G

FIGURE 2.16 Views of the basic theory of biological reactor functioning. (From Tenney, M. W. et al. 1972. Nutrients in Natural Waters. H. E. Allen and J. R. Kramer (eds.). John Wiley & Sons, New York. With permission.)

Pomeroy (1980), Rich and Wetzel (1978), Schlesinger (1977), Sibert and Naiman (1980), and Vogt et al. (1986).

Higher Plants

Higher plants, especially flowering plants, are an obvious feature of wetlands including treatment wetlands (Cronk and Fennessy, 2001). Although wetlands can be defined broadly (Cowardin et al., 1979), a general definition is that a wetland is an ecosystem with rooted, higher plants where the water table is at or near the soil surface for at least part of the annual cycle. Lower plants, such as algae, mosses, and ferns, can be important but they are usually less dominant than the flowering plants. A variety of life forms fall under the category of higher plants in wetlands, including trees, emergent macrophytes (grasses, sedges, rushes), and floating leafed and submerged macrophytes. Although their function in treatment wetlands is secondary to microbes, they do play significant roles (Gersberg et al., 1986; Peterson and Teal, 1996; Pullin and Hammer, 1991).

Figure 2.18 depicts a general model that covers many of the higher plant life-forms and illustrates several important functions. The plants themselves are composed of aboveground (stems, shoots, and leaves) and belowground (roots and rhizomes, which are underground stems) components which interact in the central process of primary production. Belowground components physically support the

Organic subtrate

Adsorbed organic material

Microflora

Microfauna

FIGURE 2.17 Two early depictions of the detritus concept. PAR: photosynthetically available radiation; DOM: dissolved organic matter. (The top part of the figure is from Goldman, J. C. 1984. Flows of Energy and Materials in Marine Ecosystems: Theory and Practice. M. J. R. Fasham (ed.). Plenum Press, New York. With permission. The bottom part of the figure is from Darnell, R. M. 1967. Estuaries. G. H. Lauff (ed.). Publ. No. 83, American Association for the Advancement of Science, Washington, DC. With permission.)

shoots and facilitate uptake of nutrients. Photosynthesis occurs in shoots and leaves that are exposed to sunlight during a portion of the year when the temperature is above freezing (i.e., the growing season). Aboveground and belowground components die and transform into litter and soil organic matter, respectively (both forms of detritus), where decomposition and recycle by detritivores takes place.

Probably the most obvious contribution of higher plants to the treatment process is uptake of nutrients. If biomass is harvested and removed from the system, uptake can play a role in removing nutrients. However, without harvest, nutrients eventually recycle providing no net treatment. This situation leads to a description of treatment wetlands as alternating sinks and sources for nutrients. They are sinks during the growing season when uptake dominates the mass balance, and they are sources during the winter and early spring when decomposition and seasonal flushing dominate the mass balance. Harvest can cause treatment wetlands to be primarily sinks,

Retaining Ring Size Chart
FIGURE 2.18 Energy circuit diagram of an aquatic plant-based system.

but this option is often expensive. Also, only small amounts of nutrients can be removed by harvesting because plant biomass contains only small percentages of nutrients (about 5% by mass).

A more important contribution of higher plants to the treatment process is their support of microbes within the rhizosphere, which is the zone adjacent to living roots in soils and sediments. Roots provide surfaces that are colonized by biofilms, and they leak organic molecules and oxygen that directly support microbes. These kinds of flows are shown in Figure 2.18 in the belowground zone. Oxygenation of sediments through air spaces that connect shoots and roots is a very important function of many macrophytes (Dacey, 1981; Gunnison and Barko, 1989; Jaynes and Carpenter, 1986; Kautsky, 1988). Wetland sediments are normally anaerobic and oxygen leakage from roots supports more efficient aerobic metabolism by microbes in the rhizosphere. The contributions of roots in creating microzones within sediments may be a critical role in supporting microbes that transform nitrogenous materials in wastewater to nitrogen gas. Removal of nitrogen through denitrification is a reliable function in treatment wetlands that is a definite tool to be used by ecological engineers.

