Responses of Diatom Communities to Heavy Metals in Streams: The Influence of Longitudinal
Variation
Author(s): C. Nicolas Medley and William H. Clements
Source: Ecological Applications, Vol. 8, No. 3 (Aug., 1998), pp. 631-644
Published by: Ecological Society of America
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Ecological Applications, 8(3), 1998, pp. 631-644
?) 1998 by the Ecological Society of America
RESPONSES OF DIATOM COMMUNITIES TO HEAVY METALS IN
STREAMS: THE INFLUENCE OF LONGITUDINAL VARIATION
C.
NICOLAS
MEDLEY
AND WILLIAM
H.
CLEMENTS1
Departmentof Fisheryand WildlifeBiology, Colorado State University,
Fort Collins, Colorado 80523 USA
Abstract. We investigatedlongitudinalvariation(e.g., fromupstreamto downstream)
in diatom communitycompositionin an unpollutedstream(the Cache la Poudre River,
Colorado, USA) and demonstratedhow naturalvariationin communitystructureand morphological growthformsinfluenceresponsesofdiatomsin metal-pollutedstreams.Upstream
communitiesin the Cache la Poudre River were physiognomicallysimple and dominated
by small, adnate species (Achnanthes minutissima and Fragilaria vaucheriae), which de-
creased in relativeabundance downstream.Responses of diatomcommunitiesto Zn in six
metal-pollutedstreamswere influencedby elevationand othervariablescorrelatedwiththe
stream's longitudinalgradient(pH, conductivity,alkalinity).Althoughclear community
responses were observed at sites where Zn concentrationexceeded 200 VLg/L,
effectsof
metals at moderatelypolluted sites were not significant.
These fieldresultswere compared to responses of periphytoncommunitiesin experimentalstreamsdosed withheavy metals.We hypothesizedthatcommunitiescollectedfrom
high elevation sites,whichwere dominatedby early successional species (A. minutissima),
would be more tolerantto metals than downstreamcommunities.To test this hypothesis,
diatomcommunitiescollected froma highelevation site (2340 m) and a low elevation site
(1536 m) on the Cache la Poudre River were exposed to a mixtureof Cd, Cu, and Zn in
experimental streams. Small, adnate species (Achnanthes minutissima and Fragilaria vau-
cheriae) were tolerantto metals and increasedin abundancein treatedstreams.In contrast,
late successional species, whichwere foundonly at thelow elevationsite (Diatoma vulgare
and Melosira varians), were highlysensitiveto metals and were eliminatedfromtreated
streamsafter24 h. These resultsindicatethatnaturalvariationin diatomcommunitiesmay
of routinebiomonitoringstudies.We suggestthatit maybe useful
complicateinterpretation
to studyperiphytoncommunityresponses to pollutantsin relationto ecological characteristics and morphologicalgrowthforms.Conductingresearchwithinthis frameworkmay
improve the predictiveability of periphytonmodels and provide a theoreticalbasis for
therelationshipsbetweencommunitypatternsand theevolutionaryprocesses
understanding
thatdetermineresponses to disturbance.
Key words: diatom communities;heavy metals; longitudinalvariation; morphological growth
forms; RockyMountainstreams.
INTRODUCTION
Understandingthe relativeinfluenceof naturalspatial and temporalvariationon theresponsesof aquatic
organismsto pollutantsis one of the most persistent
problems in streambiomonitoringstudies (Clements
and Kiffney1995). Althoughapproaches such as the
before-after-control-impact
design (BACI) have been
developed to address this problem (Stewart-Oatenet
al. 1986, Underwood 1992), theyare not applicable in
fieldinvestigationswherepre-impactdata are unavailable or where appropriatereferencesites cannot be
found. Experimentalapproaches, such as the use of
microcosms,mesocosms (Genteret al. 1987, Clements
et al. 1992, Kiffneyand Clements 1996), or natural
field experiments(Feldman and Connor 1992, Clementsand Kiffney1995), supportresultsof traditional
Manuscriptreceived 4 November 1996; revised 3 October
1997; accepted 25 October 1997.
1 Address reprintrequests to this author.
streamsurveys and allow researchersto test specific
hypothesesgeneratedby field studies (Lamberti and
Steinman 1993).
Diatoms are routinelyused to assess water quality,
and the literaturesuggests thatthey are excellent biological indicatorsformanytypesofpollutionin aquatic systems(Lowe 1974, Patrick and Palavage 1994,
Kelly et al. 1995). However,diatom communitiesare
extremelysensitiveto naturalvariationin abiotic factors, including current(McIntire 1966, Horner and
Welch 1981, Korte and Blinn 1983, Lamb and Lowe
1987, Petersonand Stevenson 1990, Poffet al. 1990),
light(Shortreedand Stockner1983, Steinmanand McIntire 1987, Stevenson et al. 1991, DeNicola et al.
1992), and nutrients(Marcus 1980, Bothwell 1988). It
is well established that longitudinalvariation (e.g.,
fromupstreamto downstream)in theseand otherphysinfluencebenicochemical characteristicssignificantly
thic communities(Vannote et al. 1980). However,unlike researchon longitudinalvariationin benthicmac-
631
632
C. NICOLAS MEDLEY
AND WILLIAM H. CLEMENTS
roinvertebrate
communities(Vannoteet al. 1980, Minshall et al. 1983, Minshall et al. 1985, Bruns and
Minshall 1985, Perry and Schaeffer 1987), little informationis available on the longitudinaldistribution
of diatoms in streams.The few studies published into upstream
dicate thatsome algal species are restricted
or downstreamreaches,but it has been difficultto find
clear longitudinalpatternsbased on algal species composition (Kawecka 1971, Ward 1986, Molloy 1992).
Rocky Mountainstreamsof Colorado providean exto investigatelongitudinalvariation
cellentopportunity
in diatom communitiesbecause they exhibit marked
altitudinalgradients,oftendroppingseveral thousand
meters across distinctvegetation and climatological
zones (Ward 1986). Communitiesin high elevation
streamsare oftenexposed to extremelyharsh winter
conditionsand intense scouring.Consequently,highelevation streamspossess simple two-dimensionalperiphytoncommunitiesdominatedby small, early successional species (e.g., Achnanthesminutissimaand
Fragilaria vaucheriae), which are regulated by frequent disturbance(Oemke and Burton 1986). In contrast,low-elevationstreamshave warmertemperatures,
highernutrients,and rarelyproduce frazil ice in the
winter.These downstreamsites are less physicallydisturbed,have longer growingseasons, and are characterizedby mature,physiognomicallycomplexperiphyton communities.
