ECOTOXICOLOGY: A Comprehensive Treatment - Chapter 31 pptx

Functional measures such as community metabolism or nutrient transport can be measured inisolated soil microbial systems or in whole forests or watersheds.. com-Although many different p

Investigations of ecosystem processes may be conducted across a range of spatial and temporalscales Functional measures such as community metabolism or nutrient transport can be measured inisolated soil microbial systems or in whole forests or watersheds However, as we move up the hier-archy of biological organization from individuals→ populations → communities → ecosystems,

we generally increase the spatial and temporal scales of our investigations Because many mental studies of ecosystem processes are often limited in spatial and temporal scale, descriptiveapproaches can provide very compelling and ecologically realistic results As discussed inChapter 23for communities, the typical trade-off is that observational or correlative investigations only provide

experi-a cexperi-atexperi-alog of potentiexperi-al cexperi-ausexperi-al explexperi-anexperi-ations A more powerful cexperi-ase for cexperi-ausexperi-ation in descriptive ies can be established by the application of strong inference (Platt 1964), other formal inferentialmethods such as stressor identification (Suter et al 2002), or Bayesian inferential techniques.The initial definition of ecological integrity proposed by Karr (1991) included both structuraland functional measures, and most ecologists would agree that assessing effects of anthropogenicstressors on ecosystems requires adequate characterization of both patterns and processes The effic-acy of using functional measures to assess ecosystem responses to contaminants has received limitedattention As a consequence, development of functional criteria as indicators of ecological integrityhas lagged behind more traditional approaches based on community structure (Bunn and Davies

stud-2000, Gessner and Chauvet 2002, Hill et al 1997) Kersting (1994) provides an excellent review

of literature on the use of functional endpoints in freshwater field tests for hazard assessment ofchemicals Some assessments of ecological integrity measure patterns of community composition as

a surrogate for ecosystem processes (Bunn and Davies 2000); however, patterns and processes arenot necessarily related in some instances, especially in systems where disturbance is relatively weak.For example, Bunn and Davies (2000) measured stream metabolism at seven sites in southwesternAustralia and related ecosystem processes to community structure Although changes in gross primaryproduction (GPP) and respiration were related to water quality, there was no relationship betweenwater quality and macroinvertebrate community structure

665

The characterization of ecological integrity based exclusively on structural measures is sistent with how most ecologists view ecosystems (Gessner and Chauvet 2002) We believe thatrestricting our analyses to mainly structural measures has provided a somewhat incomplete picture

incon-of how ecosystems respond to and recover from anthropogenic disturbances Issues such as relativesensitivity, response variability, and functional redundancy have been considered when comparingthe usefulness of structural and functional measures (Howarth 1991, Schindler 1987, 1988) Despiteconcern that changes in some ecosystem processes occur only after compositional changes and aretherefore less useful, alterations in material cycling and energy flow are such fundamental properties

of ecosystems that they should be included in ecological assessments Leland and Carter (1985)argue that some functional processes in ecosystems are easier to quantify than relationships betweenabundance and environmental variables These functional measures also integrate general charac-teristics of diverse communities, thus facilitating comparisons among different ecosystems Giventhe recent interest in making comparisons across relatively broad geographic regions, functionalmeasures may be more useful than structural measures because they are not dependent on specifictaxa that are often restricted to a specific region Finally, because causal mechanisms that controlecosystem processes are generally well understood, restoration strategies may be more obvious whenbased on functional measures (Bunn and Davies 2000)

