Wetlands: An Essential Component of Curricula in Limnology

Eville Gorham

Department of Ecology, Evolution, and Behavior

University of Minnesota, St. Paul

St. Paul, Minnesota

SUMMARY

Wetlands have long received little attention in traditional limnology courses. Yet they are a critical resource, providing habitat for important species, significant links in the cycling of nutrients and the global storage of carbon, buffering against pollutants, and other services. Limnology courses should, therefore, be broadened to cover wetlands more thoroughly. Students should be taught how wetlands are defined, categorized, and distributed locally and globally; their patterns of development; their ecological and biogeochemical functions; their values to human society; the causes of wetland degradation and destruction; concepts and techniques in wetland restoration and creation; and issues in wetland management. They should also learn about key research areas in wetland science, dealing in particular with concerns about their future in the face of increasing human disturbance.

INTRODUCTION

Wetlands are waterlogged landscapes that cover approximately 8.6 × 106 km2, which amounts to 6.4 percent fo the world's land surface (Mitsch and Gosselink, 1993). They cover, therefore, a much greater area than the 1.2 × 106 km2 of freshwater lakes and the 0.8 × 106 km2 of saline lakes (Shiklomanov, 1993). A little less than half of wetland area is peatland, with deposits of organic detritus more than 30 cm deep. About 2 percent of the global wetland area is located in polar regions, 30 percent in the boreal zone, 12 percent in the subboreal, 25 percent in the subtropics, and 31 percent in the tropics. In the conterminous United States, wetlands cover about 5 percent of land area.



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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Wetlands: An Essential Component of Curricula in Limnology Eville Gorham Department of Ecology, Evolution, and Behavior University of Minnesota, St. Paul St. Paul, Minnesota SUMMARY Wetlands have long received little attention in traditional limnology courses. Yet they are a critical resource, providing habitat for important species, significant links in the cycling of nutrients and the global storage of carbon, buffering against pollutants, and other services. Limnology courses should, therefore, be broadened to cover wetlands more thoroughly. Students should be taught how wetlands are defined, categorized, and distributed locally and globally; their patterns of development; their ecological and biogeochemical functions; their values to human society; the causes of wetland degradation and destruction; concepts and techniques in wetland restoration and creation; and issues in wetland management. They should also learn about key research areas in wetland science, dealing in particular with concerns about their future in the face of increasing human disturbance. INTRODUCTION Wetlands are waterlogged landscapes that cover approximately 8.6 × 106 km2, which amounts to 6.4 percent fo the world's land surface (Mitsch and Gosselink, 1993). They cover, therefore, a much greater area than the 1.2 × 106 km2 of freshwater lakes and the 0.8 × 106 km2 of saline lakes (Shiklomanov, 1993). A little less than half of wetland area is peatland, with deposits of organic detritus more than 30 cm deep. About 2 percent of the global wetland area is located in polar regions, 30 percent in the boreal zone, 12 percent in the subboreal, 25 percent in the subtropics, and 31 percent in the tropics. In the conterminous United States, wetlands cover about 5 percent of land area.

