Clements: “3357_c032” — 2007/11/9 — 12:39 — page 687 — #1 32 The Use of Microcosms, Mesocosms, and Field Experiments to Assess Ecosystem Responses to Contaminants and Other Stressors When factors are chosen for investigation, it is not because we anticipate that laws of nature can be expressed with any particular simplicity in terms of these variables, but because they are variables that can be controlled or measured with comparative ease. (Fisher 1960) 32.1 INTRODUCTION Results of field surveys and other descriptive approaches have provided a solid foundation by which to evaluate the effects of contaminants on ecosystem processes. These studieshaveshownthatcertain functional characteristics of ecosystems, especially productivity, nutrient flux, and decomposition, are quite sensitive to anthropogenic disturbance. However, as we noted in the previous chapter, descriptive studies are limited because of the inability to demonstrate cause-and-effect relationships and because of difficulties identifying underlying mechanisms responsible for changes in these eco- system processes. Complex interactions and indirect effects of chemicals are likely to be the rule rather than the exception in many ecosystems. In addition, community inertia, defined as the tend- ency for communities to persist under unfavorable conditions following disturbance (Milchunas and Lauenroth 1995), complicates evaluation of ecosystem responses to perturbation. Isolating causal mechanisms is particularly important in ecosystem studiesbecausetheseprocessesareoften complex and controlled by an assortment of direct and indirect effects. For example, litter decomposition in aquatic and terrestrial ecosystems is regulated by microbial processes and activity of invertebrates. Because effects of contaminants on decomposition rate are dependent on the relative sensitivity of microbial and macroinvertebrate communities, experimental approaches that isolate these dif- ferent mechanisms are necessary to predict effects. This is an ideal application of microcosm and mesocosm experiments, which are often designed to manipulate single or multiple environmental variables, providing an opportunity to isolate specific factors and identify underlying mechanisms. It is the ability to isolate and manipulate individual factors that makes application of microcosm and mesocosm experiments particularly powerful in ecotoxicological research. In this chapter we turn our attention to experimental approaches that have been employed to demonstrate effects of contaminants and other stressors on ecosystem processes. We will examine the use of both small-scale approaches such as microcosms, as well as larger, more ecologically realistic 687 © 2008 by Taylor & Francis Group, LLC Clements: “3357_c032” — 2007/11/9 — 12:39 — page 688 — #2 688 Ecotoxicology: A Comprehensive Treatment and field-based approaches such as mesocosms and whole ecosystem manipulations. Because of the limited spatiotemporal scale, measuring responses of ecosystem processes to contaminants in micro- cosms presents significant challenges. The duration of microcosm and mesocosm experiments is of critical importance when assessing effects of contaminants. For example, a common phenomenon in soil microcosms is the natural reduction in microbial biomass or activity over time. As a consequence, effects of stressors on soil microbial processes become more difficult to quantify as experiments pro- gress. Although microcosms are designed to simulate specific portions of a natural ecosystem, the most valuable experiments investigating effects of contaminants on ecosystem processes have been conducted in larger systems. 32.2 MICROCOSM AND MESOCOSM EXPERIMENTS There is an increasing belief amongst risk assessors that model ecosystems do not possess ecological advantages that were originally assumed, and that an instrumentalist approach to the prediction of toxic effects in ecosystems will yield the most cost-effective results. (Crane 1997) Current approaches to ecological risk assessment of chemicals are ecologically naive and fail to include current knowledge about effects of stressors on ecological communities. (Pratt et al. 1997) The genesis of microcosm and mesocosm research emerged from uncertainties regarding the use- fulness of single species approaches for predicting effects of contaminants in nature (Cairns 1986). While questions about ecological relevance of laboratory toxicity tests persist, microcosm and meso- cosm experiments are now routinely employed in ecotoxicological research and to test ecological principles (Fraser and Keddy 1997). The use of controlled experimental systems in aquatic ecology has clearly increased over time (Figure 32.1), and these systems have been used to address both basic and applied ecological questions. For example, if we consider contaminants as simply another form of anthropogenic disturbance, model ecosystems can be used to characterize ecological resistance and resilience. However, in his critique of model ecosystems used in ecotoxicological research, Crane (1997) argues that research priorities should shift from understanding these ecological com- plexities to questions regarding repeatability, precision, and the relationship between experimental Ye a r 1989 1990 1991 1992 1993 1994 1995 1996 1997 Number of studies 0 10 20 30 40 50 60 Limnology Toxicology Microbiology Terrestrial FIGURE 32.1 The number of studies