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SL 169 Functional Role of Wetlands in Watersheds1 William F. DeBusk2 1. This document is SL 169, a fact sheet of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Published: July 1999. Please visit the EDIS Web site at http://edis.ifas.ufl.edu. 2. William F. DeBusk, assistant professor and extension specialist, Soil and Water Science Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611-0510. The Institute of Food and Agricultural Sciences (IFAS) is an Equal Employment Opportunity Affirmative Action Employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / Larry R. Arrington, Interim Dean Introduction The purpose of this fact sheet is to introduce the basic concepts of the water quality-related functions of wetlands, and provide an overview of the role of wetlands as integral components of watersheds. Recent attention has been focused on wetland functions and values, particularly in the context of wetland mitigation and restoration. Valuation of individual wetlands, often referred to as functional assessment, is a rapidly emerging field that has evoked considerable controversy within scientific and regulatory communities. The terms wetland "function" and "value" are often used interchangeably, but there are fundamental differences between these terms, with both policy and management implications. Function refers to ecological, hydrological or other processes that contribute to the self-maintenance of the wetland and typically exert an influence (either positive or negative) on surrounding ecosystems. Values of wetlands are society's perception of the functioning of wetlands, and generally connote something worthy, desirable or useful to humans. Wetland values may change independently of function, depending on culture, technology and other market and non-market forces. Environmental Factors Controlling Wetland Function Functioning of wetlands in the landscape is strongly influenced by local and regional-scale environmental factors related to climate, geomorphology and source of water. In addition, human alteration of wetlands and the surrounding landscape can have considerable influence on wetland functioning. Climate Climate effects on wetland function include temperature and precipitation, both of which vary widely among different regions of the country. Temperature regimes control the rates of important biological processes, such as those involving organic matter decomposition, and consequently, accumulation of peat in the wetland. For example, wetlands in the northern United States tend to accumulate a greater depth of peat than wetlands of a similar type in the southeastern U.S., due to higher rates of decomposition in the latter. Precipitation has a substantial effect on wetland hydrology, particularly the water balance in the wetland. Especially important is the amount of
Functional Role of Wetlands in Watersheds 2 rainfall received relative to the amount of water lost through evapotranspiration (evaporation plus plant transpiration). Also of significance is the timing or pattern of rainfall in the region, including the proportion of annual rainfall occurring during the growing season. Rainfall patterns can affect frequency and duration of flooding, as well as the dominant type of vegetation in a wetland. Geomorphology Geomorphology, referring to landforms and landscape relief, plays a major role in wetland hydrology and ecology within a particular climatic region. Geomorphology encompasses the shape, size and location of wetlands in the landscape. The morphology of individual basins or wetlands, for example, influences flooding depth as well as hydroperiod, which refers to frequency and duration of flooding. Geomorphology of the surrounding landscape exerts a strong influence on surface and groundwater connections between the wetland and adjacent terrestrial and aquatic ecosystems. Another example of geomorphological influence on wetland function is the location of the wetland in the landscape, especially the landscape position relative to aquatic ecosystems such as rivers and lakes. Landscape position of a wetland may have a significant effect on regional water quality, by controlling the type and extent of wetland interaction with surface and groundwater flows in the watershed. Wetlands can be broadly grouped according to landscape position as either depressional, riparian or fringe wetlands (Figure 1). Figure 1. Variations in wetland geomorphology in the landscape. Depressional wetlands, as the name implies, form in depressions in the landscape and are not directly associated with rivers and lakes; consequently, these wetlands often have a minor role in surface hydrology of the watershed. Certain types of depressional wetlands also are effectively isolated from direct interaction with groundwater in the surrounding area. These are sometimes called perched wetlands, or surface water depression wetlands (Figure 2a). Some percolation of water from perched wetlands to local or regional groundwater (groundwater recharge) may occur, but is largely restricted by low soil permeability. In many cases, the water balance in perched wetlands is controlled almost exclusively by rainfall and evapotranspiration. Commonly in Florida, depressional wetlands interact with groundwater in the local or regional area, serving as hydrologic donors or receptors to shallow, unconfined aquifers. For example, cypress domes occur as shallow depressions in areas with low topographic relief and high water table, typically in pine flatwoods. For these wetlands, surface water level is maintained primarily through discharge (seepage) of shallow groundwater into the wetland (Figure 2b). When the water table drops in the surrounding uplands, surface water in the wetland recedes via infiltration into the groundwater (Figure 2c). Their dependence on regional groundwater levels for maintaining flooded conditions makes cypress domes and similar wetlands vulnerable to water table depression resulting from high-rate extraction of groundwater from municipal and private wells. Riparian wetlands are found in low-lying regions adjacent to rivers and streams that are periodically subjected to overbank flooding. Since they are hydrologically connected to both the river (downstream) and surrounding watershed (upstream), riparian wetlands are of major importance in regional hydrology. Riparian wetlands intercept surface and subsurface (groundwater) runoff from the upland regions of the watershed, and thus function as buffers for the river systems (Figure 2d). These wetlands also interact periodically with
