Status of Stream Biotic Communities 

in Relation to Metal Exposure

By John M. Besser and William G. Brumbaugh

Chapter E18 of

Integrated Investigations of Environmental Effects of Historical 

Mining in the Animas River Watershed, San Juan County, Colorado

Edited by Stanley E. Church, Paul von Guerard, and Susan E. Finger

Professional Paper 1651

U.S. Department of the Interior

U.S. Geological Survey
Contents

Abstract .......................................................................................................................................................825

Introduction.................................................................................................................................................825

Purpose and Scope ..........................................................................................................................826

Acknowledgments ............................................................................................................................826

Status of Stream Biotic Communities .....................................................................................................826

Sources of Data.................................................................................................................................826

Trout Populations ..............................................................................................................................826

Benthic Invertebrate Communities ................................................................................................828

Metal Bioaccumulation in Stream Food Webs .....................................................................................829

Methods..............................................................................................................................................829

Periphyton ..........................................................................................................................................830

Benthic Invertebrates ......................................................................................................................830

Brook Trout .........................................................................................................................................831

Food-Web Transfer of Metals .........................................................................................................832

Association of Metal Exposure with Biological Effects ......................................................................833

References Cited........................................................................................................................................834

Figures

 1. Map showing location of sampling sites for biological community surveys 

and food-web metals study .....................................................................................................827

 2–5. Graphs showing:

 2. Characteristics of benthic invertebrate communities of Animas River 

 watershed study area .......................................................................................................829

 3. Algal biomass (represented by chlorophyll-a) and iron in periphyton .....................831

 4. Concentrations of metals in caddis flies, Arctopsyche sp., from the Animas 

 River and Cascade Creek .................................................................................................832

 5. Concentrations of metals in livers of brook trout, Salvelinus fontinalis, from 

 the Animas River and reference sites ............................................................................833

Tables

 1. Sites for aquatic community sampling and food-web metals study ................................828

 2. Concentrations of metals in periphyton from the Animas River and tributaries ............830
Abstract

The fish community of the Animas River watershed 

study area is dominated by the introduced brook trout 

(Salvelinus fontinalis), which has replaced native trout 

because it can better tolerate acidity and metals contamination 

and because it is well adapted to small stream habitats. The 

distribution of brook trout in the watershed is discontinuous. 

Fish are absent from the Cement Creek and upper Mineral 

Creek drainages, which are heavily affected by acidic drainage from both natural and mining-related sources. The upper 

Animas River has reaches without fish near the headwaters, 

due to acidic conditions, and near Silverton, due to elevated 

concentrations of dissolved metals and precipitated iron and 

aluminum oxyhydroxides downstream of the mixing zone with 

Cement Creek. Self-sustaining trout populations occur in other 

reaches of the Animas River upstream of Silverton, and trout 

populations recover gradually in the Animas River canyon 

downstream of Silverton. The status of benthic invertebrate 

communities generally parallels that of trout populations: 

reductions in abundance and number of invertebrate taxa, and 

shifts from metal-intolerant to metal-tolerant taxa are evident 

in stream reaches affected by acidity and metals.

Metal concentrations in components of stream food 

webs—periphyton, benthic invertebrates, and brook trout—

reflect differences in metal bioavailability and indicate differing risks of chronic metal toxicity to stream biota. Concentrations of the toxic metals cadmium, copper, lead, and zinc 

in periphyton, benthic invertebrates, and liver tissue of trout 

from one or more sites in the Animas River in the study area 

were significantly greater than those in biota from reference 

sites. Periphyton from sites in acidic streams and in mixing 

zones contained high concentrations of aluminum and iron 

and reduced algal biomass. Metal concentrations in benthic 

invertebrates reflected differences in feeding habits and body 

size as well as gradients of metal bioavailability. Concentrations of copper and cadmium remained stable or increased 

in biota across several trophic levels, suggesting that these 

metals were more efficiently transferred via dietary exposure. 

Copper concentrations in invertebrate diets and liver tissue 

of brook trout from the Animas River were closely associated 

with differences in resident fish and invertebrate populations 

and approached levels associated with adverse effects on trout 

populations in field studies and with toxic effects in laboratory 

studies. These results suggest that chronic copper toxicity is 

an important factor limiting the distribution and abundance of 

brook trout in the study area.

