Thông tin tài liệu
691 19 Ancillary Benefits When wetlands are used to reduce pollutant concentrations and peak stormwater ows, ancillary benets can be achieved through thoughtful site selection and design. This chapter summarizes the ancillary benets of wetlands used for pol- lution control and recommends design features to optimize these benets. The primary objective of most wetland pol- lution control projects is water quality enhancement through assimilation and transformation of sediments, nutrients, and toxic chemicals. Secondary benets that can be incor- porated in wetland treatment designs include (1) vegetative biodiversity; (2) protection and production of fauna; and (3) aesthetic, recreational, commercial, and educational human uses. Realization of these benets usually requires extra land and extra expenditures. Some of the features that are added are illustrated in Figure 19.1. The potentials for including each of these benets in treatment wetlands are described in the following text. Research and manuals on wetland evaluation techniques (Golet, 1978; Greeson et al., 1978; Richardson, 1981; Kusler and Riexinger, 1986) may be used to compare the additional benets of treatment wetlands with natural wetland functions. 19.1 VEGETATIVE BIODIVERSITY Because of the presence of ample water, the wetland envi- ronment is generally characterized by a high diversity and abundance of plants. In many cases, wetland plant commu- nities include multiple vertical strata, ranging from ground- cover species to shrubs, and subcanopy trees to canopy tree species. Wetland plant diversity is important in determining wildlife diversity because of the creation of niches associated with differing vegetative structure, reproduction strategies, owering and seeding phenologies, gross productivity, and rates of decomposition (Mitsch and Gosselink, 2000b). The wetland treatment system designer should not expect to create or maintain a system with just a few known species. Such attempts frequently fail owing to the natural diversity of competitive species and the resulting high management cost associated with eliminating competition, or because of imprecise knowledge of all the physical and chemical requirements of even a few species. Rather, the successful wetland designer creates the gross environmental conditions suitable for groups or guilds of species; seeds the wetland with diversity by planting multiple species, using soil seed banks, and inoculating from other similar wetlands; and then uses a minimum of external control to guide wetland devel- opment. This form of ecological engineering results in lower initial cost, lower operation and maintenance costs, and a most consistent system performance. This section presents an overview of the oristic diversity that naturally develops in treatment wetlands as well as some details of the growth requirements of commonly occurring plant species in wetland treatment systems. These microbial and plant species are typically the dominant structural and visual components in treatment wetlands. An understand- ing of their basic ecology will provide the wetland designer or operator with insight into the mechanics of their “green” wastewater treatment unit. WETLAND PLANTS Algae There are multiple designations of the algal component of wetland vegetation. The term periphyton describes the com- munity of organisms that grows attached to emergent and submerged plants. Although periphytons usually begin colo- nization of new plant surfaces by attaching algal growth of lamentous and unicellular species, this functional compo- nent also includes mat-forming assemblages of algae, fungi, bacteria, and protozoans. Benthic algae are attached to the sediment–water interface. Planktonic or free-oating algae are generally not a large component of wetland ecosystems unless open or deep water areas are present. Plankton spend most of their life cycle suspended in the water column and are of potential concern as a source of total suspended solids (TSS). Algae are often the rst colonizers in sparsely vegetated, newly constructed wetland basins. They are capable of rapid colonization, and for some months may outcompete the mac- rophytes. Eventually, emergent plants will ll in, block the light, and reduce the algal populations. Algae are only rarely selected as the dominant plants in treatment wetlands. Exceptions include the technology known as algal turf scrubbing (Adey et al., 1993; Craggs et al., 1996; Craggs, 2000) and the use of periphyton, known in Florida as periphyton stormwater treatment (Vymazal, 1989; Kadlec and Walker, 2004). Algae are not the target of attempts at planting or management and would not be consid- ered a major factor in system diversity. Macrophytes The term macrophyte includes vascular plants that have easily visible tissues. A wide variety of macrophytic plants occur naturally in wetland environments. The U.S Fish and Wildlife Service has more than 6,700 plant species on its list of obligate and facultative wetland plant species in the United States. Godfrey and Wooten (1979; 1981) list more © 2009 by Taylor & Francis Group, LLC 692 Treatment Wetlands than 1,900 species (739 monocots and 1,162 dicots) of wet- land macrophytes in their taxonomy of the southeastern United States. Obligate wetland plant species are dened as those that are found exclusively in wetland habitats, whereas facultative species are those that may be found in upland or in wetland areas. There are many guidebooks that illustrate wetland plants, for example, Hotchkiss (1972) and Niering (1985). Lists of plant species that occur in wetlands are avail- able, for example, RMG (1992). Wetland macrophytes are the dominant structural component of most wetland treatment systems. The terms emergent, oating, and submerged refer to the predomi- nant growth form of a plant species. In the emergent plant species, most of the aboveground part of the plant emerges above the waterline and into the air. These emergent struc- tures are generally self-supporting. Emergent wetland plant species are the