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Wetlands may range in size from wet hollows a few meters square to the vast peatlands of the west Siberian Plain (Neishstadt, 1977; Walter, 1977) and the Hudson/James Bay Lowland (Wickware et al., 1980; Pala and Weischet, 1982), which between them cover more than 106 km2. They occur in a great diversity of types, which can be grouped into the seven major categories listed in Table 1 (Mitsch and Gosselink, 1993). Each category can be divided into two or more subcategories. Northern peatlands, for instance, are usually divided into circumneutral fens and strongly acid bogs (Gorham and Janssens, 1992a). In fens, the bicarbonate buffer system regulates pH, whereas in bogs, colored organic acids buffer the aqueous system at a pH of about 4. Despite the significance of wetland ecosystems, which cover 26 percent more area in the conterminous United States than do fresh and saline lakes, reservoirs, rivers, and bays (Frayer, 1991), they seldom receive much attention in curricula designed for limnologists. In six familiar limnology textbooks (Table 2), wetlands account for a little more than 1 percent of the textual material. This paper summarizes material that might be included in a limnology course encompassing wetland ecosystems (see also Gore, 1983; Mitsch and Gosselink, 1993). DESCRIPTION OF WETLANDS Wetlands are waterlogged landscape features and often develop at the margins of rivers and lakes. In the latter case the lakes may eventually—through the deposition of silt and peat—become converted wholly to wetland. The largest wetlands, however, generally form on very flat terrain in which damp mineral soils are invaded in their wettest parts by peat-forming vegetation. Peat impedes and dams up the natural drainage and brings about an expansion of the waterlogged area, eventually swamping large areas of upland forest. Very large peatlands often develop intricate and beautiful landscape patterns, which represent perhaps the most delicate mutual interaction between hydrology and vegetation on the surface of the earth (Sjörs, 1961; Heinselman, 1963; Wright et al., 1992). Ground water upwelling is a major factor in the development of these TABLE 1 Major Types of Wetlands Inland Coastal Freshwater Tidal salt marshes Riparian wetlands Tidal freshwater marshes Southern deepwater swamps Mangrove swamps Northern peatlands  

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology TABLE 2 Coverage of Streams and Wetlands in Six Familiar Limnology Texts       Pages Author Date Title Total Text Streams Wetlands Welch 1935 Limnology 394 30 4 Hutchinson 1957 A Treatise on Limnology, V.1. 902 0 0 Ruttner 1963 Fundamentals of Limnology, 3rd ed. 249 22 6 Cole 1983 Textbook of Limnology, 3rd ed. 344 22 0 Wetzel 1983 Limnology, 2nd ed. 753 0 8 Horne and Goldman 1994 Limnology, 2nd ed. 520 51 24 Total     3,162 125 41 Percentage of total     100 4.0 1.3 NOTE: The table excludes occasional brief mentions of streams and wetlands in on lakes. patterns (Siegel, 1983, 1988; Glaser et al., 1990) and is likely to be influenced strongly by global warming. The importance of hydrology in wetland studies has long been understood (Ingram, 1983), and the physiographic and climatic factors that influence it are becoming increasingly clear (Winter, 1992). Wetlands have their own distinct flora and fauna and are associated with about one-third of the rare, threatened, and endangered species in the United States (Niering, 1988). Among the showy or unusual plants of the wetlands are diverse carnivorous plants (sundews, pitcher plants, Venus flytraps, and bladderworts), numerous kinds of orchids, and the bog moss Sphagnum. Animals include a variety of invertebrates, among the most noticeable being the butterflies, and vertebrates as diverse as lemmings, moose, and alligators. Many invertebrate groups are poorly known, particularly among the insects (Danks and Rosenberg 1987), and await detailed taxonomic study. PATTERNS OF DEVELOPMENT Peat deposits provide a long-term archive of landscape dynamics, which can be complex and exhibit a great variety of pathways (e.g., Tallis, 1983; Janssens et al., 1992). Fossil assemblages, as well as physical and