Introduction

The water quality, habitats, and biota of streams in 

the Animas River watershed study area are affected by 

metal contamination associated with acidic drainage from 

inactive mines, deposits of mine and mill wastes, and naturally 

acidic soil and altered rock. Biota of streams receiving acidic 

drainage can be adversely affected by exposure to metals via 

multiple exposure routes. These adverse effects can result 

from short-term exposure to stream water (Henry and others, 

1999), suspended colloids (Smith and Sykora, 1976), and bed 

sediment (Kemble and others, 1994) containing toxic levels 

of acid or metals. Toxicity tests conducted on-site and in the 

laboratory have demonstrated that stream water and, to a lesser 

extent, fine streambed sediment from sites with impaired 

biotic communities in the Animas River watershed are directly 

toxic to fish and benthic invertebrates (Nimmo and others, 

1998; Besser, Allert, and others, 2001; Besser and Leib, this 

volume, Chapter E19). Metal-contaminated diets can be a significant source of metal bioaccumulation and chronic toxicity 

to trout in stream habitats where aqueous exposure alone does 

not cause toxicity (Miller and others, 1993; Woodward and 

others, 1994, 1995; Farag and others, 1999).

Metal bioavailability to higher order consumers such 

as trout can be substantially modified by the processing of 

metals in stream food webs. Periphyton, the community of 

attached algae, bacteria, and fungi that develops on stream 

substrata, can accumulate high concentrations of metals and 

may be an important source of metal exposure to benthic 

Chapter E18

Status of Stream Biotic Communities in Relation to 

Metal Exposure

By John M. Besser and William G. Brumbaugh
macroinvertebrates in streams affected by mining (Kiffney 

and Clements, 1993; Farag and others, 1998; Beltman and others, 1999). Differential accumulation of metals among different 

invertebrate taxa and differences in taxonomic composition of 

invertebrate communities among locations can lead to substantial variation in metal concentrations in the diets available to 

stream-dwelling trout (Moore and others, 1991; Clements and 

Kiffney, 1994; Farag and others, 1998).

Purpose and Scope

This chapter summarizes available information from 

recent surveys of stream biological communities of the 

Animas River watershed study area and from a study of metal 

bioavailability in stream ecosystems, based on metal concentrations in tissues of biota. Data from these sources are used 

to identify associations between metal exposure and biological effects and to identify factors that may limit recovery of 

stream communities of the Animas River watershed.

Acknowledgments

Tom May, Mike Walther, and Ray Weidmeyer conducted 

metal analyses. Win Wright, Ken Leib, and Paul von Guerard 

provided water chemistry data and logistical assistance in 

the field. Briant Kimball and Stan Church provided advice, 

assistance, and useful data. Barry Poulton provided taxonomic verifications of invertebrate samples. Brad Mueller, 

Chester Anderson (Animas River Stakeholders Group), and 

Bob Brantlinger (USDA Forest Service) assisted with field 

collections. Mike Japhet (Colorado Division of Wildlife), 

Robert Gallegos (Colorado Dept. of Public Health and 

Environment), and David Gerhardt (USDA Forest Service) 

provided fish and invertebrate community data. Durango & 

Silverton Narrow Gauge Railroad provided transportation to 

sites in the Animas River canyon.

Status of Stream Biotic Communities

Sources of Data

The fish and benthic invertebrate communities of 

the Animas River watershed have been surveyed repeatedly since the 1970s, with recent fish sampling conducted 

primarily by the State of Colorado, Division of Wildlife 

(Robert Gallegos, written commun., 1996), and sampling of 

benthic invertebrate communities conducted by the State of 

Colorado, Department of Public Health and Environment, 

and the Animas River Stakeholders group (Peter Butler, 

Robert Owen, and William Simon, Unpublished report to 

Colorado Water Quality Control Commission, Animas River 

Stakeholders Group, 2001). Sample sites reported in this chapter are shown in figure 1 and described in table 1.