primary concern in this section because they provide the dominant visual impression in wetland treat- ment systems. Floating and submerged vascular plant spe- cies may also occur in wetland treatment systems. Floating species have leaves and stems buoyant enough to oat on the water surface. Submerged species have buoyant stems and leaves that ll the niche between the sediment surface and the top of the water column. Floating and submerged species are more typical of deeper, aquatic habitats within Spreader swales Buried forcemain from urban area Interpretive center Picnic areas Boardwalk Islands Berm Collector swale Approximate Scale in Feet 0 250 500 Cascade aeration Marsh Open water Overlook Rip-Rap Discharge to receiving stream FIGURE 19.1 Conceptual plan for treatment wetlands with ancillary benets. (From Kadlec and Knight (1996) Treatment Wetlands. First Edition, CRC Press, Boca Raton, Florida.) the wetlands, and they may dominate in wetlands when water depth exceeds the tolerance range for rooted, emer- gent species. WATER REGIME The hydropattern, or water regime, of the water is the most important contributor to the wetland type or class (Gosselink and Turner, 1978; Gunderson, 1989). The concept of water regime includes two interdependent components: (1) the duration of ooded or saturated soil conditions (the hydrope- riod as a percentage of time with ooding), and (2) the depth of ooding (Gunderson, 1989). Whereas hydroperiod refers to the duration of ooding, the term water regime refers to hydroperiod as well as to the combination of water depth and ooding duration (depth–duration curve). The importance of this factor as a determinant of wetland ecology cannot be overstated, because an incorrect understanding of the hydro- period and water regime limitations of wetland plant species is the most frequent cause of vegetation problems and shifts in constructed wetlands. Plant survival in ooded environments is a balance between the severity of oxygen limitation and the adapta- tions available to overcome this oxygen shortage. Thus, hydrophytic plants may be adapted to survive and ourish © 2009 by Taylor & Francis Group, LLC Ancillary Benefits 693 in specic ooded conditions, for example, for three months each year, or in “clean” or owing water, which might have higher in situ dissolved oxygen concentrations (Gosselink and Turner, 1978). However, these same plants may not be able to grow or survive during ve months of ooding or in stagnant or “dirty” water conditions. Likewise, plants may have adap- tations that allow prolonged survival in 0.3 m of water, but not in 0.6 m. It may be hypothesized that this balance is tilted unfavorably at higher water levels because of reduced aerial plant stem surface area to provide oxygen to the roots through the lenticels and aerenchymous tissues. This proposed expla- nation is supported by the nding that hydrophytes generally respond to ooding by growing taller—a growth response that allows a more favorable balance between emergent and submerged plant organs (Grace, 1989). PROPAGATION Wetland plants increase their numbers and density through asexual and sexual reproduction. Asexual reproduction refers to an increase in the number of individuals of a plant spe- cies through vegetative growth and typically occurs through the growth of roots or rhizomes, with subsequent emergence of new aboveground stems and leaves. Technically, a cattail bed that developed vegetatively from a single parent plant is a single plant. However, when these rhizomes are cut or decay, the individual daughter plants may remain viable and continue to spread vegetatively. A number of woody wetland plant species can spread vegetatively through coppice growth from viable root systems. Most wetland plant species also may increase their num- bers through sexual reproduction. In sexual reproduction, two individual plants, or the male and female owers from a single plant, contribute gametes to form seeds with new com- binations of genetic material. Sexual reproduction is impor- tant in providing alternative strategies for plants to survive from year to year through seasonal extremes, to propagate the species over large distances, to rapidly colonize new hab- itats, and to provide genetic variants that can adapt to chang- ing environmental and competitive conditions. Because of the potential for year-to-year hydrological variations in natural herbaceous wetlands with large num- bers of annual plant species, many species produce seeds that remain viable for years. These seeds accumulate during pro- ductive years and remain dormant until conditions are favor- able for germination, frequently after a period of desiccation and rewetting. This storage of viable seeds is known as a seed bank and has been studied in a number of marsh ecosystems (Pederson, 1981; Leck et al., 1989). In some cases, soil from a natural wetland with a seed bank can be used to establish a new constructed wetland. Most seeds in seed banks are annuals, but in some marshes, up to 50% may be perenni- als. Numbers of seeds range from <100/m 2 to >375,000/m 2 (Leck et al., 1989). A typical freshwater marsh in Manitoba had 4,582 seeds/m 2 and 34 species in emergent areas and only 93 seeds/m 2 in open water areas (Pederson, 1981). BIODIVERSITY Constructed treatment wetlands are typically dominated by emergent marsh, oating aquatic plant, or submerged aquatic plant communities. Emergent marsh species are frequently intermingled and codominant with populations of small oating aquatic plants such as duckweed (Lemna spp.). Most constructed treatment marshes in the United States are domi- nated by cattails (Typha spp.) or bulrush (Scirpus spp.) and, in Europe, by common reed (Phragmites australis) or grasses (Phalaris arundinacea or Glyceria maxima); however, many treatment marshes are dominated by other species or by a complex admixture of plant species, which include cattails, bulrush, grasses and/or reed as the dominant species. These plants are selected because they are robust and can survive in a wide variety of environmental