Page 762 - The Toxicology of Fishes

742 The Toxicology of Fishes

of biological organism communities is a critical part of microcosm and mesocosm experiments. Adequate
time is required to establish a number of interacting functional groups (Giesy and Odum, 1980).
Colonization methods used in microcosm and mesocosm research vary as a function of system size,
type of study, whether the study is fate or effect oriented, and the endpoints of interest (Kennedy et al.,
1995). Studies using limnocorrals and littoral enclosures usually have no acclimation period because it
is assumed that these systems enclose established communities (with or without fish) (Lozano et al.,
1992; Solomon et al., 1989). In stream mesocosms, stabilization periods of 10 days (Genter et al., 1987),

4 weeks (Fairchild et al., 1992), and 1 year (Lynch et al., 1985) have been reported. Duration of the
maturation period for pond mesocosms varies from 1 to 2 months to 2 years (deNoyelles et al., 1989).
Following initial system preparation, acclimation is usually required to allow the various biotic compo-
nents to adjust to the new environment and to establish interspecific and abiotic interactions. Duration
of acclimation time depends on system size and complexity. Systems with more trophic levels will form
more complex interactions that require more time to equilibrate than do small systems with fewer species.
The time required to equilibrate will increase with initial system complexity, although the use of natural
sediments usually shortens the duration of the stabilization period because natural maturation processes
are enhanced (Kennedy et al., 1995). During this acclimation period in outdoor systems, the initial
preparation of the systems is typically controlled, and natural colonization by insects and amphibians
contributes to biotic heterogeneity and system realism. Continuous colonization, however, presents
further problems in that each system tends to follow its own trajectory through time. These trends are
most apparent in small-scale systems and in systems that have been in operation longer (Heimbach et
al., 1994). Circulation of water between the different systems has frequently been proposed as a way to
limit among-system variability during this period (Crossland, 1984; Crossland et al., 1986).

Macrophytes
Aquatic vascular plants play a key role in system dynamics within natural lakes, and their presence in
model ecosystems makes them more representative of littoral zones in natural systems; however, once
introduced into model ecosystems, macrophyte growth is difficult to control and may vary greatly among
replicates. This is of particular concern in field studies because macrophytes influence chemical fate,
occurrence, and spatial distribution of invertebrates and growth of fish. Bluegill sunfish, for example,
require vegetated areas to nest for spawning purposes, and a proper test system, if field responses on
bluegill reproduction are important, must take nest habitat into consideration. Thus, variations of plant
density and diversity in model ecosystems contribute to system variability and must be accounted for.
Macrophyte density affects chemical fate processes by increasing the surface area available for sorption
of hydrophobic compounds. The pyrethroid insecticide deltamethrin accumulated rapidly in aquatic plants
and filamentous algae during a freshwater pond chemical fate study (Muir et al., 1985). Caquet et al.
(2000) measured residues of deltamethrin and lindane in macrophyte samples for 5 weeks after treatment
but never in the sediment. Macrophytes also affect physicochemical composition in surrounding waters,
influencing the distribution of many aquatic prey organisms (Barko et al., 1988). In addition, macrophytes
provide a three-dimensional structure within constructed ecosystems that affects organism distribution and
interactions. Brock et al. (1992), in a study with the insecticide Dursban 4E, observed considerable
invertebrate taxa differences between Elodea-dominated and macrophyte-free systems. Other workers have
shown macroinvertebrate community diversity to be influenced by patchy macrophyte abundance (Street
and Titmus, 1979) and specific macrophyte types (Learner et al., 1989; Schramm et al., 1987). Cladoceran
communities are also associated with periphytic algae on aquatic macrophytes (Campbell and Clark, 1987).
Impacts of chemicals on macrophyte densities indirectly affect organisms by influencing trophic
linkages such as predator–prey interactions between invertebrates and vertebrates. Bluegill utilization of
epiphytic prey may be much greater than predation upon benthic organisms (Schramm and Jirka, 1989).
Excessive macrophyte growth may force fish that normally forage in open water to feed on epiphytic
macroinvertebrates where energy returns are not as great (Mittlebach, 1981). Fish foraging success on
epiphytic macroinvertebrates depends on macrophyte density (Crowder and Cooper, 1982) and plant
growth form (i.e., cylindrical stems vs. leafy stems) (Dionne and Folt, 1991; Gilinsky, 1984; Loucks,
1985). Dewey (1986) studied atrazine impacts on aquatic insect community structure and emergence