Trout Populations

The fish community of the Animas River watershed 

study area originally consisted of native Colorado River cutthroat trout (Oncorhynchus clarki pleuriticus) and possibly 

mottled sculpins (Cottus bairdi). Streams and tributaries in 

drainages with extensive areas of alteration (Bove and others, this volume, Chapter E3) and (or) extensive past mining (Jones, this volume, Chapter C), such as Cement Creek, 

Mineral Creek (except the South Fork) and the headwaters 

of the upper Animas River, probably never supported trout 

populations. The fish community is now dominated by the 

introduced species, brook trout (Salvelinus fontinalis), with 

cutthroat trout occurring only in high-altitude tributaries with 

good water quality, such as Cunningham Creek and Minnie 

and Maggie Gulches (Wright, Simon, and others, this volume, Chapter E10). Two other non-native salmonids, rainbow 

trout (Oncorhynchus mykiss) and brown trout (Salmo trutta), 

have been stocked in the study area in the past. Although 

healthy populations of rainbow trout and brown trout occur 

in downstream reaches of the Animas River canyon, and a few 

rainbow trout may still occur in the study area, neither species 

has maintained self-sustaining populations in the study area. 

The current distribution of brook trout in the study area is discontinuous (table 1), with reaches supporting fish separated by 

reaches that are fishless due to the influence of acidic drainage 

and metal contamination (Unpub. report to Colorado Water 

Quality Control Commission, ARSG, 2001).

The replacement of cutthroat trout by brook trout 

reflects some combination of the ability of brook trout to compete for habitat in small streams, and the greater tolerance of 

brook trout for acidity and metal contamination. The principal 

current populations of brook trout in the study area occur in 

South Fork Mineral Creek and in two segments of the Animas 

River: from Minnie Gulch downstream to near Silverton, 

and from approximately Molas Creek downstream (fig. 1). 

Greatest densities of brook trout occur in the Animas River 

downstream from Minnie and Maggie Gulches and in South 

Fork Mineral Creek (table 1). Current brook trout populations 

in the Animas River upstream of Silverton are substantially 

greater than those reported in the 1970s (N.F. Smith, unpub. 

report, Colorado Division of Wildlife, 1976), and populations 

in the Animas River canyon, downstream from Silverton, 

have increased since 1992 (table 1). These changes suggest that anthropogenic degradation of water quality and 

(or) habitat changes have been partially reversed by reduced 

mining and milling activity and by reclamation activities in 

the watershed.

826 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Stream Biotic Communities and Metal Exposure 827

Animas R vi er

Cunn ni gham

C reek

Elk Park

M

ni eral Creek

Animas

River

Cement Cree

Chattanooga k

Eureka

Silverton

Animas

Forks

Howardsville

Animas River

watershed 

study area

C O L O R A D O

Durango

0

0 1

1 2 MILES

2 KILOMETERS

107°50'30" 107°45' 107°37'30" 107°30'

37°

52'

30"

37°

45'

37°

58'

ElkCreek

Fishless site

Recovery site

Reference site

SMC CEM

CAS

AR6

AR7

UMC

LMC

AR3

AR2

CUN

AR2a

AR1

AR4

AR5

EXPLANATION

South F ork Mineral Creek

Figure 1. Location of sampling sites for aquatic biological community surveys and food-web metals study. Not shown on map: AR6, 

Animas River above Needle Creek (lat 37°37.845' N., long 107°41.702' W.); AR7, Animas River above Cascade Creek (lat 37°35.909' N., 

long 107°46.331' W.); and CAS, Cascade Creek (lat 37°39.573' N., long 107°48.714' W.).
For the purposes of this chapter, sites were classified into 

three categories based on their populations of brook trout and 

degree of influence of upstream mining or acid rock drainage: fishless sites (no fish present; high levels of acidity and 

metals); recovery sites (fish present; downstream of fishless 

sites); and reference sites (fish present; relatively little influence 

of upstream acidity or metals) (table 1; fig. 1). Fishless sites 

included acidic headwater streams (Cement Creek, CEM; and 

upper Mineral Creek, UMC) and neutral-pH sites downstream 

of mixing zones of acidic tributaries (lower Mineral Creek, 

LMC; Animas River upstream of Eureka, AR1; and Animas 

River downstream of Silverton, AR4). Recovery sites on the 

Animas River (Howardsville, AR2; upstream of Silverton, 

AR3; Elk Park, AR5; and Needleton, AR6) had circumneutral 

pH and supported varying densities of brook trout and benthic 

invertebrates. Reference sites (South Fork Mineral Creek, SMC; 

Cunningham Creek, CUN; and Cascade Creek, CAS) supported 

reproducing populations of brook trout and relatively diverse 

and abundant benthic invertebrate communities. Cascade Creek 

flows into the Animas River in northern La Plata County, south 

of the Animas River watershed study area.

