2
Ecology of Wetland Ecosystems

INTRODUCTION

Many kinds of wetland ecosystems are found within the United States (Table 2.1). These range from small, discrete sites, such as Thoreau's Bog in Massachusetts or Four Holes Swamp in South Carolina, to large, spatially complex ones, such as the Great Dismal Swamp in Virginia and North Carolina or the peatlands of northern Minnesota. The characteristics and functions of any given wetland are determined by climate, hydrology, and substrate, as well as by position and dominance in the landscape. In many cases, wetlands occupy a small portion of the total landscape (usually less than 10%), but have extensive boundaries with both terrestrial and aquatic ecosystems. In some cases, they occupy virtually the entire landscape. Despite their great range in size and other features, wetlands share specific characteristics, some of which are structural (water, substrate, biota), while others are functional (nutrient cycling, water balance, organic production). Analysis of these characteristics shows how wetlands are distinct from other kinds of ecosystems, and illustrates the reasons for variation among wetlands.

In very large wetlands, such as extensive peatlands, marshlands, bottomlands, and river floodplains, internal spatial variation can be great. Examples include the Great Dismal Swamp, which consists of at least four major wetland plant communities integrated with lakes and streams (Kirk, 1979); the Everglades, which includes sloughs, sawgrass prairies, and wet shrub islands (Kushlan, 1990; Davis and Ogden, 1994); the Mississippi delta, which has swamps, marshes, lakes, and rivers (Day et al., 1977); and peatlands of northern Minnesota



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Wetlands: Characteristics and Boundaries 2 Ecology of Wetland Ecosystems INTRODUCTION Many kinds of wetland ecosystems are found within the United States (Table 2.1). These range from small, discrete sites, such as Thoreau's Bog in Massachusetts or Four Holes Swamp in South Carolina, to large, spatially complex ones, such as the Great Dismal Swamp in Virginia and North Carolina or the peatlands of northern Minnesota. The characteristics and functions of any given wetland are determined by climate, hydrology, and substrate, as well as by position and dominance in the landscape. In many cases, wetlands occupy a small portion of the total landscape (usually less than 10%), but have extensive boundaries with both terrestrial and aquatic ecosystems. In some cases, they occupy virtually the entire landscape. Despite their great range in size and other features, wetlands share specific characteristics, some of which are structural (water, substrate, biota), while others are functional (nutrient cycling, water balance, organic production). Analysis of these characteristics shows how wetlands are distinct from other kinds of ecosystems, and illustrates the reasons for variation among wetlands. In very large wetlands, such as extensive peatlands, marshlands, bottomlands, and river floodplains, internal spatial variation can be great. Examples include the Great Dismal Swamp, which consists of at least four major wetland plant communities integrated with lakes and streams (Kirk, 1979); the Everglades, which includes sloughs, sawgrass prairies, and wet shrub islands (Kushlan, 1990; Davis and Ogden, 1994); the Mississippi delta, which has swamps, marshes, lakes, and rivers (Day et al., 1977); and peatlands of northern Minnesota

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Wetlands: Characteristics and Boundaries TABLE 2.1 Major Classes of Wetlands in the United States and Some of Their Characteristics Common Term Distribution and Hydrology Biota Freshwater marsh Widespread; seasonal to permanent flooding Grasses, sedges, frogs Tidal salt and brackish marsh Intertidal zones; semidiurnal to fortnightly flooding Salt-tolerant grasses and rushes, killifish, crabs, clams, snails Prairie pothole Northern plains states; temporary to permanent flooding; fluctuating water levels Grasses, sedges, herbs Fen Associated with mineral-rich water; permanently saturated by flowing water Sedges, grasses, shrubs, trees Bog Abundant in recently glaciated regions; precipitation principal desmids source of water Sphagnum moss, shrubs, trees, desmids Swamp Prolonged saturation and flooding Cypress, gum, red maple Bottomland Seasonal flooding; annual dry periods Oaks, sweetgum, other hardwoods Mangrove Subtropical, tropical regions; intermittent flooding by seawater through tidal action Red, black, white mangroves (Heinselman, 1970; Glaser et al., 1981). For such large areas, the gain, loss, and transformation of elements takes on continental or biospheric proportions (Elder, 1985; Gorham, 1991). For large river and floodplain systems, wetland complexes become landscape entities that rival major biomes in the context of global change (Lewis et al., 1990). This chapter provides an overview of wetlands. It does not address specific questions about delineation. Instead, it serves as background for the analysis of the delineation issues that are discussed in other chapters. It begins with an overview of the nature of wetland ecosystems and the response of wetlands to various alterations, progresses to a summary of the functions of wetlands, and it closes with a consideration of boundaries between wetlands and terrestrial ecosystems. THE NATURE OF WETLANDS Because wetlands are neither aquatic nor terrestrial, they have not been easily assimilated by the well-established scientific disciplines of terrestrial and aquatic ecology. Wetlands have some of the same features as deepwater systems, including frequently anoxic substrate and some species of algae, vertebrates, and