published between 1990 and 1996 that included the words microcosm or mesocosm in the title or abstract. (Data from Table 1 in Fraser and Keddy (1997).) © 2008 by Taylor & Francis Group, LLC Clements: “3357_c032” — 2007/11/9 — 12:39 — page 689 — #3 Microcosms, Mesocosms, and Field Experiments to Assess Ecosystem Responses 689 and natural systems. He notes the “dangers of allowing model ecosystem studies to be driven by ecological theory” and argues that model ecosystems are most appropriate as a tool to provide envir- onmentally realistic exposure conditions. Because microcosm and mesocosm experiments often provide complex responses across levels of biological organization, interpreting results can be chal- lenging. Recall that problems with data interpretation were provided as a primary justification for the decision by the U.S. EPA to drop mesocosm testing for pesticide registration (Touart 1988, Touart and Maciorowski 1997, Chapter 23). Stay et al. (1988) argued that a lack of correspondence between population, community, and ecosystem-level responses observed in their experiments indicated that measurements at one hierarchical level may not be useful for predicting effects at other levels. Sorting out these complex responses and relating alterations in community structure to ecological processes should be a priority for microcosm and mesocosm research. We also believe that a critical area of research is to determine the extent to which these experimental systems reflect natural con- ditions. One of the challenges associated with the use of microcosms and mesocosms is the change in functional measures over time, independent of the effects of contaminants. These changes, which are often a result of container artifacts, compromise our ability to make comparisons among treat- ments. Williams et al. (2002) compared structural characteristics of microcosms and natural ponds, and recommended refinements to the design of model systems to improve their ecological realism. Unfortunately, few studies have made this comparison based on functional measures or ecosystem processes. Kurtz et al. (1998) measured reproducibility and stability of structural and functional processes in estuarine microcosms. Both structural (relative abundance and density of sulfate redu- cing bacteria) and functional (CO 2 assimilation, sulfate reduction) measures in microcosms were similar to conditions in natural sediments after 7 days. However, the authors cautioned against longer term experiments without modifying the system. Suderman and Thistle (2003) examined changes in structural and functional measures in sediment microcosms derived from a shallow estuary. Chloro- phyll a, primary production and most measures of meiofauna community composition remained relatively stable over the 3-month period. Another potential criticism of microcosm and mesocosm experiments is their relatively limited temporal scale. Because long-term mesocosm experiments (e.g., >1 year) are rare, our understand- ing of prolonged exposure to stressors is incomplete. This is an especially important issue when considering ecosystem processes that often show delayed responses compared to structural altera- tions. Bokn et al. (2003) exposed rocky intertidal communities to long-term nutrient enrichment in marine mesocosms. Despite large inputs of N and P (maximum target concentrations were 32 and 2.0 µM, respectively) and significant increases in periphyton biomass, there were essentially no effects on NPP, GPP, or respiration. These unexpected results were attributed to competition among macroalgal species, grazing by herbivores, and physical disturbance. Although this study was con- ducted for 2.5 years, a relatively long time period when compared with most mesocosm studies, this was not a sufficient amount of time for opportunistic algal species to become established and respond to nutrient enrichment (Bokn et al. 2003). In addition to comparing processes in microcosms and mesocosms with those in natural eco- systems and assessing changes in controls over time, additional research is necessary to optimize experimental designs and to evaluate the statistical power of these systems (Kennedy et al. 1999). In Chapter 23, we discussed strengths and weaknesses of different experimental designs (e.g.,ANOVA versus regression; assignment of replicates to experimental units) for community-level assessments. Consideration of statistical power is especially critical for assessments of ecosystem processes because variability of these measures is often greater than for structural measures. Kraufvelin (1998) estimated the number of replicates necessary to detect significant differences for 50 different variables derived from land-based, brackish water mesocosms. Although calculations were based on population and community-level variables, the results have important implications for meso- cosm experiments designed to assess functional endpoints. Relatively few of the structural variables examined had coefficients of variation (CV) less than 20%. Using an endpoint with a modest CV of approximately 30%, 24 replicates were necessary to detect a statistically significant difference © 2008 by Taylor & Francis Group, LLC Clements: “3357_c032” — 2007/11/9 — 12:39 — page 690 — #4 690 Ecotoxicology: A Comprehensive Treatment (α = 0.05) of 25% between control and treatment mesocosms. Kraufvelin (1998) also noted large differences in the amount of variation among response variables. Assuming that ecosystem processes will show a similar or greater level of variation, statistical power will obviously be an important consideration when selecting functional endpoints. 