Wetlands Loss and Degradation
In the 1600's, over 220 million acres of wetlands existed in the lower 48 states (Dahl and Johnson 1991). Since then, extensive losses have occurred, with many of the original wetlands drained and converted to farmland. Today, less than half of the nation's original wetlands remain. Activities resulting in wetlands loss and degradation include: agriculture; commercial and residential development; road construction; impoundment; resource extraction; industrial siting, processes, and waste; dredge disposal; silviculture; and mosquito control (USEPA 1994b; USEPA 1993a). The primary pollutants causing degradation are sediment, nutrients, pesticides, salinity, heavy metals, weeds, low dissolved oxygen, pH, and selenium (USEPA 1994).
Twenty-two states have lost at least 50 percent of their original wetlands. Indiana, Illinois, Missouri, Kentucky, and Ohio have lost more than 80 percent of their original wetlands and California and Iowa have lost nearly 99 percent (USEPA 1995). Since the 1970's, the most extensive losses of wetland acreages have occurred in Louisiana, Mississippi, Arkansas, Florida, South Carolina, and North Carolina (Dahl and Johnson 1991). Between the mid-1970's and the mid-1980's, approximately 4.4 million acres of inland freshwater wetlands (-4%) and 71,000 acres (-1.5%) of coastal wetlands were destroyed (Dahl and Johnson 1991). Inland forested wetlands were impacted the most during the mid-1970's to the mid-1980's, with a loss of 3.4 million acres (-6.2%), primarily in the Southeast (Dahl and Johnson 1991). Approximately 900,000 acres were converted from forested wetlands to other wetland types. Conversion to agricultural usage of land was responsible for 54 percent of the losses of both freshwater and coastal wetlands; drainage for urban development for 5 percent and "unspecified usage" (planned development) was responsible for 41 percent of the losses. This is in contrast to the mid-1950's to mid-1970's, when agricultural drainage of wetlands was responsible for 87 percent of the losses and urban development for 8 percent.
To see a map, Wetland Loss Measured by National Wetlands Inventory
Although wetlands can improve watershed water quality, their
capacity to process pollutants without becoming degraded can
be exceeded. Many wetlands have suffered functional
degradation, although it is difficult to calculate the
magnitude of the degradation. Wetlands are threatened by air
and water pollutants and by hydrologic alteration (USEPA
1994b). Some researchers believe that a significant percentage
of the nation's remaining wetlands has been substantially
compromised hydrologically (Whigham 1988; Dahl and Johnson
1991). Measurements of the frequency or magnitude of such
degradation have not been attempted to any significant degree
in the United States.
Photo courtesy of
Wetlands form as a result of certain hydrologic conditions which cause the water table to saturate or inundate the soil for a certain amount of time each year.
The frequent or prolonged presence of water at or near the soil (hydrology) is the dominant factor determining the nature of soil development and the types of plant and animal communities living in the soil and on its surface. Wetlands can be identified by the presence of those plants (hydrophytes) that are adapted to life in the soils that form under flooded or saturated conditions (hydric soils) characteristic of all wetlands (Mitsch and Gosselink 1993). Thus alteration of wetland hydrology can change the soil chemistry and the plant and animal community. Alteration which reduces or increases the natural amount of water entering a wetland or the period of saturation and inundation can, in time, cause the ecosystem to change to an upland system or, conversely, to a riverine or lacustrine system. This alteration can be natural, such as through the successional process of stream impoundment by beavers or climate change.
Wetland loss and degradation through hydrologic alteration by man has occurred historically through such actions as: drainage, dredging, stream channelization, ditching, levees, deposition of fill material, stream diversion, ground water withdrawal, and impoundment.