Although papers that report relative sensitivity of community metrics to contaminants are mon in the literature (Carlisle and Clements 1999, Kilgour et al 2004), surprisingly few studieshave compared responses across levels of biological organization (Adams et al 2002, Bendell-Young et al 2000, Cottingham and Carpenter 1998, Niemi et al 1993, Sheehan 1984, Sheehanand Knight 1985) There is also the perception that quantifying ecosystem responses is logisticallychallenging compared to structural measures (Crossey and La Point 1988), an idea that has notbeen rigorously examined in the literature Thus, we believe that it is premature to conclude thatecosystem processes are less sensitive or less reliable indicators of stress In fact, some studies havereported that changes in ecosystem processes may occur in the absence of alterations in communitystructure (Bunn and Davies 2000) Niemi et al (1993) reported that functional measures such asGPP were more sensitive indicators of recovery than structural measures Similarly, Clements (2004)observed that community respiration was generally more sensitive to heavy metals than commonstructural measures such as abundance and species richness Because alterations in community struc-ture are not necessarily related to ecosystem processes, we view these as complementary measuresfor assessing ecological integrity More important, simultaneous assessment of pattern and processcan provide insight into the mechanistic linkages between stressors and responses Studies by Wal-lace and colleagues (Wallace et al 1996) provide some of the best examples demonstrating howcontaminant-induced alterations in structural characteristics (e.g., elimination of macroinvertebrateshredders) directly influence ecosystem processes (e.g., litter decomposition and export) There isalso some evidence that functional measures may be more directly related to specific types of stressors(Gessner and Chauvet 2002)

com-Although many different processes could be used to assess ecosystem integrity, we will focus inthis section on three functional measures: ecosystem metabolism (respiration, primary and second-ary production), litter decomposition, and nutrient cycling As described inChapter 30, a significantamount of background information characterizing these processes is available, although the level ofdevelopment varies among ecosystem types For example, lake ecologists have historically relied

on functional measures, especially primary production, whereas lotic ecologists have tended torely on structural measures (Gessner and Chauvet 2002) These differences have resulted in diver-gent approaches used to assess effects of contaminants in aquatic ecosystems Similarly, studies ofbiogeochemical processes in terrestrial habitats, especially in agricultural systems, focus primarily onfactors that increase primary production, whereas aquatic ecologists have been more concerned withunderstanding factors that limit production as a way to control eutrophication (Grimm et al 2003).The methodological approaches used to assess effects of contaminants on ecosystem metabolism areconsequently different in aquatic and terrestrial ecosystems

31.2 DESCRIPTIVE APPROACHES IN AQUATIC

ECOSYSTEMS

31.2.1 ECOSYSTEMMETABOLISM ANDPRIMARYPRODUCTION

Energy flow and metabolism are fundamental properties of ecosystems that are also closely related

to the transport of contaminants Many of the same physical, chemical, and biological processesthat influence the flow of energy between biotic and abiotic compartments also regulate the fate ofchemicals Primary production in aquatic ecosystems is particularly sensitive to many anthropogenicstressors The effects of nutrient subsidies and input of organic materials on productivity havereceived considerable attention in streams, lakes, and marine ecosystems In their description ofsubsidy-stress gradients, Odum et al (1979) contrast the ecosystem-level effects of “utilizable”inputs such as nutrients and organic materials with toxic materials (Chapter 25) This analysiscontrasts the role of nutrients such as N and P as both regulators of ecosystem production as well asstressors when threshold levels are exceeded

Input of nutrients associated with agricultural, domestic, industrial, and atmospheric sources arewidely regarded as major stressors of aquatic ecosystems (National Research Council 1992) Wholeecosystem nutrient budgets calculated for several ecosystems reveal that inputs often exceed outputs,resulting in large amounts of nutrients being stored in a watershed (Bennett et al 1999, Jowarski et al

1992, Lowrance et al 1985) Bennett et al (1999) used a mass-balance approach to estimate P storagebased on inputs and outputs in the Lake Mendota (Wisconsin, USA) watershed They reported thatapproximately 50% of the P entering the watershed was retained and could be readily mobilized byclimatic, geologic, or hydrologic events These increased nutrient levels in aquatic ecosystems areoften associated with toxic algal blooms, increased plant growth, oxygen depletion, fish kills, andmajor shifts in community composition Land-based inputs of nutrients also increase eutrophicationand have negative effects on primary production of macrophytes in coastal areas Using data compiledfrom an extensive literature survey, Valiela and Cole (2002) reported a strong inverse relationshipbetween N loading and primary production of seagrass meadows in coastal marine areas The percent

of seagrass cover lost reached 100% as N loading approached 100 Kg N/ha/y These effects resultedfrom reduced light supply associated with increased phytoplankton production The damaging effects

of N enrichment were significantly reduced in areas protected by salt marshes and mangroves