conditions—in other words, they are survivors. They are inexpensive, plentiful in supply, and easy to propagate. However, they may also be viewed as a nuisance species owing to their ability to crowd out other species and assume visual dominance. In the United States, species such as Phalaris arundina- cea, Lythrum salicaria, Typha spp., and Phragmites austra- lis are considered by many to be undesirable and aggressive invaders. These may take over natural wetlands and also be a component of treatment wetlands. Pollutant removal pro- cesses do not require these species. They are often selected on the basis of cost and survivability. Unfortunately, many people now erroneously equate free water surface (FWS) treatment wetlands with the low biodiversity and nuisance species. Many constructed treatment wetlands undergo plant suc- cession during their operational life. Constructed marshes tend to remain as marshes as long as ooding is nearly con- tinuous and water depths exceed about 5 cm. However, if the water is too deep, then some emergent species will give way to submerged and oating plants. In any case, new plants nd their way into the system. For instance, Schwartz et al. (1994) found numerous volunteer species in a Florida constructed wetland. The system was established by planting a diverse set of 21 species. After one year, 185 species were found in the system. A side-by-side study of wetland vegetation changes was conducted at the Olentangy River wetlands site in Ohio (Mitsch et al. , 2004). As seen in Table 19.1, the effect of the i nitial planting strategy (planted or unplanted) was gone in three years. The volunteer cattails were then eaten out by muskrats and replaced by volunteer bulrushes in both wet- lands. The Tres Rios, Arizona, wetlands were planted in an elegant mosaic with Scirpus validus (soft-stem bulrush) and Scirpus olneyi (three-square bulrush), having varying num- bers of deep zones and islands (CH2M Hill, 1995). During the spring and summer of the third year after startup, almost all the vegetation died in all four wetlands for reasons that have not been fully resolved. The three possible causes were drowning, starvation, and sulde toxicity (Kadlec, 2006b). Thus, it is probable that the species patterns and types set by ecological or botanical designers will not survive in many circumstances. Such “designer” wetlands are likely to evolve © 2009 by Taylor & Francis Group, LLC 694 Treatment Wetlands that hundreds of plant species occur in a variety of treatment wetlands. Even when treatment wetlands are dominated by cattails or bulrush, dozens of other herbaceous and woody plant species are typically present. Thus, the issue is not so much of diversity as it is of dominance. It is possible to start the treatment wetland with a variety of regionally prevalent plants so that these have an opportu- nity to compete during the successional development of the system. This philosophy has been implemented at a number of treatment wetlands, including 21 species planted at the Orange County Eastern Service Area in Florida (Schwartz et al., 1994), 20 species at Victoria, Texas (Reitberger et al., 2000), and 14 species at Wakodahatchee, Florida (Bays et al., 2000). The Oregon Gardens treatment wetland used doz- ens of native wetland plants (Figure 19.2), and the Lapeer, Michigan, wetland used four species that did not include treat- ment wetland species, although bulrushes were used on the fringes (Figure 19.3). 19.2 WILDLIFE Numerous wildlife species of all taxonomic orders depend on wetlands as habitat. Plant structures that are lost into the aquatic portion of the food chain are typically degraded by a complex assemblage of small aquatic organisms that include inverte- brates (protozoans, worms, molluscs, arthropods, and others). These organisms, in turn, serve as the basis of the food chain for other invertebrates, and for diverse vertebrate groups such as sh, amphibians, reptiles, birds, and mammals. Above-water plant materials may also be processed as food by nonaquatic fauna that have no direct contact with the water in the wetland. We know from qualitative observation that a diver- sity of microscopic invertebrates such as protozoans and into other species mixtures and patterns. Wetlands that rely on antecedent seed banks and natural importation will develop more slowly, but are more likely to be regionally adapted. “Self-designed” wetlands may not be avoidable. It seems rea- sonable that new wetlands should be established with a spe- cies mix that provides a headstart for a number of regional species that can tolerate the water quality and hydropatterns deemed proper for water quality improvement. Table 3.1 summarizes the number of plant species reported for the treatment wetlands in the North American Treatment Wetland Database, v.2 (NADB database, 1998). Almost 600 species of macrophytic plants have been reported in constructed treatment wetlands. Of these observed plant species, 501 are emergent herbaceous macrophytes, 31 are oating aquatic species, 12 are submerged aquatics, 17 are shrubs, 25 are trees, and 5 are vines. While the NADB v.2 data collection effort provides some insight into the plant diversity at constructed treatment wetland sites in North America, the level of investigation varied widely between sites, and these numbers should be considered as the mini- mum number of plant species that actually occur in treatment wetlands. Wetland plant diversity is a poorly understood subject both in unaffected natural wetlands and in treatment wet- lands. Wetlands are frequently dominated by a few plant spe- cies (for example, cattail, sedge, or sawgrass marshes) that are best adapted to stressful environmental conditions, such as low nutrient or soil oxygen levels and uctuating water levels. Other unaffected natural wetlands have higher plant diversity and greater evenness between multiple dominant plant species. Constructed treatment wetlands cover the same range of plant dominance and diversity as unaffected natu- ral wetlands. Information collected for NADB