Clements: “3357_c030” — 2007/11/9 — 18:39 — page 635 — #1 30 Overview of Ecosystem Processes A major stumbling block in the study of ecosystems is their bewildering complexity. (O’Neill and Waide 1981) 30.1 INTRODUCTION The perspective offered by O’Neill and Waide (1981) in the above quote illustrates one of the more significant challenges faced by ecotoxicologists when attempting to understand the potential impacts of anthropogenic stressors on ecosystem processes. This perspective may also partially explain the relative infrequency with which ecosystem processes are measured in biological assess- ments. Because ecosystem “surprises” (sensu Paine et al. 1998) may result from focusing on isolated components, one potential solution to this “bewildering complexity” is to develop a comprehensive understanding of emergent ecosystem properties (O’Neill and Waide 1981). For example, we can readily quantify the contributions of decomposers to nutrient cycling or the influence of predators on energy flow; however, it is unlikely that we can predict ecosystem consequences based exclusively on abundance or biomass estimates of these functional groups. As described in the previous chapters, the idea that behavior of a complex system often cannot be understood solely by analysis of its com- ponents is a major thesis of hierarchy theory. The order that emerges from complex systems and the constraints placed on the range of potential interactions in these systems are fundamental differ- ences between randomly assembled populations and a stable ecosystem. The functional redundancy of ecosystems that results from species replacement is a good example of our inability to predict ecosystem responses based on understanding of components. In one of the earlier theoretical treatments of ecosystem ecotoxicology, O’Neill and Waide (1981) provide several recommendations for research programs in this emerging field: 1. Focus on functionally intact systems that reflect ecosystem-level properties. 2. Focus on integrative properties that reflect interactions among physical, chemical, and biological properties. 3. Treat the ecosystem as a biogeochemical system that focuses on movement of energy and materials. 4. Rate processes (e.g., mineralization, decomposition, and nitrification) may be better indic- ators of contaminant effects than the amount of materials or energy stored in ecosystem pools. Thus, before we can understand how ecosystems respond to contaminants and other anthropo- genic perturbations, it is necessary to develop an appreciation for the complex ecosystem processes that are most likely to be affected by physical and chemical stressors. In the previous chapter, we characterized ecosystems in terms of energy flow and materials cycling. Much of our discussion of how ecosystems respond to stressors will focus on these processes. Although general ecology textbooks and much of the ecological literature treat energy flow and materials cycling through an ecosystem separately, it is important to realize that these processes are intimately related. Patterns of 635 © 2008 by Taylor & Francis Group, LLC Clements: “3357_c030” — 2007/11/9 — 18:39 — page 636 — #2 636 Ecotoxicology: A Comprehensive Treatment primary and secondary production in ecosystems are often limited by the amount of available nutri- ents. Biogeochemical processes, the size of nutrient pools, and the rate of materials cycling in an ecosystem can, in turn, be regulated by primary productivity. Finally, while our focus in this section will be on characterizing functional attributes of ecosystems, recent findings that demonstrate strong links between species richness, diversity, and ecosystem processes require that we also consider structural features. 30.2 BIOENERGETICS AND ENERGY FLOW THROUGH ECOSYTEMS In addition to viewing ecosystems within a hierarchical context, contemporary ecologists routinely characterize ecosystems based on bioenergetic and biogeochemical processes. Captured solar radi- ation stored in chemical bonds by autotrophic organisms is made available to heterotrophs. As described in the previous chapter, the perception that ecosystems are energy-transforming systems emerged relatively early in the history of ecology. Elton’s (1927) depiction of a tundra food web and his recognition that a large number of herbivores are necessary to support a smaller number of pred- ators preceded Tansley’s definition of ecosystem by a decade. Elton’s description of this relatively simple food web also made ecologists aware of the difficulties associated with accurately character- izing ecosystem energetics. Although the use of calories or other units of energy as the currency to integrate Elton’s trophic levels did not occur for several decades, these early investigations helped to formalize contemporary perspectives of ecosystem dynamics. Elton’s (1927) food web became Lindeman’s (1942) food cycle that was eventually formalized as a universal energy model by Odum (1968) that also included a material-cycling component. 