As described inChapter 30, availability of N and P can directly regulate primary productionand biomass accrual in aquatic ecosystems (Biggs 2000) However, the direct effects of nutrients

on primary production are complex and may be mediated by other factors such as hydrologic acteristics and abundance of grazers Riseng et al (2004) used covariance structure analysis (CVA)

char-to examine effects of hydrologic regime and nutrients in 97 midwestern U.S streams Increasednutrients in streams with high hydrologic variability resulted in greater algal abundance becausegrazers were reduced In contrast, in more stable streams where grazers were abundant, algal pro-duction was limited and the net effect was an increase in herbivore production Because hydrologiccharacteristics of a watershed are dependent on watershed physiography and climate, these factorsmay ultimately control responses to nutrient additions (Riseng et al 2004)

Alterations in primary productivity and respiration have been measured in response to chemicalstressors other than nutrients in aquatic ecosystems Crossey and La Point (1988) could not detectdifferences in GPP between metal-impacted and reference sites, but when data were normalized toalgal biomass (as chlorophyll a), GPP was higher at the reference site Hill et al (1997) measuredvariance and sensitivity of several functional measures in the Eagle River, a Colorado (USA) streamimpacted by metals Results showed that measures of community metabolism (GPP, NPP, and res-piration) were lower at stations located downstream from heavy metal inputs (Figure 31.1) These

functional measures were correlated with mortality of Ceriodaphnia dubia, and inhibition

concen-trations (IC50 values) for respiration were comparable to LC50 values derived from these moretraditional toxicological approaches These results suggest that functional measures were about

Station ER-1r ER-3 ER-5 ER-12 ER-12a 0

2 4 6 8 10 12 14

FIGURE 31.1 Effects of Zn on community respiration measured at different stations in the Eagle River,

Colorado, United States (Data from Tables 2 and 3 in Hill et al (1997).)

as sensitive as acute toxicity for quantifying effects of heavy metals Clements (2004) comparedstructural (species richness and abundance of metal-sensitive mayflies) and functional (communityrespiration) responses of benthic macroinvertebrate communities to a mixture of Cd and Zn in streammicrocosms (Figure 31.2) Both structural and functional measures were significantly related to metalconcentration, but effects on community respiration were generally greater than effects on speciesrichness or abundance of metal-sensitive mayflies

Unlike studies conducted with diatoms and attached algae, research investigating effects ofcontaminants on emergent macrophytes has shown that primary production and photosynthesis ofthese groups are relatively insensitive Bendell-Young et al (2000) compared the response of severalstructural and functional endpoints (mutagenic responses, morphological deformities, mortality,community structure, and plant productivity) measured in wetlands receiving oil sands effluents

Photosynthetic rates of cattails (Typha latifolia L.), measured as CO2uptake, were actually greater inwetlands receiving processed water from oil sands, a response that contradicted expectations Theseresearchers concluded that structural changes in benthic communities and blood chemistry of fishwere more sensitive indicators of stress than functional measures Photosynthetic rate of salt marsh

plants (Spartina alterniflora) was measured at reference and contaminated sites in the southeastern

United States (Wall et al 2001) Although significant negative effects on benthic detritivores were

observed at a site heavily contaminated by mercury and PCBs, photosynthesis of Spartina was not

affected

31.2.2 SECONDARYPRODUCTION

In addition to direct effects on primary producers, contaminants and other stressors may alter theamount and rate of energy flow to higher trophic levels The utilization of available energy in anecosystem is thus an important measure of ecological integrity Perhaps the most common functionalresponse related to energetics measured in aquatic ecosystems is the abundance of different functionalfeeding groups (Rawer-Jost et al 2000, Wallace et al 1996) In part, the utility of functional feedinggroups as a metric in ecological assessments is based on the assumption that specialist feeders such asscrapers and shredders are more sensitive to contaminants than generalist feeders such as collector-gatherers and filterers (Barbour et al 1996) Although data on functional feeding groups are generallypresented as abundance or density per unit area, and therefore not strictly a functional measure,