Many of the mountainstreamsin Colorado are also
pollutedby heavymetalsfromabandonedmines(Smith
1987, Clements1994, Clementsand Kiffney1995), the
effectsof whichmaypersistformanykilometersdownstream(Moore et al. 1991, Clements 1994). Dilution
bindingof metals to sediments,and upby tributaries,
take by organismscreate a gradientof metal concentrationsfromupstreamto downstream,which is superimposedon the naturallongitudinalgradientof the
stream.Assessing effectsof heavymetalson thesehigh
gradient streams is complicated by longitudinal
changes in streamstructureand function,which may
occur over relativelyshortdistances (Clements 1994,
Clements and Kiffney1995).
Species life historytraitsare influencedby natural
disturbanceregimes(Odum 1969, Pianka 1970), which
may also vary fromupstreamto downstream.Headwater streamsexhibitrelativelysmall fluctuationsin
which are hypothewater chemistryand temperature,
sized to be importantfactorsinfluencingbenthiccommunitystructure(Ward and Stanford1983). Species
inhabitingheadwaterstreamsmay be moresusceptible
to anthropogenicdisturbancethan species adapted to
the fluctuatingconditionsin mid-orderstreams(Ward
and Stanford1983, Clements and Kiffney1995). Alternatively,
periphytoncommunitiesin high gradient,
headwater streamsexperience greaterturbulenceand
shear stress, especially during spring runoff.These
conditionsmay have a dramaticinfluenceon morphological growthformsand life historycharacteristicsof
Ecological Applications
Vol. 8, No. 3
diatoms (Oemke and Burton 1986, Petersonand Stevenson 1990, Molloy 1992). We hypothesizethatspecies fromthesephysicallyharshenvironments
possess
traitsthatconfertoleranceto both naturaland anthropogenic disturbance.
In this studywe used field biomonitoringand laboratoryexperimentsto examine the responses of diatom communitiesto heavy metals in Colorado Rocky
Mountainstreams.The specificobjectives of the study
were: (1) to describethe naturallongitudinalvariation
in diatom communitiesin an unpollutedstream;(2) to
investigate the confoundingeffects of longitudinal
variation in communitystructureand morphological
growthformson streambiomonitoringstudies;and (3)
to compare responses of diatoms observed at metalpolluted sites to those measured experimentallyin
streammicrocosms.
METHODS
Longitudinalvariationin the Cache la Poudre River
Natural longitudinalvariation of epilithic diatom
communitieswas investigatedin the Cache la Poudre
River,a relativelypristinestreamthatoriginatesfrom
Poudre Lake (elevation 3267 m) in Rocky Mountain
National Park, Colorado (Fig. 1). The stream flows
freelythroughthe Cache la Poudre Canyon for -100
km beforebecominga plains rivereast of FortCollins
(elevation
-
1500 m). The river has a similar topology
to otherwesternstreams(Ward 1986) and showstypical
changes in streamwidth(7-24 m), temperature(8.920.5?C), and waterchemistryfromupstreamto downstream(Table 1).
Periphytonsamples were collected fromthe Cache
la Poudre River in August 1992. To reduce sampling
variability,all samples were collected fromlarge cobble substrate(128-256 mm) in shallow,unshadedriffle
areas (-50 cm deep) withsimilarcurrentvelocity,substratecomposition,and canopycover.Individualstones
were gentlyremoved fromthe streamand periphyton
samples were collected using a neoprenecuffedsampler (7.1 cm2),whichwas pressedfirmlyon thesmooth
uppersurfaceof thesubstrate.A small amountof water
was placed in the sampler,periphytonwas loosened
fromthe rock surface with a battery-operated
dental
plaque remover(model 4726, Braun NorthAmerica,
Woburn,Massachusetts),and the resultingslurrywas
removedwitha pipette.Periphytonremovedfromthree
rocks was combined,diluted to a standardvolume of
100 mL, and preservedin 1% lugol's solution.Three
samples were collected fromeach site.
In the laboratory,periphytonsamples were homogenized for 60 s in a blender.Cell densities were enumeratedby countinglive diatom cells in 50 random
fields,definedby a Whipple gridin the ocular lens, at
400X magnificationusing a Palmer-Maloneycounting
chamber.Data are reportedas live cells per unit area
of substratesampled.
August 1998
DIATOM RESPONSES
TO HEAVY METALS
633
Cache la Poudre River
Forkofthe
North
Cache la PoudreRiver
Seaman Reservoir
PR
PR5
v~~~~~~R
c_
PR8
Flo
ChambersLake
>
/
Joe Wright
Creek
PR3
SouthForkof the
Litt~~~~le
..
FortCollins
Cache la PoudreRiver
N
PoudreLake
10km
FIG. 1.
P
R9
-
PR4
FortCollins
COLORADO
Map of samplinglocations on the Cache la Poudre River (Colorado) in August 1992.
Relative abundance of diatom species was determined by examinationof permanentslides of cleaned
diatom frustulesusing phase contrastmicroscopy at
IOOOX magnification.Diatom frustuleswere counted
in lengthwisestrips across the mount,definedby a
Whipple grid, until at least 250 cells were counted.
Studies have shown that this numberis sufficientto
characterize the dominant species in a community
(Johnsonand Lowe 1995).
Water temperature,dissolved oxygen (YSI, model
57), pH (Jenco, model no. 6009), and conductivity
(YSI, model 33) were measuredat all sites in thefield.
Watersamples foralkalinityand hardnesswere placed
on ice, returnedto the laboratory,and analyzed by
titrationfollowingstandardmethods(APHA 1989).
Communityresponses to metals in thefield
To assess longitudinalvariation in metal-polluted
streams,diatom communitystructurewas measuredat
reference,impacted,and recoverysites (n = 34) in six
Rocky Mountainstreams(Arkansas River,Blue River,
Chalk Creek, Eagle River,Snake River,and the South
Platte River) in north-central
Colorado (Fig. 2). Between four and seven stations were located on each
stream,upstreamand downstreamfromknown metal
inputs.All streamswere qualitativelysimilarand characteristicof streamsin the southernRocky Mountain
ecoregion. Each stream originatesfrom small headwatersabove treeline,high on the ContinentalDivide,
and is characterizedas cold, clear, high-gradient,
and
oligotrophic.Additionally,each streamis polluted by
heavymetals,usuallyfroma discretesourcein itsupper
reaches, and metal concentrationsdecrease downstream(Clements and Kiffney1995).