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Wetlands: Characteristics and Boundaries FIGURE 2.1A Wetlands can be part of a continuum between terrestrial and deepwater aquatic systems. Source: Mitsch and Gosselink, 1993. invertebrates. Most wetlands share with terrestrial ecosystems a flora dominated by vascular plants, although the species composition of wetlands generally differs from that of uplands. Wetlands often are found at the interface of terrestrial ecosystems (such as upland forests and grasslands) and aquatic systems (such as lakes, rivers, and estuaries, Figure 2.1A,B). Some are isolated from deepwater habitats, and are maintained entirely by ground water and precipitation. Even though they show structural and functional overlap and physical interface with terrestrial and aquatic systems, wetlands are different from these other ecosystems in so many respects that they must be considered a distinctive class. Hydrology as a Driving Force Hydrology controls the abiotic and biotic characteristics of wetlands (Figure 2.2). Abiotic characteristics such as soil color, soil texture, and water quality depend on the distribution and movement of water, as do the abundance, diver-

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Wetlands: Characteristics and Boundaries FIGURE 2.1B Isolated from connections with water bodies. Source: Mitsch and Gosselink, 1993. Copyright Van Nostrand Reinhold, with permission. sity, and productivity of plants, vertebrates, invertebrates, and microbes. Control is not unidirectional, however. For example, the biotic component of a wetland also can affect hydrology by increasing or decreasing water level or flow. Low rates of decomposition in some types of wetlands can cause basins to fill with undecomposed plant material, thus altering hydrologic conditions. Also, the water tables of some forested wetlands are held down in part by evapotranspiration; if trees are removed, standing water and marsh vegetation can develop. Muskrats, beavers, and alligators also can change hydrologic conditions in wetlands (Johnston, 1994a). Thus, wetland ecosystems are more than simple mixtures of water, soil, and organisms. Water flows and levels in most wetlands are dynamic (Kusler et al., 1994). The temporal pattern of water level, or hydroperiod, for an individual wetland is part of its ecological signature (Mitsch and Gosselink, 1993). Water level fluctuates daily in coastal marshes and seasonally in almost all wetlands, as shown in Figure 2.3 on arbitrary scales referenced to the surface of the substrate. It also

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Wetlands: Characteristics and Boundaries FIGURE 2.2 The relationships among hydrology, physicochemical environment, and biota in wetlands. Vegetation provides important feedback to hydrology through evapotranspiration and increase in flow resistance and to the physicochemical environment by affecting soil properties (organic content, dissolved oxygen) and elevation (accumulating organic matter, trapping sediment). Animals such as beaver, muskrat, and alligators can also significantly affect hydrology, soils, and other biota. varies significantly from year to year in some wetlands, such as prairie potholes. For these reasons, Fredrickson and Reid (1990) criticize the practice of stabilizing water level in managed wetlands. They point out that resource managers can be misled by the notion that most wetland wildlife species require year-round standing water for their life cycles. In fact, dry periods are often important for reasons that are less obvious but no less important. Moisture gradients vary temporally and spatially at the margin of a wetland, and plants, animals, and microbes often orient in predictable ways to the gradient. Figure 2.4 illustrates the zonation of vegetation that develops in four wetland types. It is this gradient, and particularly the junction between upland and wetland, that is central to the wetland characterization issue in the United States. The junction between wetlands and deepwater systems, while also incorporating a gradient, raises fewer regulatory issues. Causes of Variation Three factors explain many of the differences among wetlands (Figure 2.5): geomorphic setting (for example, floodplain, estuary fringe), water source, and hydrodynamics (such as unidirectional flow, reversing flow); these have been called hydrogeomorphic characteristics (Brinson, 1993a). Hydrogeomorphic