Habitat loss and fragmentation
In Louisiana, coastal areas are subsiding as a result of the redirection of sediment by the Mississippi River levees, subsurface withdrawals of water, oil, gas, sulfur, and salt, from under wetlands, channelization of wetlands, and drainage of wetlands for development (Carney and Watson 1991; Boesch 1983; Duffy and Clark 1989). As the coast subsides, sea levels rise, essentially, to cover the land. The loss of $300 million worth of coastal real estate in the next 50 years is possible if subsidence continues (Carney and Watson 1991). The cost of the loss of wetland habitat as the sea levels rise to cover the land has not been determined. Land subsidence also allows saltwater intrusion into freshwater wetlands and causes shifts in the plant and animal community (Pezenski et al. 1990). Saltwater intrusion and the subsequent modification of wetlands habitat threaten the billion dollar fishery industry as well as the multi-million dollar trapping business (Boesch 1983; Duffy and Clark 1989).
Habitat fragmentation, as wetlands are drained or hydrologically altered, may result in changes in species composition as wetlands species are replaced by upland species; loss of large, wide-ranging species; loss of genetic integrity when isolated habitats are too small to support viable populations; reduced populations of interior species that can only reproduce in large tracts; and increased numbers of competitor, predator, and parasite species tolerant of disturbed environments (Harris 1988; Fleming e t al. 1994).
Water diversion structures
Water diversion structures, such as canals (channels), ditches, and levees have been used to modify wetlands to achieve flood control, drainage, mosquito control, irrigation, timber harvest, navigation, transportation, and industrial activity (Mitsch and Gosselink 1993). Canals and channelization change the hydrology of wetlands and increase the speed with which water moves into and through wetlands. As a result, patterns of sedimentation are altered and wetland functions and values that depend on the normal slow flow of water through a wetland can be affected. High sediment loads entering wetlands through channels, irrigation ditches and drainage ditches can smother aquatic vegetation, shellfish beds and tidal flats, fill in riffles and pools, and contribute to increased turbidity (USEPA 1993a). However, normal sedimentation rates in coastal wetlands are necessary to reduce land subsidence. Channelization and channel modification alter instream water temperature and diminish habitat suitable for fish and wildlife (USEPA 1993a). Normal sheet flow through wetlands is inhibited by the spoil banks that line a canal and by road embankments. Spoil banks and embankments also increase water stagnation. Channels often connect low-salinity areas to high-salinity areas, resulting in saltwater intrusion upstream, and causing species change and mortality of salt-intolerant vegetation.
Impoundment of natural wetlands for stormwater management or wildlife and habitat management may exploit one function of wetlands at the expense of others (USEPA 1993a; Mitsch and Gosselink 1993). Impoundment alters the natural wetlands' hydrology and decreases water circulation. Decreased water circulation causes increased water temperature, lower dissolved oxygen levels, and changes in salinity and pH; prevents nutrient outflow; and increases sedimentation (USEPA 1993a). Sedimentation reduces the water storage capacity, smothers vegetation, reduces light penetration, reduces oxygen content and affects the entire ecosystem richness, diversity, and productivity. Toxic substances, adhering to sediments, may accumulate in impoundments as a result of decreased water circulation and bioaccumulation of contaminants by wetland biota may occur.
Impoundment of coastal wetlands reduces the exchange of tidal
water in salt marshes and can impede the movement of fish that
use the marsh for a part of their life cycle. Impoundments are
often invaded by non-native plant species such as common reed
(Phragmites) and purple loosestrife (Lytherium) which
outcompete the native species and change the wetland community
Urbanization is a major cause of impairment of wetlands (USEPA 1994b). Urbanization has resulted in direct loss of wetland acreage as well as degradation of wetlands. Degradation is due to changes in water quality, quantity, and flow rates; increases in pollutant inputs; and changes in species composition as a result of introduction of non-native species and disturbance. The major pollutants associated with urbanization are sediment, nutrients, oxygen-demanding substances, road salts, heavy metals, hydrocarbons, bacteria, and viruses (USEPA 1994b). These pollutants may enter wetlands from point sources or from nonpoint sources. Construction activities are a major source of suspended sediments that enter wetlands through urban runoff.