Periphytonand watersamples were collected in August 1992 fromeach stationusing the same procedures
describedpreviously.Watersamples fordetermination
of totalrecoverablemetals(Cd, Cu, Zn) werepreserved
by acidificationwithnitricacid to a pH of <2. Samples
fordissolved metals were filteredin the fieldthrough
a 0.45 ALmfilterbefore acidification.Metals were analyzed using flameor furnaceatomic adsorptionspecat the Colorado Division of Wildlife,
trophotometry
Fort Collins, Colorado. Analyticallimitsof detection
refor Cd, Cu, and Zn were 0.1, 1.0, and 5.0 VLg/L,
spectively.
Community response to metals in experimental
streams
To testthe hypothesisthatdifferencesin periphyton
communitycompositioninfluencedresponsesto heavy
metals, we conducted experimentsin stream microcosms. Periphytoncommunitieswere collected from
Cache la Poudre River stationsPR5 and PR9 (Fig. 1)
using unglazed ceramic tiles (5 x 5 cm) secured to a
0.6-M2sectionof concrete.Substratecomposition,gradient,flow,and canopy cover were similarat the two
sites;however,thelowerelevationsite(PR9) had greater temperature,
hardness,alkalinity,and conductivity
(see Table 1). Because of differencesin abundance of
dominanttaxa between these sites, this approach allowed us to compare effectsof metals on two distinct
periphytoncommunitiesfromthe same stream.
Tiles (n = 108) were placed at each site in unshaded
riffleareas, -50 cm deep on 7 September1993. Tiles
were removedfromthe streamafter42 d colonization
and transportedon shelves in large coolers to the
Stream Research Laboratory at Colorado State University.Descriptionsoftheexperimentalstreamsin this
facilityhave been published previously(Clements et
al. 1989, Kiffneyand Clements 1994). The 18 oval,
streams(76 x 46 x 14 cm) are located
flow-through
in a greenhouseand are supplied withuntreatedwater
fromnearbyHorsetoothReservoir,a deep, cold-water
634
AND WILLIAM H. CLEMENTS
C. NICOLAS MEDLEY
Ecological Applications
Vol. 8, No. 3
TABLE 1. Characteristicsat sites sampled on the Cache la Poudre River (Colorado), August 1992.
Site
Elevation
(m)
pH
PRI
PR3
PR4
PR5
PR6
PR7
PR8
PR9
3267
2938
2566
2340
2133
1780
1603
1536
8.44
7.66
7.52
7.91
8.18
8.07
8.52
8.67
Tempera- ConducDissolved
ture
tivity AlkalinityHardness oxygen
(OC)
(mg/L)
([iS/cm) (mg/L) (mg/L)
92
37
8.9
40
9.5
47
13
12
9.4
8.0
42
13
14
9.4
8.0
48
19
16
12.8
9.0
59
23
15.0
20
10.0
54
20
15.5
18
10.0
115
46
42
18.3
8.0
227
73
95
20.5
9.5
Density
(cells/mm2) Richness Diversity
32
27
22
34
26
28
36
34
201
81
167
29
220
14
167
11 190
2.70
1.74
1.49
2.28
2.32
2.03
2.61
2.87
Note: Locations of sampling stationsare shown in Fig. 1.
impoundment.Waterwas delivered to each streamat
a rateof 1.0 L/min,resultingin a turnovertimeof -13
min. Currentin each streamwas maintainedby motordrivenpaddlewheels at 30 cm/s.
Nine tiles were randomlyassigned to each of the 18
experimentalstreams.Aftera 24-h acclimationperiod,
streamswere randomlyassigned to one of threemetal
treatments:
O.Ox,0.5x, and 5.Ox,wherex was a mixture
of Cd, Cu, and Zn at 1.1 jIg/L, 12 jig/L and 110 pLg/
were
L, respectively.Metal levels in the0.5x treatment
comparable to the U.S. EPA chronic criterionvalues
(at waterhardnessof 30 mg/L),whereas levels in the
N
5.Ox treatmentare withinthe range of concentrations
measuredat highlypolluted sites in Colorado streams
(Clements and Kiffney1995). Metals were delivered
to treatedstreamsby peristalticpumps at a rate of 10
mL/minfromstocksolutionspreparedin 25 L carboys.
The final experimentaldesign was a completelyrandomized 2 x 3 factorialdesign, with location (two
levels) and metal treatment(threelevels) as the main
effects.Each treatmentcombinationhad threereplicates, with individual streammicrocosmsas the unit
of replication.
After10 d, periphytonwas removedfromthreeran-
2. Eagle River
4. SnakeRiver
DillonRes.
~~~~~~ER4
4
SR
~~~Edwards
esone
COLORADO
:V,ail
5
~~
~~~BR
ER3
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
I. ArkansasRiver
Minturn:ER2
DC1
BR3
ERI1
EF5
SR
lSR2
~~~Breckenridge
R
10km
C
BRI
ARIl
AR2 A
3. South Platte River
5. Blue River
9
;
|A8tR5
MCI
MC2
S
1SPR1
AC3SPR2
|M|
6. Chalk Creek
1
Fairplay
Buena Vista
L10km
K
10km
|
CC3
SPR3 |
`AR8
10 km
______
CC4
Nathrop,)
CC5
CC2
ccl
10km
FIG. 2. Map of samplingstationsin the metal-pollutedstreams(Colorado) in August 1992. Arrowsindicatethelocations
of known metal inputsto each stream.
August 1998
DIATOM RESPONSES
TO HEAVY METALS
635
domly selected tiles witha sharpedged plastic scraper respectively.Canonical discriminantanalysis (CDA)
and a hardbristledtoothbrush.
Materialfromthreetiles was used to examine overlap and separationof stations
was combinedand dilutedto a standardvolume of 100 withineach category,based on proportionsof the 10
mL. This procedurewas repeatedforthe six remaining dominantspecies (e.g., those species that comprised
tiles and these subsamples were used forspecies enu- >1% total abundance). Loading coefficientswere exmeration,determination
of ash-freedrymass (AFDM),
amined to determinewhich species were responsible
and metalbioaccumulation(notreportedhere) foreach fortheseparationofZn categoriesalong each canonical
stream.
axis. Stepwise multipleregressionanalysis was conWater samples were collected fromboth field sites ducted to examine the relationshipbetween diatom
at the beginningand end of the colonization period, communitymeasures and abiotic variables (Zn conand on days 0, 2, 6, and 10 fromthe experimental centration,water hardness, conductivity,alkalinity,
streams. Routine water quality parameters(tempera- dissolved oxygen,pH, and elevation).