v.2 indicates TABLE 19.1 Progression of Two Side-by-Side Wetlands at the Olentangy River Wetlands Park over a Start-Up Period of Nine Years Wetland 1 (Planted) Wetland 2 (Unplanted) Year 1 (1994) Algal pond with few macrophytes Algal pond with few macrophytes Year 2 (1995) Plants developed, particularly around the perimeter, to about 13% macrophyte cover Essentially no macrophyte cover Year 3 (1996) Continued to develop in vegetation cover, with about 39% cover Developed to about 35% macrophyte cover by August, essentially catching up with the planted wetland within 3 growing seasons Year 4 (1997) About 54% cover About 58% cover Year 5 (1998) Wider diversity of cover; not dominated by Typha Began to be dominated by Typha Year 6 (1999) Dominated by Typha Dominated by 3–4 of the planted species Year 7 (2000) Similar to 1999, except muskrats began to have a dramatic effect Similar to 1999, except muskrats began to have a dramatic effect Year 8 (2001) Maximum muskrat impact, vegetation cover was lower than 1995 Maximum muskrat impact, vegetation cover was lower than 1995 Year 9 (2002) Drawdown from April through June; developed cover of Schoenoplectus tabernaemontani Drawdown from April through June; developed cover of Schoenoplectus tabernaemontani Source: Data from Mitsch et al. (2004) In Olentangy River Wetland Research Park at The Ohio State University, Annual Report 2004. Zhang and Tuttle (Eds.), Columbus, Ohio, pp. 59–68. © 2009 by Taylor & Francis Group, LLC Ancillary Benefits 695 microarthropods is present, but there are no comprehen- sive lists of even the most common species in these groups. The macroinvertebrate groups most commonly studied are annelids (worms), arachnids (spiders and mites), crustaceans (amphipods, isopods, copepods, water eas, and craysh), insects, and molluscs. Fish are an obvious component of most aquatic ecosystems, but knowledge of their role in wetlands is limited. Amphibian and reptile (herptile) populations in wet- lands tend to be variable and secretive and, therefore, difcult to quantify. Birds are extremely mobile, and many species include wetlands in some part of their life history. Because birds are one of the fauna groups that people appreciate the most, study of this wildlife group in treatment wetlands has been more extensive than for any other. Many mammal spe- cies use wetlands as an important component of their total habitat. Some depend greatly on wetlands, whereas others use wetlands for specic and narrow life history requirements. FIGURE 19.3 The Durakon treatment wetland, near Lapeer, Michigan. Most of the plants selected for the wetland are not in common use in treatment wetlands (Carex lacustris, Sagittaria latifolia, Alisma plantago-aquatica, Pontederia cordata, and Sparganium eurycarpum). Only narrow fringe zones were planted with species with established treatment performance (Scirpus spp.) The wastewater inow is lagoon efuent. FIGURE 19.2 The Oregon Gardens treatment wetland, near Silverton, Oregon. A tremendous diversity of vegetation was introduced. The wastewater is better than secondary at the wetland inow. MACROINVERTEBRATES Information concerning wetland macroinvertebrate popu- lations is infrequent in the published literature. A number of specic ndings from wetland treatment systems are described in the following text to provide an indication of typical macroinvertebrate population densities and diversi- ties in these systems. A total of 518 species of aquatic invertebrates have been recorded from treatment wetlands in NADB v.2 (Table 19.2). These include 14 species of aschelminthes (cavity worms), 55 species of crustaceans, 10 species of arachnids, 23 spe- cies of molluscs, and 343 species of insects. Thirty-two treat- ment wetland systems listed in NADB v.2 have invertebrate data, although, in most cases, only species lists are available. The average Shannon-Weiner diversity (H’) of benthic mac- roinvertebrate collections was 1.36 units (with a total of 342 © 2009 by Taylor & Francis Group, LLC 696 Treatment Wetlands species reported) for constructed treatment wetlands. Benthic macroinvertebrate evenness averaged 0.56 for constructed wetlands. Average benthic populations in the NADB v.2 are 6,083 individuals per square meter for constructed treatment wetlands. Total populations of mosquito larvae and pupae in treatment wetlands from a few projects had an average of 1,144 individuals per cubic meter for constructed treatment wetlands (Hemet and Sacramento, California). FISH The number of sh families that typically occur in wetland environments in North America is small compared to pond, TABLE 19.2 Number of Invertebrate Species by Group in the NADB v.2 Database for Constructed Wetlands Benthos Subgroup Number of Species Recorded Annelida 47 Arachnida 10 Aschelminthes 14 Bryozoa 1 Coelenterata — Crustaceans 55 Entognatha 2 Insects 343 Mollusk 23 Platyhelminthes — Tardigrads 1 Unknown 22 Total 518 Source: Data from NADB database (1998) North American Treatment Wetland Database (NADB), Version 2.0. Compiled by CH2M Hill. Gaines- ville, Florida. TABLE 19.3 Number of Fish Species by System in the NADB v.2 Database for Constructed Wetlands Site Name System Name Number of Species West Jackson County, Mississippi WJC System 2 Brookhaven, New York Meadow Marsh Pond System 1 2 Brookhaven, New York Marsh Pond System 2 2 Hayward, California Hayward 16 Hillsboro, North Dakota American Crystal Sugar Co. 3 Everglades, Florida ENRP 18 Sacramento, California Sacramento Demo Wetlands 2 Champion, Florida Champion Pilot 3 Mt. View Sanitary District, California Mt. View Marsh 33 Source: Data from NADB database (1998) North American Treatment Wetland Database (NADB), Version 2.0. Compiled by CH2M Hill. Gainesville, Florida. lake, and stream environments. The sh species typical of wetlands are either adapted to obtaining oxygen in low- oxygen waters or only visit wetlands on a seasonal or shorter basis. The most widely distributed sh species in warmer temperate, subtropical, and tropical wetland treatment sys- tems is the mosquitosh (Gambusia afnis) in