As roads, buildings, and parking lots are constructed, the amount of impervious surface increases. Impervious surfaces prevent rainfall from percolating into the soil. Rainfall and snowmelt carry sediments; organic matter; pet wastes; pesticides and fertilizers from lawns, gardens, and golf courses; heavy metals; hydrocarbons; road salts; and debris into urban streams and wetlands (USEPA 1993a; USEPA 1993c). Increased salinity, turbidity, and toxicity; and decreased dissolved oxygen, all affect aquatic life and, therefore, the food web (Crance 1988). Excessive inputs of nutrients can lead to eutrophication or result in the release of pollutants from a wetland into adjacent water resources (USEPA 1993a).
As runoff moves over warmed impervious surfaces, the water temperature rises and dissolved oxygen content of the runoff water decreases (USEPA 1993c). Increased water temperature, as well as the lower dissolved oxygen levels, can cause stress or mortality of aquatic organisms. Rising water temperatures can trigger a release of nutrients from wetland sediment (Taylor et al. 1990). For example, as temperature rises, sediments release phosphorus at an exponential rate. Thus water temperature increases can lead to eutrophication.
Impervious surfaces decrease ground water recharge within a watershed and can reduce water flow into wetlands (USEPA 1993c). Significant increases in stormwater peakflow rates, and longer-term changes in wetland hydrology, as a result of stormwater discharge, can cause erosion and channelization in wetlands, as well as alteration of species composition and decreased pollutant removal efficiency (USEPA 1993a; USEPA 1993c). Changes in frequency, duration, and timing of the wetland hydroperiod may adversely affect spawning, migration, species composition, and thus the food web in a wetland as well as in associated ecosystems (Crance 1988; USEPA 1993c).
Wastewater treatment plant effluent and urban stormwater are a source of pollutants that continue to degrade wetlands (USEPA 1994b). The "aging" of wetlands can occur when wetlands filter organic matter. "Aging" is the saturation of the ecosystem by nutrients and heavy metals over time that results in the reduced effectiveness and degradation of the wetland (Mitsch and Gosselink 1986). Wastewater and stormwater can alter the ecology of a wetland ecosystem if high nutrient levels cause extended eutrophication and metals cause plant and aquatic organism toxicity (Ewel 1990). Iron and magnesium, in particular, may reach toxic concentrations, immobilize available phosphorous, and coat roots with iron oxide, preventing nutrient uptake.
Over one-third of shellfish waters can not be harvested because of habitat degradation, pollutants, algal blooms, and pathogens. To a large extent, this degradation is caused by urban pollution (NOAA 1995b; NOAA 1990b; USEPA 1994b).
Heavy metals may bioaccumulate in estuarine wetlands, causing deformities, cancers, and death in aquatic animals and their terrestrial predators. Heavy metal ingestion by benthic organisms (including many shellfish) in estuarine wetlands occurs because the metals bind to the sediments or the suspended solids that such organisms feed on or settle on the substrate where such organisms live.
Urban and industrial stormwater, sludge, and wastewater treatment plant effluent, rich in nitrogen and phosphorus, can lead to algal blooms in estuaries. Algal blooms deplete dissolved oxygen, leading to mortality of benthic organisms. Some algae are toxic to aquatic life (Kennish 1992). Excess algae can shade underwater sea grasses (part of the coastal wetland ecosystem), preventing photosynthesis and resulting in sea grass death (Batiuk et al. 1992; USEPA 1994b). Because sea grass meadows reduce turbidity by stabilizing sediments and provide critical food, refuge, and habitat for a variety of organisms, including many commercially harvested fish, the death of these plants profoundly impairs the estuarine ecosystem. (Dennison et al. 1993; USEPA 1994 b; Batiuk et al. 1992).