We used two-wayANOVA in the microcosmstudy
ture, dissolved oxygen, conductivity,hardness,alkalinity,and pH) and heavy metal concentrationswere to test the effectsof site (PR5 and PR9), metal treatmeasured. Additional water samples were collected ment(0.0x, 0.5x, and 5.Ox),and theinteraction
between
fromthe two field sites and the experimentalstreams site and metal treatmentfor diatom communityvariat the end of the experimentforanalysis of dissolved ables. If the overall F testwas significant(P < 0.05),
organic carbon.
Ryan's Q multiplecomparisontestwas used to testfor
Biomass was estimatedby measuringash-freedry differencesbetweenmetal treatments.
This testis recmass (AFDM), using standardmethods(APHA 1989) ommendedbecause of its strictcontrolof the experiand therecommendationsof Aloi (1990). Species were mentwiseerrorrate (Day and Quinn 1989).
enumeratedin a similarmannerto the fieldstudy,with
RESULTS
one important
exception.Most fieldstudiesassume that
all species withina communitywill have a similarratio Longitudinalvariationin the Cache la Poudre River
of live: dead cells, and that an accurate estimateof
Except for station PRI, hardness, alkalinity,contheirrelative abundance can be obtainedfromexamination of the cleared diatoms. However,in short-term ductivity,pH, and temperaturewere greater at the
toxicitytestsdifferential
sensitivityof individualspe- downstreamstationscomparedto theupstreamstations
cies may resultin different
ratios of live: dead cells. (Table 1). Water chemistryat stationsPR8 and PR9
Accordingly, counts obtained from the permanent was distinctfromother stations,as conductivity,almountswere correctedat the genericlevel using live: kalinity,and waterhardnessincreased4x betweenstadead ratios of dominantgenera obtained by counting tions PR7 and PR9. Due to the influenceof Poudre
at least 250 cells in wet mountsat 1000X magnifica- Lake, waterchemistryat PRI was anomalous relative
tion.We distinguishedlive diatomsfromdead diatoms to other upstream stations and was more similar to
by visual inspection(live diatoms appeared more lus- lower elevation sites.
Differencesin water chemistrybetween upstream
trous and translucent,with the cytoplasmcompletely
sites
(PR3 to PR7) and downstreamsites(PR8 and PR9)
occupyingthe cell).
were reflectedin changes in diatom communitycomStatisticalanalyses
position(Table 1; Fig. 3). Cell densitywas muchgreatAll statisticalanalyses were performedusing a PC er at the lowest elevation site (PR9) comparedto upversionof StatisticalAnalysis Systems(SAS 1988). In streamsites. Most upstreamsites were dominatedby
the studyof the six metal-pollutedstreams,each of the Achnanthesminutissima,Fragilaria vaucheriae, Han34 stationswas assignedto one of fourmetalcategories naea arcus, and/orSynedra rumpens.Relative abunbased on the concentrationof Zn (total recoverable), dance of A. minutissimaand F. vaucheriae decreased
thedominantmetalsampledat all sites.Concentrations downstream,and stationsPR8 and PR9 were characat background,low, medium,and highZn stationswere terized by higherspecies diversityand greaterabun<20, 21-50, 51-200, and >200 jig Zn/L,respectively. dance of Cymbella,Gomphonema,Navicula, and MeThese fourcategorieswere chosen because theybrack- losira varians.The diatomcommunityat PRI was more
different
fromotherupstream
eted the U.S. EPA chroniccriterionvalue for Zn (50 diverse and structurally
pig/L)and because theyresultedin an approximately sites, probablydue to the influenceof Poudre Lake.
equal numberof stationswithineach category.
Communityresponses to metals in thefield
Fixed effects,one-wayANOVA was used to testfor
measuredin the
Routinewaterqualitycharacteristics
differencesbetween Zn categories in the field study.
To check the assumptionsof normalityand heteroge- six metal-pollutedstreamsare shownin Table 2. Other
neityof variance, we examined scatterplots of resid- habitat characteristicsof these sites (e.g., elevation,
uals and performedFmax tests. Where necessary,den- currentvelocity,depth,canopy cover) have been resities and relative abundance of dominanttaxa were ported previously (Clements and Kiffney1995). All
transformed
using the log and arcsine transformation, streamswere cold, oligotrophic,and characterizedby
636
C. NICOLAS MEDLEY AND WILLIAM H. CLEMENTS
Ecological Applications
Vol. 8, No. 3
m vMelosira
1
varians
Navicula
CZ 0.8
0.8
CZ
E
_
6 0.6
0.6
0
.
0.4
0.4
o
o
0~
II
0.2
0.2
;..lX g
0
E
[7
g
I I* 1 * Id
|
g
110
* I lIl I I
*
_
E
S
t
t
g
-
|
|
S
W
=|
E
| g
SPJ.
Gomphonema
Synedra
spp.
Cym.bella
Hannaea
FIG. 3. Proportionalabundance of dominant
taxa at sites sampled on the Cache la Poudre
~~Fragilari
River (Colorado)
Iarcus
vaucheriae
Achnanthes
Lj~~~~~~~~iE~ii
in August 1992.
minutissima
0
Co
mt
LQ0
N
CO
oC)
Site
low to moderate conductivity(range: 40-240 RS/
cm), hardness (range: 10-126 mg/L), and alkalinity
(range: 10-119 mg/L). Temperature,conductivity,alkalinity, and hardness increased from upstream to
downstreamin most streamsand were inverselycorrelated with elevation (temperature,r = -0.52, P =
0.0016; conductivity,r = -0.52, P = 0.0015; alkalinity, r = -0.36, P = 0.036 and hardness, r = -0.41,
P = 0.015). The pH was neutralor slightlyalkaline at
all sites, with the lowest values (6.7) measured in the
Snake River.
Zn was the dominantmetal measuredat all polluted
sites and ranged frombelow detection (<5 Rwg/L)
to
1735 Rg/L. Cu and Cd were presentat some polluted
sites, althoughusually in much lower concentrations.