the topmin- now family. This species has been widely disseminated in North America and abroad in shallow wetlands and ponds because of its noted ability to consume the larvae and pupae of mosquitoes. In more northern climates, the black-striped topminnow (Notrophus fundulus) is a mosquito predator, and the central mudminnow (Umbra limi) is an important wet- land sh species. Seventy-seven sh species are reported from 13 treat- ment wetland sites in NADB v.2 (70 species from constructed treatment wetlands and 15 species from natural treatment we tlands). Table 19.3 summarizes sh data by site and system in NADB v.2. Fish enter treatment wetlands from inlet and outlet streams as well as from other sources, such as raptor drops. For example, smallmouth bass (Micropterus dolomieui) and sunsh (Lepomis macrochirus) swam up the outlet structures at the Hillsdale, Michigan, treatment wetland, and took up residence in all deep zones. Drawdown of the wetlands, for purposes of maintenance and retrotting, killed the sh. They were effectively precluded after the deep zones were lled for muskrat control. Bass and sunsh also are found in the Incline Village, Nevada, constructed wetlands, which are abutted by natural wetlands with deep water habitats. Tila- pia (Oreochromis spp.) found their way into the Tres Rios, Arizona, wetlands, although there appeared to be no route open to them from either downstream with entering water (WWTP) or upsteam from receiving water (drop boxes and cascades). Drawdown of the wetlands, for research purposes, killed the sh. Tilapia also are prolic in the Lakeland, Flor- ida, constructed wetlands—again with no obvious route for them to have emigrated. © 2009 by Taylor & Francis Group, LLC Ancillary Benefits 697 Fish may be problematic in treatment wetlands, particu- larly those that feed or nest on the bottom. For instance, at the Des Plaines constructed wetlands in Illinois, carp (Cypri- nus carpio) were unintentionally introduced into the wetland from the pumped source water (Des Plaines River), presum- ably as very small individuals. Over the course of two years, the sh grew in size to about 15 cm. These larger sh were observed to stir bottom sediments and impair suspended solids removal. The carp were removed from the wetland through a winter drawdown and freeze. A similar phenom- enon occurred at the Tarrant County constructed wetlands in Texas, where oodwaters from the source river brought carp (Cyprinus carpio), shad (Dorosoma cepedianum), and bull- heads (Ictalurus spp.) into the system. The activity of the sh caused severe bioturbation of the sediments during feeding and spawning activities (APAI, 1994). The cells were dewa- tered, and egrets and herons effectively removed the stranded sh. A 21-ha constructed wetland has been used for nal pol- ishing of an industrial efuent since 1999 in Victoria, Texas. Fish populations in the eight wetland cells have been used as an indicator of environmental condition at or near the top of the wetland aquatic food chain (Reitberger et al., 2000). This constructed treatment wetland includes three different types of cells arranged in parallel and in series. Treated efuent from a conventional activated sludge treatment facility is distributed into the stage 1 wetland cells, which are colonized primarily by cattails (Typha spp.) and contain transverse deep water zones colonized by submerged and oating aquatic plants. Water from these cells ows into the stage 2 cells containing a diverse marsh plant community and transverse deep water zones. Water ows nally into a habitat cell with diverse water depths (up to 3 m). Six sh species have been collected; mosquitosh (Gambusia af- nis) have the highest density, and populations of other spe- cies are variable from year to year, presumably in response to climatic conditions and water quality conditions. Sun- fish (Lepomis macrochirus and Lepomis gulosus), bluefin killifish (Lucania goodie), silversides (Menidia beryllina), and gizzard shad (Dorosoma cepedianum) populations are sporadic in the habitat cell. AMPHIBIANS AND REPTILES Amphibians and reptiles (known jointly as herptiles) are part of the top consumer structure of wetlands, including treat- ment wetlands. The four most important groups are frogs (anurans), turtles, snakes, and alligators. Twenty-one anuran species are reported from six constructed and three natural treatment wetlands in NADB v.2 (ten from the constructed treatment wetlands). Turtles are found primarily in wetland treatment systems that have areas of deeper, open water, and especially in pretreatment lagoons. Species of turtles fre- quently found in these habitats include snapping turtles (Che- lydra serpentina), mud turtles (Kinosternon subrubrum), and painted turtles (Chrysemys picta). A wide variety of snakes are associated with wetlands and wetland treatment systems and play an important ecological role by feeding on sh, invertebrates, birds, and small mammals. In North American treatment wetlands, the snakes that receive the most inter- est are the venomous species, including the cottonmouth or water moccasin (Agkistrodon piscivorus), the copperhead (Agkistrodon contortrix), and timber rattlesnakes (Crotalus horridus), which occasionally inhabit wetlands. In fact, most snakes in wetlands are nonpoisonous species of water snakes (Genus Natrix), swamp snakes (Liodytes and Seminatrix), rat snakes (Elaphe), and king snakes (Lampropeltis). Alligators (Alligator mississippiensis) have been observed at relatively high densities in wetland treatment systems having open water areas and high sh populations (Figure 19.4). Densi- ties in the Florida stormwater treatment constructed wetlands have reached proportions sufcient to allow hunting. BIRDS About 600 different bird species (one third of the total resi- dent bird species) are either partially or wholly dependent on wetlands for some part of their life history in North America (Kroodsma, 1978). The diversity and abundance of birds in and around wetlands attract many bird watchers, who repeat- edly have observed that