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology chemical data, analyzed in dated peat cores allow reconstruction of historical records of the successional development of the deposit, as well as of past levels of acidity and water table depth, that can serve as baselines against which to assess recent and current impacts of human activity (Gorham and Janssens, 1992b). Pathways of development are driven both by external environmental factors and by the internal dynamics of the system itself. ECOLOGICAL AND BIOGEOCHEMICAL FUNCTIONS Wetland productivity ranges from a few hundred grams of dry matter per square meter per year in many peatlands of the northern boreal zone to thousands of grams in coastal salt marshes and inland freshwater marshes (Mitsch and Gosselink, 1993). In northern peatlands, about 8 percent of the 296 grams of carbon fixed annually from the atmosphere is preserved as peat, which forms a carbon pool of 412 × 1015 g (Woodwell et al., 1995). The average annual accumulation rate over the postglacial period is currently thought to be 96 × 1012 g per year, or 230 kg per hectare per year (Gorham, 1995). About 7 percent of carbon in northern peatlands is exported in streams as dissolved organic carbon, with the potential to acidify downstream waters (Gorham et al., 1986; Kerekes et al., 1986), and about 1.4 percent is emitted to the atmosphere as methane (Gorham, 1995). The rest, 84 percent, returns to the atmosphere as carbon dioxide. Wetlands are important in the cycling of nitrogen and sulfur to the atmosphere because their anoxic soils and peats are habitats well suited to microbes capable of reducing nitrate and sulfate. Denitrification occurs readily in circumneutral waterlogged soils (Kaplan et al., 1979) but less so in acid oligotrophic peatlands (Gorham, 1995). Nitrogen fixation takes place in a variety of ways, more particularly in circumneutral wetlands. Shrubs such as Alnus and Myrica have actinomycete nodules on their roots, and cyanobacteria (blue-green algae) are often important in marshes with standing water (Dickinson, 1984; Mitsch and Gosselink, 1993). Sulfate reduction in waterlogged soils results in the emission of volatile sulfur gases, but relatively few measurements have been made (Castro and Dierberg, 1987; Faulkner and Richardson, 1989). Northern peatlands can be important sinks for nitrogen and sulfur, with a nitrogen stock of about 16 × 1015 g and a sulfur stock of about 1.3 × 1015 g (Gorham, 1991). Their average accumulation rates during the postglacial period are estimated at 10 and 0.83 kg per hectare per year, respectively. Wetlands are extremely efficient sinks for a diverse array of chlorinated hydrocarbons, including DDT, toxaphene, and PCBs (polychlorinated biphenyls; Rapaport and Eisenreich, 1988). Mercury, on the other hand,

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology is more mobile. St. Louis et al. (1994) report that despite relatively low total yields of mercury in waters flowing from catchments dominated by wetlands, yields of methylmercury were relatively high compared to yields from upland catchments. A purely upland catchment appeared to retain or demethylate methylmercury, whereas catchments containing wetlands were net producers of methylmercury. Rudd (1995) has reviewed the sources of methylmercury to freshwater ecosystems. HUMAN VALUES In recent times wetlands—whether pristine or not—have been seen to have a broad array of values to human society (reviewed by Greeson et al., 1979, and Sharitz and Gibbons, 1989; see also Richardson, 1994). Among the physical values are such properties as shoreline stabilization, flood-peak reduction, and ground water recharge. On the other hand, peatlands can also serve as physical barriers to human movement, as they do in northern Canada and Russia and as they did for Napoleon in his Russian campaign of 1812 (Chandler, 1966). Environmental problems of transport, pipeline building, and waste disposal have been described by Radforth and Brawner (1977). Wetlands function chemically to improve water quality as filters, transformers, and sinks for materials delivered to them by human activities. For instance, they can filter 60-90 percent of suspended solids from added wastewater and as much as 80 percent of sediment in runoff from agricultural fields (Richardson, 1989). Wetlands to which nitrogen fertilizer is added can transform it by microbial denitrification to gaseous molecular nitrogen and nitrous oxide at rates substantially higher than those in similar unfertilized wetlands (Martikainen et al., 1992). As mentioned earlier, wetlands are sinks for carbon, sulfur, and nitrogen, and they retain in their peat deposits or volatilize a good deal of these materials when added from human by-products such as acid deposition (Bayley et al., 1987), agricultural runoff, and sewage wastewater (Richardson, 1989). Wetlands also provide a variety of biological benefits to human society. In the natural state, these include substantial forest resources and minor agricultural resources such as marsh hay, wild rice, and cranberries. After drainage, both forestry and agriculture are possible on a broader scale. Waterfowl and furbearers such as muskrat, beaver, and mink inhabit many natural wetlands, and many riparian wetlands serve as protective nurseries and food sources for young fish prior to moving out into open water. Aesthetic values of wetlands include unique plants, animals, and scenery. The patterned nature of many large northern peatlands as seen from the air also provides an aesthetically as well as scientifically valuable experience (see, for example, the illustrations in Wright et al., 1992). In