Roads and bridges are frequently constructed across wetlands since wetlands have low land value. It is often considered to be more cost effective to build roads or bridges across wetlands than around them (Winter 1988). Roads can impound a wetland, even if culverts are used. Such inadvertent impoundment and hydrologic alteration can change the functions of the wetland (Winter 1988). Road and bridge construction activities can increase sediment loading to wetlands (Mitsch and Gosselink 1993). Roads can also disrupt habitat continuity, driving out more sensitive, interior species, and providing habitat for hardier opportunistic edge and non-native species. Roads can impede movement of certain species or result in increased mortality for animals crossing them. Borrow pits (used to provide fill for road construction) that are adjacent to wetlands can degrade water quality through sedimentation and increase turbidity in the wetland (Irwin 1994).
The maintenance and use of roads contribute many chemicals into the surrounding wetlands. Rock salt used for deicing roads can damage or kill vegetation and aquatic life (Zentner 1994). Herbicides, soil stabilizers, and dust palliatives used along roadways can damage wetland plants and the chemicals may concentrate in aquatic life or cause mortality (USEPA 1993a). Runoff from bridges can increase loadings of hydrocarbons, heavy metals, toxic substances, and deicing chemicals directly into wetlands (U SEPA 1993a). Bridge maintenance may contribute lead, rust (iron), and the chemicals from paint, solvents, abrasives, and cleaners directly into wetlands below.
Innovative methods of constructing roads and bridges, and end-state (master) planning that reduces the need for new roads, can reduce the impacts of urbanization on wetlands.
Landfills can pose an ecological risk to wetlands. Landfill construction may alter the hydrology of nearby wetlands. Leachate from solid waste landfills often has high biological oxygen demand (BOD), and ammonium, iron, and manganese in concentrations that are toxic to plant and animal life (Lambou et al. 1988). Sanitary landfills may receive household hazardous waste and some hazardous waste from small quantity operators, as well as sewage sludge and industrial waste. Although regulated (under RCRA Subtitle D), these facilities may not always be properly located, designed, or managed, in which case some surface water contamination may occur. Researchers who conducted a study of the proximity of 1,153 sanitary landfills to wetlands in 11 states, found that 98 percent of the sanitary landfills were 1 mile or less from a wetland, and 72 percent were 1/4 mile or less from a wetland (Lambou et al. 1988).
As a result of disturbance and habitat degradation, wetlands can be invaded by aggressive, highly-tolerant, non-native vegetation, such as purple loosestrife (Lythrum salicaria), water hyacinth (Eichornia crassipes), and salvinia (Salvinia molesta), or can be dominated by a monoculture of cattails (Typha spp.) or common reed (Phragmites spp.) (McColligan and Kraus 1988; Weller 1981; Mitsch and Gosselink 1993). Particularly in constructed wetlands, including restored wetlands, non-native and tolerant native species may outcompete other species leading to a reduction in species diversity.
Non-native species may be introduced on purpose. For example, water hyacinth has been noted for its ability to sequester nutrients and is used for wastewater purification (Mitsch and Gosselink 1993). Water hyacinth and similar species can rapidly fill a wetland and are a threat to water quality in some areas.
Carp and nutria are two introduced exotic animal species that degrade wetlands (Mitsch and Gosselink 1993). Carp, introduced for recreational fishing, severely increase the turbidity of water resources. Nutria, introduced for their pelts, are rodents that voraciously eat, as well as destroy, freshwater and coastal wetland vegetation. Domestic and feral cats can be extremely damaging as they prey on wetland birds.
Mosquito control efforts in urbanized and resort communities has resulted in wetlands loss and degradation through drainage, channelization, and use of toxic pesticides.
? Information about methods of mosquito control that do not degrade wetland ecosystems
Marina construction and dredging activities can contribute suspended sediments into waters adjacent to wetlands. Intense boating activity can also increase turbidity and degradation of wetlands.