32.2.1 MICROCOSMS AND MESOCOSMS IN AQUATIC RESEARCH Microcosms and mesocosms have been employed extensively to assess the effects of contaminants on processes in aquatic ecosystems. Although the majority of these investigations have focused on changes in primary production, respiration and other aspects of ecosystem metabolism, end- points related to nutrient processing and decomposition rates have also been considered. Most experiments conducted in stream microcosms and mesocosms have focused on the response of periphyton. Because of their small size, rapid rate of development, and diverse taxonomic composi- tion, periphyton are sensitive indicators of water quality in natural and experimental streams (Lowe et al. 1996). Changes in the structure and function of epilithic assemblages exposed to contamin- ants can occur very rapidly. Colwell et al. (1989) attributed increased respiration in outdoor stream microcosms treated with Zn to the establishment of Zn-tolerant bacteria and algae. We should also remember that structural characteristics of ecosystems may directly or indirectly influence ecological processes. For example, physicochemical characteristics in macrophyte-dominated systems are often controlled by biological processes that are directly related to ecosystem metabolism (Brock et al. 1993). Kersting and van den Brink (1997) describe a dissolved oxygen–pH–alkalinity–conductivity syndrome, in which each of these variables is expected to respond to toxic substances in parallel. These interrelated responses may result in feedback between contaminants and ecosystem processes. For example, it is likely that alterationsin community metabolism resulting from exposureto contam- inants may affect pH and thereby modify contaminant bioavailability. To improve our understanding of the complex responses frequently observed in microcosm and mesocosm experiments, sampling protocols should be designed to quantify relationships between these physicochemical and biological variables. 32.2.1.1 Separating Direct and Indirect Effects One of the important applications of microcosm and mesocosm research has been to separate the direct effects of contaminants from the indirect or secondary effects on ecosystem processes. Select- ive application of contaminants that have specific effects on one group of organisms but relatively limited effects on another group is a useful approach for quantifying these direct and indirect effects (Pratt et al. 1997, Slijkerman et al. 2004). Pearson and Crossland (1996) measured photosynthesis and respiration in outdoor experimental streams exposed to the herbicide atrazine and the insect- icide lindane. Atrazine had a direct inhibitory effect on photosynthesis at 100 µg/L. In contrast, photosynthesis increased in lindane-treated streams due to the elimination of grazing inverteb- rates. Because the direct toxicological effect on invertebrates is limited, atrazine has been used to identify bottom-up responses in model systems (Pratt et al. 1997) (Figure 32.2). In this example, reduced food availability to higher trophic levels would be considered an indirect effect of herb- icide exposure. Conversely, exposure of model systems to insecticides can have a direct effect on invertebrate grazers, resulting in increased algal biomass and production. Predicting effects of contaminants on intermediate trophic levels where elimination of one group may impact both lower and higher trophic levels presents special challenges. Boyle et al. (1996) quantified indir- ect effects of the insecticide diflubenzuron, a chitin inhibitor, on ecosystem processes in 0.1 ha mesocosms. Significant declinesin abundanceof grazinginsects andzooplankton followingdifluben- zuron treatment resulted in increased chlorophyll a biomass and GPP (a top-down response due to reduced grazing) and reduced biomass of juvenile bluegill (a bottom-up response due to reduced food supply). Brock et al. (1993) reported similar increases in periphyton and phytoplankton when © 2008 by Taylor & Francis Group, LLC Clements: “3357_c032” — 2007/11/9 — 12:39 — page 691 — #5 Microcosms, Mesocosms, and Field Experiments to Assess Ecosystem Responses 691 Atrazine level (µg/L) Control 3.2 10 32 110 337 Dissolved oxygen (mg/L) 0 2 4 6 8 10 Number of species 20 30 40 50 60 70 Dissolved oxygen Species richness FIGURE 32.2 Changes in structural (species richness) and functional (dissolved oxygen concentration) variables in microcosms treated with the herbicide atrazine. (Data from Table 1 in Pratt et al. (1997).) mesocosms were treated with the insecticide chlorpyrifos, which reduced abundance of grazing invertebrates. The relationships between structural and functional components of model ecosystems are often complex and may be dependent on contaminant concentration. Slijkerman et al. (2004) observed that at intermediate concentrations of the fungicide carbendazim (17 µg/L), structural changes were observed but there were no corresponding effects on ecosystem function. Functional impairment occurred at higher exposure concentrations (219 µg/L), indicating that functional redundancy could not compensate for changes in community structure. Similar results were