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Wetlands: Characteristics and Boundaries characteristics are interdependent. For example, geomorphic setting is in part a product of water source and hydrodynamics, but it also places constraints on water source and hydrodynamics. Hydrogeomorphic classes are distinctive combinations of the three factors (Figure 2.5). Depressional wetlands are maintained by overland flow, ground water, and precipitation rather than channelized flow. Riparian wetlands show seasonal or periodic pulses of water level (Fixture 2.3) that are delivered from overbank flows carrying nutrients and organic matter. Estuarine fringe wetlands are pulsed hydrologically by daily tides. Slope wetlands, such as the seeps that occur where ground water reaches the surface, are maintained by relatively constant sources of water. Peatlands occur in many settings, but can be maintained entirely by precipitation. Organic Matter The saturation of soils with water generally slows decomposition, which often causes wetlands to accumulate organic matter in the substrate. The organic-rich soil of wetlands, including peat in some wetlands (Glaser et al., 1981), is evidence of this accumulation. Even so, not all organic matter that enters or is formed by photosynthesis in wetlands remains within the wetland boundary. Many wetlands export organic carbon to streams and estuaries at a rate substantially higher than that of terrestrial ecosystems (Mulholland and Kuenzler, 1979). In this way, wetlands can make large contributions to the support of organisms that consume nonliving organic matter. For example, in Arctic tundra, where most of the landscape is wetland underlain by peat, aquatic food webs are supported to a significant extent by fossil peat (Schell, 1983). Natural Disturbance Many wetlands are maintained in part by natural disturbances such as flood, fire, or herbivory. Ewel and Mitsch (1978), for example, found that fire prevents pines and hardwoods from invading cypress (Taxodium) swamps in Florida (Appendix B, Florida pine flatwoods). By occurring alternately, fire and inundation together maintain the characteristic plant communities of these wetlands (Figure 2.6). Riverine and riparian wetlands commonly change as meanders undercut banks to form point bars that can be colonized by hydrophytes. Muskrats (Errington, 1963) and Canada geese (Jefferies et al., 1979) can clear vegetation from large portions of freshwater marshes. Beavers can flood stands of upland vegetation, thus causing the development of wetland vegetation (Johnston, 1994a). Nutrient Transformation Nutrients such as nitrogen and phosphorus are carried into wetlands by precipitation, overbank flow from streams, lunar fides, movement of surface and

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Wetlands: Characteristics and Boundaries

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Wetlands: Characteristics and Boundaries FIGURE 2.3 Hydroperiods for wetlands of several classes and physiographic regions. Source: Mitsch and Gosselink (1993); Brinson (1993a).

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Wetlands: Characteristics and Boundaries

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Wetlands: Characteristics and Boundaries FIGURE 2.4 Zonation of vegetation in four kinds of wetlands. Source: Mitsch and Gosselink, 1993.

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Wetlands: Characteristics and Boundaries

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Wetlands: Characteristics and Boundaries FIGURE 2.6 Interaction of hydroperiod and fire frequency for wetlands in Florida.  Adapted from Ewel (1990); copyright University of Central Florida Press, Orlando. ground water and, in the case of nitrogen, biological fixation from the atmosphere. Nutrients are exported by channelized and surface flows, seepage, and gas transfer via denitrification. Intrasystem nutrient cycling is to a large extent embedded in the pathways of primary production, food chain transfer, and decomposition. When production and decomposition rates are high, as is especially likely in flowing water or in wetlands that have pulsed hydroperiods, nutrient cycling is rapid. When rates of production and decomposition are low, as is most likely in nutrient-poor wetlands such as ombrotrophic bogs, nutrient cycling can be slow. All wetlands, including those with high flows of water, tend to recycle nutrients repeatedly (Faulkner and Richardson, 1989). Wetlands can be sources, sinks, or transformers of nutrients (Figure 2.7). A wetland is a sink for a specific substance if it shows net retention of the substance, and it is a source if it shows net loss of the substance. If a wetland changes a substance from one oxidation state to another or from dissolved to particulate form, it is acting as a transformer. A given wetland can perform different functions for different substances. For example, a wetland could be a sink for phosphorus, at steady state for nitrogen, and an exporter of organic carbon (Figure 2.7). Primary production can be limited by the availability of nutrients. Both low-nutrient (some bogs, cypress domes) and high-nutrient wetlands (floodplain wet-