their species lists are longer and their counts higher when they include wetlands in their counting areas. Where these water bodies are enriched by nutrients and FIGURE 19.4 Alligators are at the top of the food chain in wetland systems across the American south. (Photo courtesy J. Bays.) © 2009 by Taylor & Francis Group, LLC 698 Treatment Wetlands organic matter from wastewater and stormwater discharges, bird watchers usually nd even better success in their sport. A number of wetland bird species are game species and are hunted. Treatment wetlands frequently provide good waterfowl habitat. In a few cases, these treatment wetlands have been used for duck hunting as a recognized secondary benet. The stormwater treatment constructed wetlands in Florida are open to duck hunting and are deemed the best hunting spots in the entire state. In other cases, wastewater has been found to improve the waterfowl breeding and feed- ing habitat as a secondary or nonessential side effect (Wil- helm et al., 1989). A few quantitative studies of bird populations in wet- lands treating wastewaters are available. If the wetland is constructed on previous upland, increases in bird species and abundance have been observed, compared to the previ- ous upland habitat (Hickman, 1994). During the summer of 1991, the U.S. EPA conducted a study of the environmen- tal condition of six constructed wetland treatment systems in the United States (McAllister, 1992; 1993a; 1993b). This inventory included two systems in the arid west (Incline Vil- lage, Nevada, and Show Low, Arizona), two systems along the coastal plain of Mississippi (Ocean Springs and Collins), and two systems in peninsular Florida (Orlando Easterly and Lakeland). Bird surveys were conducted by local ornitholo- gists using slightly different methods. Table 19.4 summarizes the major ndings of this research. The EPA bird surveys at constructed wetland treatment systems conrmed that these systems have high species rich- ness and population densities compared to control wetlands. The total number of bird species observed at each site dur- ing the 1991 study ranged from 33 to 63, with average daily population densities of all wetland-dependent species rang- ing from about 7 to 19 birds/ha. Densities of wading birds at the two central Florida constructed wetland treatment sys- tems averaged 0.29 birds/ha at Orlando and 0.38 birds/ha at Lakeland. These densities were as high as, or higher than, comparison data from marshes along the St. Johns River, the central Everglades, and Central America. Highest total bird densities were noted at the arid region sites where there are few natural wetlands available to compete as alternative hab- itats. The EPA studies also concluded that the constructed treatment wetlands are important habitats for a number of endangered or threatened wetland-dependent bird species. TABLE 19.4 Results from Constructed Wetland Treatment System Bird Surveys by U.S. EPA during 1991 Site Constructed Wetland Area (ha) Total Species Density (#/ha) [Average (Range)] Incline Village, Nevada 198 47 19.1 (0.8–42.2) Show Low, Arizona 284 42 13.8 (7.8–21.7) Collins, Mississippi 4.5 35 7.2 (5.9–8.5) Ocean Springs, Mississippi 22 35 10.4 (6.4–14.5) Orlando Easterly, Florida 494 141 0.29 (0.11–0.54) Lakeland, Florida 498 63 0.38 (7.7–13.5) Bird usage of the 16-ha DUST (Demonstration Urban Stormwater Treatment) marsh in Coyote Hills Regional Park near Fremont, California, was studied by Dufeld (1986). Weekly or biweekly bird censuses were conducted in the DUST marsh and a nearby 18.3-ha control from mid-Janu- ary 1984 through mid-June 1985. The mean abundance of wetland birds in the marsh areas ranged from 100 to 300 (6.25 to 18.75 birds/ha) in the DUST marsh and 90 to 420 (4.91 to 23.0 birds/ha) in the control marsh. The mean num- ber of species per census ranged from 14 to 23 in the DUST marsh and 10 to 18 in the control area. Highest mean species counts were observed in both marshes during the winter and spring seasons. During all seasons, the dabbling and diving ducks preferred the control marsh, and shorebirds, gulls, and terns preferred the DUST marsh. Individual sh-eating wading birds were generally present in greater numbers in the DUST marsh than in the control area. These populations were enhanced by the presence of roosting black-crowned night herons (Nycticorax nycticorax). Shorebirds preferred the mudat areas of the overland ow cell in the DUST marsh. The most common shorebirds were American avocets (Recurvirostra americana), black-necked stilts (Himantopus mexicanus), and marbled godwits (Limosa fedoa). Bird populations at the Victoria, Texas, constructed treat- ment wetland have been quantied by designed studies and by birdwatcher counts (Reitberger et al., 2000). A total of 188 bird species had been observed using the wetland as of June 2000. Annual species numbers ranged from 106 to 144, with average numbers of species per count from 34 to 53; average annual bird densities ranged from 19 to 26 birds per hectare. Bird population numbers were dominated by grebes, cormo- rants, ibis, waterfowl, vultures, coots, rails, stilts, sandpip- ers, gulls, doves, swallows, wrens, warblers, and blackbirds. This site is open to school groups and the public, and bird watching continues to be one of the most important ancillary benets of the wetland. The Wakodahatchee, Florida, wetlands are attractive to wildlife (Bays et al., 2000). The numbers of bird species observed within the wetlands totaled 119 and 142 in 1997 and 1998, respectively. Thirteen of these species are con- sidered commercially exploited, threatened, or endangered by state and federal agencies. Bird densities estimated for 1997 averaged 53 per hectare. At least 13 species of birds were observed to have nested on the site, indicating that the © 2009 by Taylor & Francis Group, LLC Ancillary Benefits 699 system is providing suitable avian nesting habitat for certain species. Bird use and water quality treatment are not always totally compatible uses of constructed wetlands. There may be con- icts