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FIGURE 4.11 Scales of experimental units from the pelagic-benthic research at the Multi-scale Experimental Ecosystem Research Center (MEERC) at the University of Maryland's Center for Environmental Science. (Adapted from Petersen, J. E. 1998. Scale and Energy Input in the Dynamics of Experimental Estuarine Ecosystems. Ph.D. dissertation, University of Maryland, College Park, MD.)

FIGURE 4.11 Scales of experimental units from the pelagic-benthic research at the Multi-scale Experimental Ecosystem Research Center (MEERC) at the University of Maryland's Center for Environmental Science. (Adapted from Petersen, J. E. 1998. Scale and Energy Input in the Dynamics of Experimental Estuarine Ecosystems. Ph.D. dissertation, University of Maryland, College Park, MD.)

inter-year variability can be examined. One approach to accommodate this issue of time scaling is to study communities of protozoans and other microorganisms whose generation times are short. These kinds of microcosms have been called biological accelerators (Lawton, 1995) because they allow the examination of long-term ecological phenomena, such as predator-prey cycles and succession, with short realtime durations. These kinds of microcosms are essentially scaled on a one-to-one basis with their real-world analogs and thus they have been commonly used for ecological experimentation. A major challenge of microcosm work is to design and operate experimental systems that allow for reproduction of larger animals, such as fish, and for completion of complex life cycles, as exhibited by organisms that have planktonic larvae and sedentary adults (e.g., oysters and corals). In some cases this may require simply enlarging the size of the experimental unit (from flasks or tanks to ponds), but there is also a need for pumps and water circulation systems that do not destroy larvae. As demonstration of this need, for the short time that the EPA required aquatic mesocosm screening of pesticides, they mandated that mesocosms be large enough to include a reproducing population of bluegill sunfish (Lepomis macrochirus) (Kennedy et al., 1995).

The other category of scaling concern has been termed artifacts of enclosure (Petersen et al., 1997, 1999), which includes wall effects and missing components. The first aspect of wall effects is the composition of the walls of the container themselves. A wide variety of wall materials has been used in microcosms. Most are rigid (such as fiberglass), but flexible walls (such as plastic) are used for limno-corrals or other large in-situ enclosures. Schelske (1984) has covered possible chem ical effects of walls that must be considered in design decisions: (1) walls should be nontoxic, (2) nutrients should not leach out of the walls, and (3) walls should not sorb substances added in experiments. An example of the latter issue of sorbtion was discussed by Saward (1975) who found that copper absorbsion was very low for fiberglass walls of an aquatic microcosm whereas absorbsion of oils and orga-nochlorine was high. The other aspect of wall effect is that walls act as substrate for a biofilm of attached microorganisms (bacteria, algae, fungi, and protozoans). This biofilm, which begins to develop within hours to days, can have dramatic and undesirable effects on an experiment, especially if it is designed to study a plankton system suspended in a water column (Dudzik et al., 1979; Pritchard and Bourquin, 1984). As noted by Margalef (1967):

When experiments are performed with a wide assemblage of species taken from natural populations, the systems develop a flaw — a fortunate flaw, because it throws light on the dynamics of populations in estuaries and in other natural environments. Species able to attach themselves to the walls of the culture vessels become more successful in competition. ...

The adherence of organisms to the walls is a most serious inconvenience in the use of chemostats as analogues of plankton systems. Species that are used often as models of planktonic algae, as Nitzchia closterium, and even some small species of Chaetoc-eros, are found attached in some way. Propensity to attachment seems to be different according to conditions of nutrition, to accompanying bacterial flora, and to the time elapsed from the start of the experiment. The role of possible mutants cannot be excluded. Stirring does not check attachment of algae to the walls. The design of a reliable chemostat for experimenting with complex planktonic populations awaits the improbable discovery of a bottle without walls. Ice walls do not help.

Can ecological engineers design a microcosm without walls, as mentioned by Margalef? Remarkably, he seems to have tried. Although he doesn't elaborate, Margalef's ice walls presumably were intended to reduce biofilm growth and thereby eliminate the wall effect. The biomass and metabolism of the biofilm on walls can quantitatively dominate a microcosm, thereby significantly influencing normal biogeochem-ical and toxin cycling. In general this kind of wall effect is proportional to wall surface area and inversely proportional to container volume. To the extent that artificial surface area in a microcosm exceeds that area found in the intended natural analogs, the microcosm represents a new system and may not be appropriate for extrapolation of experimental results. Many workers have recognized this problem and devised methods of removing the biofilm from the walls during experiments. The study by Chen et al. (1997) in the MEERC tanks (Figure 4.11) may be the most detailed study of wall effects. They found a number of relationships between biofilm growth and design factors of estuarine plankton tanks, along with quantifying the dominance of biofilm metabolism over plankton metabolism. Figure 4.12 is an energy circuit diagram of their system showing the dimensional effects of microcosm wall area (A) and volume (V) on biofilm and plankton components, respectively. Also shown is a new pathway that emerged with zooplankton, which are normally pelagic, feeding on the wall growth of the system. These kinds of wall effects are

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FIGURE 4.12 Energy circuit diagram of the influences of wall area and tank volume on the MEERC microcosms.