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Wetlands: Characteristics and Boundaries FIGURE 2.7 Wetlands alter the flow of nutrients, the magnitude of which is depicted by the width of the dark arrows. Wetlands can be: (A) net sink, (B) net source, or (C) in steady state with respect to a given nutrient. Reproduced with permission from Mitsch and Gosselink (1993), Van Nostrand Reinhold.

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Wetlands: Characteristics and Boundaries lands, tidal marshes) occur in nature, and each has special characteristics. Low-nutrient wetlands often support plant species that cannot compete with plants in high-nutrient wetlands. Consequently, low-nutrient wetlands support some of the rarest and most diverse plant communities (Keddy, 1990). WETLAND FUNCTIONS Functions of wetlands can be defined broadly as all processes and manifestations of processes that occur in wetlands. For example, denitrification is a function of wetlands that arises from a microbial process; maintenance of waterfowl populations, which results from production of food and cover by wetlands, is also a function of wetlands. Most functions fall into three broad categories: hydrologic, biogeochemical, and maintenance of habitat and food webs. Examples of each are listed in Table 2.2, although the table does not include all wetland functions, nor are all the functions shown in the table characteristic of every wetland. Functions of wetlands often have effects beyond the wetland boundary. For example, wetlands store surface water, and the effect of this function downstream is a reduction in flood peak. Indicators often correspond to specific functions (Table 2.2), which can vary with wetland class, physiographic region, and degree of disturbance. Information on functions of wetlands has numerous uses, as explained in Chapter 10, but functional analysis is not necessary for the delineation of wetlands, as shown by Chapters 3 through 5. Relationship to Value Society does not necessarily attach value to all functions. Value is usually associated with goods and services that society recognizes. Thus, a connection can be made between the functions of wetlands, which are value-neutral, and to goods and services, which have value to society. Because value is a societal perception, it often changes over time, even if wetland functions are constant. It also can change over time, for example, as economic development changes a region. The value of a wetland in maintaining water quality near a drinking-water source can be great even if the wetland is small (Kusler et al., 1994). Some values can be mutually incompatible if they involve direct or indirect manipulation, exploitation, or management of a wetland. For example, production of fish for human consumption could conflict with the use of a wetland for improving the quality of water that contains toxins, if the toxins reduce fish production or contaminate fish flesh. The alteration of wetland functions can impair the capacity of a wetland to supply goods and services. Alternatively, if the functions of the wetland are protected, many goods and services will be sustainable for the life of the wetland. The cost of functional protection of wetlands can be large, however, because it

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Wetlands: Characteristics and Boundaries TABLE 2.2 Functions, Related Effects of Functions, Corresponding Societal Values, and Relevant Indicators of Functions for Wetlands Function Effects Societal Value Indicator Hydrologic Short-term surface water storage Reduced downstream flood peaks Reduced damage from floodwaters Presence of floodplain along river corridor Long-term surface water storage Maintenance of base flows, seasonal flow distribution Maintenance of fish habitat during dry periods Topographic relief on floodplain Maintenance of high water table Maintenance of hydrophytic community Maintenance of biodiversity Presence of hydrophytes Biogeochemical Transformation, cycling of elements Maintenance of nutrient stocks within wetland Wood production Tree growth Retention, removal of dissolved substances Reduced transport of nutrients downstream Maintenance of water quality Nutrient outflow lower than inflow Accumulation of peat Retention of nutrients, metals, other substances Maintenance of water quality Increase in depth of peat Accumulation of inorganic sediments Retention of sediments, some nutrients Maintenance of water quality Increase in depth of sediment Habitat and Food Web Support Maintenance of characteristic plant communities Food, nesting, cover for animals Support for furbearers, waterfowl Mature wetland vegetation Maintenance of characteristic energy flow Support for populations of vertebrates Maintenance of biodiversity High diversity of vertebrates includes not just the expense of regulatory programs, but also ''opportunity costs'' and "replacement costs" associated with a reduced range of economic choices. Costs of this type can be estimated (Farber and Costanza, 1987; Costanza et al., 1989; Gren, 1995), but they are beyond the scope of this report. Unique Functions Some of the functions of wetland ecosystems are shared by uplands. Even so, wetlands perform some functions, such as maintenance of breeding habitat for some bird species (Brinson et al., 1981), that are either unique or particularly efficient in proportion to their size. Also, wetlands are often the last portions of