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology addition, wetlands are often the least disturbed among urban ecosystems and provide locally valuable aesthetic and educational experiences for city dwellers. The values of wetlands to scientists interested particularly in ecology and biogeochemistry should not be ignored. As noted earlier in connection with ecological functions, wetlands are important on both local and global scales in the cycles of carbon, nitrogen, and sulfur (Deevey, 1970, 1973), and their patterned development—often driven by upwelling ground water (Siegel, 1983, 1988; Glaser et al., 1990)—is a matter of intense interest to peatland ecologists (Janssens et al., 1992). DEGRADATION AND DESTRUCTION OF WETLANDS Until recently, wetlands have been regarded generally as wastelands fit for nothing until ''reclaimed" by human manipulation (Patrick, 1994). The largest human uses of wetlands are for forestry and agriculture (Kivinen, 1980). For northern peatlands the percentage of use varies from as high as 97 percent in Denmark and 94 percent in Poland to as low as 1 percent in Canada and 9 percent in the former Soviet Union, the two countries possessing the largest areas of peatland in the world. Wetland protection is very limited. Usually, at most, a few percent of wetlands are protected, New Zealand being a striking exception with 33 percent (Kivinen and Pakarinen, 1980). In the contiguous United States, 54 percent of the original wetlands had been lost by the 1970s (Frayer, 1991), primarily to agriculture. In some states, such as California, Ohio, and Iowa, less than 10 percent of the original wetlands remains, with consequent losses of wildfowl, furbearers, and fish. In the decade from the 1970s to the 1980s, the United States lost 2.6 percent of its wetlands (Frayer, 1991). Fuel mining is an important use for peat in northern countries, particularly in Russia and to a lesser degree in Ireland and Finland (Kivinen, 1980). Peat has scarcely been exploited in North America, although after the first "oil shock," consideration was given to the possibilities of converting peat in northern Minnesota to natural gas. Some peat has also been used as a chemical feedstock (Botch and Masing, 1984). Urban development has led to the dredging and filling of many wetlands in urban areas. Others have been flooded or drained by highway construction with inadequate provision for culvert drainage. Stormwater and wastewater diversion to wetlands (Environmental Protection Agency, 1984) has added both nutrients and toxins, the former especially leading to major alterations of flora and fauna. Air pollution has also affected many wetlands, in particular through acidification by sulfuric acid and the addition of nitrate as a significant plant nutrient (Gorham et al., 1984). The subject of wetland ecotoxicology has been reviewed by Catallo (1993).

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Biochemical indicators of environmental stress in wetland plants, whether natural or anthropogenic, have been investigated recently by Mendelssohn and McKee (1992). Human activities have led to the decline or extinction of certain wetland plants and animals; as mentioned earlier, wetlands are habitats for a disproportionate share of rare, threatened, or endangered species. Humans have also facilitated the spread of exotic species into wetlands, a notable current example being the invasion of many North American wetlands by Eurasian purple loosestrife (Lythrum salicaria) with consequent loss of wildlife habitat (Thompson et al., 1987). WETLAND RESTORATION According to the National Research Council (NRC, 1992), restoration should involve the return of a given ecosystem to a state approximating that in which it existed prior to disturbances; the NRC states that, "the goal is to emulate a natural, functioning, self-regulating system that is integrated with the ecological landscape in which it occurs." As presently practiced, wetland restoration is a very imperfect science (Mitsch and Gosselink, 1993). The conservation reserve program in the United States has encouraged farmers to restore wetlands previously drained for crop production, but the degree to which they approximate their former condition is largely unknown. Restoration of forested wetlands is difficult, given the long lives of trees. Early successional wetlands (e.g., cattail marshes) are the best candidates for restoration, whereas restoration of the large patterned peatlands of the boreal zone—developed over millennia of interactions of local and regional hydrology (Siegel, 1988)—must be regarded as impossible once they are seriously degraded. In the intermediate case of natural prairie wetlands, Galatowitsch and van der Valk (in press) have observed that the efficient-community hypothesis—"all plant species that can become established and survive under the environmental conditions found at a site will eventually be found growing there and/or will be found in its seed bank"—cannot be accepted completely as a basis for restoration, particularly for the species of sedge meadows. WETLAND CREATION New wetlands are often constructed in the United States to replace nearby wetlands destroyed by development, following rules laid down in Section 404 of the Clean Water Act (Kusler and Kentula, 1989). The process is described as mitigation and is intended to replace lost wetland functions in the landscape. Unfortunately, there is often no record of what the destroyed wetland was like, and there is seldom any follow-up