16 18 20 22 24

1.5

200 250 300

FIGURE 31.2 Relationship between structural (total number of species; abundance of metal-sensitive

hepta-geniid mayflies) and functional (community respiration) endpoints and heavy metal (Cd and Zn) concentration

in stream microcosms Heavy metal concentration was expressed as the cumulative criterion unit (CCU), defined

as the ratio of the measured metal concentration to the hardness adjusted chronic criterion values for Cd and Zn.(Data from Clements (2004).)

the assumption is that composition of different feeding groups reflects important ecosystem processes.For example, abundance of grazers, organisms that feed directly on periphyton and algae, is related

to primary productivity in streams Similarly, abundance of shredders, organisms that process leaflitter, regulates downstream transport of coarse particulate organic material (Wallace et al 1982).Secondary production, which we have defined as the production of heterotrophic organisms,has been used to document effects of several stressors in aquatic ecosystems, including hydrologicmodification (Raddum and Fjellheim 1993), pesticides (Whiles and Wallace 1995), urbanization(Shieh et al 2002), and heavy metals (Carlisle and Clements 2003) Because secondary productionintegrates individual growth rates and population dynamics, it captures in a single measure severalimportant aspects of energy flow through ecosystems Although integration of these measures acrosslevels of biological organization is a laudable goal in ecosystem ecotoxicology, measures of second-ary production are rarely included in biological assessments Even measuring secondary production

of individual species is highly labor intensive because it requires sampling populations with sufficientfrequency to quantify individual growth rates, mortality, immigration, and emigration The logistical

challenges associated with measuring secondary production will likely deter some researchers fromusing this endpoint in ecological assessments Indeed, France (1996) argues that because secondaryproduction is dependent on abundance, little additional information is gained by including thesemore labor-intensive approaches However, because secondary production is a composite of indi-vidual mortality, growth rate, population abundance, and biomass, it represents a potential holisticindicator of ecosystem bioenergetics that is not reflected in these individual measures.

Because most studies of secondary production are based on detailed analysis of individual species

or groups of related species, our understanding of energetics from the perspective of entire ecosystems

is somewhat limited (Shieh et al 2002) To be useful as an ecosystem-level indicator, a measure ofsecondary production should include a significant number of dominant species in an ecosystem andshould also be combined with data on trophic interactions Sheehan and Knight (1985) comparedpatterns of community composition and secondary production at several sites along a gradient ofmetal contamination Chironomids dominated benthic communities at metal-polluted sites, a findingcommonly reported in the literature However, despite large shifts in community composition amongsites, relatively little difference in secondary production of chironomids was observed

Another challenge associated with using secondary production as an indicator of ecosystemintegrity is that production may either increase or decrease, depending on the nature of the stressor.The theoretical basis for the difference in responses between subsidizing and toxic stressors wasfirst described by Odum et al (1979), but there have been relatively few empirical studies docu-menting this pattern Shieh et al (2002) estimated energy flow based on secondary production andtrophic interactions at polluted and reference sites in a Colorado stream receiving urban discharges.Secondary production, which was primarily supported by detritus, increased by more than two times

at the most impacted site due to the input of nutrients and organic materials (Figure 31.3a) In contrast

to these patterns, Carlisle and Clements (2003) reported a decline in secondary production along agradient of heavy metal contamination (Figure 31.3b) Differences in production among these streamswere primarily a result of lower population abundance of metal-sensitive species, especially grazingmayflies The large reduction in secondary production of herbivores likely had important cascadingeffects on trophic interactions and energy flow through this ecosystem Results of these studies areconsistent with predictions of the subsidy-stress hypothesis (Odum et al 1979) and illustrate thecontrasting effects of subsidizing materials and toxic chemicals on ecosystem processes