30.2.1 PHOTOSYNTHESIS AND PRIMARY PRODUCTION Flux of energy through an ecosystem is determined by the rate at which plants assimilate energy by photosynthesis, the transfer of this energy to herbivores and other consumers, and the efficiency of these conversions. Because contaminants and other stressors can affect any of these processes, energy flux through an ecosystem is an important indicator in ecosystem-level assessments. Photo- synthesis in plants is the conversion of light energy and raw materials (carbon dioxide and water) to carbohydrates and oxygen: 6CO 2 +6H 2 O +Light energy → C 6 H 12 O 2 +6O 2 (30.1) Although this stoichiometricallybalanced chemical reaction to describe photosynthesis is correct, it is not especially satisfying from an ecological perspective and should be expanded to include both elemental and energy components to reflect biomass accrual in the following way (Sterner and Elser 2002): Inorganic carbon +Nutrients +Light energy → Biomass +Heat (30.2) The energy necessary for the conversion of CO 2 to a reduced state in carbohydrates is provided by visible light and the total amount of energy fixed by plants is referred to as gross primary production (GPP). Plants require a portion of this fixed energy for their own metabolic needs (e.g., respiration) and the difference between GPP and these metabolic costs is called net primary production (NPP). NPP is defined as the total amount of energy available to the plant for growth and reproduction after accounting for respiration: NPP = GPP −Respiration (30.3) © 2008 by Taylor & Francis Group, LLC Clements: “3357_c030” — 2007/11/9 — 18:39 — page 637 — #3 Overview of Ecosystem Processes 637 30.2.1.1 Methods for Measuring Net Primary Production Methods for measuring NPP in terrestrial and aquatic ecosystems are diverse, but typically focus on assessing changes in biomass, CO 2 ,orO 2 . The most direct method for estimating NPP in terrestrial ecosystems is the harvest method, which generally involves measuring the increase in plant standing crop or biomass (B) over a growing season. B = B 2 −B 1 (30.4) where B 2 is the biomass at time 2 and B 1 is the biomass at time 1. Note that primary production of an ecosystem is a functional measure of the instantaneous rate of biomass generation, generally expressed as dry weight of plant material (or carbon) per unit area per unit time (g/m 2 /year). In contrast, biomass is a structural measure of the amount of plant material present at one particular point in time. A more energetically appropriate measure may be obtained by converting dry weight of plant material to calories. Estimates of both NPP and biomass have been used as endpoints in assessing stressor impacts on ecosystem energetics. Other approaches for estimating primary production involve measuring gas exchange (e.g., uptake of CO 2 or release of O 2 ) and the use of radioactive carbon isotopes, 14 C. Although harvest methods provide the most direct measure of NPP in terrestrial ecosystems and have been employed to estimate production of larger marine plants (macrophytes, kelp), they are less com- mon in aquatic ecosystems because of small size and rapid turnover of primary producers. Three approaches have been employed in aquatic ecosystems to estimate primary productivity: light and dark bottles oxygen techniques, radioisotopes such as 14 C, and in situ diel approaches. The traditional approach for aquatic systems uses light and dark bottles containing water with ambient phytoplank- ton populations. This approach compares changes in dissolved oxygen concentration ([O 2 ]) in bottles held in the light with changes in the dark. Because [O 2 ] in the light is a result of both GPP and respiration whereas change in the dark bottle is a result of respiration only, GPP can be estimated by the difference between these measures: GPP = [O 2 ]light −[O 2 ]dark (30.5) A similar approach has been used to estimate metabolism in stream ecosystems in which cobble substrate collected from the streambed is placed in light and dark chambers. This approach provides an estimate of whole community metabolism because the cobble substrate typically includes both autotrophic and heterotrophic organisms (e.g., algae, bacteria, fungi, and invertebrates). The carbon-14 ( 14 C) technique provides a considerably more sensitive estimate of primary productivity, which may be necessary in oligotrophic systems where GPP is very low. The 14 C tech- nique is also preferred by some ecologists because it allows researchers to explicitly follow carbon flow through an ecosystem (Howarth and Michales 2000). Clear bottles with water and ambient phytoplankton are incubated with a tracer amount of 14 C—labeled dissolved inorganic carbon. The accumulation of carbon in organic matter relative to the dissolved inorganic fraction provides a measure of primary production. Note that the light and dark bottle technique and the 14 C incubation technique may be comprom- ised by container artifacts. Isolation of primary producers from natural systems by placement in bottles may result in