods (Ewel, 1997). Additional benefits of wetlands include the return of nitrogen to the atmosphere (denitrification), which counter-balances to some extent human introductions of nitrogen in fertilizers, as well as the reduction of sulfates into insoluble complexes, which partially mitigates human introductions of sulfur via acid rain. Wetlands have the potential to be a net sink for carbon (through accumulation of peat), but in recent years due to extensive wetland conversions they have become a net source (Mitsch & Gosselink, 1993).
Profound impacts on freshwater ecosystems could result from a doubling of atmospheric C02. In addition to increases in mean ambient temperature, climate change models predict declining levels of soil moisture, changes in timing and intensity of rainfall, shifting of storm tracks, and increasing frequency and intensity of drought periods. Shriner and Street (1998) suggest that North American non-forested ecosystems could experience losses of migratory waterfowl and mammal breeding and forage habitats, invasions of exotic species, and increased sediment loading into rivers and lakes. Novel assemblages of plant and animal species could result as the ranges of some species expand while other ranges decline (McKinney & Lockwood, 1999; Lockwood & Duncan, 1999).
Flood conveyance - Riverine wetlands and adjacent floodplain lands often form natural floodways that convey floodwaters from upstream to downstream areas.
Protection from storm waves and erosion - Coastal wetlands and inland wetlands adjoining larger lakes and rivers reduce the impacts of storm tides and waves before they reach upland areas.
Flood storage - Inland wetlands may store water during floods and slowly release it to downstream areas, lowering flood peaks.
Sediment control - Wetlands reduce flood flows and the velocity of floodwaters, reducing erosion and causing floodwaters to release sediment.
Habitat for fish and shellfish - Wetlands are important spawning and nursery areas and provide sources of nutrients for commercial and recreational fin and shellfish industries, particularly in coastal areas.
Habitat for waterfowl and other wildlife - Both coastal and inland wetlands provide essential breeding, nesting, feeding, and refuge habitats for many forms of waterfowl, other birds, mammals, and reptiles.
Habitat for rare and endangered species - Almost 35% of all rare and endangered animal species either are located in wetland areas or are dependent on them, although wetlands constitute only about 5% of the nation's lands.
Recreation - Wetlands serve as recreation sites for fishing, hunting, and observing wildlife.
Source of water supply - Wetlands are becoming increasingly important as sources of ground and surface water with the growth of urban centers and dwindling ground and surface water supplies.
Food production - Because of their high natural productivity, both tidal and inland wetlands have unrealized potential for food production from harvesting of marsh vegetation and aquaculture.
Timber production - Under proper management, forested wetlands are an important source of timber, despite the physical problems of timber removal.
Preservation of historic, archaeological values - Some wetlands are of archaeological interest. Indian reservations were located in coastal and inland wetlands, which served as sources of fish and shellfish.
Education and research - Tidal, coastal, and inland wetlands provide education opportunities for nature observation and scientific study.
Source of open land and contribution to aesthetic values - Both tidal and inland wetlands are areas of great biological diversity and beauty, and provide open space for recreational and visual enjoyment.
Water quality improvement - Wetlands contribute to improving water quality by removing excess nutrients and many chemical contaminants. They are sometimes used in tertiary treatment of urban wastewater.
Source: National Research Council, 1992, Table 6.1, adapted from Kusler (1983).
In addition to solar energy and wind, the most important driving forces for wetlands are hydrologic, including tides, streamflow, surface runoff, and groundwater flow (Mitsch & Gosselink, 1993). Wetland management involves active attention to a project or region over time. The time dimension is critical to preserving a wetland's biodiversity and ecosystem services because a region's rate of environmental change may directly affect the qualitative biological outcome of such change (Kingsolver et al., 1993).
California's San Luis National Wildlife Refuge Complex, commonly known as "the Grasslands," offers insight to the management and coordination of water supply necessary for wetlands to remain viable over time. The Grasslands is located along California's San Joaquin River and is the state's largest remaining inland wetland. It encompasses more than 59,000 hectares straddling Merced and Fresno Counties in the San Joaquin Valley. It is a critical stopover for migrating and wintering waterfowl, with more than 50% of all San Joaquin Valley shorebirds in residence during the peak spring migration season. The Grasslands provides habitat to 46 plants and animals that are endangered, threatened, or are candidates for listing as a special status species.
Ownership and management of the Grasslands involve a complex mosaic of state, federal and private entities. The U.S. Fish and Wildlife Service owns and/ or manages 32,100 hectares, part of which includes perpetual conservation easements donated from private parties. Another 13,600 hectares is owned and managed by the California Department of Fish and Game as the Los Banos, Mendota, Mud Slough, North Grasslands, and Volta Wildlife Areas, and Grasslands State Park. Private duck hunting clubs add to the mosaic, owning and managing 13,900 hectares.
Three phases in the evolving institutional structure of water supply to the Grasslands can be identified. The first was simply the historical pattern of capturing unclaimed flood flows from the San Joaquin River. The second phase began when the Grasslands was cut off from its historical source of San Joaquin River water in 1944 with the completion of the Friant Dam, part of the federal Central Valley Project. Water managers for the Grasslands began securing water delivery though multiple contracts with irrigation districts and the federal government. In an average year, 37 million cubic meters are delivered to the Grasslands from the San Luis Canal Company (a neighboring irrigation district), which receives its water via the Delta-Mendota Canal and the California Aqueduct. The Merced Irrigation District is contracted to supply groundwater to the Grasslands, but due to this water's high cost, it is reserved for dry years. In addition, private duck clubs purchase approximately 18.5 million cubic meters per year from the Grassland Water District, whose sole purpose is to deliver water to the wetlands.
The third phase was launched when revisions to the water-delivery system for the Grasslands were mandated by the 1992 Central Valley Project Improvement Act. This Act, which seeks in part to mitigate the long-term environmental impacts of the Friant Dam and other federally-owned water facilities in California, when fully implemented will assure firm water supplies to the Grasslands for the first time in its history. The federally-guaranteed supplies will provide a basic quantity to the Grasslands while other existing sources will continue to provide supplemental supplies.
The Grassland's water supply has thus transferred from its original (unmanaged) linkage to flood flows of the San Joaquin River to a mosaic of uncoordinated and often insecure water agreements involving state, federal, and local water agencies, as well as private organizations, to subsequently include a basic level of supply guaranteed by federal legislation. The intermediate phase of multiple uncoordinated agreements may not have been effective at preserving biodiversity and ecological services in a post C02-doubled era. For example, demand for dry-year groundwater supplies from the Merced Irrigation District is likely to intensify in the coming decades. Water supplies for wetlands or other ecologically-important areas may not remain financially competitive with demand from urban and agricultural regions. State or federal legislation may be required to guarantee available funding to purchase water supplies for wetlands and other regions of ecological value. To the extent Californians make wetland preservation and restoration a priority, costs would involve water procurement, development of conveyance infrastructure, as well as more intensive research into and oversight of ecologically-important areas to evaluate the effectiveness of existing management practices.7 Benefits would include those provided by the wetlands themselves (Table 3) as well as existence values.
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