Reduced secondary production of zooplankton in lake ecosystems may result from increasedmortality, lower growth rates, and/or shifts in size composition of dominant species Hanazato(2001) reviewed effects of pesticides on zooplankton across levels of organization, from individuals

to ecosystem-level responses A general trend observed in lakes receiving pesticides was a reduction

in mean body size of zooplankton as a result of differential sensitivity among species Hanazato(2001) speculated that one potential ecosystem-level consequence of altered size distributions was areduction in the amount of energy transferred from primary producers to higher trophic levels Thisreduced transfer efficiency was associated with a variety of anthropogenic stressors, including heavymetals, acidification, and nutrient enrichment

31.2.3 DECOMPOSITION

Litter decomposition is a fundamental ecological process that has been studied extensively, cially in lotic ecosystems (Chapter 30) It integrates responses of a variety of biota, from bacteriaand fungi to shredding macroinvertebrates (Niyogi et al 2001, 2003) There is a large databaseavailable reporting decomposition rates of leaves and quantifying biotic and abiotic factors thatinfluence litter decay under a variety of environmental conditions Gessner and Chauvet (2002)provided an excellent and comprehensive review of numerous studies that used litter breakdown toquantify effects of physical and chemical stressors in streams They make a compelling argumentfor the use of decomposition as an ecosystem indicator and provide specific criteria for assessing

espe-ecological integrity Breakdown rate coefficients (k ), measured by regressing remaining mass of

Ref erence

0.0 0.5 1.0

1.5

2 ° Production Nitrogen

Station

Ref erence Low Zn

Inter med Zn High Zn

0 50 100 150 200 250 300

2 ° Production Zinc

(a)

(b)

FIGURE 31.3 Contrasting effects of nutrients (a) and heavy metals (b) on invertebrate secondary production.

(Effects of nutrients from Tables 1 and 4 in Shieh et al (2002) Effects of Zn from Tables 1 and 2 in Carlisleand Clements (2003).)

litter against time, are generally reduced in disturbed ecosystems Effects of contaminants on litterdecay may result either from alterations in microbial processes or reduced abundance of macroin-vertebrate shredders (Figure 31.4) It is also necessary to distinguish effects of contaminants onbiological processes, such as the elimination of shredders or reduced microbial activity, from effectsdue to physical characteristics of the system Methodological approaches that exclude or includedifferent groups of organisms can be used to separate the relative importance of these processes, thusallowing ecologists to isolate underlying mechanisms Because of the diversity of approaches used

to quantify litter decay and the large number of environmental factors that influence k, development

of standardized techniques for assessing effects of contaminants is essential

The most comprehensive applications of leaf litter methodologies to investigate effects of taminants have been conducted in metal-polluted and acidified streams Schulthesis and Hendricks(1999) and Schulthesis et al (1999) measured macroinvertebrate community composition and leafdecomposition at sites upstream and downstream from an abandoned pyrite mine in southwest-ern Virginia (USA) Shredder abundance was greater and decomposition rates were 1.4–2.7 timesfaster at the reference site compared Cu-polluted sites Remediation activities initiated during the

con-Physicochemical characteristics

Microbial processes

Contaminants and other stressors

Shredder biomass and production

Leaf decomposition

FIGURE 31.4 Conceptual model showing the effects of contaminants and physicochemical characteristics

on microbial processes, shredder biomass, and leaf litter decomposition

TABLE 31.1 The Influence of Aqueous Zn Concentration, Metal Oxide Deposition, and Nutrient Concentrations on Structural and Functional Endpoints Measured at 27 Stream Sites in the Rocky Mountains, Colorado (USA)

Variable Independent Variables R2 P -Value

Leaf breakdown rate (k ) [Zn], oxide deposition 72 0001 Shredder biomass [Zn], oxide deposition 64 0001 Microbial respiration Oxide deposition, nutrients 54 0001 Data from Table 2 in Niyogi et al (2001).