depleted nutrient concentrations, decreased turbulence and mixing, and growth of organisms on the sides of the container. In situ approaches that measure diel changes in O 2 or CO 2 eliminate these bottle effects and provide ecosystem-level estimates of GPP and respiration. Changes in CO 2 or O 2 during daylight are a result of GPP, whole ecosystem respiration, and exchange with the atmosphere. Thus, whole ecosystem GPP and respiration can be estimated by measuring changes in O 2 or CO 2 during the daylight and at night, and correcting for atmospheric exchange. A variation of this approach is used in streams, where whole ecosystem metabolism is determined by comparing O 2 © 2008 by Taylor & Francis Group, LLC Clements: “3357_c030” — 2007/11/9 — 18:39 — page 638 — #4 638 Ecotoxicology: A Comprehensive Treatment or CO 2 concentrations at upstream and downstream locations and measuring the travel time between stations. 30.2.1.2 Factors Limiting Primary Productivity Numerous abiotic factors limit primary productivity in both terrestrial and aquatic ecosystems; however, light, temperature, nutrients, and moisture (in terrestrial habitats) are generally considered the most important limiting factors. Becauseplants differ inthe efficiency withwhich they captureand convert incident sunlight, an understanding of factors that limit the efficiency of GPP is necessary to understand how contaminants may influence theseprocesses. In general, phytoplankton communities have very low efficiencies (<1%), whereas higher values are observed in forests (2–3.5%) (Cooper 1975). Most of the energy fixed by plants, approximately 50–70%, is used for plant respiration. Sufficient light levels are necessary for primary production; however, intense light can saturate pigments and inhibit photosynthesis. Similarly, the rate of photosynthesis generally increases with temperature, up to some optimal value, and then declines. Because respiration also increases with temperature, optimal temperatures for NPP and photosynthesis will likely differ, complicating our ability to predict the precise relationship between temperature and NPP. Finally, the amount of available nutrients, especially nitrogen (N) and phosphorus (P), limit primary production in many ecosystems. Primary production of aquatic ecosystems is particularly sensitive to nutrient limitation, as evidenced by studies showing that even slightly increased levels of nutrients can significantly increase algal productivity (Ryther and Dunstan 1971). Although most of the research on nutrient limitation has focused on N and P, some ecosystems may be limited by other materials. Studies conducted using water collected from the Sargasso Sea, a highly oligotrophic ecosystem, showed that enrichment with N and P had relatively little effect on phytoplankton (Menzel and Ryther 1961). Primary productivity in much of the open ocean is limited by iron, which has stimulated interest in the use of iron to fertilize the oceans as a measure to increase sequestration of anthropogenic CO 2 . Not surprisingly, many of the most comprehensive studies demonstrating effects of nutrients on productivity have been conducted in lakes where the association between primary productivity and abiotic factors has been documented experimentally (Schindler 1974). Because light is rapidly attenuated in aquatic ecosystems, the amount of light available to primary producers decreases as a function of depth according to the following equation: dI/dz =−kI (30.6) where I = amount of solar radiation, z = depth, and k = extinction coefficient. The extinction coefficient varies among ecosystems, from about 0.02 in pure water to 0.10 in open seawater. The amount of light at 10 m depth in open seawater is about 50% lower than at the surface. Because of greater amounts of light absorbing materials, values of k in lakes and other productive ecosystems are considerably greater. In deep rivers, lakes, and marine ecosystems, the reduction in light limits the depth at which many plants can occur to a narrow band called the euphotic zone, and is defined as the area near the surface where photosynthesis is greater than respiration. 30.2.1.3 Interactions Among Limiting Factors In addition to the direct effects of these limiting factors, combined and interactive effects of light levels, nutrients, and other abiotic factors can affect primary production in aquatic ecosystems. In a large-scale comparison across several ecoregions in North America, Bott et al. (1985) concluded that the combined effects of photosynthetically active radiation (PAR), chlorophyll a, and water tem- perature accounted for >70% of the variation in community metabolism among streams. Similarly, Fleituch (1999) reported that benthic community metabolism along a river continuum was primar- ily influenced by physical factors, including solar radiation, riparian canopy, water temperature, © 2008 by Taylor & Francis Group, LLC Clements: “3357_c030” — 2007/11/9 — 18:39 — page 639 — #5 Overview of Ecosystem Processes 639 and conductivity. It is well