A final note on higher plants in treatment wetlands deals with the special features of individual species. Paradoxically, some of the most useful species in treatment wetlands, such as water hyacinth (Eichhornia crassipes), common reed (Phragmites sp.) and cattail (Typha sp.), are considered pests that sometime require control when they occur in natural wetlands. These species are characterized by fast growth, which is a positive feature in treatment wetlands but a negative feature in natural wetlands where they outcompete other plant species. Several of these species are nonnative or "exotic" in North America and the ecology of these kinds of species is covered in Chapter 7. Duckweed (family Lemnaceae) is another type of plant with fast growth that has been used for wastewater treatment (Culley and Epps, 1973; Harvey and Fox, 1973). Species from this plant family are small, floating-leaved plants that can completely cover the water surface of a pond or small lake. In an early reference Hillman and Culley (1978) envision a dairy farm system in which 10 acres (4 ha) of duckweek lagoons could treat the wastewater of a herd of 100 cattle. One more recent design for domestic wastewater treatment uses this species exclusively (Bud-dhavarapu and Hancock, 1991), which was developed by a company named appropriately the Lemna Corporation. Other species, not now used in treatment wetlands (Symplocarpus foetidus), may have special qualities that preadapt them for this use. Skunk cabbage is one example that has adaptation for growth during early spring when temperatures are too low for other species. Heat generated by enhanced respiration (Knutson, 1974; Raskin et al., 1987) supports the early growth. Perhaps this species could be manipulated and managed to extend the growing season of treatment wetlands, which is now a limiting factor for their implementation in cold climates.

Protozoans

Protozoans are microscopic animals found primarily in soils and sediments. A variety of groups are known, roughly separated by locomotion type: amoebae, flagellates, and ciliates, along with foraminifera. Their primary role in treatment wetlands is as predators on the bacteria. This predation controls or regulates bacteria populations by selecting for fast growth. Predation is always selective, with the predators choosing among alternative prey individuals. This is true for all organisms from protozoans to killer whales and has important consequences. Predators affect and improve the genetic basis of prey populations by selecting against individuals that are easy to catch (i.e., the sick, the dumb, the weak, and the very young or old) and selecting for those individuals that are hard to catch (i.e., the healthy, the smart, the strong, and the middle-aged). As an example, Fenchel (1982) simulates a predator-prey model for protozoans and bacteria which generate a classic oscillating pattern over time (see also Figure 4.5). The interaction between predators and prey is an important topic in ecological theory (Berryman, 1992; Kerfoot and Sih, 1987), and knowledge of the subject will provide ecological engineers with an important design tool (see also the discussion of top-down control in Chapter 7).

Because bacteria and other microbes metabolize the organic matter in wastewa-ter, protozoans indirectly control treatment effectiveness through their predation. Treatment of BOD is thus a "bacterial-protozoan partnership" (Sieburth, 1976), and this interaction is illustrated in Figure 2.19. This highly organized food web also has been called the microbial loop because of the strong and fast interconnections between components in the system. The microbial loop was first discussed for the oceanic plankton (Azam et al., 1983; Pomeroy, 1974b), but it may well apply to treatment wetlands as well. Numbers of organisms are very high in these mixed microbial systems — on the order of 105-106 individuals/gram for bacteria and 103-104 individuals/gram for protozoans (Chapman, 1931; Spotte, 1974; Waksman, 1952). The importance of protozoans is well known in both natural ecosystems (Bick

FIGURE 2.19 The role of protozoans in regulating decomposition processes. (From Caron, D. A. and J. C. Goldman. 1990. Ecology of Marine Protozoa. G. M. Capriulo (ed.). Oxford University Press, New York. With permission.)

and Muller, 1973; Stout, 1980) and conventional wastewater treatment systems (Barker, 1946; Bhatla and Gaudy, 1965; Curds, 1975; Kinner and Curds, 1987). Experiments with seeding protozoans into wastewater treatment systems have been shown to improve treatment (Curds et al., 1968; McKinney and Gram, 1956), and it is possible that protozoans may be able to be manipulated by ecological engineers in treatment wetlands.