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology evaluation of created wetlands, so that the success of the mitigation effort cannot be evaluated (Erwin, 1989; K. Bettendorf, personal communication, 1993). Erwin (1991), in a rare follow-up study, reports a very limited record of success for wetland mitigation in South Florida. According to Bedford (in press), reviews of hundreds of mitigation projects have demonstrated that hydrology has not been given adequate attention, nor has the reestablishment of specific types of plant communities. According to D'Avanzo (1989), efforts at wetland creation by the U.S. Army Corps of Engineers have been among the most successful. The current U.S. policy of "no net loss" of wetlands has stimulated their creation as an effort at mitigation where wetlands have been destroyed. As discussed above, this can be successful where early successional wetlands are replaced. However, replacing mature wetlands that have taken centuries or millennia to develop with cattails goes against the no-net-loss concept because it results in a major shift in biodiversity and ecological function. As argued cogently by Bedford (1996), mitigation should be evaluated on the basis of hydrologic equivalence on a landscape scale. WETLAND MANAGEMENT Management of wetlands (Mitsch and Gosselink, 1993) may involve a wide range of practices, including draining and converting them to agriculture or forestry, using them for wastewater processing, "improving" them for enhancement of wildlife for hunters and trappers, or maintaining them in a relatively undisturbed state. For all these purposes, but particularly for the last, a major legal objective is to delineate exactly the boundaries of wetlands. Guidelines to assist such delineation have been prepared by federal agencies; they include characteristics of wetland hydrology, vegetation, and soils, along with a variety of field indicators. Short courses on wetland delineation are now widely available and are used in particular by consulting firms engaged in the process. Ecosystem management is, of course, closely tied to assessment of ecological risk and to the consequent determination of public policy (Pastor, 1995). It also requires, for its basis, a sound program of basic research; see, for example, Mendelssohn and McKee (1989). Whether such management is sustainable in the long run remains uncertain (Stanley, 1995). MAJOR QUESTIONS CONCERNING WETLANDS For all wetlands, major questions include the following: What have been their patterns of development in time and space? To what degree has that development been controlled by environmental factors, hydrology in particular (Siegel, 1988; Winter, 1992), and

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology to what degree has it been a consequence of autogenic processes such as peat accumulation? Do changes in the environment generally cause slow and steady changes in wetlands, or do threshold phenomena dictate relatively sudden responses to environmental stresses that build up over time (see Gorham and Janssens, 1992b)? For areas of extensive peatland, which constitute a global carbon pool of about 412 × 1015 g as compared to about 694 × 1015 g in plants, 1,600 × 1015 g in soils (including peat), and more than 700 × 1015 g in the atmosphere (Woodwell et al., 1995), the following are major questions: How has this pool accumulated over time and space? What environmental factors have controlled its accumulation? What is the role of these peatlands in the global carbon cycle, particularly with regard to the greenhouse gases carbon dioxide and methane (Billings, 1987; Gorham, 1995)? For areas of extensive peatland, future concerns center around what may happen to the very large pool of carbon locked up in peat as global warming occurs: Will falling water tables lead to oxidation of surface peats, large emissions of carbon dioxide to the atmosphere, and a strong positive feedback to global warming—particularly in the southern boreal zone where fire frequency will increase considerably and peatland fires may smolder for years in remote areas? Will such emissions of carbon dioxide be offset (or more than offset) by a shutdown of methane emissions from previously waterlogged anoxic peats? How will the melting of permafrost in the north temperate zone affect the carbon cycle in its peatlands (i.e., how much of the landscape will be flooded and how much will be dried out by runoff to the ocean)? Can northward migration of peat-forming plant communities into the Arctic, as global warming provides suitable conditions there, take place as rapidly as peatland degradation along the southern boundary of the boreal zone (Gorham, 1991, 1994, 1995)? For smaller wetlands, major questions for the future include the following: What will be the effects of global warming (Poiani and Johnson, 1991), acid rain (Gorham et al., 1984), and nutrient enrichment by atmospheric nitrogen deposition upon them. Can the pace of wetland drainage for human activities (agriculture, forestry, urban development, etc.) be slowed sufficiently to allow a reasonable degree of wetland protection and preservation?