study period allowed these researchers to compare recovery of structural and functional responses.Although community composition and abundance of shredders increased following improvements

in water quality, the rate of leaf processing did not increase as expected, suggesting some ual effects of Cu on microbial processes In contrast to these studies, Nelson (2000) reported little

resid-effects of Zn contamination on decomposition rates of aspen (Populus tremuloides) in a Colorado

Rocky Mountain stream, despite significant changes in community composition These ers speculated that the lack of a response in their study resulted from the relative insensitivity ofmicrobes, especially fungi, to the moderate levels of Zn contamination Alternatively, because micro-bial activity is limited by cold temperatures in Rocky Mountain streams, leaf processing may be moredependent on invertebrate shredders (Niyogi et al 2001), which were unaffected by Zn in this study(Nelson 2000)

research-Comparative studies in streams across a gradient of heavy metal pollution provide the best tunity to quantify effects of stressors relative to other factors that regulate leaf decomposition Niyogi

oppor-et al (2001) measured decomposition rates at 27 stream sites (8 reference and 19 moppor-etal-polluted) inthe Rocky Mountains of Colorado, USA In addition to its broad spatial scale, this study is uniquebecause researchers quantified the relative influence of several stressors associated with miningpollution, including acidification, elevated Zn concentration, and metal oxide deposition Litter

decay coefficients (k) and shredder biomass decreased with increasing aqueous Zn concentration

and deposition of metal oxides (Table 31.1) In contrast, microbial respiration was more influenced

by metal oxide deposition and nutrients Because decay coefficients were more closely related to

shredder biomass than microbial respiration, results of this study suggest that macroinvertebrateswere more important than microbial processes in regulating leaf litter processing in streams (Niyogi

et al 2001) Carlisle and Clements (2005) related leaf decomposition to shredder secondary tion and microbial respiration in reference and metal-polluted streams in Colorado, USA Becauseleaf decomposition was measured as a function of shredder secondary production instead of shredderbiomass, this study provided a unique opportunity to quantify effects of stressors on energy flowthrough an allochthonous food web Results showed that shredders disproportionately contributed

produc-to leaf litter decay, and that species-specific differences in sensitivity produc-to metals among shreddershelped explain differences among streams

Stream acidification by atmospheric deposition or other sources can have direct effects on litterdecomposition (Dangles et al 2004, Griffith and Perry 1993, Tuchman 1993, Webster and Benfield1986) Griffith and Perry (1993) attributed slower processing rate of litter in acidic streams tolower biomass of shredders In contrast, differences in community composition of shredders wereprimarily responsible for differences in processing rates between neutral and more alkaline streams.Tuchman (1993) reported that declines in invertebrate shredders in acidified lakes were correlatedwith decreased litter breakdown rates As with studies of metal-polluted streams, the most convin-cing evidence demonstrating a relationship between acidification and leaf decomposition has beenobtained from spatially extensive surveys Dangles et al (2004) measured microbial respiration,litter decay, and shredder abundance and composition in 25 streams along a gradient of acidification

in the Vosges Mountains, France Breakdown rates varied 20-fold between acidified and neutralstreams, with alkalinity and aluminum concentration explaining 88% of the variation Reduced leafdecomposition in acidified streams was related to lower abundance and biomass of the amphipod,

Gammarus fossarum, a functionally important and acid-sensitive species The greater breakdown

rate observed in coarse mesh bags (5.0 mm), which allowed shredder colonization, compared to finemesh bags (0.3 mm), which excluded shredders, supported the hypothesis that microbial processeswere relatively unimportant in this investigation (Dangles and Guerold 2001)