Benthic Invertebrate Communities

The status of benthic invertebrate communities provides 

additional resolution of differences in water quality and habitat 

conditions throughout the Animas River watershed study 

area. A study of benthic invertebrate communities conducted 

over three sampling periods in 1996–98 (Unpub. report to 

Colorado Water Quality Control Commission, ARSG, 2001) 

demonstrated both dramatic differences in the status of benthic 

communities of different portions of the watershed and more 

subtle longitudinal shifts in communities resulting from 

gradual changes in water quality and stream habitats (fig. 2). 

Average taxonomic richness in these samples was relatively 

low, even at reference sites, reflecting both the expectation 

for low taxonomic richness in high-altitude streams and the 

small area sampled by individual Surber samplers (0.093 m2

). 

Benthic communities at sites with persistent acidic conditions 

and high metal concentrations averaged fewer than five taxa 

per sample and fewer than 100 organisms per square meter, 

whereas communities of reference sites consistently had 15 or 

more taxa and more than 1,000 organisms per square meter. 

Communities also differed qualitatively between sites: fishless sites were dominated by tolerant taxa such as fly larvae 

(Diptera; for example, Chironomid midges); whereas recovery 

and reference sites had greater representation of more metalsensitive taxa such as mayflies (Ephemeroptera), stoneflies 

(Plecoptera), and caddis flies (Trichoptera).

Longitudinal changes in benthic invertebrate communities of the Animas River were generally consistent with 

the patterns in the distribution of brook trout. Communities 

exhibited moderately high diversity and abundance in the 

reach between Eureka Gulch and Arrastra Creek, but they 

showed evidence of increasing adverse effects in the reach that 

includes the confluences of Cement and Mineral Creeks near 

Silverton, followed by a gradual recovery in the downstream 

Table 1. Sites for aquatic community sampling and food-web metals study.

[Brook trout population data from Colorado Department of Public Health and Environment (1992; Rob Gallegos, Denver, Colo., written commun., 1992) and 

Colorado Division of Wildlife (Mike Japhet, Durango, Colo., written commun., 1998). No fish sampling occurred at sites indicated by --]

Site ID

(fig. 1) Site Elevation(m)

Site category

(see text)

Brook trout

(number/300 m)

1992 1997–98

Animas River

AR1 Animas at Eureka 3,006 Fishless -- --

AR2 Animas at Howardsville 2,940 Recovery 110 270

AR2a Animas below Cunningham Creek 2,910 Recovery 24 --

AR3 Animas above Silverton 2,854 Recovery 57 2a

AR4 Animas below Silverton 2,805 Fishless 0 0

AR5 Animas at Elk Park 2,707 Recovery 13 23

AR6 Animas at Needleton 2,500 Recovery -- --

AR7 Animas above Cascade Creek 2,353 Recovery 36 102

Tributaries

CEM Lower Cement Creek 2,835 Fishless 0 --

UMC Upper Mineral Creek 2,903 Fishless 0 --

LMC Lower Mineral Creek 2,817 Fishless 0 --

CUN Cunningham Creek 3,008 Reference -- --

SMC South Fork Mineral Creek 2,975 Reference 66 --

CAS Cascade Creek 2,664 Reference -- --

a

1997 fish sampling upstream of Arrastra Creek.

828 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Stream Biotic Communities and Metal Exposure 829

reach between Molas and Cascade Creeks. Comparison of 

these data to the 1992 invertebrate survey conducted by the 

Colorado Department of Public Health and Environment 

shows relatively little change in benthic communities at most 

sites during this period, but it suggests that some recovery 

of benthic invertebrate communities is occurring in certain 

reaches of the Animas River, both upstream and downstream 

of Silverton.

Metal Bioaccumulation in 

Stream Food Webs

Methods

Samples of stream biota of the Animas River watershed 

study area were collected for analysis of metals during fall 

low-flow periods in 1997 and 1998 (Besser, Brumbaugh, and 

others, 2001). Samples of periphyton, benthic invertebrates, 

and fish tissue were collected at some or all of the study sites. 