established that enrichment of aquatic ecosystems caused by excessive nutrients often stimulates primary production and causes excessive plant growth, including blooms of potentially toxic blue-green algae. Because these dense populations of algae limit light penetra- tion, dramatic shifts in the structure and function of major primary producers may occur. Attached macrophytes, which are dependent on sufficient light levels penetrating from the surface, are often replaced by phytoplankton communities that are capable of remaining near the surface. Because of its association with global climate change, ecologists have recently given special attention to the influence of CO 2 and other abiotic factors on primary productivity. Researchers hypothesize that if elevated CO 2 increases primary productivity, some of the excess anthropogenic carbon releasedfrom burningfossil fuels and land usechanges maybe sequestered into plant biomass. This response remains uncertain because of the potential for other factors (e.g., nutrients, light, temperature) to limit primary production in terrestrial and aquatic ecosystems. Melillo et al. (1993) used aterrestrial ecosystemmodel (TEM)to predictthe effects ofclimate changeand elevated CO 2 on NPP. Spatially referenced information on climate, soils, vegetation, water availability, and elevation were used to predict current NPPvalues fora widevariety ofecosystems. Model predictionsof current NPP were very close to values based on field measurements. The model was then run to simulate responses of NPP to a doubling of CO 2 and associated changes in temperature, precipitation, and cloud cover as predicted by general circulation models (Figure 30.1). Overall global NPP increased by approximately 23%, but there was considerable variation among ecosystems. This geographic variation reflects not only how different ecosystems will respond to climate change but also the underlying mechanisms. For example, moist temperate ecosystems responded primarily to elevated temperature and increased nitrogen cycling whereas dry temperate ecosystems responded primarily to elevated levels of CO 2 . 30.2.1.4 Global Patterns of Productivity Productivity is not evenly distributed among regions of the world, and comparisons of NPP and biomass estimated by Whittaker and Likens (1973) for some of the world’s major biomes reveal Ecosystem type Alpine tundra Short grassland Desert Boreal forest Temp. conif. Temp. decid. Temp. savan. Trop. savan. Trop. decid. 0 1 2 3 4 5 6 7 Current NPP Predicted NPP NPP (10 15 gC/y) FIGURE 30.1 Results from a TEM used to predict the effects of a 2×increase in CO 2 and associated changes in temperature, precipitation, and cloud cover on NPP of different terrestrial ecosystems. (Data from Table 2 in Melillo et al. (1993).) © 2008 by Taylor & Francis Group, LLC Clements: “3357_c030” — 2007/11/9 — 18:39 — page 640 — #6 640 Ecotoxicology: A Comprehensive Treatment TABLE 30.1 Estimates of Net Primary Productivity and Bio- mass in the Earth’s Major Biomes NPP (g/m 2 /y 2 ) Biomass (kg/m 2 ) Terrestrial ecosystems Tropical forest 1800 42 Temperate forest 1250 32 Boreal forest 800 20 Temperate grassland 500 1.5 Alpine and tundra 140 0.6 Desert scrub 70 0.7 Aquatic ecosystems Algal beds and reefs 2000 2 Estuaries 1800 1 Lakes and streams 500 0.02 Continental shelf 360 0.01 Open ocean 125 0.003 Source: Data from Whittaker and Likens (1973). several interesting patterns (Table 30.1). Although sunlight is necessary for primary production, it is evident from Table 30.1 that other factors contribute to global patterns. If adequate moisture or nutrients are not available, as in arid ecosystems or the open ocean, NPP will be low regardless of the levels of sunlight. In forest ecosystems, a general decrease in NPP is seen as we move to colder and more arid climates. The combination of sufficient sunlight, warm temperature, and abundant moisture results in very high productivity for tropical forests. Despite the generally low productivity of open ocean ecosystems, estuaries, algal beds, and coral reefs are among the most productive aquatic habitats. Biomass also varies among these different habitats and reflects different growth forms of the major primary producers. Biomass in terrestrial habitats is generally much greater than in aquatic ecosystems, and this large terrestrial biomass represents an important pool of global carbon. The lower biomass in aquatic environments results from the relatively small body size of dominant primary producers (e.g., phytoplankton), which has important implications for trophic dynamics. Because small primary producers in aquatic ecosystems are capable of very rapid turnover, they can support a relatively large biomass of consumers compared to terrestrial ecosystems. The ratio of productivity to biomass (P:B) also varies greatly among different biomes and ecosystems. As shown in Table 30.1, P:B ratios for terrestrial ecosystems, especially forests, are relatively low, reflecting the large amount of nonphotosynthetic biomass in these ecosystems (e.g., bark, trunk, and branches). In contrast, P:B ratios in aquatic ecosystems, especially those dominated by phytoplankton, are much higher because of their small size and rapid turnover rates. The production values of lentic and marine phytoplankton reflect multiple and overlapping generations, but biomass is measured at a specific point in time. These differences in growth forms and turnover rates between terrestrial and aquatic ecosystems may also have important consequences for responses to anthropogenic disturbances. Because of their rapid growth rates, we expect that primary producers in aquatic ecosystems would respond more rapidly to contaminants than in terrestrial ecosystems. 