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Wetlands: Characteristics and Boundaries a landscape converted to alternative uses (Brinson, 1988, 1993b). Because many wetlands are adjacent to surface waters, they often represent the best opportunity for natural improvement of water quality because of their filtering and transformation capacity. Uplands also can provide retention and transformation, but they are often preferentially allocated to other land uses—such as agriculture and urban development—that generate nutrients and sediments, and are more remote from surface waters. When wetlands are seasonally dry, they can be temporarily cease some functions, such as support of aquatic habitat, but retain others, such as the capacity to store surface water. Because the return of functions associated with saturation can be contingent on maintenance of the physical and hydrologic conditions under which the wetland developed, alteration of wetlands during dry phases is likely to be detrimental to their functional integrity. Landscape Perspective Individual wetlands function in part through interaction with the adjacent portions of the landscape and with other wetlands. For example, flyway support for waterfowl is a collective function of many wetlands. Likewise, no single wetland or aquatic site could support anadromous fish. The connections between individual wetlands, aquatic systems, and terrestrial systems are critical to the support of many organisms. Furthermore, flood control and pollution control are determined by the number, position, and extent of wetlands within watersheds. Thus, the landscape gives proper context for the evaluation of some wetland functions. Maintenance of biodiversity, water quality, and natural hydrologic flow regimes in part depends on the total wetland area and on the types of wetlands within regions (Preston and Bedford, 1988). As wetland acreage declines within a watershed, some functional capacities, such as maintenance of water quality or waterfowl populations, also decline. In this way, cumulative loss of wetland gradually impairs some landscape-level functions (Gosselink and Lee, 1989; Gosselink et al., 1990; Preston and Bedford, 1988). This occurs not only through loss of surface area, but also through reduction in average size, total number, linkage, and density of wetlands (Johnston, 1994b). Many wetland functions and their associated value to society depend on the connections among wetlands and between wetlands and adjacent aquatic and terrestrial systems. For example, river floodplain wetlands form natural corridors for the migration of fish, birds, mammals, and reptiles (Brinson et al., 1981). Uses of uplands can affect the physical, chemical, and biotic characteristics of wetlands. Paving or agricultural uses, for example, affect the amount and quality of water that reaches adjacent wetlands. Where the use of uplands is intensive, as in urban areas, wetlands often show signs of stress (Ehrenfeld and Schneider, 1993). Scarcity may magnify the value of wetlands. For example, in an urban