Periphyton samples were collected at all 12 study sites and 

analyzed for metals, chlorophyll-a (a measure of algal biomass), and total organic carbon (TOC). Benthic invertebrates 

were collected at eight sites, and dominant taxa at each site 

were sorted into separate samples for metal analysis. Samples 

of the mayfly, Rhithrogena sp. (Heptageniidae), and the caddis 

fly, Arctopsyche sp. (Hydropsychidae), were collected from six 

sites. Samples of the stonefly, Megarcys sp. (Perlodidae), were 

collected from five sites, and samples of a second stonefly, 

Zapada sp. (Nemouridae), were collected from one site. Adult 

brook trout were collected with a backpack electrofishing unit 

at six sites (AR2, AR3, AR6, CUN, SMC, and CAS). Livers 

were dissected for metal analysis as an indication of chronic 

metal exposure (Crawford and Luoma, 1993). Biological 

samples were analyzed by inductively coupled plasma–mass 

spectroscopy after microwave digestion with concentrated 

nitric acid (May and others, 1997; Besser, Brumbaugh, and 

others, 2001).

Metal concentrations in tissues of stream biota were 

log-transformed before statistical analysis to improve normality and homogeneity of variance. Associations among metal 

concentrations and other characteristics of sediment and 

periphyton were evaluated by Pearson correlation analysis. 

Differences in metal concentrations among locations were 

Figure 2. Characteristics of benthic invertebrate communities of Animas River 

watershed study area. Mean number of taxa and mean abundance from three 

sample dates in 1996–97. See table 1 and figure 1 for site identifications and 

locations. Data from Peter Butler, Robert Owen, and William Simon, Unpublished 

report to Colorado Water Quality Control Commission, Animas River Stakeholders 

Group (2001).

EXPLANATION

Number of taxa

Abundance
evaluated by analysis of variance (ANOVA), with differences among means compared by Duncan’s multiple range 

test (Snedecor and Cochran, 1980). Statements of statistical 

significance indicate a probability of Type I error of 5 percent 

or less (p≤0.05).

Periphyton

Concentrations of metals in periphyton differed significantly among sites. Greatest concentrations of cadmium, copper, manganese, lead, and zinc occurred in the Animas River 

upstream of Silverton (table 2). Periphyton samples from this 

reach contained zinc concentrations approaching 2 percent on 

a dry-weight basis. Concentrations of copper in periphyton 

from the Animas River decreased gradually from upstream to 

downstream, whereas concentrations of zinc, cadmium, and 

lead were consistently high at sites upstream of Silverton and 

dropped off sharply downstream. As a result, concentrations of 

copper, but not other metals, in periphyton from downstream 

recovery sites in the Animas River remained greater than those 

at reference sites.

Metal concentrations in periphyton samples can reflect 

both trapping of metal-rich colloids and uptake of dissolved 

metals, by bioaccumulation or sorption (Newman and others, 

1985). Concentrations of most metals in periphyton were 

significantly correlated with concentrations in fine streambed sediment, although these associations usually explained 

less than half of the overall variation (Besser, Brumbaugh, 

and others, 2001). Periphyton from the upstream reach of the 

Animas River (AR1-AR3) contained both high concentrations 

of several metals and concentrations of chlorophyll and TOC 

that were similar to those of reference sites (fig. 3). Concentrations of chlorophyll and TOC were positively correlated 

with concentrations of zinc and copper in periphyton, suggesting that the presence of algae and other organic constituents 

resulted in increased accumulation of metals. In contrast, 

chlorophyll and TOC were negatively correlated with concentrations of iron and aluminum. Samples of periphyton from 

two acidic sites, with pH <4.5 at low flow (CEM and UMC; 

Wright, Simon, and others, this volume), and two neutral-pH 

sites downstream from acid tributaries (AR1, LMC) contained little algal biomass and were dominated by precipitated 

iron and aluminum oxyhydroxides (fig. 3). These minerals 

made up approximately 50–90 percent of periphyton dry 

mass at the sites most affected by precipitates, compared to 

3–10 percent at the reference sites. This finding is consistent 

with experimental studies that found copper and cadmium to 

be preferentially sorbed by algal biomass, relative to Fe, Al, 

or Mn oxyhydroxides (Calmano and others, 1988). Overall, our data suggest that periphyton in stream reaches with 

circumneutral pH and low levels of iron or aluminum precipitates can provide a substantial source of metal-contaminated 

food for stream food webs.

Benthic Invertebrates

Concentrations of metals in benthic invertebrates differed significantly among taxa and among sites. Concentrations of cadmium, copper, lead, and zinc differed significantly 

among invertebrate taxa (Besser, Brumbaugh, and others, 

Table 2. Concentrations of metals in periphyton from the Animas River and tributaries.