Because we observedlittledifferencebetweentotaland
dissolved Zn concentrationsduringthe low flow conditions when this studywas conducted,only total Zn
concentrationwas used in these analyses.
Over 200 species of diatomswere collected fromthe
studyarea. Ten dominanttaxa (Achnanthesminutissima, Fragilaria vaucheriae, F. pinnata, F. constreuens,
Hannaea arcus, Cymbella minuta, Nitzschia palea,
Synedra rumpens,S. ulna, Cocconeis placentula) typically represented>80% of the diatom communityat
all stations.Althougha few otherspecies were found
in highrelativeabundance,these species were restricted to a few sites or foundonly withinone riverbasin.
Differencesin Zn concentrations
amongbackground,
low, medium,and highZn sites were highlysignificant
(Table 3). Species diversitywas significantly
lower and
relativeabundanceofAchnanthesminutissimawas significantlygreaterat the high Zn sites. Cell density,
species richness,and abundanceof otherdominanttaxa
did not vary significantlyamong Zn categories (P >
0.05).
Canonical discriminantanalysis, based on relative
abundance of the 10 dominantspecies, clearly separatedthe highZn sites frombackground,low, and mediumZn sites (Fig. 4). Wilk's lambda statisticindicated
thatdifferencesbetweenZn categorieswerehighlysignificant(F3,30= 2.32; P = 0.0037). Canonical variables
one and two explained 55 and 36% of the total variation, respectively.Inspection of loading coefficients
indicatedthatseparationalong canonical variable one
resultedprimarilyfromgreaterabundance of Achnanthesminutissimaat highZn sites. Separationalong canonical variable two resultedfromgreaterabundance
of Fragilaria vaucheriae and Nitzschia palea at medium Zn sites.
Stepwise multiple regression showed that Zn concentration,elevation, pH, and conductivityexplained
most of the variationin diatom communitystructure
in the metal-pollutedstreams(Table 4). Zn concentration was the mostimportantpredictorvariable forspecies richnessand diversity,and explained 23 and 33%
of the total variationin these measures, respectively.
Zn was also includedin thestepwiseregressionmodels
forrelativeabundance of Achnanthesminutissimaand
Fragilaria vaucheriae; however,unlike the effectson
species richnessand diversity,Zn had a positive effect
on relative abundance of these taxa. The positive relationshipbetweenZn concentrationand relativeabundance of A. minutissimaand F. vaucheriae resulted
fromthe eliminationof metal-sensitivediatomsat polluted stations. Zn was not included in the stepwise
multipleregressionmodel forany otherdominantspecies.
Communityresponse to metals in experimental
streams
Periphytoncommunitieson tiles collected fromstationsPR5 and PR9 forthemicrocosmstudyweremark-
August 1998
TABLE
1992.
2.
DIATOM RESPONSES
Physicochemical
637
characteristics and diatom community variables from the six metal-polluted streams in August
pH
Dissolved
oxygen
(mg/L)
Conductivity
([LS/cm)
Eagle River
ERI
ER2
ER3
ER4
8.5
8.0
8.0
8.8
8.4
8.0
8.0
7.8
180
145
150
240
11
14
15
18
119
52
54
77
Blue River
FC1
FC2
BRI
BR2
BR3
BR4
BR5
7.8
7.4
7.5
8.1
7.9
7.3
7.8
8.6
8.8
8.2
8.1
8.3
8.3
8.4
55
130
75
85
100
105
100
9
8
11
12
12
11
9
Snake River
DC1
PCI
SRI
SR2
SR3
7.5
7.3
6.7
6.8
6.8
7.6
8.1
7.5
7.6
7.3
65
70
100
90
90
Arkansas River
EFI
7.8
8.4
EF5
ARI
8.2
AR2
8.0
AR3
7.9
8.8
AR5
AR8
7.8
7.4
8.8
7.6
7.6
7.6
8.2
7.3
Chalk Creek
CC1
CC2
CC3
CC4
CC5
8.2
7.3
7.3
7.4
7.9
South Platte River
MCI
7.8
MC2
7.9
MC3
8.3
SPRI
8.5
SPR2
8.7
SPR3
8.2
Stream/
Station
TO HEAVY METALS
Tempera- Alkalinity Hardness
ture (?C)
(mg/L)
(mg/L)
[Zn]
([Lg/L)
Species
richness
122
84
86
126
2
407
336
104
35.3
21.7
20.7
16.7
2.27
1.25
1.15
1.62
0.37
0.48
0.35
0.02
30
41
43
52
42
52
51
42
84
50
60
66
64
64
7
1735
3
9
691
72
56
19.0
5.7
26.0
27.3
14.3
22.0
21.3
1.70
0.63
2.05
1.98
1.04
2.19
2.10
0.17
0.76
0.44
0.43
0.60
0.22
0.17
10
8
11
14
16
10
17
11
16
18
10
48
52
48
48
13
72
364
399
276
34.0
23.7
28.7
29.3
10.7
2.20
2.06
1.90
1.88
0.85
0.43
0.36
0.44
0.53
0.61
90
170
155
150
170
170
100
12
8
13
15
15
17
18
32
70
64
67
69
65
34
48
108
94
92
100
98
44
39
28
24
26
181
79
27
14.0
26.3
23.0
26.3
27.0
22.7
27.0
1.40
2.18
1.83
1.88
2.02
1.94
2.10
0.36
0.35
0.51
0.50
0.46
0.13
0.33
8.0
7.8
7.6
7.6
6.8
55
75
75
80
140
8
10
11
15
21
23
23
28
31
50
32
40
36
42
52
31
452
224
76
26
38.0
22.0
23.3
25.0
42.0
2.38
1.43
1.37
1.75
2.83
0.31
0.64
0.68
0.44
0.21
7.6
7.3
7.2
7.5
7.3
6.8
40
75
155
130
150
210
11
14
16
15
16
20
17
35
69
64
62
89
18
40
96
80
86
110
2
12
168
24
52
10
20.7
7.0
7.7
18.7
11.3
24.0
1.53
0.76
0.80
1.66
0.98
1.89
0.57
0.41
0.12
0.12
0.15
0.10
Diversity Proportion
H'
Achnanthes
Note: Locations of sampling stations are shown in Fig. 2.
3.
1992).