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Wetlands: Characteristics and Boundaries environment, the remaining wetlands may provide the only refuge for many kinds of wildlife, protect large amounts of valued property against flooding, serve as the main remaining mechanism for natural improvement of water quality, and recharge groundwater (Tiner, 1984). In this way, the significance of wetland functions derives partly from the surrounding landscape (Chapter 10). Relationship to Biodiversity Reduction of the area of wetland in a landscape often reduces biodiversity because many organisms depend on the wetlands and riparian zones with which they are frequently associated. For example, Hudson (1991) concluded that about 220 animal and 600 plant species are threatened with serious reduction in California, and the state's high rate of wetland loss (91% since the 1780s) is in part responsible. Blem and Blem (1975) showed the importance of river bottomlands to wildlife relative to adjacent uplands in Illinois. Ohmart and Anderson (1986) have shown that availability of large riparian areas, which include wetlands, is the primary factor that explains the number of birds that breed at high elevations in central Arizona. Weller (1988) views wetlands as islands in a terrestrial sea, and suggests that bird diversity follows the rules of island biogeography (more species with larger island area), as shown for prairie potholes. Similarly, Leibowitz et al. (1992) conclude that many waterfowl species are sensitive to reductions in area, patch size, wetland density, and proximity to other wetlands. They cite work that supports the need for many small wetlands as well as for large ones. Harris (1988) also points out that data on waterfowl, which provide some of the best long-term records of species that depend on wetlands, show steady declines (mallard down 35% from 1955 to 1985, pintail down 50%). Fish, which are good surrogates for aquatic biodiversity (Moyle and Yoshiyama, 1994), are sensitive to alteration of habitat, including wetlands. In the United States, 41 fish species have become extinct in the past century (Minckley and Douglas, 1991), and an estimated 28% of freshwater fish species in North America are seriously reduced in abundance or distribution. In addition, studies (Hickman, 1994; Weller, 1995) are beginning to document the extensive increase in biodiversity that occurs when wetlands are created or restored in a disturbed landscape. Factors other than reduction of area can cause a decline in biodiversity. For example, Moyle and Sato (1991) found that habitat heterogeneity is closely related to species diversity of fish communities, presumably because a more variable habitat provides a wider range of biological niches. A large number of both invertebrates and vertebrates show some association with wetlands, but species vary widely in the nature of this association. Some taxa, including certain species of aquatic invertebrates and amphibians, may be confined to wetlands or dependent upon them for specific stages of the life cycle. Waterfowl and mammals also have a range of dependencies on wetlands for food and habitat. For individual species, the suitability of a particular wetland for

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Wetlands: Characteristics and Boundaries habitat or for food may be critically dependent on the duration and time of year at which the wetland is inundated or saturated with water. In particular, species that require the presence of water for extended intervals will obviously not be able to live in a wetland that is inundated or saturated for a couple of weeks per year, but might be well suited to wetlands that show constant inundation. Table 2.3 gives some information on the great range of temporal dependencies for species that might be associated with wetlands. For example, some invertebrates, such as fairy shrimp, are adapted to spring inundation, as is typical of vernal pools, and require only two weeks of standing water for the completion of the life cycle. In contrast, maturation of amphibians in some cases may extend over more than a year. Even among organisms of a particular group, there is considerable variation. For example, Wright (1907) found that the shortest period from hatching to metamorphosis of amphibian species in the Okefenokee Swamp was 15 days, the next shortest was 24 days, and the other 19 taxa required more than 30 days. Thus in evaluating the role of wetlands in maintenance of biodiversity, the duration of inundation is a significant consideration. Longer inundation does not necessarily increase biodiversity, however, because wetlands that are characteristically inundated for only brief intervals offer support organisms that are unable to withstand the competitive and predatory forces of environments that are inundated for longer intervals. Removal of Nutrients and Sediments The geology of most wetlands is depositional. In general, uplands lose mass that accumulates in wetlands. In many watersheds, wetlands process dissolved and suspended materials from an area much greater than their own, which explains their disproportionately strong influence on water quality. In watersheds subject to human activities, the importance of wetlands on water quality is exaggerated by two factors: disturbances to uplands that increase erosion and augment the fertility of the landscape, and reduction of wetland area through filling, diking, and draining. Research has demonstrated repeatedly that natural wetlands enhance water quality by accumulating nutrients, trapping sediments, and transforming a variety of substances (Mitsch et al., 1979; Lowrance et al., 1984a,b; Whigham et al., 1988; Kuenzler, 1989; Faulkner and Richardson, 1989; Johnston, 1991). Whigham et al. (1988) observe that wetlands in different parts of a watershed improve water quality in different ways. For example, nitrogen processing and retention of large sediment particles might be more important functions of wetlands that border uplands where large particles are abundant, whereas phosphorus retention and trapping of fine particles might be more important in floodplain wetlands farther downstream (Mitsch et al., 1979; Cooper and Gilliam, 1987; Cooper et al., 1987). Stromberg et al. (1993) documented sediment accretion averaging 5-15 cm within a floodplain 150-200 m wide along the Hassayampa