[Means, with standard errors in parentheses (n=3–7). For each metal, means followed by the same letter are not significantly different]

Site

Metal concentration 

(μg/g dry wt.)

Cadmium Copper Lead Zinc

Animas River

AR1 33.3 (0.5)a 6,587 (371)a 1,051 (40)abc 18,950 (743)a

AR2 25.3 (1.3)a 1,031 (64)bc 2,023 (33)a 8,520 (465)ab

AR3 27.8 (1.8)a 952 (45)bc 1,597 (24)ab 11,903 (314)a

AR4 5.6 (0.4)b 764 (54)bc 680 (80)abc 2,179 (124)cd

AR5 6.1 (0.3)b 362 (36)c 465 (26)abc 2,656 (272)cd

AR6 9.5 (0.6)b 709 (72)bc 480 (9)abc 3,673 (243)bc

Fishless sites

CEM 0.2 (0.04)e 31 (1)e 207 (9)c 44 (8)e

UMC 0.2 (0.1)e 105 (7)d 753 (54)abc 40 (13)e

LMC 5.2 (0.9)b 1,571 (296)b 605 (150)abc 1,135 (3,830)e

Reference sites

CUN 2.7 (1.4)c 40 (36)f 266 (265)d 338 (300)e

SMC 6.7 (0.3)b 89 (5)d 276 (5)bc 1,783 (87)cd

CAS 0.4 (0.01)d 9 (1) 9 (0.5)abc 84 (6)cd

830 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Stream Biotic Communities and Metal Exposure 831

Figure 3. Algal biomass (represented by chlorophyll-a) and iron in periphyton. Mean 

and range for chlorophyll (n=2); mean and standard error for iron (n=4).

2001). Concentrations of all four metals were significantly 

greater in two smaller taxa (Rhithrogena and Zapada), 

consistent with previous reports that metal accumulation 

by invertebrates decreases with increasing body size (Smock, 

1980). However, differences in metal concentrations among 

taxa also reflected differences in feeding ecology. Of the two 

small taxa, the periphyton grazer, Rhithrogena, accumulated 

greater concentrations of zinc and cadmium, and the detritivore, Zapada, accumulated greater concentrations of lead. 

Two larger taxa, Megarcys and Arctopsyche, with diets that 

include substantial components of invertebrate tissue, accumulated proportionally greater concentrations of copper. Metal 

concentrations in invertebrates also differed significantly 

among sites, reflecting variation in metal bioavailability in 

the watershed. Lowest concentrations of all metals occurred 

in invertebrates at Cascade Creek (CAS), the reference site 

outside the highly mineralized portion of the watershed 

(Yager and Bove, this volume, Chapter E1), but metal concentrations at the other reference sites (SMC and CUN) were not 

consistently less than those from recovery sites in the Animas 

River. Of the metals studied, only copper occurred at consistently greater concentrations in invertebrates from fishless 

and recovery sites in the Animas River, relative to reference 

sites.

Because it is widely distributed and tolerant of elevated 

metal burdens, the caddis fly Arctopsyche grandis has been 

widely used to monitor metal bioavailability in streams 

of the western United States (Cain and others, 1992; Kiffney 

and Clements, 1993; Farag and others, 1998). Arctopsyche 

occurred at five of six sites sampled in the Animas River, 

and samples from these sites were used to evaluate longitudinal patterns of metal bioavailability. Concentrations of 

copper in Arctopsyche were significantly greater downstream 

of Cement and Mineral Creeks, at the fishless site AR4 and 

at recovery sites AR5 and AR6, than at sites upstream of 

Silverton. In contrast, concentrations of zinc, lead, and cadmium in Arctopsyche did not show consistent increases at sites 

downstream of Cement and Mineral Creeks, and were generally lower at AR4 than at other Animas River sites (fig. 4).

Brook Trout

Concentrations of lead, zinc, and copper in liver tissue of brook trout differed significantly among sites (fig. 5). 

Concentrations of both zinc and copper in liver tissue corresponded to general trends in the status of stream biota, 

although both differences between Animas River sites and 

reference sites and longitudinal differences among Animas 

River sites were more pronounced for copper than zinc. 

Lead concentrations in trout livers were significantly lower 

at Cascade Creek than at other sites, but differences between 

reference and recovery sites were not evident. Previous 

studies have reported elevated concentrations of copper and 

zinc in livers of trout from metal-contaminated sites (Wilson 

and others, 1980; Moore and others, 1991; Farag and others, 

1995).