30.2.2 SECONDARY PRODUCTION Secondary production is defined as the rate of productivity of consumers such as herbivores and predators that obtain their energy from plant or animal biomass. Consumers such as bacteria and © 2008 by Taylor & Francis Group, LLC Clements: “3357_c030” — 2007/11/9 — 18:39 — page 641 — #7 Overview of Ecosystem Processes 641 TABLE 30.2 Measures and Definitions of Ecosystem Energetics and Efficiencies Measure Definition Consumption (C) Total amount of energy consumed Egestion (E) Total amount of energy lost to egestion Assimilation (C–E) Total amount of energy available for production and respiration Production (C–A) Total amount of energy available for growth and reproduction Assimilation efficiency (A/C ×100%) Portion of consumed food that is assimilated Net production efficiency (P/A ×100%) Portion of assimilated food that is converted to new biomass Gross production efficiency (P/C ×100%) Portion of consumed food that is converted to new biomass Trophic level efficiency (A n /A n−1 ×100%) Efficiency of transfer of assimilated energy between two trophic levels n, a consumer, and n −1, the resource fungi, organisms that obtain energy from decomposing plant and animal material, should also be included in measures of secondary production. Secondary productivity is similar to primary pro- ductivity in that we must distinguish between the portion of energy for growth and reproduction (and thus available to higher trophic levels) and the portion associated with maintenance costs of the consumer (Table 30.2). As noted above, the amount of energy available to consumers is ultimately determined by NPP and the efficiency with which fixed energy is converted to biomass. Similar to the bioenergetic approaches described for populations, ecosystem ecologists have identified several processes that limit efficiency of secondary production. Only a portion of the biomass consumed by herbivores or predators is actually assimilated. Because food quality for predators is generally greater than herbivores (i.e., proteins vs. recalcitrant cellulose and lignin), assimilation efficiency, which is defined as the fraction of consumed biomass that is assimilated (e.g., available for growth, reproduc- tion, respiration, and maintenance), is generally greater in predators. In addition to the recalcitrant materials in plant tissue, herbivores must also contend with a diverse assortment of defensive chem- icals produced by plants, which also limits consumption. Interestingly, coevolutionary responses to these natural defensive chemicals may also explain the well-developed detoxification systems in herbivores, which coincidentally provide protection against some xenobiotics. In contrast to ter- restrial herbivores, assimilation efficiency is relatively high for zooplankton and other herbivores feeding on unprotected phytoplankton or algae. 30.2.2.1 Ecological Efficiencies Only a small fraction of the assimilated energy in consumers is available for growth and reproduction; the remaining is necessary for maintenance and respiration. Because metabolic costs are generally greater in homeotherms than in poikilotherms, net production efficiency, defined as the amount of assimilated food available for new biomass, is generally lower in “warm-blooded” organisms (1–2%) than in “cold-blooded” organisms (5–10%). Gross production efficiency, defined as the amount of consumed food available for biomass, is a function of both assimilation and production efficiencies. Finally, trophic level efficiency (also called Lindeman’s efficiency) is the efficiency of energy transfer between two trophic levels. Although trophic level efficiency averages around 10%, there is considerable variation among ecosystems (Pauly and Christensen 1995). The important point is not the universality of the figure but the relative inefficiency of ecological systems. The inefficiency of energy transfer also limits food chain length and the number of trophic levels in an ecosystem (Table 30.3). Ricklefs (1990) estimated the average length of food chains based on NPP, ecological efficiency, and energy flux of top predators for several different ecosystems using © 2008 by Taylor & Francis Group, LLC Clements: “3357_c030” — 2007/11/9 — 18:39 — page 642 — #8 642 Ecotoxicology: A Comprehensive Treatment TABLE 30.3 Comparison of Average NPP, Predator