Comments from the Literature about Muskrats in Treatment Wetlands

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The proponents of reeds argue for a monoculture of reeds, while others argue that bulrushes are superior. Survivability of each species will depend on other factors, such as plant pests. Muskrats love cattails and bulrushes, while reeds apparently are inedible ...


Campbell and Ogden, 1999

Muskrats can damage dikes by burrowing into them. Although muskrats generally prefer to start their burrows in water that is more than 3 ft deep, they can be a problem in shallower waters. Muskrats can be excluded by installing an electric fence low to the ground or by burying muskrat-proof wire mats in the dikes during construction.

Davis, no date

I should point out that for pest control, we do muskrat trapping to prevent destruction of berms .

Wile et al., 1985

Muskrats are also problematic in constructed wetlands because they burrow into dikes, creating operational headaches and potential for system failure .

Kadlec and Knight, 1996

1982; Richardson, 1983; Ringold, 1979). Aeration generally has a positive effect on metabolism and biodiversity, though it inhibits denitrification, which requires anaerobic conditions.

Perhaps the most important general contribution of muskrats in marsh ecosystems is generating spatial heterogeneity, which is not shown in Figure 2.22. Through their various construction activities muskrats create a mosaic pattern of open areas within dense marsh vegetation. Spatial heterogeneity increases diversity through a number of mechanisms (Hutchings et al., 2000; Kolasa and Pickett, 1991; Shorrocks and Swingland, 1990; Smith, 1972). This quality provides redundancy in system design, which is similar to a safety factor in engineering. The spatial heterogeneity caused by muskrats in treatment marshes also may help explain why this system does not match with the "paradox of enrichment" (Scheffer and DeBoer, 1995), as noted above.

The cumulative impact of muskrats on treatment wetlands is unknown, though both positive and negative effects have been noted. Although it is not completely clear, the obvious negative effects seem to dominate over the less obvious positive effects. For example, at the treatment wetland studied by Latchum (1996), muskrats were judged to be negative because they became trapped in some of the mechanical parts of the system. Overall, the fact that muskrats can act as positive, keystone species in natural marshes but as negative, pest species in treatment marshes is a paradox. However, active design and management through ecological engineering

FIGURE 2.21 Views of the problems that muskrats cause by burrowing. (From World Wide Photos. New York, NY. With permission.)

may shift this balance. Perhaps their ecological role can be used to improve treatment capacity. One strategy might be to take advantage of their concentration of biomass in mounds by harvesting the mounds in the spring to remove nutrients. This might reduce the cost of harvesting because the muskrats would be doing some of the collection work for free, and the mound material might be used, like compost, as a soil amendment. In a sense the muskrat is a basic element in the ecological and hydrologic self-organization of temperate, humid landscapes. They have evolved to spread water around and regulate wetland processes in marsh ecosystems. It would be a significant accomplishment of ecological engineering if their adaptations could be used productively. Ultimately, a treatment marsh without muskrats is an incomplete ecosystem.

Aquaculture Species

The aquacultural production of useful species from domestic wastewater is related to the topic of treatment wetlands. Allen (1973) called these systems "sewage farming" and many examples exist (Allen and Carpenter, 1977; Costa-Pierce, 1998;

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FIGURE 2.22 Causal diagram of direct and indirect effects of muskrats in treatment wetlands. (Adapted from Latchum, J. A. 1996. Ecological Engineering Factors of a Constructed Wastewater Treatment Wetland. M.S. Thesis, University of Maryland, College Park, MD.)

Drenner et al., 1997; Edwards and Densem, 1980; Gordon et al., 1982; Roels et al., 1978). An interesting example of a sequential treatment system using "controlled eutrophication" (Ryther et al., 1972) is shown in Figure 2.23. The system was a constructed marine food chain capable of producing several kinds of biomass along with clean water. An obvious risk of this kind of system is the incorporation of pathogenic microbes or chemical toxins into food products grown in wastewater. Korringa (1976) reviewed this issue and suggested that strict control over the use of these artificial food chains through monitoring systems, quarantine measures, and purification plants is possible. However, these actions are expensive and they reduce the economic viability of sewage farming. Other techniques, such as the production of species that provide nonfood products (i.e., ornamental plants or aquarium fishes) may be more viable but may lack extensive markets (see "living machines" in Chapter 8). Ecological engineering designs will certainly continue to be tested in the future to take advantage of sewage as a resource, including the production of many kinds of species that yield value to humans.

Coprophagy and Guanotrophy

One goal of ecological engineering is to design and test new treatment ecosystems that have high biodiversity and more effective treatment efficiency. Organisms that utilize feces may be good candidates for these ecosystems since they may be

Secondary effluent - 20% Filtered sea water - 80%

Secondary Marine effluent phytoplankton supply tank growth system

Secondary effluent - 20% Filtered sea water - 80%

Phytoplankton Invertebrate feed growth tank system

Daily batch phytoplankton harvest phytoplankton Food - 5% Filtered sea water - 95%

Secondary Marine effluent phytoplankton supply tank growth system phytoplankton Food - 5% Filtered sea water - 95%

Invertebrate system effluent

Phytoplankton Invertebrate feed growth tank system d

Invertebrate system effluent

Final effluent to coastal water outfall

Final effluent to coastal water outfall

FIGURE 2.23 View of the Woods Hole wastewater treatment-aquaculture system. (From Goldman, J. C. et al., 1974a. Water Research. 8:45-54. With permission.)

preadapted to sewage treatment. An ecology of feces exists in the ecological literature (see, for examples, Angel and Wicklow, 1974; Booth, 1977; Wotton and Malmqvist, 2001), and the many terms used for feces to some extent indicate the broad range of this literature: guano, frass, scat, fecal pellets, dung, droppings, and coprolites (fossil feces). The consumption of feces by animals is termed coprophagy or gua-notrophy. Animals consume feces because of its relatively high nutritive value (see, for examples, Hassall and Rushton, 1985; Rossi and Vitagliano-Tadini, 1978; Wotton, 1980). Mohr (1943) described a diversity of species involved in successional stages during the breakdown of cattle droppings in an Illinois pasture. This and other guano-rich environments (Leentvaar, 1967; Poulson, 1972; Ugolini, 1972) could be searched for food chains that might be transformed into treatment ecosystems through biodiversity prospecting. For example, dung beetles (Scarabaeidae) (Hanski and Cambefort, 1991; Waterhouse, 1974) might be ideal as the basis for a sewage sludge recycling system.

The quantitative design analysis of treatment wetlands has followed, and in fact copied, the approach traditionally used in sanitary engineering for design of other types of wastewater treatment systems. In an abstract sense, a wastewater treatment system is considered to be a bioreactor, or "... a vessel in which biological reactions are carried out by microorganisms or enzymes contained within the reactor itself.

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