Mosquitoes

Mosquitoes are biting flies of the insect family Culicidae. They are well known to be associated with wetlands since their larvae are aquatic. For many people, mosquitoes are a negative form of biodiversity because of their biting behavior and because some species can transmit diseases. Worldwide there are nearly 3000 species of mosquitoes and they range from the arctic to the tropics. They are remarkable animals with a very short breeding cycle and with an adult stage characterized by acute sensory perception and strong flight capability. Of course, the main problem with mosquitoes is that in certain species the females prey on human blood. They do this in order to acquire a concentrated dose of protein needed for the development of eggs in the reproductive cycle. The main genera are Anopheles, Aedes, and Culex and they are considered to be "man's worst enemy" (Gillett, 1973) because of their association with diseases. The most important diseases are mainly tropical and include malaria, yellow fever, dengue, encephalitis, and filariasis (Foote and Cook, 1959). The latest concern in the U.S. is the West Nile strain of encephalitis which was first recorded in New York City but which has recently spread across the country.

Large-scale, organized control of mosquitoes has been evolving for more than 100 years (Hardenburg, 1922) and it involves many of the issues now associated with invasive exotic species (see Chapter 7). The main methods of control are to restrict breeding habitats and to use chemical pesticides and biological control agents. In general, control of mosquito populations is difficult to achieve because of their dispersal abilities and short breeding cycle. Also, some species such as the Asian tiger mosquito (Aedes albopictus) are preadapted to live in human habitats by breeding in manmade containers (old tires, flower pots, clogged gutters, children's swimming pools, etc.). Of special interest, mosquitoes provided one of the first examples of the development of resistance to pesticides when DDT use became widespread after World War II (King, 1952). Because of the importance of the problem, organized mosquito control districts have been formed in many parts of the U.S., which are supported by local taxes. Lichtenberg and Getz (1985) provide a thorough review of the economics of mosquito control for one particular situation that is probably applicable in a more general sense.

Mosquitoes often are associated with treatment wetlands when ponded water provides habitat for larvae. They can participate in the treatment process because the larvae feed on organic matter, but their contribution probably is quite minor. The most important issue is whether or not treatment wetlands are a significant source of mosquito production. Kadlec and Knight (1996) have reviewed the subject and indicate that this is generally not a major concern. However, others are not convinced, as noted by Martin and Eldridge (1989): "We have the basic knowledge to design and operate created wetlands systems today. The major drawback is mosquito problems, which must be solved before created wetlands can be universally accepted by public health officials and the general public." Designs for controlling mosquito production in treatment wetlands are discussed by Anonymous (1995a), Russell (1999), and Stowell et al. (1985).

As a final aside, a significant amount of ecological engineering has been involved in mosquito control efforts in natural saltmarshes, especially along the U.S. East Coast (Carlson et al., 1994; Dale and Hulsman, 1991; Resh, 2001). In this habitat mosquito species lay eggs on moist soil surfaces rather than in standing water. Larvae develop in the eggs but will hatch only when covered by tidewater or rainwater. Historically, control efforts in this case dealt with water level manipulations, either drainage with ditches or canals or impoundment where water is contained behind dikes (Clark, 1977; Provost, 1974). Thus, construction activities often were necessary for control of saltmarsh mosquitoes. This work represented an interesting example of ecological engineering since it involved knowledge of mosquito biology, saltmarsh hydrology, and the design of impoundment and drainage systems (Figure 2.20). Skill was required to combine these areas into new saltmarshes with altered hydrology and fewer mosquitoes, and many efforts failed by actually producing more mosquitoes. These practices are no longer undertaken, partly because many systems have already been constructed and partly because of the environmental impacts caused by the changes to natural saltmarshes. In fact, some old mosquito control systems are currently being restored, which is another ecological engineering challenge (Axelson et al., 2000). A related problem is the design of irrigation systems in regard to pest populations (Jobin and Ippen, 1964).

Chain Causation Examples

Mosquitoes of Wintering

Waterfowl

FIGURE 2.20 Causal diagram of interactions involved in mosquito control through water level manipulations in tidal marshes. (Adapted from Montague, C. L., A. V. Zale, and H. F. Percival. 1985. Technical Report No. 17. Florida Cooperative Fish and Wildlife Research Unit, University of Florida, Gainesville, FL.)