reported by Carman et al. (1995) for meiofauna exposed to PAHs in sediment microcosms. Despite significant changes in meiofaunal community composition in high PAH treatments, there were no effects on bacterial or microalgal activity. Elimination of invertebrates by chlorpyrifos in experimental ditches had modest effects on eco- system metabolism by decreasing respiration and increasing oxygen concentration; however, effects on community structure and decomposition rates were much more dramatic (Kersting and van den Brink 1997). Exposure of macroinvertebrates to chlorpyrifos reduced abundance of shredders and resulted in decreased litter decomposition (Cuppen et al. 1995). These researchers also speculated that elimination of grazing invertebrates by insecticides may enhance effects of eutrophication by reducing top-down control of primary producers. Detenbeck et al. (1996) measured biomass, GPP, and respiration in mesocosms treated with the herbicide atrazine in wetland mesocosms. GPP was reduced at the lowest exposure level (15 µg/L), but respiration was either reduced (25 µg/L) or enhanced (75 µg/L). An increase in ammonium, dissolved N, and dissolved P in treated mesocosms was attributed to reduced nutrient uptake by periphyton. Bester et al. (1995) observed significant reductions inprimary productionat low levels of atrazineexposure (0.12 µg/L) in marinemesocosms. An increase in concentrations of dissolved organic N and P in treated microcosms was attributed to release from damaged cell walls. Mesocosm experiments also allow investigators to compare responses across levels of biological organization, thereby providing opportunities to examine underlying mechanisms and relate struc- tural changes to functional alterations. As noted in Chapter 31, reduced decomposition rate observed in contaminated streams may result from either lower abundance of macroinvertebrate shredders or changes in microbial activity. Stream mesocosm experiments have been used to assess the relative importance of these two explanations. Newman et al. (1987) measured litter processing rates, shred- der abundance, and microbial colonization in outdoor experimental streams dosed with chlorine. Although no effects were measured at intermediate concentrations (64 µg/L total residual chlorine), © 2008 by Taylor & Francis Group, LLC Clements: “3357_c032” — 2007/11/9 — 12:39 — page 692 — #6 692 Ecotoxicology: A Comprehensive Treatment lower rates of decomposition in streams receiving 230 µg/L were attributed primarily to reduced abundance of amphipod shredders. The bacterial insecticide Bacillus thuringiensis significantly increased microbial respiration and decreased decomposition in laboratory microcosms (Kreutz- weiser et al. 1996). Although a similar trend was observed in outdoor stream channels, this trend was not significant because of high variation among replicates. 32.2.1.2 Stressor Interactions The key strengths of microcosm and mesocosm experiments are the opportunity to assess effects of chemical mixtures and to quantify interactions among stressors under controlled experimental condi- tions. Cuppen et al. (2002) observed significant effects of a mixture of insecticides (chlorpyrifos and lindane) on decomposition rates of particulate organic matter in litterbags. Despite rapid dissipation of both insecticides (t 1/2 = 9–22 days), elimination of shredders and reduced microbial activity res- ulted in lower decomposition rates. Results of experiments measuring interactions between nutrients and agricultural contaminants (e.g., herbicides, insecticides, sediments) are especially enlighten- ing because these stressors frequently co-occur. More importantly, bioavailability of contaminants may vary depending on the nutrient status and the amount of organic material in an ecosystem. For example, sorption of contaminants is likely to be greater in more productive ecosystems. Barreiro and Pratt (1994) used a factorial experimental design to measure the interactive effects of nutrient enrichment and the herbicide diquat on primary productivity in microcosms. Although structural variables responded to nutrients, there was no effect of diquat on algal biovolume, chlorophyll a,or protein levels. In contrast, GPP was significantly reduced in treated microcosms. These researchers also reported that recovery was greater in systems with higher nutrient levels, most likely due to faster contaminant dissipation (Pratt and Barreiro 1998). The influence of nutrient concentration on community resistance and resilience was also reported by Steinman et al. (1992), indicating that functional responses to chemical perturbations are often context-dependent. Comparatively few studies have examined effects of contaminants on N cycling and flux in aquatic microcosms. Petersen et al. (2004) compared the effects of two antifouling biocides (zinc pyr- ithione, ZPT; and copper pyrithione, CPT) on nitrification and denitrification processes in sediments. Flux of nitrate from sediment increased significantly after additions of ZPT and CPT (Figure 32.3). This increase was a result of increased nitrification (NH 4 → NO 2 → NO 3 ) and/or a decrease in denitrification (NO 3 → N 2 ). The greater sensitivity of nitrification observed in this experiment was Control Low High 0 20 40 60 80 100 120 140 ZPT CPT (a) NO 3 flux (µmol/m 2 /day) TreatmentTreatment Control Low High 0 100 200 300 400 500 (b) NH 4 flux (µmol/m 