Ingestion Rates, Ecological Effi- ciencies, and Number of Trophic Levels in Marine and Terrestrial Ecosystems Community Type NPP (kcal/m 2 /y 2 ) Predator Ingestion (kcal/m 2 /y 2 ) Ecological Efficiency (%) Number of Trophic Levels Open ocean 500 0.1 25 7.1 Coastal marine 8000 10.0 20 5.1 Temperate grassland 2000 1.0 10 4.3 Tropical forest 8000 10.0 5 3.2 Source: Data from Ricklefs (1990). the following equations: E(n) = NPP Eff n−1 (30.7) n = 1 + log[E(n)]−log(NPP) log(Eff) (30.8) where n = number of trophic levels, E(n) = energy available to a predator at a trophic level n, and Eff = geometric mean of the ecological efficiencies of transfer between each level. Results showed that the number of trophic levels was more closely related to ecological efficiency than overall NPP. 30.2.2.2 Techniques for Estimating Secondary Production Estimates of secondary production for some species can be derived from measures of feeding rates, assimilation efficiencies, and respiration in the laboratory (Fitzpatrick 1973) or under controlled conditions (West 1968). However, determining secondary production in natural populations is more challenging and generally requires estimates of consumption, growth, and reproduction. Sophist- icated bioenergetics models have been developed for some aquatic species such as large-mouth bass (Kitchell 1983). These individual-based models generally use laboratory-derived estimates of consumption, respiration and elimination, and then solve for growth. Consumption = Respiration +Wastes +Growth (30.9) Several practical issues complicate our ability to estimate whole ecosystem production using these individual-based models. While estimates of secondary production for individual species, especially those for which we have a thorough understanding of natural history (Jordan et al. 1971, Kilgore and Armitage 1978), have been developed, integrating this information to derive secondary production estimates for whole ecosystems or even major components of ecosystems is challenging. Wiens (1973) estimated secondary production of grassland bird communities, and Chew and Chew (1970) examined energy relationships of dominant mammals in a desert shrub community. Perhaps the best examples have been developed in aquatic ecosystems where researchers have derived community- level estimates of secondary production for major taxonomic or functional feeding groups (Benke and Wallace 1980, 1997, Carlisle 2000, Fisher and Gray 1983). Secondary production (P) in benthic macroinvertebrates is obtained from estimates of biomass and growth rate using the following © 2008 by Taylor & Francis Group, LLC Clements: “3357_c030” — 2007/11/9 — 18:39 — page 643 — #9 Overview of Ecosystem Processes 643 simple relationship: P = B i ×g i (30.10) where B i and g i are biomass and growth rates of the ith species. While the methodology for estimating secondary production in aquatic ecosystems is well estab- lished, these are labor-intensive efforts. Estimating biomass for macroinvertebrates is relatively straightforward; however, the intensive sampling frequency necessary to determine growth rates of macroinvertebrates often limits application of this technique. Because of these methodological challenges, with few exceptions, secondary production has not received significant attention in the ecotoxicological literature.An example of one such exception, Carlisle (2000) constructed food webs based on quantitative analyses of macroinvertebrate secondary production for six different streams along a gradient of heavy metal pollution. Difficulties quantifying the role of detritus and the imprecise assignment of organisms to different trophic groups are also impediments to studies of secondary production. Early attempts to quantify the relationship between NPP and secondary production should be evaluated cautiously because of the failure to appreciate the dominant role of decomposers and microbial production. The opinion of O’Neill et al. (1986) that “the trophic level concept is most useful as a heuristic device and tends to obscure, rather than illuminate, organizational principles of ecosystems” is likely shared by many ecosystem ecologists. The use of stable isotopes, described in Section 34.4.4, is one potential solution to this problem; however, relatively few studies have employed this technique in ecosystem-level studies of secondary production. 