Mosquitoes of Wintering

Waterfowl

FIGURE 2.20 Causal diagram of interactions involved in mosquito control through water level manipulations in tidal marshes. (Adapted from Montague, C. L., A. V. Zale, and H. F. Percival. 1985. Technical Report No. 17. Florida Cooperative Fish and Wildlife Research Unit, University of Florida, Gainesville, FL.)

Muskrats

Muskrats (Ondatra zibethicus) are large, semiaquatic rodents that are distributed throughout most of temperate North America and, as exotic species, in Europe. They are primarily herbivorous, feeding on rhizomes of emergent macrophytes in marshes. However, their indirect effects on marsh ecosystems are probably more significant than their direct effect of grazing (Kangas, 1988). These indirect effects include construction activities that result in mounds, which they use for overwintering, and in burrows and paths, which they use to facilitate movements within the often dense vegetation of marshes. The ecology and natural history of muskrats are well known (Errington, 1963; Johnson, 1925; O'Neil, 1949) and include complex patterns of population oscillations (Elton and Nicholson, 1942), which occurred at least historically before landscapes became fragmented by human development. The network of direct and indirect effects performed by muskrats makes them keystone species in natural marsh ecosystems because of their important control functions (see Chapter 7 for discussion of the keystone species concept).

Muskrats can occur in treatment wetlands, which provide ideal habitats due to a dominance of preferred food plants and stable water levels. In fact, Latchum (1996) found the highest density of muskrat mounds ever reported at 62.5/ha in a treatment wetland in central Maryland. This is particularly significant because mounds are often used as an index of population size for muskrats (Danell, 1982). High densities of muskrats can build up in natural marshes and cause "eat-outs" where a large amount of plant biomass is harvested over a short period of time, leading to a crash in the muskrat population. This is reflected in the oscillatory dynamics commonly reported for muskrats. However, muskrat populations may be stabilized at high levels in treatment wetlands due to the steady supply of nutrients and optimal habitat conditions, creating a kind of "sustainable eat-out." This result does not match with the "paradox of enrichment" described in theoretical ecology (Rosenzweig, 1971), where enrichment of a predator-prey system causes it to become unstable or, in extreme cases, to collapse. In treatment wetlands the marsh vegetation (as prey)-muskrat herbivore (as predator) system is enriched with sewage nutrients, but it becomes stabilized at higher levels rather than destabilized. This may be an exception to the paradox, like other counter examples (Abrams and Walters, 1996; McAllister et al., 1972) due to additional complexities in a real-world example. Muskrats are close relatives of the lemmings (Lemmus sp. and Dicrostonyx sp.), which have been suggested to be components in homeostatic networks of the tundra (Schultz, 1964, 1969). Enrichment in treatment marshes may push the marsh vege-tation-muskrat herbivore system into an alternative stable state with a new homeo-static structure.

A small amount of literature exists on the effect of muskrats on treatment wetlands, though most seem to feel it is negative (Table 2.4). There is an obvious negative effect due to their burrowing into dikes or berms that enclose constructed wetlands (Figure 2.21), which is part of the natural role of the muskrat in spreading water over the landscape. Latchum (1996) traced out many more possible impacts as shown in the causal diagram in Figure 2.22. This diagram illustrates a network of positive and negative and direct and indirect effects that muskrats may have in a treatment wetland. The most significant effects seem to come from the construction activities that cascade through a number of processes to influence treatment capacity. Mounds are the most obvious construction feature of muskrats, and they act like compost piles (see Chapter 6) in accelerating the decomposition rate (Berg and Kangas, 1989; Wainscott et al., 1990). Overall, mounds may have a direct negative effect on primary production since plant materials are used in construction, but they have several indirect positive effects through increasing decomposition.

Paths and burrows constructed by muskrats can be extensive in marshes. At high water levels, they can act as channels or macropores (Beven and Germann, 1982) and they probably increase water flow. This can have a negative impact on treatment capacity if some of the pollutant load passes through the wetland without treatment. In general, treatment effectiveness is directly related to retention time. The preferential flow in channels or macropores reduces retention time and therefore also treatment effectiveness. Muskrat burrows increase aeration of the sediments and they probably have many similar effects as burrowing crustaceans (Montague, 1980,

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