TABLE
Zinc concentration and diatom community variables at a range of Zn sites in the six metal-polluted streams (August
Zn class
Variable
Number of stations
Zn concentration
Cell density (cells/mm2)
Species richness
Species diversity
Relative abundance of
Achnanthes minutissima
Background
8
7.3a
(0.6)
2905
(1471)
24.1
(3.2)
1.80a
(0.17)
0.37a
(0.05)
Low
Medium
8
9
28.Ib
95.5c
(0.8)
4095
(3597)
26.9
(3.3)
2.03a
(0.16)
0.34a
(0.05)
(5.7)
6263
(4789)
19.7
(2.2)
1.72ab
(0.17)
0.23a
(0.05)
High
9
542.6d
(51.8)
1539
(1293)
19.6
(2.7)
1.28b
(0.14)
0.56b
(0.04)
F valuet
94.90***
0.73 ns
1.61 ns
3.94*
7.97**
Notes: Results of one-way ANOVA are also shown. Means with the same letter were not significantly different. Standard
errors are shown in parentheses. * P < 0.05, ** P < 0.01, *** P < 0.001, ns = not significant.
t df = 3,30.
C. NICOLAS MEDLEY
638
AND WILLIAM H. CLEMENTS
Ecological Applications
Vol. 8, No. 3
3
2
LI .Packground
-2
Low
CD
.
CZ
~~~~High
+
E~~~~~~~~
CO
CZ
co
o 21
-2-20
Canonical Variable1
FIG. 4. Canonical discriminantanalysis showingseparationand overlap of background,low, medium,and highZn stations
in metal-pollutedstreams.The analysis was based on the relativeabundance of the 10 dominantspecies, e.g., those species
thatcomprised >1% total abundance. The primarytaxa responsibleforseparationalong each canonical axis is also shown.
edly different
(Table 5). Cell density,species richness,
and species diversitywere lower at the upstreamsite,
Frawhichwas dominatedbyAchnanthesminutissima,
gilaria vaucheriae,and Gomphonemaspp. In contrast,
the downstream communitywas physiognomically
more complex and characterizedby colonies of Diatoma vulgare, long filamentsof Melosira, and several
species of Nitzschia, Navicula, Gomphonema, and
Cymbella.
Physicochemicalcharacteristicsin the experimental
streamswere withinthe range of those observed at
stationsPR5 and PR9 in the Cache la Poudre River,
with the exception of temperaturethatwas higherin
the experimentalstreams(Table 6). Hardness,alkalinity, conductivity,pH, temperature,and dissolved organic carbon (DOC) were greaterat PR9 comparedto
PR5. Mean concentrationsof Zn, Cu, and Cd at the
two fieldsitesand in controlexperimentalstreamswere
low or below detection.Mean Zn and Cd concentrations approximatedtarget concentrationsin treated
streams.Mean Cu concentrations
approximatedthetarget levels in O.5x streams,but were only about half of
the targetvalue in the 5.Ox streams.Total recoverable
and dissolved metal concentrationswere similaron all
sampling occasions.
Communitylevel responses of diatomsto metals in
experimentalstreamsvaried between sites and among
metal treatments(Fig. 5). Significantmetal and site
effectswere observedforall variables except cell density.Metal X site interactiontermswere significant
for
all variables except species richness,indicatingthat
effectsof heavymetalson periphyton
communitiesdifferedbetween stations.For example, cell densitywas
similar between sites in control streams,but the re-
4. Results of stepwisemultipleregressionanalyis showingtherelationshipsbetweenperiphyton
communityvariables,
Zn concentration,and otherabiotic variables in the six metal-pollutedstreams(August 1992).
TABLE
Dependent
variable
Species richness
df
Overall
F value
Overall r2
Overall
P value
3,30
5.43
0.35
0.0042
Partial r2
P value
(-)Zn
(-)Elevation
(-)pH
0.23
0.06
0.06
0.0036
0.1230
0.1100
Model variable
Shannon-Weinerdiversity
1,32
15.49
0.33
0.0004
(-)Zn
0.33
0.0004
Relative abundance of
Achnanthesminutissimna
3,30
11.56
0.53
0.0001
(-)pH
(+)Zn
(-)Conductivity
0.36
0.13
0.04
0.0002
0.0072
0.1200
Relative abundance of
Fragilaria vaucheriae
3,30
3.4
0.25
0.0300
(+)Zn
(+)pH
(+)Elevation
0.13
0.07
0.05
0.0350
0.1230
0.1460
Note: The table shows the F values and
effect(+, -) of the independentvariable.
r2
for the complete model, r2 for each independentvariable, and the directional
August 1998
DIATOM RESPONSES
5. Diatom communityvariables and relative abundance of dominanttaxa on tile substrateat stations PR5
and PR9 in October 1993.
TO HEAVY METALS
TABLE
Variable
PR5
PR9
Cell density(cells/mm2)
Species richness
Shannon diversity
13 380
23
2.01
62500
38
2.81
Achnanthesminutissima
Achnanthesdefiexa
Cocconeis placentula
Cymbellaminuta
sinuata
Cymnbella
Cymbella turgidula
Diatomnavulgare
Fragilaria vaucheriae
Gomphonemaparvulumn
Gomphonemasubelavatum
Hannaea arcus
Melosira sp.
Navicula cryptocephela
Nitzschia sp.
Synedra sp.
45.4
1.4
1.1
10.8
12.8
8.0
2.8
2.0
1.1
2.5
25.6
4.3
3.9
8.3
4.3
3.1
8.7
+
3.5
5.6
5.6
8.1
3.3
5.8
3.1
Notes: Data were collected on the same day thattiles were
collected forthe experimentalstreamstudy.Minus signs (-)
denote absent; plus signs (+) denote <1.0%.
duction in cell densitywas greaterfor communities
fromthe downstreamsite (PR9) in the O.5x streams.
Exposure to heavy metals significantly
altereddensities of dominanttaxa at bothsites,and the5.Ox treatmentwas acutelytoxic to all diatom species (Fig. 6).
Abundance of Achnanthesminutissimaincreased by
-60% in the O.5x treatments
comparedto controlsfor
both sites; however,the responsesof othertaxa varied
between sites. Fragilaria vaucheriae and Cymbella
minuta were relativelytolerantto metals, and populations fromthe downstreamsite (PR9) increased in
theO.5x streams.Nitzschiapalea and Synedrarumpens
from both sites exhibited intermediatesensitivityto
metals. The two most sensitivespecies, Diatoma vulgare and Melosira varians, were found only at the
downstreamsite. Large colonies of these species detached fromthe substratesoon afterdosing was initiated, and both species were eliminated fromtreated
streamswithin24 h.