30.2.3 THE RELATIONSHIP BETWEEN PRIMARY AND SECONDARY PRODUCTION Numerous studies have reported a direct quantitative relationship between primary productivity and secondary productivity or biomass of consumers (Coe et al. 1976, Cyr and Pace 1993, McNaughton et al. 1989). A major emphasis of the International Biological Program described in Chapter 29 was to understand the biological basis of productivity and to quantify relationships between primary and secondary production. Much of this research focused on understanding the underlying mechanisms and consequences of interactions between plants and consumers. Some of the strongest evidence to support the relationship between NPP and secondary production has been obtained from exper- imental introductions of nutrients to whole ecosystems. The predictable increases in both primary and secondary production illustrate the need to consider energy flow and nutrient cycling together when investigating ecosystem energetics. Intuitively, we would expect that herbivore biomass or production would increase with NPP; however, the nature of this relationship will likely vary among ecosystems and herbivore types. For example, because grassland herbivores consume a larger por- tion of NPP than forest herbivores (Whittaker 1975), the relationship between NPP and herbivore abundance in forest ecosystems is relatively weak (Figure 30.2). Concentrations of structural com- pounds, such as lignins and other recalcitrant materials that limit herbivory in terrestrial ecosystems, are generally lower in aquatic primary producers. Consequently, grazers in many aquatic ecosystems consume a large fraction of available biomass (30%–40%) and the relationship between primary and secondary production in these systems is generally much stronger. Because a greater fraction of NPP is removed in aquatic ecosystems, we also predict that predators would play a more important role in energy flow here than in terrestrial ecosystems (Cebrian and Lartigue 2004). These expectations are supported by studies showing the relative importance of top-down effects in aquatic ecosys- tems compared to terrestrial ecosystems (Strong 1992). Predator control over lower trophic levels, termed trophic cascades, has been frequently observed in aquatic ecosystems but only rarely in ter- restrial ecosystems. Similarly, bottom-up control of herbivores and other consumers by nutrients and primary producers is quite common in many lentic ecosystems and is the mechanism responsible © 2008 by Taylor & Francis Group, LLC Clements: “3357_c030” — 2007/11/9 — 18:39 — page 644 — #10 644 Ecotoxicology: A Comprehensive Treatment Primary productivity Secondary productivity Forest ecosystems Grassland ecosystems Aquatic ecosystems FIGURE 30.2 Hypothetical relationship between net primary productivity (NPP) and secondary productivity in aquatic and terrestrial ecosystems. Stronger relationships are expected in aquatic ecosystems because grazers consume a larger portion of plant biomass compared to terrestrial ecosystems. for cultural eutrophication. Understanding the relationship between NPP and secondary production in ecosystems is important for predicting potential contaminant effects. It is possible that some of the variation in this relationship may account for differences in contaminant transfer rates among ecosystems. These issues will be explored in Chapter 34. 30.2.4 THE RIVER CONTINUUM CONCEPT The movement of materials and energy in ecosystems has been investigated using a variety of descriptive, theoretical, and empirical approaches. Attempts to develop comprehensive explanatory models that connect physical, chemical, and biological processes have been especially successful in aquatic ecosystems. Vannote’s classic paper “The river continuum concept” (Vannote et al. 1980) recognized that patterns and processes in streams change predictably from headwaters to the mouth. In addition to linking geomorphologic characteristics of a watershed to biological processes, this paper elucidated mechanisms responsible for the downstream transport, utilization, and storage of energy and materials. The major tenets of the river continuum concept (RCC) can be summarized by considering longitudinalchanges inthe sourcesof energy andmaterials fromupstream todownstream (Figure 30.3). The relative importance of allochthonous and autochthonous sources of energy shift from upstream to downstream, resulting in changes in the ratio of NPP to respiration and structural alterations in the composition of stream communities. Shaded headwater streams are generally heterotrophic (P/R<1) because the dense riparian canopy in these systems limits primary productivity and contributes significant amounts of allochthonous materials. Further downstream, as the canopy opens, shading and the relative input of organic materials from riparian areas is reduced, and the stream becomes autotrophic (P/R>1). Finally, large rivers may return to heterotrophic conditions (P/R<1) because of increased depth and greater light attenuation. Longitudinal changes in the abundance and composition of macroinvertebrate functional feeding groups (Cummins 1973) along the river continuum are hypothesized to reflect the relative import- ance of allochthonous and autochthonous inputs. The abundance of organisms that utilize coarse particulate organic material (CPOM) (e.g., leaf litter) is greatest in headwater streams and decreases downstream. Grazers, organisms that consume attached algae and periphyton, are more important in mid-order streams where light levels are highest. Finally, organisms that collect fine particulate organicmaterial (FPOM), includingcollector-gatherersand collector-filterers, dominate larger rivers. Tests of thepredictions of the RCC indifferent geographicregions have provided good supportfor the major tenets in NorthAmerica (Bott et al. 1985, Minshall et al. 1983) and Europe (Fleituch 1999). Minshall et al. (1983) measured benthic organic matter, community metabolism, decomposition, © 2008 by Taylor & Francis Group, LLC [...]