Functional Role of Wetlands in Watersheds 3 floodwaters originating from rivers and streams; these hydrologic interactions can have a significant effect on river water quality. Figure 2. Examples of wetland hydrology and surface water-ground water interaction: a) wetland perched above the water table; b) groundwater discharge wetland; c) groundwater recharge wetland; d) riparian wetland with groundwater discharge (adapted from Mitsch and Gosselink ). Riparian wetlands have been shown to be highly effective in the reduction of non-point source (NPS) loading of nutrients and sediments to rivers and streams. As a result, many agricultural (including forestry) Best Management Practices (BMPs) are based on the premise that riparian buffer zones, which include wetlands and non-wetland areas, are essential components of the watershed that should be preserved or restored. Of particular significance to downstream water quality are riparian wetlands associated with low-order (smaller) streams, because of the large hydrologic throughput in these wetlands relative to the flow in the river or stream. These wetlands generally occur in the upper reaches of watersheds. Although the riparian zone of a single low-order stream may seem insignificant to water quality in the watershed, the cumulative impact of the multitude of riparian wetlands along low-order streams can be extremely significant. Fringe wetlands adjacent to lakes and estuaries are also hydrologically connected with uplands and aquatic ecosystems. However, the range of hydrologic influence in the landscape is somewhat limited for lake fringe wetlands compared to riparian wetlands, the difference being the reduced throughput of water in lake systems relative to rivers and streams. Source of water The source of water to a wetland can be a major determinant of water chemistry as well as overall ecological structure and function of the wetland. Rainfall-dominated wetlands receive minimal input from surface water and regional groundwater; not surprisingly, most wetlands of this type are depressional. These wetlands are typically characterized by acidic, poorly-buffered water, and low levels of biologically-available nutrients. Common examples are bogs, pocosins and wet prairies. Also included in this group are the Okefenokee Swamp in southern Georgia and the Everglades of south Florida, in its original condition. (The Everglades are an exception to the "rule" of acidic surface water, because of the abundance of calcium carbonate in the underlying bedrock). The ecological functioning of rainfall-dominated wetlands is probably more sensitive to nutrient loading than other wetland types, due to the adaptations of the native flora and fauna to low-nutrient conditions. Groundwater-dominated wetlands, such as fens, hydric seepage slopes and seepage meadows, often are more buffered (neutral pH) and nutrient-rich than rainfall-dominated wetlands. Deep groundwater normally contains relatively high concentrations of dissolved minerals, especially calcium and magnesium. Many depressional wetlands are fed primarily by groundwater, most notably fens in northern climates. Cypress domes, which are widely scattered throughout Florida, may receive most of their water from very shallow groundwater in surrounding pine flatwoods, therefore their water chemistry often resembles that of a rainfall-dominated wetland. Many riparian wetlands
Functional Role of Wetlands in Watersheds 4 receive groundwater inputs from the surrounding watershed, in addition to surface water from river flooding. Another common type of groundwater-dominated wetland is the seepage wetland, also called seepage slope, which, as the name implies, is formed along a slope where groundwater seeps upward through the soil and along the soil surface. Surface water-dominated wetlands are frequently characterized by high throughput of water and nutrients, thus they are considered to be "open" systems in terms of elemental cycling. Riparian wetlands, such as riverine marsh and floodplain forest, are dominated by periodic influx of surface water, often nutrientand sediment-laden. Lake fringe and tidal marsh wetlands are dominated by constantly fluctuating surface water. Tidal wetlands, in particular, experience regular flushing of surface water, and are thus considered to be open systems. Biogeochemical Functions of Wetlands One of the most widely-recognized functions of wetlands is the ability to reduce or remove nutrients from surface water or groundwater passing through the wetland. A wide array of physical and biogeochemical processes in wetlands interact to provide a natural filtering mechanism in the watershed to maintain or enhance downstream water quality. The term "biogeochemical" refers to the partitioning and cycling of nutrients and other compounds between the biotic (living) and abiotic (non-living, such as soil minerals and organic matter) components of an ecosystem. Among the important biogeochemical functions and values provided by wetlands are sediment deposition, nitrogen and phosphorus removal and transformation or inorganic nutrients to organic forms. It is their unique combination of structural and functional attributes that sets wetlands apart from terrestrial and aquatic ecosystems in their ability to remove or sequester nutrients and toxic environmental contaminants. For example, wetlands provide favorable conditions