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Wetlands: Characteristics and Boundaries TABLE 2.3 Name and Length of Limiting Portion of the Life Cycle for Invertebrates Commonly Associated with Wetlands (after Niering, 1985) Species Inundation Requirements Corixa spp. Eggs hatch 7-15 days; adults feed on water Lethocerus americanus Adult and nymph stage Notonecta undulata Entire life cycle Notonecta kirbyi Entire life cycle Hydrophilus spp. All but a few weeks Dineutus spp. All but pupal and part of adult stages Thermonectes marmoratus All but pupal stage Chrysomela lapponica None Labidomera clivicollis None Xylotrechus insgnis None Brachinus spp. None Chalenius seiceus None Donacia spp. 10 months Gerris remigis All but winter Ranatra brevicollis Entire life cycle Chauliodes spp. Larval stage Corydalus cornutus Two or three years Acroneuria californica One year or more Grammotaulius bettenii All but short adult stage Nannothemis bella Nymphs develop slowly Libellula luctuosa Egg to nymph Boyeria vinosa Egg to nymph Anax junius Overwinter Argia spp. Egg through naiad Pachydiplax longipennis Egg through nymph Libellula pulchella Egg through nymph Celithemis elisa Nymphs overwinter Sympetrum illotum Nymphs overwinter Lestes congener Eggs and nymphs overwinter until July Tipula spp. None Agathos comstocki All but adult Baetis spp. Eggs hatch 2-5 weeks; nymphs Culex pipiens Eggs hatch 1-5 days; larvae pupate 1-2 weeks Chlorion cyaneum none Tabanus americanus Two years Simulium spp. Egg and pupal stage Drosophila melanogaster None Dolomedes triton Most of life Nephila clavipes None Limnochares americana Much of life cycle

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Wetlands: Characteristics and Boundaries River in Arizona during a flood of 10-year recurrence in 1991. Unlike previous, smaller floods in which accretion was greatest adjacent to the main channel, the 1991 maxima were at elevations 1-2 m above the water table, indicating the functional importance of the broad floodplain for sediment retention during the larger, less frequent streamflow events. As uplands become more intensively managed and as the area of wetlands is reduced, nutrient processing and retention become impaired. Cumulative effects are discussed by Gosselink and Lee (1989), who point out that concentrations of nitrogen and phosphorus increase as watersheds are cleared and that clearing bottomlands changes them from sediment sinks to sediment sources. They also note that there can be a long lag between clearing and sedimentation downstream. Similarly, Jones et al. (1976) report that in Iowa, nitrate concentrations in streams are inversely related to the percentage of total watershed area in wetlands. Greatly increased sediment and nutrient transport from watersheds that experience urbanization or conversion of forest to agriculture can alter plant and animal species composition and even destroy wetlands. This is particularly true for isolated (depressional) and lakeshore (fringe) wetlands that have not historically received large amounts of sediment or nutrients. Many of the pothole wetlands in the glaciated regions of northern states are at particular risk from excessive sedimentation and nutrients because they lack flushing mechanisms. The original, heavily vegetated natural landscapes contributed small amounts of sediment and nutrients to these wetlands. Nutrient-poor wetlands, such as bogs, are also particularly vulnerable to watershed changes (Guntenspergen and Stearns, 1985). Other systems, such as riverine marshes, can better tolerate additional nutrients (Mitsch, 1992). Too little sediment also can also be damaging. Decreased sediment transport downstream of reservoirs along rivers and streams can threaten delta and estuarine wetlands. This is particularly true for the wetlands of the Mississippi delta, where sediment deprivation caused by reservoirs throughout the Mississippi system and levees along the lower end of the river have changed natural sediment transport to the point that accretion no longer maintains coastal and estuarine marshes. An estimated area of 25,000 acres (10,000 ha) of marsh in the delta disappears annually because of land subsidence, sediment starvation, and saltwater intrusion (Kusler et al., 1994). Changes in land use and water diversions that decrease freshwater flows in rivers and streams similarly threaten many estuarine wetlands by reducing the quantity of fresh water. Wetlands as Hydrologic Features of Watersheds The hydrology of a regional landscape is often affected by the area and position of the wetlands within it. For example, peak flow in a stream leaving a watershed is directly related to the total amount of wetland in the watershed or to the amount of wetland in headwater reaches. The relationship of peak flow and