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Freshwater Ecosystems: Revitalizing Educational Programs in Limnology Can techniques of mitigation, restoration, and wetland creation (Kusler and Kentula, 1989) be placed on a firm scientific footing to prevent further degradation, rehabilitate damaged wetlands, and replace those that must inevitably be destroyed? ACKNOWLEDGMENT This review owes much to the textbook of Mitsch and Gosselink (1993), which should be consulted for further details on most of these topics. I thank also the many colleagues who have sent me, for the past 45 years, reprints of their articles on wetland ecology and biogeochemisty. REFERENCES Bayley, S. E., D. H. Vitt, R. W. Newbury, K. G. Beaty, R. Behr, and C. Miller. 1987. Experimental acidification of a Sphagnum-dominated peatland: First year results. Can. J. Fish. Aquat. Sci. 44 (Supplement 1):194–205. Bedford, B. L. 1996. The need to define hydrologic equivalence at the landscape scale for freshwater wetland mitigation. Ecol. Appl. 6:57–68. Billings, W. D. 1987. Carbon balance of Alaskan tundra and taiga ecosystems: Past, present, and future. Quaternary Sci. Rev. 6:165–177. Botch, M. S., and V. V. Masing. 1984. Mire ecosystems in the U.S.S.R. Pp. 95–152 in Ecosystems of the World, 4B, Mires: Swamp, Bog, Fen and Moor, Regional Studies, A.J.P. Gore, ed. Amsterdam: Elsevier. Castro, M. S., and F. E. Dierberg. 1987. Biogenic hydrogen sulfide emissions from selected Florida wetlands. Water Air Soil Pollut. 33:1–3. Catallo, W. J. 1993. Ecotoxicology and wetland ecosystems: current understanding and future needs. Environ. Toxicol. Chem. 12:2209–2224. Chandler, D. G. 1966. The campaigns of Napoleon. New York: Macmillan. Cole, G. A. 1983. Textbook of Limnology, Third Edition. St. Louis: Mosby. Danks, H. V., and D. M. Rosenberg. 1987. Aquatic insects of peatlands and marshes in Canada: Synthesis of information and identification of needs for research. Mem. Entomol. Soc. Can. 140:163–174. D'Avanzo, C. 1989. Long-term evaluation of wetland creation projects. Pp. 75–84 in Wetland Creation and Restoration: The Status of the Science, vol. 2, J. A. Kusler and M. E. Kentula, eds. Report 600/3-89/038. Corvallis, Ore.: Environmental Protection Agency, Environmental Research Laboratory. Deevey, E. S. 1970. In defense of mud. Bull. Ecol. Soc. Am. 51:5–8. Deevey, E. S. 1973. Sulfur, nitrogen and carbon in the biosphere. Pp. 182–190 in Carbon and the Biosphere, G. M. Woodwell and E. V. Pecan, eds. CONF-720510. Washington, D.C.: U.S. Atomic Energy Commission, Technical Information Center, Office of Information Services. Dickinson, C. H. 1984. Micro-organisms in peatlands. Pp. 225–245 in Ecosystems of the World, 4A, Mires: Swamp, Bog, Fen and Moor, Regional Studies, A. J. P. Gore, ed. Amsterdam: Elsevier. Environmental Protection Agency (EPA). 1984. The Ecological Impacts of Wastewater on Wetlands: An Annoted Bibliography. Report 905/3-84-002. Chicago: EPA. Erwin, K. L. 1991. An Evaluation of Wetland Mitigation in the South Florida Management District, Vol. 1. West Palm Beach: South Florida Management District. Faulkner, S. P., and C. J. Richardson. 1989. Physical and chemical characteristics of freshwater

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