EXPLANATION

Chlorophyll

Iron
Food-Web Transfer of Metals

Comparisons of trophic transfer factors (TTFs), ratios 

of metal concentrations between aquatic animals and their 

available diets, suggested that the efficiency of transfer via the 

stream food web differed among the metals studied (Besser, 

Brumbaugh, and others, 2001). Overall TTFs for a hypothetical stream food chain (periphyton-invertebrate-trout liver) 

suggest that cadmium (TTF=4.8) and copper (TTF=4.6) are 

transferred most efficiently via diet. In contrast, zinc was 

apparently transferred efficiently from periphyton to invertebrates, but not to trout (overall TTF=0.54), and lead was transferred least efficiently (overall TTF=0.005). Efficient trophic 

transfer of cadmium from low levels in water and sediment 

may explain the occurrence of elevated cadmium concentrations in biota from reference sites and the persistence of 

elevated cadmium concentrations at downstream recovery sites 

in the Animas River. This hypothesis is consistent with findings of a study of a Montana watershed, that higher levels of 

cadmium in stream food chains persisted farther downstream 

than concentrations of other metals (Moore and others, 1991). 

Similarly, the hypothesis of more efficient trophic transfer may 

partially explain the greater enrichment of copper in tissues 

of invertebrates and brook trout, despite low concentrations of 

copper (relative to zinc and lead) in sediment and periphyton. 

Although the presence of relatively high levels of cadmium 

EXPLANATION

Cadmium

Copper

Lead

Zinc

A

B

Figure 4. Concentrations of metals in caddis flies, Arctopsyche sp., from the 

Animas River and Cascade Creek. A, cadmium and lead; B, copper and zinc. 

Means with standard error (n=4). For each metal, means with same letters are 

not significantly different.

832 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Stream Biotic Communities and Metal Exposure 833

and copper in trout livers reflects at least in part typical patterns of enrichment of these metals in liver tissue (Wilson and 

others, 1980; Farag and others, 1995), internal distribution of 

metals cannot explain the greater concentrations of copper in 

liver tissue of trout from recovery sites, relative to reference 

sites (fig. 5).

Association of Metal Exposure 

with Biological Effects

The distribution and abundance of trout in the Animas 

River watershed study area and adjoining area downstream are 

closely associated with patterns of metal bioaccumulation. The 

absence of brook trout at some fishless sites such as AR4 and 

LMC probably reflects direct toxicity of aqueous copper and 

(or) other metals (Besser, Allert, and others, 2001; Besser and 

Leib, this volume), but results of the food web study suggest 

that brook trout at sites with lower aqueous metal concentrations may also be subject to chronic toxicity via combined 

aqueous and dietary exposure. Concentrations of copper in 

invertebrate diets are substantially greater at Animas River 

sites with reduced densities of brook trout than at reference 

sites. However, mean concentrations of zinc and lead in invertebrates from these Animas River sites overlapped broadly 

with those in reference sites.

Concentrations of copper in diets and liver tissue of 

brook trout from the Animas River approach levels associated 

with chronic toxicity (Besser, Brumbaugh, and others, 2001). 

Dietary toxicity has been reported for other salmonid species 

fed diets containing elevated concentrations of copper and 

other metals (Woodward and others, 1994, 1995; Farag and 

others, 1999). Brook trout from affected sites in the Animas 

River had copper concentrations in liver tissue equal to or 

greater than those associated with toxicity to brook trout and 

rainbow trout in laboratory tests (McKim and Benoit, 1974; 

Lanno and others, 1985) or associated with reduced population density of brook trout in field studies (Moore and others, 

1991). The reduced abundance of brook trout at sites where 

tissue copper concentrations approach levels associated with 

toxicity supports the hypothesis that chronic copper toxicity is 

an important factor controlling the distribution and abundance 

of brook trout in the Animas River watershed. The effects of 

copper toxicity on brook trout populations are probably exacerbated by toxic effects of other metals and by degradation of 

benthic habitats, leading to reduced biological diversity and 

reduced productivity in stream habitats of the Animas River 

watershed.

A

B

C

D

Figure 5. Concentrations of metals in livers of brook trout, 

Salvelinus fontinalis, from the Animas River and reference sites. 

A, lead; B, cadmium; C, zinc; D, copper. Means with standard 

errors (n=4). For each metal, means with same letters are not 

significantly different.
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