Wetlands can be adversely affected by pollutants released from
boats and marinas. Pollutants include: hydrocarbons, heavy
metals, toxic chemicals from paints, cleaners, and solvents
(USEPA 1993a). Dumping of wastes from fish cleaning and
discharge of human waste from marinas and boats can increase
the amount of nutrients and organic matter in a wetland. The
increased organic matter and nutrients can lead to
Adverse effects of industry on wetlands can include: reduction of wetland acreage, alteration of wetland hydrology due to industrial water intake and discharge, water temperature increases, point and nonpoint source pollutant inputs, pH changes as a result of discharges, and atmospheric deposition.
Saline water discharges, hydrocarbon contamination, and radionuclide accumulation from oil and gas production can significantly degrade coastal wetlands (Rayle and Mulino 1992). Most petroleum hydrocarbon inputs into coastal wetlands are either from coastal oil industry activities, from oil spills at sea, from runoff, or from upstream releases (Kennish 1992). Oil can alter reproduction, growth, and behavior of wetland organisms, and can result in mortality. Plants suffocate when oil blocks their stomata (Dibner 1978).
Polynuclear aromatic hydrocarbons (PAHs) are extremely toxic compounds that can enter estuarine wetlands through industrial effluent and atmospheric deposition. PAHs concentrate in sediments and thus contaminate benthic organisms (Kennish 1992). Fish contaminated with PAHs exhibit external abnormalities, such as fin loss and dermal lesions.
Toxic, radioactive, or acidic compounds and high concentrations of metals in abandoned industrial wastes at Superfund sites, or in operative (RCRA) waste sites, may be an ecological risk to wetlands fauna and flora. Many sites are close enough to directly or indirectly (through water flow) impact wetlands (Magistro and Lee 1988). Clean-up activities at Superfund and RCRA sites can degrade adjacent wetlands as well through disturbance of hydrology, introduction of contaminants, and degradation of habitat by equipment.
Metals and radionuclides tend to naturally concentrate in wetlands sediments and peat (Owen 1992). Such concentrations can be released in a flush from the wetland into surface water or ground water as a result of pollutant inflow or hydrologic alteration of the wetland (Owen 1992). Such a release of toxic compounds could generate serious environmental consequences. Intake of very low concentrations of radionuclides, such as uranium, from a water supply, for instance, will cause kidney failure and death . If radioactive peat or peat with a high metal concentration is used for gardening or agricultural activities, it can pose a human health risk as well (Owen 1992).
Photo courtesy of USDA NRCS
Historically, agriculture has been the major factor in freshwater and estuarine wetland loss and degradation. Although the passage of the Food Security Act of 1985 "Swampbuster" provision prevented the conversion of wetlands to agricultural production, certain exempted activities performed in wetlands can degrade wetlands:
Wetlands provide critical habitat for waterfowl populations. The drainage of U.S. and Canadian prairie potholes for agricultural production has been linked to a concomitant 50% - 80% decline in waterfowl populations since 1955 (USEPA 1995; DU 1995). Since the Swampbuster legislation was promulgated, the waterfowl population has begin to increase. Swampbuster rendered drainage of prairie potholes costly, and encouraged farmers to allow prior converted wetlands to revert to their previous natural wetland state and to construct farm ponds or restore marshes. Duck populations in 1994 increased by 24% over 1993 populations, and were the highest since 1980, when duck populations had plunged to a low (USEPA 1995).
Irrigation ditching can increase contamination of wetlands receiving irrigation drainage water, particularly where soil is alkaline or contains selenium or other heavy metals (Deason 1989). Untreated runoff containing extremely high concentrations of selenium led to mortality and deformities in bird, amphibian, and fish embryos and the disappearance of species from wetlands in California (USEPA 1995).
Agricultural pesticides entering wetlands in runoff, as well as through atmospheric deposition, may bioaccumulate in fish and other aquatic organisms (Kennish 1992).