Although pesticides and other organic contaminants are likely to have significant effects onlitter decomposition by altering microbial processes and shredder communities, these stressors havereceived considerably less attention than heavy metals and acidification (Gessner and Chauvet 2002).Delorenzo et al (2001) provided a comprehensive review of the effects of pesticides on microbialprocesses related to decomposition The best examples showing the effects of organic chemicals

on shredder biomass and subsequent alterations in leaf processing involve long-term experimentalstudies (Whiles and Wallace 1992, 1995) and stream mesocosm experiments (Stout and Cooper1983), which will be described in Chapter 32 Swift et al (1988) examined effects of dimilin,

an insect growth regulator used for control of gypsy moths, on litter decomposition Althoughlaboratory bioassays with shredders showed significant mortality when shredders were fed dimilin-treated leaves, decomposition rates of treated leaves in the field were actually greater than controls.The faster processing rate of treated leaves was attributed to the potential carbon source that dimilinprovided for bacteria

Most investigations of leaf litter decomposition report that decay coefficients are reduced incontaminated streams However, stressors that subsidize an ecosystem (e.g., nutrients or organicmaterials) may have the opposite effect Niyogi et al (2003) measured breakdown of tussock

grass (Chionocloa rigida) in 12 New Zealand streams along a gradient of agricultural

develop-ment Nutrients (N and P), the predominant stressors in this system, increased along this gradientand stimulated microbial respiration, invertebrate abundance, and the rate of litter decomposition Incontrast, the macroinvertebrate community index (MCI), a biotic index of organic pollution, showedincreased stress along this same gradient Similar findings were reported by Pascoal et al (2001)for a stream in Portugal receiving elevated nutrients Despite reductions in abundance of shredders

at polluted sites, leaf breakdown rates were greater These results serve to illustrate the importance

of understanding mechanistic linkages among stressors, microbial processes, and macroinvertebratecommunity composition when using leaf decomposition to assess ecological integrity

In addition to the direct effects of contaminants on the rate of litter decay, the concentrations

of toxic chemicals may increase in decomposing of plant material, thus providing a direct link todetritus-based food chains Windham et al (2004) measured reduced decomposition of marsh grass

(Spartina alterniflora) in a metal-contaminated marsh as compared to a reference site The 10–

100 times increase in metal concentrations in decomposing litter was attributed to adsorption andmicrobial processes

31.2.4NUTRIENTCYCLING

The majority of studies investigating effects of contaminants on nutrient cycling in aquatic tems have focused on nitrification, denitrification, and other processes associated with N flux (Kempand Dodds 2002, Kemp et al 1990, Royer et al 2004) Most of this research has been conductedwithin the context of understanding effects of nutrient enrichment, especially N and P, on freshwaterand estuarine ecosystems Eutrophication, caused by the release of excess nutrients, is regarded asthe major threat to freshwater and coastal ecosystems in the United States (U.S EPA 1990) Greaterthan 50% of the impaired lake area and river reaches in the United States result from excess nutri-ents Most of this impairment is associated with nonpoint source inputs from agricultural and urbanactivities (Carpenter et al 1998), although atmospheric deposition is considered an important source

ecosys-of N to some areas In particular, N inputs from the upper Midwest to the Gulf ecosys-of Mexico haveincreased dramatically in the past several decades, and excess nutrients have had severe effects onwater quality and community composition A significant portion of the N from nonpoint sources

is retained in aquatic ecosystems by biological processes such as microbial uptake as well as eral exchange with the hyporheic zone However, despite N retention in some aquatic ecosystems,

lat-a llat-arge lat-amount of excess N is trlat-ansported downstrelat-am Royer et lat-al (2004) melat-asured denitrificlat-ation

in headwater stream sediments located in agricultural areas Because denitrification rates were low

in these streams, there was relatively little influence on instream concentrations and therefore most

of the NO3–N was transported downstream These researchers concluded that previous estimates ofdenitrification rates may have overestimated N loss to the sediments

Because rates of nitrification and denitrification in aquatic ecosystems are dependent on trations of ammonium (NH4) and nitrate (NO3), these processes are likely to increase in areas receiv-

concen-ing anthropogenic inputs Kemp and Dodds (2002) measured effects of anthropogenic N on rates ofnitrification and denitrification in pristine and agriculturally influenced watersheds Whole streamnitrification and denitrification rates were greater at agriculturally influenced sites, most likely due

to greater input of N Despite greater denitrification, the large amount of anthropogenic N exceededthe natural retentive ability of the stream and a significant amount was transported downstream