2 /day) FIGURE 32.3 Flux of NO 3 and NH 4 in microcosms exposed to zinc pyrithione (ZPT) and copper pyrithione (CPT). Low and high treatments in the ZPT and CPT experiments were: 1.0 and 10.0 nmol ZPT/g and 0.1 and 1.0 nmol CPT/g, respectively. (Data from Table 1 in Petersen et al. (2004).) © 2008 by Taylor & Francis Group, LLC Clements: “3357_c032” — 2007/11/9 — 12:39 — page 693 — #7 Microcosms, Mesocosms, and Field Experiments to Assess Ecosystem Responses 693 likely a result of greater functional redundancy of denitrification processes (Petersen et al. 2004). Nitrification is a process performed by a limited number of bacteria, whereas denitrification is a general process performed by many species. Microcosm and mesocosm experiments can also be used to compare functional responses of communities derived from different sources, thereby providing an opportunity to understand how intrinsic features of an ecosystem may influence susceptibility to contaminants. Stay et al. (1988) reported that effects of fluorene on respiration and rates of recovery differed among communities depending on the source of these organisms. Fate of the insecticide chlorpyrifos in microcosms and its effects on community metabolism, decomposition, and nutrient cycling was influenced by the presence of macrophytes (Kersting and van den Brink 1997). Balczon and Pratt (1994) compared effects of Cu on littoral and open water communities. Effects of Cu on oxygen production and respiration were reduced in microcosms with an established littoral zone, most likely because of greater adsorption and complexation by macrophytes and sediments. Although these results showing variable responses in different ecosystems complicate our ability to make broad generalizations, understanding the underlying mechanisms responsible for this variation may ultimately improve our predictive ability. Interactions between biotic and abiotic factors may also influence the response of primary pro- ducers to contaminants. Steinman et al. (1992) observed that the physical structure and integrity of periphyton mats influenced resistance and resilience of carbon fixation rates (a measure of primary productivity) to chlorine exposure. Hill et al. (2000) measured bioaccumulation of Cd by periphyton and subsequent effects on photosynthesis. Effects of Cd on photosynthesis were regulated by periphyton biomass, with greater effects observed in treatments with less bio- mass. Although there were differences in community composition among biomass treatments, reduced effects in high biomass treatments were attributed to contaminant dilution and lower Cd bioavailability. 32.2.1.3 Ecosystem Recovery Although the short duration of many microcosm and mesocosm experiments precludes assessment of recovery, some researchers have used these experimental systems to evaluate improvements in ecosystem processes when contaminants are reduced or eliminated. Oviatt et al. (1984) measured recovery of benthic respiration and nutrient flux for 21 months in mesocosms containing sediments collected along a pollution gradient. Within 5 months, water quality characteristics (nutrients, chloro- phyll a, and dissolved oxygen) and net system production were similar among treatments, indicating that recovery may occur rapidly after pollutants are eliminated. Rapid recovery (4 weeks) of pho- tosynthesis following exposure of marsh plants to crude oil was also reported by Pezeshki and Deluane (1993). Similarly, periphyton productivity in outdoor experimental stream channels dosed with the herbicide, hexazinone, was reduced by 80%, but recovered within 24 h following treatment (Schneider et al. 1995). The estimated LC50 of hexazinone for periphyton production (3.6 µg/L) was reported to be less than published values based on single species tests, demonstrating the greater sensitivity of this functional measure. 32.2.1.4 Comparisons of Ecosystem Structure and Function The majority of published microcosm and mesocosm experiments measure either structural or func- tional characteristics. Because of concerns over sensitivity, variability, and the rate of response of some functional indicators, we suggest that a practical application of these experimental systems is to compare the efficacy of structural and functional endpoints. Questions such as the number of replicates required to detect statistical differences between reference and treated microcosms and the rate at which structural and functional variables respond to chemical stressors are of particular importance. Rigorous control over exposureconditions and the ability to manipulate several variables © 2008 by Taylor & Francis Group, LLC Clements: “3357_c032” — 2007/11/9 — 12:39 — page 694 — #8 694 Ecotoxicology: A Comprehensive Treatment simultaneously in microcosm and mesocosm experiments provide a unique opportunity to compare effects of stressors on structural and functional characteristics (Culp et al. 2003). The conventional wisdom is that, because of functional redundancy and greater variability of functional measures, changes in community composition are likely to occur before alterations in eco- system processes are observed (Schindler 1987, Schindler et al. 1985). However, like many examples of conventional wisdom, there are exceptions to these generalizations in the literature. Some studies have reported that functional measures are equally sensitive or even more sensitive than measures