TABLE
639
DISCUSSION
Longitudinalvariationin the Cache la Poudre River
Changes in communitycompositionobservedin the
Cache la Poudre River fromupstreamto downstream
were similarto the successional sequence observed in
westernstreamsfollowingphysicaldisturbance(Fisher
et al. 1982). Achnanthesminutissima,whichwas most
abundantat upstreamsites, is oftendominantimmediately after scour events (Peterson and Stevenson
1990, Stevenson 1990). Fragilaria vaucheriae and
Hannaea arcus showed similartrendsand oftendominate in high altitudestreamsor in early successional
communities (Horner and Welch 1981, Poff et al.
1990). Conversely,species with greatervertical orientationthat are more common in later successional
communities(e.g., Navicula, Gomphonema,Cymbella,
Melosira, and Nitzschia)increasedin abundancedownstream.
Althoughsome researchershave proposed the general applicabilityof this periphytonsuccessional sequence on a temporalscale (Hoagland et al. 1982, Korte
and Blinn 1983), few studies have foundclear spatial
patternsand none have related succession to longitudinal patternsin physicochemicalcharacteristics
(Jones
and Barrington1985, Moss and Bryant1985). Molloy
(1992) found a relationshipbetween morphological
growthformsand streamsize in threeriversin Kentucky. Small adnate species (Achnanthesspp.) were
more abundantin headwaterstreams,whereas centric
and filamentousspecies occurredwithgreaterfrequency downstream.Ward (1986) found similar trendsin
Saint Vrain Creek, Colorado, wherelater successional
species were foundin higherabundanceat downstream
sites.
The complexrole of abiotic variablesin determining
is clearlyillustratedat
periphytoncommunitystructure
PRl. We expected this high elevation site would have
the greatestproportionof early colonizing species and
be dominated by Achnanthesminutissima.However,
species compositionat PR1 was similarto thatfound
at PR9, the furthestdownstreamsite. The hydrologic
6. Physicochemicalcharacteristicsof field sites (PR5 and PR9) and in experimentalstreamsin October 1993.
Field sites
Variable
PR5
PR9
Experimentalstreams
0.0x
0.5x
5.Ox
12
Temperature(?C)
2
7
12
12
Dissolved oxygen (mg/L)
12.4
10.1
8.1
7.8
7.3
81
Conductivity([LS/cm)
38
115
81
81
18
70
31
31
31
Hardness (mg/LCaCO3)
32
32
32
Alkalinity(mg/LCaCO3)
20
57
pH
7.6
8.8
7.6
7.5
7.3
2
Dissolved organic carbon (mg/L)
2
272
2
2
547.0 (218.0)
Zinc ([Lg/L)
7.0
6.0
5.6 (1.4)
54.0 (12.5)
bd
36.0 (17.0)
Copper ([Lg/L)
1.0 (0.4)
6.7 (1.5)
bdt
bd
6.7 (2.9)
Cadmium ([g/L)
0.18
bd
0.8 (0.2)
Notes: Field data were collected on the same day that tiles were collected for the experimentalstreamstudy.Standard
errorsare shown in parentheses.
t bd = below detection.
C. NICOLAS MEDLEY
640
FTT]
3
PR5
U
PR9
AND WILLIAM H. CLEMENTS
MetalIP=0.0038
~5
cn 2
~
o
~ ~
~
~
~
~
~
MetalP = 0.0001
Site P = 0.117
Metalx SiteP= 0.016
80
P = 0.0081
~~~Site
x SiteP =0.0004
~~~Metal
~
.
r)
Ecological Applications
Vol. 8, No. 3
60
~~~~40
20
C)~~~~~~~~~~~~~......
40
00
P 0.0002
~~~~~Metal
SiteP =0.0326
MetalxSiteP 0.13
cn
....Site
0.004
30
C~
MetalP 0 0001
P =0.0001
Metalx SiteP 0.046
0.003
0
....
20
0.002
CL
10
0
.........~~~~~~~~~~~U.
..............
0.001
0.0
0.5
Treatment
5.0
0
--
.......
0.0
0.5
Treatment
5.0
FIG. 5. Mean (-1-1SE) Shannon-Wienerdiversity,species richness, cell density,and ash-freedry mass (AFDM) of
fromthe experimentalstreamstudy.Results of two-wayANOVA
periphytoncommunitiesin O.Ox,O.5x, and 5.Ox treatments
(site X metals treatment)are shown in each panel.
regime and water quality at PR1 were influencedby
Poudre Lake, 0.3 km upstreamfromthe samplingsite.
We hypothesizethatthestablewaterqualityconditions
and flow regime at PR1 created an environmentthat
was favorable to developmentof a midsuccessional
community.
In summary,diatom communitiesare sensitiveto a
host of environmentalvariables that change along a
stream's longitudinalgradient. We hypothesize that
these physicochemicalconditionsconstitutea general
productivitygradientfromupstreamto downstream,
which influencesecological succession and produces
later successional communitiesat downstreamsites.
We suggest that this natural variation in periphyton
communitystructuremay complicate biomonitoring
studies in Colorado mountainstreamswherephysicochemical changes occur overrelativelyshortdistances.
Although"metal treatments"in the six metal-polluted
streamsin our studywere not assigned randomly,we
feelthatour studydesignis an improvement
oversingle
streamstudies and thatour resultshave more general
applicability.
Periphyton communities in Colorado mountain
streams showed little response to metal levels at or
below the U.S. EPA waterqualitycriterionforZn (50
Kg/L), a findingthat was also reportedfor macroinvertebratecommunities(Clements and Kiffney1995).
One-way ANOVA and canonical discriminantanalysis
(CDA) indicated thatdifferencesin communitycomposition were apparent at sites where Zn levels exceeded 200 Kg/L.Communitiesin these streamswere
dominatedby Achnanthesminutissimaand Fragilaria
vaucheriae, a patternfound in otherstreamspolluted
by heavy metals (Leland and Carter 1984, Deniseger
et al. 1986, Takamuraet al. 1990, Kelly et al. 1995).
Communityresponses to metals in thefield
We suggestthatourresultsrepresenta generalresponse
Most field studies that investigatethe response of of periphyton
communitiesto minedrainage,whichare
aquatic communitiesto contaminantsare confinedto a applicable to otherColorado streamspollutedby heavy
single stream,whereupstreamreferencesites are com- metals.