As described inChapter 30, retention of nutrients and organic materials is dependent on a number

of physical, chemical, and biological characteristics Headwater streams often represent the largestportion of the linear dimension of a watershed and are closely connected to surrounding riparian andterrestrial ecosystems These systems are generally considered to be highly retentive of nutrients(Peterson et al 2001) A similar situation exists in coastal marine ecosystems Meta-analysis of datacollected in coastal areas demonstrated that denitrification by fringing wetlands (e.g., salt marshes,mangroves) serves to intercept excess nutrients and protect seagrass meadows from anthropogenic

N (Valiela and Coe 2002) Physical disturbances such as removal of vegetation and reduced habitatcomplexity may decouple denitrification and nitrification processes in aquatic ecosystems, therebyexacerbating the effects of nutrient enrichment (Kemp and Dodds 2002)

31.3 TERRESTRIAL ECOSYSTEMS

31.3.1 RESPIRATION ANDSOILMICROBIALPROCESSES

Functional measures of ecosystem processes have also been used to characterize the impacts ofcontaminants in terrestrial ecosystems In particular, effects of contaminants on microbial and soil

ecosystem processes, especially soil respiration, have been examined in considerable detail (Aceves

et al 1999, Dai et al 2004, Megharaj et al 1998, 2000, Zimakowska-Gnoinska et al 2000) There

is also evidence suggesting that ecosystem processes in soils are significantly more sensitive to taminants than the plant communities they support (see review byGilleret al 1998) The emphasis

con-on soil processes in terrestrial ecosystems is at least partially a result of taxcon-onomic difficulties ciated with characterizing microbial communities Consequently, most soil ecologists tend to focus

asso-on functiasso-onal measures instead of structural characteristics such as biodiversity and species ition Dai et al (2004) reported a strong inverse relationship between heavy metal concentrations

compos-in soils and respiration rates An compos-increase compos-in the accumulation of organic C, total N, and the C:Nbiomass ratio was attributed either to reduced microbial activity in soils or to changes in microbialcommunity composition Similar results were reported by Edvantoro et al (2003) where microbialrespiration was greatly reduced in soils contaminated by DDT and arsenic Interestingly, althoughmicrobial biomass was significantly lower in polluted soils, bacterial populations measured usingplate counts showed little difference between reference and contaminated sites

Shifts in community structure, such as an increase in biomass of fungal populations and a decrease

in bacterial populations, can also result in changes in soil ecosystem function Megharaj et al (1998)reported that changes in community composition of soil microalgae were associated with large(>90%) reductions in microbial activity (dehydrogenase, nitrate reductase) at field sites contaminated

by pentachlorophenol (PCP) Megharaj et al (2000) measured changes in soil microbial function atsites contaminated by DDT and its metabolites Because sensitive bacteria were replaced by DDT-tolerant microorganisms, total microbial biomass was found to be less sensitive than dehydrogenaseactivity, a measure of total microbial activity that correlates with respiration (Brookes 1995) Changes

in respiration can result in carbon accumulation in ecosystems and therefore specific respiration rate,measured as the ratio of CO2production to biomass C, may be a better indicator of stress thaneither measure alone (Brookes 1995) For example, Megharaj et al (2000) reported that specificactivity, defined as the ratio of dehydrogenase activity to microbial biomass C, decreased with DDTconcentration (Figure 31.5) Aceves et al (1999) also determined that biomass-specific respirationrate was a better indicator of heavy metal contamination in soils than either microbial biomass orrespiration

Level of soil contamination Control Low Medium High

Specific activity Total [DDT]

FIGURE 31.5 Relationship between DDT concentration in soil and specific microbial activity Specific

activ-ity was calculated as the ratio of dehydrogenase activactiv-ity (mg 2,3,5, triphenyltetrazoluim formazan/kg) to soilmicrobial biomass (mg/kg) (Data from Tables 2 and 3 in Megharaj et al (2000).)