reminiscent of the classic concept of edge effects in natural ecosystems. Edge effect is the "tendency for increased variety and density at community junctions" (E. P. Odum, 1971). Community junctions are also known as ecotones (Risser, 1995a). The edge effect concept was coined by Aldo Leopold (1933) in relation to wildlife species that take advantage of qualities in communities along both sides of the ecotone; for example, foraging in one community and nesting or roosting in the other. Studies of species distributions along community transitions have identified some as "edge species" and others as "interior species," especially in terms of birds (Beecher, 1942; Kendeigh, 1944). Because some of the edge species are game animals, such as deer, wildlife managers have historically tried to maximize the amount of edge in landscapes. However, this wisdom is being questioned, especially for plants and nongame wildlife that seem to be negatively affected by edge (Harris, 1988). The classic concept of edge effect is related to wall effects in microcosms in the way the walls represent a discontinuity. A true edge effect occurs when two communities or habitats are in juxtaposition. Few microcosm studies have tried to model this situation of a true ecotone, which seems to represent a significant design challenge (John Petersen, personal communication). Metcalf's microcosm (Figure 4.9) was intended to include ecotones of an agricultural landscape (cropland and farm pond), but it was too simple to represent the concept.

The other aspect of artifacts of enclosure is that certain characteristic species or phenomena are left out of microcosms due to closure. Walls of a microcosm act as a barrier to movements of organisms and thus they limit genetic diversity inside the system. In some cases characteristic organisms are just too large or difficult to maintain within the confines of a microcosm. For example, sharks simply won't fit inside small marine microcosms even if they are the characteristic top predators in

FIGURE 4.13 Experimental burning of the marsh mesocosms at the MEERC facility, in Cambridge, MD.

the pelagic system of the natural analog marine ecosystems. Some species are always left out of experimental microcosms, and their absence can cause artifacts to arise, such as larger than normal prey populations in the absence of predators. Human actions are sometimes required to simulate top predators by removing prey individuals from a microcosm in order to maintain specified conditions (Adey and Loveland, 1998). Another important class of missing features in microcosms is the large-scale disturbances that influence ecosystems. Some workers have simulated disturbances such as fire (Figure 4.13; Schmitz, 2000; see also Richey, 1970) and storm events (Oviatt et al., 1981), but more research is required to test microcosm responses. Disturbances are large-scale phenomena in that they occur infrequently and act over large areas. They may be appropriately left out of short-term experiments, but their inclusion in micrcocosms can add to the accuracy of modelling of real ecosystems.

The Energy Signature Approach to Design

The use of energy signatures is one approach for the physical scaling of microcosms. The concept can be used to design microcosms by matching, as closely as possible, the energy signature of the natural analog system with the energy signature of the microcosm. The most straightforward approach to this matching of energies is to construct the microcosm in the field where it is physically exposed to the same energies as natural ecosystems. Examples are the pond ecosystems commonly used in ecotoxicology and in situ plastic bags floated in pelagic systems (called limno-corrals when used in lakes). In the lab the challenge of matching energies is greater. Significant effort is usually taken to match sunlight with artificial lighting whose intensity, spectral distribution, and timing can be controlled. Perhaps the most abstract examples of laboratory scaling are the origin-of-life microcosms (Figure 4.14). Here the challenge is to bring together the prebiotic physical-chemical conditions on the earth in a bench-scale recirculating systems in order to examine the chemical reactions that may have led to the origin of life. As an example of this

FIGURE 4.14 Miller's origin of life microcosm. (From Schwemmler, W. 1984. Reconstruction of Cell Evolution: A Periodic System. CRC Press, Boca Raton, FL. With permission.)

kind of study, Miller (1953; 1955; Miller and Urey, 1959; Bada and Lazcano, 2003) used an energy signature of the earth as a guide for designing their microcosm. In Miller's experiments, electrical discharge into a simulated prebiotic atmosphere produced a number of organic molecules including amino acids. This was a significant breakthrough, but there was still nothing alive in the microcosm after the experiments. Obviously creating a microcosm that generates life from nonliving components is the greatest design challenge!

A more modest but still difficult design challenge is providing turbulent mixing in pelagic microcosms. Turbulence is important in pelagic systems in providing physical-chemical mixing and reducing losses from sinking for phytoplankton and, to a lesser extent, for zooplankton. Turbulent mixing is reduced or eliminated when enclosing a water column with a microcosm because it is driven by larger-scale processes of water circulation and wind that are excluded. These larger-scale processes that generate turbulence represent auxiliary energy inputs to the plankton system. Early studies of pelagic microcosms, especially the floating bags in lakes and marine waters, completely excluded mixing energies, and artificial successions of phytoplankton occurred with dominance of motile species and losses of heavier, nonmotile species such as diatoms (Bloesch et al., 1988; Davies and Gamble, 1979; Takashashi and Whitney, 1977). This led to criticism of these studies; for example Verduin (1969) stated, "... before a lot of people buy a lot of polyethylene, I suggest that such companion experiments be performed and their validity versus the big bag be assessed and reported." Recognition of the problem also led to designs that generated turbulence in pelagic microcosms, including bubbling the water column with compressed air within floating bags (Sonntag and Parsons, 1979) and mechanical mixing with plungers or propellers in fixed tanks (Estrada et al., 1987; Nixon

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