for settling of particulate matter (sediments): shallow water, low current velocity and the physical filtering action of plant stems and leaves. Wetlands also provide physical support, or substrates, for a multitude of chemical and microbiological processes, promoting nutrient removal and storage within the complex maze of micro-sites in the soil and vegetation cover. The total surface area available for microbial activity in the soil and the overlying dead plant material (litter or detritus) is extremely high in wetlands. Also, in contrast to terrestrial ecosystems, wetlands facilitate physical, chemical and microbiological processes by retaining water for extended periods within this biologically active zone. Another important characteristic of wetlands is the presence of anaerobic (oxygen-depleted) soils during periods of flooding, which gives rise to an aerobic-anaerobic interface, or boundary, near the soil surface. This juxtaposition of aerobic and anaerobic conditions provides an environment for unique chemical and microbiological reactions that greatly enhance the removal of nutrients from the inflowing water. Loading of nutrients to surface waters can result in undesirable environmental and economic impacts to aquatic ecosystems. The fate of nutrients in wetlands is controlled by biogeochemical cycling in the soil and water, and by the capacity of the soil and vegetation to assimilate and store N and P. However, nutrient-retention efficiency varies widely among wetlands. Much of this variability can be attributed to the environmental factors discussed earlier, i.e., those related to climate, geomorphology and water source. Environmental factors also exert significant influence on wetland biogeochemistry through their control of plant species composition, vegetation density and soil type. Human activities in and around wetlands can also influence or regulate wetland functioning, and may override the effects of environmental factors. Examples of human activities that can affect wetland biogeochemical cycling are alteration of hydroperiod (either shorter or longer) by ditches or berms, sediment and nutrient loading, and timber harvest in or immediately adjacent to a wetland. A wetland may function in the watershed as either a nutrient sink or source, providing net retention or release of nutrients to downstream
Functional Role of Wetlands in Watersheds 5 waters (Figure 3). Whether a wetland serves as sink or source depends on the biogeochemical characteristics, including soil and vegetation, within the wetland, as well as the rate of nutrient input from surface or groundwater sources. If the export of nutrients from the wetland is lower than the incoming nutrient load, the wetland is considered a net sink for nutrients. On the other hand, if the export of nutrients is greater than the nutrient inflow, the wetland will be a net source of nutrients. Net export of nutrients may occur as a result of high nutrient loading rate to a wetland followed by reduced loading rate. In many cases, some of the nutrients that accumulated under high loading rates continue to be exported from the wetland. Thus, a wetland that historically functioned as a nutrient sink may become a nutrient source as a result of chronic nutrient loading. Figure 3. Summary diagram of hydrologic and biogeochemical funcitons of wetlands in the watershed (adapted from Mitsch and Gosselink ). Wetlands can also function as net transformers of nutrients from inorganic to organic forms. In many cases, nutrients entering wetlands are predominantly in dissolved inorganic form (e.g., nitrate, ammonium, phosphate). In contrast, nutrients exported from wetlands are predominantly in organic form, a consequence of the copious production of dissolved and particulate organic matter within a wetland. This net transformation of nutrients by wetlands is ecologically significant because organic forms of nutrients must undergo decomposition to inorganic forms prior to biological utilization. Because a large portion of the wetland nutrient export is not immediately bioavailable, nutrient-related impacts in downstream surface waters, such as excessive growth of algae, may be greatly reduced or eliminated. The nutrient transformation function of wetlands, coupled with their ability to buffer pulses of nutrients in the watershed by storing and slowly releasing nutrients to downstream waters, provides a significant measure of ecological stability to contiguous aquatic systems. Summary Wetland functions give rise to a number of societal benefits, or values. Among the most widely cited values of wetlands is their potential for maintaining or improving water quality in downstream areas of the watershed. Wetlands perform a variety of biogeochemical functions, including sediment deposition, nitrogen and phosphorus removal, and transformation of inorganic nutrients to organic forms. The overall functioning of wetlands with respect to water quality is governed by several factors related to climate, geomorphology and the source of water to the wetland. The landscape position of wetlands, and their interaction with surrounding surfaceand groundwater, is of particular significance to regional water quality. Riparian wetlands are generally considered to have the most important water quality role in watersheds, due to their strategic location between upland and aquatic ecosystems. Nutrient removal and storage capacity in wetlands is controlled by the interaction of a number of physical, chemical and biological processes in the soil and biota. The net result of these processes determines the potential of a wetland to serve as a filter or sink for nutrients. References Brinson, M.M. 1993. Changes in the functioning of wetlands along environmental gradients. Wetlands 13:65-74. Mitsch, W.J., and J.G. Gosselink. 1993. Wetlands. Van Nostrand Reinhold, New York. Richardson, C.J. 1994. Ecological functions and human values in wetlands: a framework for assessing forestry impacts. Wetlands 14:1-9.