Grazing livestock can degrade wetlands that they use as a food and water source. Urea and manure can result in high nutrient inputs. Cattle traffic may cause dens and tunnels to collapse. Overgrazing of riparian areas by livestock reduces streamside vegetation, preventing runoff filtration, increasing stream temperatures, and eliminating food and cover for fish and wildlife. As vegetation is reduced, streambanks can be destroyed by sloughing and erosion. Streambank destabilization and erosion then cause downstream sedimentation (Kent 1994b). Sedimentation reduces stream and lake capacity, resulting in decreased water supply, irrigation water, flood control, hydropower production, water quality, and impairment of aquatic life and wetland habitat (USEPA 1993b).
The economic losses attributed to the reduced quality and quantity of water and habitat from overgrazing of riparian wetland vegetation is more than $200 million (USEPA 1993b). The depletion of vegetation from riparian areas causes increased water temperatures and erosion and gully formation, prevents runoff filtration, and eliminates food and cover for fish and wildlife (USEPA 1993b). If stocking of livestock is well managed, grazing can coexist with wetlands, benefiting farmers and increasing habitat diversity.
If best management practices are used and careful monitoring occurs, silviculture and timber removal may only minimally affect some wetland functions. Habitat and community structure, however, still may be seriously degraded.
Drainage, clearing, haul road construction, rutting, and ditching of forested wetlands, all may affect wetlands in some way, although the impact may only be temporary. Since timber removal generally occurs in 20-50 year rotations, careful harvest may not be a permanent threat to wetlands. Adverse effects of timber harvest can include a rise in water table due to a decrease in transpiration, soil disturbance and compaction by heavy equipment, sedimentation and erosion from logging decks, skid trails, roads, and ditches, and drainage and altered hydrology from ditching, draining, and road construction (Shepard 1994). By utilizing best management practices, hydrology and biogeochemical processes of wetlands may be altered for only one to three years following timber harvest (Shepard 1994).
Pesticides and fertilizers used during silvicultural
operations can enter wetlands through runoff as well as
through deposition from aerial application. Fertilizers may
contribute to eutrophication of wetlands.
Peat is mined for agricultural and horticultural uses on a relatively small scale in the United States (Mitsch and Gosselink 1993). Wetlands that are mined for peat are significantly modified, often being transformed into open water habitat (Camp Dresser and McKee 1981). Peat mining not only removes peat but requires clearing of vegetation, drainage of the wetland, and creation of roads for equipment access to harvest the peat. These activities destroy the portion of the wetland selected for harvest and degrade adjacent areas.
An alternative to mining peat in pristine wetlands is to mine in former wetlands or wetlands that have been severely degraded through conversion to other uses.
Phosphate mining has resulted in the loss of thousands of acres of wetlands in central Florida (Mitsch and Gosselink 1993). Other types of mining operations can also degrade wetlands through hydrologic alterations, high metal concentrations, and/or decreased pH.
Acid drainage from active and abandoned mines causes extensive ecological damage. Acid mine drainage introduces high levels of acidity and heavy metals into the wetland environment through runoff and through direct drainage from mines into wetlands. The acidity and the high metal concentrations alter the biotic community composition and can result in mortality (Lacki et al. 1992; Mitsch and Gosselink 1993). Although natural wetlands may have the capacity to buffer some of the acidity and absorb a certain amount of the pollutants, over time, the assimilative capacity will be saturated (Kent 1994; Weider 1993).
Nitrous oxides, sulfurous oxides, heavy metals, volatilized pesticides, hydrocarbons, radionuclides, and other organics and inorganics are released into the atmosphere by industrial and agricultural activities, and from vehicles. These compounds can enter wetlands through wet and dry atmospheric deposition and can adversely affect aquatic organisms and the terrestrial organisms that feed on them.
? Information on particular pollutants and BMPs for pollutant sources
Regulations and other forms of protection for wetlands
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