... Juvenile salmon growth and survival Riparian vegetation Primary production Anadromous salmon Terrestrial scavengers Salmon carcasses FIGURE 30. 9 The influence of anadromous salmon on production in aquatic and terrestrial ecosystems Although we have traditionally considered the transport of nutrients and other materials in streams as a one-way process, in some instances nutrients exported downstream may be... water, U = uptake rate of nutrients from water, and w = average stream width Uptake length generally increases with stream discharge and decreases with temperature, the amount of riparian vegetation, and biomass of detritus and algae In relatively small streams, attached algae, fungi, bacteria, and periphyton are responsible for most of the uptake of nutrients, which generally follows Michaelis–Menten... oxidation states Five basic processes drive the N cycle: nitrogen fixation, nitrification, assimilation, ammonification, and denitrification (Figure 30. 8) The vast majority of N occurs in the atmosphere as molecular N2 , a form that is unavailable to plants Nitrogen-fixing bacteria in soil (Rhizobium) and cyanobacteria (blue-green algae) in aquatic environments convert atmospheric N2 to ammonia Nitrification is... The goal of this large-scale comparative study was to relate inter-biome variability in stream metabolism and nutrient uptake to physical, chemical, and biological characteristics Stream metabolism (i.e., autotrophic primary production, and autotrophic and heterotrophic respiration) was measured using the upstream–downstream diurnal dissolved oxygen technique To measure ammonium uptake, 15 NH4 was injected... isotopic tracers of 15 N and 15 C have quantified the amount of marine-derived N and C delivered to streams and adjacent riparian habitats Because salmon are enriched with heavier isotopes of N and C, comparisons of primary producers, consumers, riparian vegetation, and wildlife in streams with and without spawning salmon have revealed the importance of these subsidies In addition to stimulation of primary... terrestrial and aquatic ecosystems In a comprehensive analysis of >800 aquatic and terrestrial systems, Cebrian and Largitue (2004) examined factors that controlled herbivory and decomposition rates Although NPP varied greatly within aquatic and terrestrial ecosystems, there was surprisingly little variation between these ecosystem types when analyzed across all studies Nutritional quality of primary producers... producers and bottom-up effects on higher trophic levels (Bilby et al 1996, Wipfli et al 1998), nutrients and organic material from salmon carcasses increase productivity and diversity of riparian vegetation, and provide up to 25% of the N to riparian plants and 30 90% of the N to the diet of terrestrial scavengers (Naiman et al 2002) Increased primary and secondary productivity associated with salmon carcasses... other materials are assimilated from soil or water by autotrophic organisms (plants and autotrophic bacteria), passed on to consumers, and released back to abiotic compartments The amount and availability of nutrients are among the most important factors that limit primary productivity In addition to limiting growth rates of primary producers and heterotrophic microbes, nutrient availability also influences... simple because the atmosphere plays a relatively small role Consequently, transport of P in ecosystems is primarily sedimentary and at a local scale The major source of P to ecosystems is from underlying rocks (Figure 30. 8), and loss from soils is usually balanced by releases of inorganic P from weathering Plants assimilate phosphorus as phosphate (PO3− ), and availability and rate of uptake are dependent... spiral reflects uptake and turnover and is dependent on a variety of biotic and abiotic factors including the rate of microbial mineralization, stream temperature, stream velocity, the shape of the stream channel, and the number of snags and other woody debris that reduce downstream transport Uptake length, defined as the distance that a molecule travels before sorption to particulate matter or uptake . tundra streams. The goal of this large-scale comparative study was to relate inter-biome variability in stream metabolism and nutrient uptake to physical, chem- ical, and biological characteristics N 2 , a form that is unavailable to plants. Nitrogen-fixing bacteria in soil (Rhizobium) and cyanobacteria (blue-green algae) in aquatic envir- onments convert atmospheric N 2 to ammonia. Nitrification. usually balanced by releases of inorganic P from weathering. Plants assimilate phos- phorus as phosphate (PO 3− 4 ), and availability and rate of uptake are dependent on pH. Herbivores Plant, animal