between maintenance activities and the lifestyles of threatened or endangered species. For example, the nesting habits of stilts and burrowing owls lead them to prefer the levee areas of drained or dewatered wetland cells. This has interfered with system restarts in the stormwater treatment constructed wetlands in Florida. Huge ocks (thousands) of yellow-headed blackbirds (Xanthocephalus xanthocephalus) have been impli- cated in recontamination of water by pathogens in Arizona (Orosz-Coghlan et al., 2006) (Figure 19.5). Waterfowl have been implicated in turbidity increases at the Columbia, Mis- souri, constructed wetlands (Knowlton et al., 2002). Waterfowl, particularly geese, can also be devastating on new transplants and can effectively wipe out an entire planting. MAMMALS A wide variety of mammals reside in or visit treatment wet- lands. Some of the more common for North America are listed in Table 19.5. Shrews (Family Soricidae) are found along the edges of wetlands and moist elds and feed on insects. But by far the largest and most important group of mammals associ- ated with wetlands are rodents. Small rodents are also found in FWS systems, such as mice and voles, most of which are herbivorous species that graze on plants and seeds and are prey to sh, wading birds, and raptors. However, it is the larger rodents that have proven problematic in many treat- ment wetlands. These rodents are briey discussed herein. Muskrats Muskrats (Ondatra zibethica) cut large numbers of emergent herbaceous plants, primarily cattails (Latchum, 1996), and build feeding platforms and nests (mounds). This grazing can change treatment wetland areas from densely vegetated to a FIGURE 19.5 Thousands of blackbirds move in and out of the Sweetwater constructed wetlands in Tucson, Arizona. patchwork of open and emergent areas (Kadlec et al., 2007). Muskrats consume a portion of the annual net primary pro- ductivity, principally rhizomes, but their mounds represent a greater share of this production. Densities of 20 or more animals per hectare have been found, which can destroy the majority of the macrophyte standing crop in a given year. At such an exacerbated scale, muskrat herbivory may be termed as an “eatout,” and it is evidenced by the removal of essentially all emergent plant parts (Figure 19.6). Destruc- tion of the wetland vegetative infrastructure may create an attendant loss of some water quality functions, but may not harm others. No designed research studies have been con- ducted to quantify the effects of muskrat eatout on wet- land water quality performance, but potential impacts may TABLE 19.5 Mammals in Treatment Wetlands in North America Rodents Shrews Sorex spp. Mice Peromyscus spp. Voles Microtus spp. Muskrat Ondatra zibethica Beaver Castor canadensis Nutria Myocastor coypus Herbivores Rabbits Sylvilagus spp. Deer Odocoileus virginianus Elk Cervus canadensis Carnivores Raccoon Procyon lotor Opossum Didelphus marsupialis Skunks Mephitis spp. Mink Mustela vison Otter Lutra canadensis Bobcat Lynx rufus Coyote Canis latrans © 2009 by Taylor & Francis Group, LLC 700 Treatment Wetlands be speculatively identied. Two water quality measures are very liable to change because of denudation: dissolved oxygen (DO) and total suspended solids (TSS). Increased potential for atmospheric reaeration is present in the plant-free wetland, and plankton can generate oxygen within the water column because there is sufcient light. A 2-ha wetland in Commerce Township, Michigan, provided data to support these intuitive concepts. In an early, fully vegetated phase, this wetland had an annual average outow of 6 mg/L TSS and 9.8 mg/L DO. After a 100% muskrat eatout, TSS rose to 12 mg/L and DO rose to 10.4 mg/L. There were also possible muskrat impacts at the Olentangy, Ohio, wetlands. An eatout occurred in the year 2000, ve years after system start-up (Mitsch et al., 2004). Thereafter, turbidity increased upon passage, in contrast to decreases that occurred prior to the eatout. A similar response for total phosphorus was observed. However, such results are not conclusive because of other factors in system operation. The integrity of berms may be threatened by burrowing. Impacts on wetland hydraulics are also possible. In all cases, loss of emergent vegetation has been viewed with dismay by owners, regulators, and the general public. The damage caused by muskrats, primarily by burrowing in containment and separation berms, is not a matter of conjecture. There are several examples of compromised parallel cells in which divider berms have been breached by burrows: Estevan, Saskatchewan (Duncan et al., 1999); Corcoran, California (Gao et al., 2003); and Sacramento, California (Nolte and Associates, 1998b). At the Manitoba Interlake site 1, it was found that muskrat burrows were extensive and were threat- ening to breach the dikes at several locations. Roads have been damaged by burrow collapse at Saginaw, Michigan (unpublished), and at Sacramento. When muskrats eat all the emergent plant parts, there is a large visual impact, regardless of physical or water quality damage. A common perception is that wetlands should be lush and green to be effective in treatment. This is an intui- tive carryover from agriculture, where lush green elds are a FIGURE 19.6 Hillsdale, Michigan, wetland before (July 18, 1999) and after (June 29, 2001) eat-out by muskrats. (b) strong indicator of a good crop. Consequently, owners, regu- lators, and the general public equate good emergent stands of plants with the highest treatment capability. This may or may not be the case, but the urge to “x it” is irrepressible in many cases. From the point of view of treatment efciency, an eatout calls for careful scrutiny of performance to ascer- tain if the damage has, in fact, impaired pollutant removal. If not, then muskrat control becomes an expense associated with the ancillary benet of a green emergent appearance. At Brighton, Ontario, where water quality was relatively un- affected by extensive muskrat herbivory, the aesthetic aspect became a