of abundance, biomass, or community composition. Functional measures (periphyton productivity) were considerably more sensitive than structural measures (periphyton biomass; macroinvertebrate abundance and drift) to the herbicide hexazinone in outdoor stream mesocosms (Schneider et al. 1995). Concentration–response relationships between copper and several functional endpoints were established by Hedtke (1984) in laboratory microcosms. GPP and respiration were reduced at 9.3 µg Cu/L, but changes in community composition were observed only at higher concentrations of Cu (30 µg/L), suggesting that ecosystem processes were more sensitive than structure in these experi- ments. Clements (2004) reported that EC 10 values for heavy metals based on community respiration and abundance of metal-sensitive species were similar. Jorgensen et al. (2000) calculated no effect concentrations (NECs) for a variety of structural and functional measures in large pelagic meso- cosms exposed to anionic surfactants (linear alkylbenzene sulfonates). Biomass (as chlorophyll a) and biovolume of the dominant taxonomic groups were affected only at the highest concentrations tested, whereas phytosynthetic activity was the most sensitive parameter for phytoplankton. After 4.5-day exposure, NECs for photosynthetic activity were similar to values for structural characterist- ics (abundance of protozoans, crustaceans, and diatoms). Detenbeck et al. (1996) reported that gross productivity of periphyton was significantly reduced in microcosms exposed to 15 µg/L of atrazine, a concentration that significantly reduced survival of Daphnia but had no effect on other response variables measured (biomass of cattails; growth of tadpoles and fathead minnows). Fairchild et al. (1987) compared community composition, nutrient dynamics, leaf decomposition, and primary pro- duction in experimental streams exposed to clean and contaminated (triphenyl phosphate) sediments. Sediment exposures altered patterns of macroinvertebrate drift and increased nutrient retention, but had no effects on leaf decomposition. Some stream microcosm experiments have been conducted specifically to validate results of laboratory toxicity tests and provide an opportunity to compare ecosystem functional measures with more traditional toxicological endpoints. Exposure of stream mesocosms to relatively high levels of Cd (143 µg/L) resulted in reduced abundance of grazing snails and increased periphyton biomass, but had no effects on gross or net primary productivity (Brooks et al. 2004). Concurrent single species toxicity tests with Ceriodaphnia dubia and Pimephales promelas showed that survival was significantly reduced at this concentration. The lack of a response at lower Cd concentrations (15 µg/L) was attributed to high concentrations of dissolved organic materials in these effluent- dominated streams, which likely reduced metal bioavailability. Richardson and Kiffney (2000) compared structural and functional measures in outdoor experimental streams dosed with mixtures of metals. Significant concentration–response relationships were developed for several measures related tomayfly abundance anddrift, but noeffects ofmetals on algal biomass or bacterial respiration were observed. These researchers recommended that regulatory agencies should include estimates of mayfly abundance and richness as indicators of metal impacts in streams. Balczon and Pratt (1994) derived maximum allowable toxicant concentrations (MATCs) for littoral and aquatic microbial microcosms exposed to Cu. The MATCs were generally greater for process (photosynthesis, respiration) as compared to measures of community composition (species richness, chlorophyll a biomass), indicating greater sensitivity of structural responses. Similar res- ults were reported by Melendez et al. (1993) in which microbial communities were exposed to the herbicide diquat. Changes in productivity and respiration were observed only at the two highest con- centrations (10 and 30 mg/L), and these ecosystem-level responses recovered after 2 weeks exposure. In contrast, the MATC for protozoan species richness and bacterial cell density was 0.32 mg/L, and © 2008 by Taylor & Francis Group, LLC Clements: “3357_c032” — 2007/11/9 — 12:39 — page 695 — #9 Microcosms, Mesocosms, and Field Experiments to Assess Ecosystem Responses 695 these responses showed little evidence of recovery. Barreiro and Pratt (1994) observed that gross community productivity of periphyton was considerably more sensitive than chlorophyll a to diquat. The lowest observable effect concentration (LOEL) for P/R in planktonic communities exposed to fluorene, a polycyclic aromatic hydrocarbon (0.12 mg/L), was comparable to chronic toxicity values based on single species tests with cladocerans, chironomids, and bluegill (Stay et al. 1988). How- ever, the magnitude of change in ecosystem processes did not reflect the near complete elimination of most zooplankton at concentrations exceeding 2 mg/L. Exposure of microbial communities derived from natural sediments to a fungicide, herbicide, or insecticide reduced microbial biomass but had no significant effects on respiration or denitrification (Widenfalk et al. 2004). These differences among experiments suggest that not only is the relative sensitivity of structural and functional meas- ures contaminant-specific, it may also vary with level of contamination and characteristics