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Wetlands: Characteristics and Boundaries wetland area may be nonlinear, however, in the sense that progressive loss of wetland may have an escalating influence on flood peaks (Novitzki, 1979; Gosselink and Lee, 1989; Johnston, 1994b). For example, in the Minneapolis metropolitan area, runoff per unit area of watershed increased rapidly when wetland area decreased to less than 10 percent of total watershed area (Johnson et al., 1990). NATURE OF BOUNDARIES WITH UPLANDS Wetlands frequently are bounded by uplands, but the boundary often lies within a broad transition zone. For gentle gradients, or where microtopography causes wetlands to be interspersed with uplands on very fine scales, the boundary of a wetland can be especially difficult to determine (Appendix B, hydric pine flatwoods of southwest Florida, and Chapter 8). Because vegetation analysis often is used in locating boundaries, the response of plant communities to environmental gradients is fundamentally important to the characterization of wetlands. Curtis (1959) and Whittaker (1967) introduced the continuum concept, which holds that vegetation changes gradually in response to environmental gradients because of the differing environmental optima among species. The continuum concept is now widely applied in vegetation analysis (Cox and Moore, 1993), and it is a useful basis for analyzing wetland boundaries. Change in the plant community at the boundary of a wetland is determined not only by differing adaptations of plant species to abiotic conditions, but also by competition among species. The importance of competition has been demonstrated by Pennings and Callaway (1992) and by Bertness (1991), but there is little information on interactions among species along the wetland-upland transition, where boundary determinations of wetlands are of great practical importance. Beals (1969) suggests that competition between species can be a cause of more discrete boundaries on steeper environmental gradients than on gentle ones. He reasons that the individuals of any two species are closest together on steep slopes, where they will compete most strongly. It follows that the steepest environmental gradients at the margins of wetlands will show the most distinct vegetation boundaries. The wetland boundary as judged by vegetation is not always stable. Natural hydrologic changes from year to year or from one decade to the next may cause the vegetation to shift. Changes in boundaries of wetland vegetation have been documented for prairie potholes (van der Valk and Davis, 1976; Weller, 1981; Kantrud et al., 1989a; Appendix B), for the riparian ecosystems of add zones (Stromberg et al., 1991), for salt marshes (Morris et al., 1990; Zedler et al., 1992), and for vernal pools (Zedler, 1987). Although wetland plants respond to changing environmental conditions, they might not do so immediately. For example, forested wetlands respond more slowly than do marshes because of the long lives

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Wetlands: Characteristics and Boundaries of trees. Soil morphology is less responsive than is vegetation and thus tends to integrate conditions over decades. CONCLUSIONS Wetlands have strong connections to adjacent uplands and deepwater environments. The interdependence between wetlands and associated aquatic ecosystems provides strong scientific justification for policies that make a connection between clean water and the protection of wetland ecosystems. Wetlands and associated terrestrial ecosystems are also interdependent, but alterations in terrestrial ecosystems usually affect wetlands more than the reverse. Watersheds and water bodies associated with wetlands control the quantity and quality of water reaching wetlands, and thus affect wetland functions. For this reason, regulation of activities within a wetland boundary is not always sufficient to maintain all wetland functions. Not all functions occur in all wetlands, nor are wetlands structurally uniform, but classification of wetlands into groups that share hydrogeomorphic and other properties clarifies similarities and differences in function. Wetlands often occupy only a small proportion of the watershed in which they lie, yet they often maintain exceptional biodiversity and process a large proportion of the dissolved and suspended materials leaving uplands, which typically occupy greater areas. When wetlands are removed, their collective functions are likely to decrease faster than the rate of reduction in surface area. RECOMMENDATION More intensive and regionally diverse studies of the following basic wetland phenomena should be undertaken in support of a stronger foundation for identification, delineation, and functional protection of wetlands: maintenance of biodiversity by wetlands, improvement of water quality by wetlands, flood abatement by wetlands, contributions of wetlands to functions occurring at the landscape scale, and effects of various kinds of land use of adjacent wetlands.