concern. Residents of the community viewed the partially vegetated wetland as ineffective and expressed con- cern that there might be environmental consequences. Nutria Nutria (Myocastor coypus) are an introduced species from South America, but they now range across the southern and northwestern United States. Nutria cause all the same problems as do muskrats, except that the animals are larger, and each individual is capable of more damage to vegeta- tion. Mature nutria are large and have fewer natural preda- tors. Nutria are strictly herbivorous and feed on a broad range of plants in treatment wetlands, including cattails, grasses, water hyacinth, duckweed, and young tree seedlings. Nutria cut vegetation in a manner similar to muskrats and build feed- ing platforms and nest mounds. They are prolic and com- monly reach damaging population densities unless they are controlled by trapping or shooting. Nutria have occurred in problematic numbers in many treatment wetlands, including Halsey, Oregon; Victoria, Texas; and West Jackson County, Mississippi. Beavers Beavers (Castor canadensis) are found in wetland treatment systems from Texas to the Canadian provinces. Beavers feed © 2009 by Taylor & Francis Group, LLC (a) [...]... FIGURE 19. 10 The Whangarei, New Zealand, treatment wetlands are very visitor friendly (a) (b ) FIGURE 19. 11 The Wakodahatchee Wetlands in Palm Beach County, Florida, has a very user-friendly boardwalk FIGURE 19. 12 Even this short boardwalk at the University of Michigan Matthaei Botanical Gardens treatment wetland is a popular place to stop and look © 2009 by Taylor & Francis Group, LLC 710 Treatment Wetlands. .. existing wetlands CONFLICTS PASSIVE ACTIVITIES Nonconsumptive uses of wetlands constructed primarily for water quality treatment include recreation, nature study, aesthetics, and education Increasingly, designs of treatment wetlands have incorporated attractive and informative parklike areas For instance, stormwater treatment wetlands in urban Although most reasonable human use is acceptable in treatment wetlands, ... limitation of access to treatment wetlands Insurance against public injury has led to exclusion fencing at many treatment wetlands This is more important for wetlands treating waters that may be hazardous to human health, which include some remediation sites as well as wetlands treating primary effluents Nevertheless, there are no known instances of human injury in a treatment wetland 19. 5 DESIGN FOR ANCILLARY... worth of furs are harvested from wetlands annually (Chabreck, 197 8) Constructed treatment wetlands at Orlando (Florida), the stormwater treatment wetlands of Florida, and the Incline Village (Nevada) Ancillary Benefits 705 FIGURE 19. 9 The Tollgate urban stormwater wetland in Lansing, Michigan system are used for waterfowl hunting An ancillary benefit of the stormwater treatment areas (STAs) is that vegetation... FIGURE 19. 13 Wetland observation tower at the Hennepin–Hopper (Illinois) site FIGURE 19. 14 Public viewing shelter at the Tucson, Arizona, Sweetwater wetlands FIGURE 19. 15 Boardwalks and boat launching sites are features of the South Florida Stormwater Treatment Areas © 2009 by Taylor & Francis Group, LLC Ancillary Benefits 711 FIGURE 19. 16 Scientific visits are an important human use of treatment wetlands, ... therefore an important aspect of design for human use (Figures 19. 10 and 19. 11) Although clearly marked walking trails suffice for wetlands of limited size (Figure 19. 10), boardwalks are the preferred means of protecting both the visitors and the wetlands in large systems (Figure 19. 11) These are not restricted, however, to large systems (Figure 19. 12) Boardwalks facilitate nature study and outdoor recreation,... landscape (Figure 19. 13) Wildlife viewing blinds can greatly enhance the ability to observe wildlife with minimal disturbance to the animals An interpretive center allows small groups to assemble with shelter from sun and rain (Figure 19. 14) In some very large treatment wetlands, boat launching facilities are provided (Figure 19. 15) It is not uncommon for scientists to visit treatment wetlands This human... and is nationally known.” STA-5 was opened for the first year for alligator hunting in 2006 (SFWMD, 2006) settings such as Greenwood Park in Orlando (Florida), the Tollgate Wetlands in Lansing (Michigan), and Coyote Hills east of San Francisco Bay are used frequently for field trips and other educational purposes (Figure 19. 9; Table 19. 6) Wetlands constructed for wastewater treatment in Arcata, California;... enough to fend for themselves (Figure 19. 16) It is noteworthy that there are no reports of accidents involving humans and treatment wetlands EXAMPLES The popularity of treatment wetlands for public use is very high Perhaps the best way to illustrate the design features that contribute to public use is to describe examples Palm Beach County, Florida Originally conceived in 199 2 as part of a cooperative effluent... solids can smother plant growth in inflow areas (Kuenzler, 199 0) Generally, solid loads can be controlled by conventional pretreatment In stormwater systems, the problem can be minimized with a forebay or pond prior to the wetland (Livingston, 198 9) If the pretreatment area traps the mineral-suspended solids (clays, silt, and sand), the wetland treatment system will need to be maintained less frequently . unaffected natu- ral wetlands. Information collected for NADB v.2 indicates TABLE 19. 1 Progression of Two Side-by-Side Wetlands at the Olentangy River Wetlands Park over a Start-Up Period of. wetlands are likely to evolve © 2009 by Taylor & Francis Group, LLC 694 Treatment Wetlands that hundreds of plant species occur in a variety of treatment wetlands. Even when treatment wetlands. 500 Cascade aeration Marsh Open water Overlook Rip-Rap Discharge to receiving stream FIGURE 19. 1 Conceptual plan for treatment wetlands with ancillary benets. (From Kadlec and Knight (199 6) Treatment Wetlands. First Edition,
Ngày đăng: 18/06/2014, 22:20
Xem thêm: TREATMENT WETLANDS - CHAPTER 19 pot