of the exposure system. In a comprehensive analysis of structural and functional responses of outdoor aquatic mesocosms to the insecticide diflubenzuron, Boyle et al. (1996) observed relatively little effects on GPP, but a significant increase in chlorophyll a, and reduced abundance and species richness of secondary consumers (zooplankton, insects, and bluegill) in treated mesocosms (Figure 32.4). Although com- munity metabolism and decomposition rates were affected in microcosms treated with chlorpyrifos, these processes were generally less sensitive and occurred only after changes in structural measures, suggesting functional redundancy of these systems (Brock et al. 1993). Cuppen et al. (2002) repor- ted that no observable effects concentrations (NOECs) for decomposition rate of Populus leaves and abundance of several macroinvertebrate shredders were similar. Interestingly, the structural and Abundance 0 100 200 300 400 500 Richness 0 2 4 6 8 10 12 Zooplankton Insects Treatment Control Monthly Biweekly Biomass (µg/L) or production (mg O 2 /L /day) 0 10 20 30 40 50 Chlorophyll a Production Treatment Control Monthly Biweekly Biomass (kg/ha) 0 20 40 60 80 100 120 Bluegill (adults) Bluegill (recriuts) Zooplankton Insects FIGURE 32.4 Effects of the insecticide, diflubenzuron, on structural (abundance, biomass, richness) and functional (primary production) measures in lentic mesocosms. Increased chlorophyll a biomass in mesocosms treated monthly and biweekly compared with controls was attributed to reduced grazing pressure. (Data from Boyle et al. (1996).) © 2008 by Taylor & Francis Group, LLC Clements: “3357_c032” — 2007/11/9 — 12:39 — page 696 — #10 696 Ecotoxicology: A Comprehensive Treatment functional NOECs derived from this microcosm experiment were considerably less than the LC50 value derived from standard toxicity tests using known sensitive organisms. Because of the com- plex and often unpredictable relationship between structural and functional measures observed in some studies, we suggest that an appropriate strategy will be to include endpoints reflecting both pattern and process when designing microcosm and mesocosm experiments. We agree with Brock et al. (1993) that an understanding of contaminant effects on ecosystem function cannot be fully appreciated without an understanding of community structure. 32.2.1.5 Effects of Contaminants on Other Functional Measures Although we traditionally consider changes in community composition to be a structural measure, some researchers consider alteration in the abundance of groups that play an important functional role (e.g., abundance of shredders in streams; abundance of grazing zooplankton in lakes) to be intimately related to ecosystem processes and therefore an appropriate surrogate functional measure (Gruessner and Watzin 1996, Wallaceet al. 1996). Field (Wallaceet al. 1982) and stream microcosm experiments (Carlisle and Clements 1999) have assessed the effects of contaminants on functional feeding group composition. The export or loss of materials from an ecosystem is an important functional process that has received relatively little attention in the ecotoxicological literature. Similarly, emergence of adult insects represents a net transfer of energy from aquatic to terrestrial habitats and therefore could be considered a functional response. Gruessner and Watzin (1996) reported increased emergence of insects in stream microcosms treated with atrazine. Culp et al. (2003) measured increased algal biomass and changes in taxonomic composition in stream mesocosms dosed with 5% or 10% pulp mill effluents. Although most measures of benthic macroinvertebrate community composition were similar between treatments, emergence of mayflies was significantly reduced in treated streams. Increased nutrient loading from nonpoint sources is expected to have significant impacts on aquatic ecosystem structure and function. Elevated levels of nutrients are likely to produce excess organic matter, which will result in greater biomass or increased export. An understanding of the ability of an ecosystem to assimilate this excess production is necessary to predict the potential negative effects of nutrient enrichment. Barron et al. (2003) observed no change in GPP, NPP, respiration, or biomass following 27 months of nutrient addition in marine rocky intertidal mesocosms. Carbon budgets calculated in this system showed that the lack of a response to nutrient enrichment resulted from increased export of dissolved organic carbon. The ability of an ecosystem to export relatively large amounts of excess carbon may offer some protection from nutrient enrichment in coastal areas. 32.2.2 MICROCOSMS AND MESOCOSMS IN TERRESTRIAL RESEARCH While aquatic ecotoxicologists have long recognized the value of microcosms and mesocosms as research tools for investigating effects of contaminants on ecosystem processes, these systems have received considerably less attention in terrestrial ecotoxicology (Figure 32.1). Fraser and Keddy (1997) reported that despite a general increase in the use of microcosms and mesocosms to address basic and applied research questions during the mid-1990s, <5% of the studies were conducted in terrestrial ecosystems. For practical reasons, much of the research using terrestrial microcosms and mesocosms has focused on soil microbial systems. As described in previous chapters, alterations in abundance and activity of soil microbes can have significant effects on decomposition rates and nutrient processing. By examining both structure and function of soil communities, it is possible to link direct and indirect effects of contaminants, and identify important regulating mechanisms (Bogomolov et al. 1996). Although the vast majority (>95%) of soil respiration in terrestrial ecosys- tems is a result of microbial activity, nematodes, arthropods, annelids, and other organisms contribute significantly to decomposition. Experiments have been conducted to determine the relative contri- butions of microbes and invertebrates to detrital food webs. Salminen et al. (2001) measured the © 2008 by Taylor & Francis Group, LLC [...]... microcosm and mesocosm experiments is the ability to manipulate several independent variables or site characteristics to quantify factors that determine contaminant effects and bioavailability Khan and Scullion (2000) examined effects of heavy metals on microbial biomass, respiration, and mineralization in soils with varying clay and organic content Metal bioavailability and effects were generally greater... These researchers speculated that alterations in abundance of nitrifying bacteria are likely to have long-term consequences for nutrient dynamics in soils and may disrupt the balance among nitrogen fixation, denitrification, and nitrification Riparian habitats are located at the interface between terrestrial and aquatic ecosystems, and often function as buffers to protect lakes and streams from anthropogenic... a small headwater stream significantly reduced leaf litter decomposition (Figure 32. 8) and downstream export of particulate organic material (Cuffney et al 1984, Wallace et al 1982) Results showed significant variation among leaf species and that litter decomposition was a sensitive indicator of contaminant effects These alterations in detritus dynamics were not associated with changes in microbial activity... the water column is generally valid Although most experiments conducted with soil microcosms attempt to achieve a relatively homogeneous distribution of contaminants, chemicals in natural soils are often patchily distributed In addition, small-scale spatial variation in the physicochemical characteristics of soils may alter chemical bioavailability (Salminen and Sulkava 1997) Some of the characteristics... that this approach differs in an important way from simple descriptive or comparative studies because the systems are actually manipulated Avariety of statistical techniques, including intervention analyses, multivariate autoregressive models, dynamic linear models, and repeated measures, have been employed to analyze changes in unreplicated ecosystem experiments 32. 3.1 AQUATIC ECOSYSTEMS Experimental... program of observational and small-scale experimental approaches to identify critical questions that can only be addressed using large-scale experiments This strategy will allow researchers to focus limited resources on those issues that are central to understanding how ecosystems respond to anthropogenic perturbations 32. 4.1 SUMMARY OF FOUNDATION CONCEPTS AND PARADIGMS • Isolating causal mechanisms... Steinman, A. D., Mulholland, P.J., Palumbo, A. V., DeAngelis, D.L., and Flum, T.E., Lotic ecosystem response to a chlorine disturbance, Ecol Appl., 2, 341–355, 1992 Stewart, J.S., Wang, L.Z., Lyons, J., Horwatich, J .A. , and Bannerman, R., Influences of watershed, ripariancorridor, and reach-scale characteristics on aquatic biota in agricultural watersheds, J Am Water Res Assoc., 37, 1475–1487, 2001 Suderman,... However, large-scale experimental manipulation of some classes of stressors, including elevated CO2 , UV-B radiation and atmospheric N, present significant logistical problems and will be examined in a later chapter We believe that a more efficient strategy for generalizing among ecosystem and stressor types is to integrate what we already know about ecosystem responses to perturbations with a well-designed... microbial biomass and estimates of microarthropod and nematode abundance Soil respiration was reduced in all contaminant treatments, and effects increased with chemical concentration Changes in respiration were accompanied by decreases in microbial biomass and abundance of soil organisms resulting from direct toxicity in most treatments Because some of these responses were not observed until late in the... published after 16 years of P enrichment reported a dramatic increase in abundance of the bryophyte Hygrohypnum, an aquatic moss that replaced epilithic diatoms, resulting in significant changes in the benthic habitat and a four times increase in NH+ uptake (Slavik et al 2004) The authors concluded that even 4 relatively long-term experimental manipulations (e.g., 4–8 years) were not adequate to predict . contaminants, chemicals in natural soils are often patchily distributed. In addition, small-scale spatial variation in the physicochemical characteristics of soils may alter chemical bioavailability. (Salminen and Sulkava 1997). Some of the characteristics of soils that modify chemical bioavailability, such as particle size and amount of organic material, are analogous to properties of aquatic. nutrient dynamics in soils and may disrupt the balance among nitrogen fixation, denitrification, and nitrification. Riparian habitats are located at the interface between terrestrial and aquatic ecosystems,