Results of our studyof metal-pollutedstreamsare
pared to downstreampollutedand recoverysites. Conclusions drawnfromthese studies are applicable only limitedbecause samplingwas restricted
to late summer,
to thespecificstreamunderstudy(Hurlbert1984, Feld- low-flowconditions.Diatom communitycomposition
man and Connor 1992, Clements and Kiffney1995). in streamsoften shows high temporalvariation(Ste-
August 1998
DIATOM RESPONSES
TO HEAVY METALS
PR5----------50
Achnanthes
minutissima
40 -
641
PR9
TrtP < 0.0001
Site P < 0.039
Trtx Site P <0.0001
4
TrtP < 0.35
Site P < 0.84
TrtxSiteP<0.33
Frilana
Falarea
302
20
10
8ontrol
10 -
Low
Cymbella
minuta
8
|
)
lx
10
TrtP<0.0001
SiteP < 0.011
Synedra
High
TrtP < 0.0001
SiteP < 0.0001
Trtx Site P < 0.0001
6
4T
1t
2
75
Low
rumpens
8
Trt Site P < 0.12
6
E
ntrol
High
4
2
*
0
>1
0.0
0.5
-
Nitszchiapa/ea
0
O
1, h-
0.0
15
25-
1
0.5
Diatoma
0.0
5.0
15,0
TrtP < 0.0032
< 0.24
x Site F < 0.33
SiteP
~~~~~~~~~~~~Trt
0
?
0.5
MeIo.sira
5
1~~~~~~~~~~~~~~~,
o
< 0.53
FP<
0.38
ieF<03
6
TrtF < 0.005
5.0
Gomphonena
parvulum TrtF
1TrtxSite
Tr12,5
5.0
20v0^ul7are
5
0.0
0.5
5.0
TrtF < 0.0001
0
0.5
5.0
0.0
0.5
5.0
Treatment
FIG. 6. Mean (+1 SE) abundance of dominantspecies in
0 .0x, O.5x, and 5.Ox treatmentsfromthe experimentalstream
study.Results of two-wayANOVA (site x metals treatment)are shown in each panel.
642
C. NICOLAS MEDLEY
AND WILLIAM H. CLEMENTS
venson 1990), which may complicate the use of diatoms as indicatorsof water quality (Pan et al. 1996).
It is likely thatseasonal variationin periphytoncommunitycompositionand physicochemicalcharacteristics will influenceresponses to heavy metals in Colorado streams.For example, we would expect thatthe
earlysuccessional communitiespresentin springwould
be more tolerantof heavy metals thanthe late successional communitiesof summer.Seasonal variationin
heavy metal concentrationscould also influenceperiphytoncommunitycomposition.Previous studies of
Colorado's metal-pollutedstreamshave reportedthe
highest metal levels during spring runoff(Clements
1994). The influenceof seasonal variationin diatom
communitycompositionand physicochemicalcharacteristicscould be examined by conductingmicrocosm
experimentswithspringand summercommunities.
Results of our fieldstudymay have been influenced
by the indirecteffectsof invertebrategrazers,particularly heptageniidmayflies.Our previous work has
shown thatheptageniidsare highlysensitiveto heavy
metalsand are usuallyabsentin metal-pollutedstreams
(Clements 1994, Clementsand Kiffney1995). It is possible thateliminationof these metal-sensitivegrazers
may have altered diatom communitycompositionin
our study.In a reviewof grazer-periphyton
interactions
in streams,Feminellaand Hawkins(1995) reportedthat
the most consistentresponse of algal assemblages to
grazers was reduced abundance of numericallydominant species, includingA. minutissima.Therefore,the
greaterabundanceofA. minutissimaobservedat metalpolluted sites in our studymay have been influenced
by reduced abundanceof grazingmayflies.We suggest
that additional experimentalstudies are necessary to
understandthe complex interactionsbetween invertebrate grazersand heavy metal pollutionin streams.
Communityresponses to metals in experimental
streams
Ecological Applications
Vol. 8, No. 3
low-elevationstreamsare naturallydominatedby late
successional species, then effectsof metals on these
communitieswill be greater.Consequently,it may be
necessary to account fordifferencesin sensitivitybetweenlocationswhenestablishingwaterqualitycriteria
to protectaquatic organisms (Kiffneyand Clements
1996).
Some attemptshave been made to relate morphological growthformsand reproductivestrategiesof algae to theirability to dominatedisturbedhabitatsor
duringearly stages of succession (Fisher et al. 1982,
McCormick and Stevenson 1991, Stevenson et al.
1991, Molloy 1992, Petersonand Stevenson 1992). To
our knowledge no attemptshave been made to relate
life historycharacteristicsand morphologicalgrowth
formsof algae to theirtoleranceof contaminants.We
suggest that the same morphological and ecological
characteristicsthatallow a species to dominateduring
early stages of primarysuccession also influencetolerance to heavy metals. In our field studies, communitiesfrombothreferencesites and metal-pollutedsites
weresimilarand dominatedbyAchnanthesminutissima
or Fragilaria vaucheriae. The natural dominance of
these metal-tolerant
species at referencesites may also
explain why we did not observe large shiftsin communitycompositionat moderatelypolluted sites (e.g.,
whereZn levels werebetween51 and 200 Kg/L)in our
study of metal-pollutedstreams.Our hypothesisthat
morphologicaland life historycharacteristicsof diatomsinfluencetoleranceto contaminantscould be tested by exposing communities collected at different
stages of succession to heavy metals. We predictthat
communitiescollected duringthe early stages of succession would be dominatedby small, adnate species,
which would be more tolerantto metals than those
collected duringlater stages.
It may be possible to correctfor the influenceof
naturallongitudinalchangesin communitystructure
by
developing predictiverelationshipsbetween communitycompositionand elevationfromregionalreference
sites (sensu Fausch et al. 1984); however,because of
the sensitivityof diatom communitiesto manyabiotic
variables,locatingreferencesites withinadjacentriver
drainagesmaybe difficult.
Adjacent streamswithinthe
same watershedmay have very differentdiatom assemblages because of differencesin geology, water
chemistry,and land use (Kutka and Richards 1996).
Because routinebiomonitoringstudiescannotestablish
direct cause-and-effectrelationships between communitypatternsand anthropogenicstressors,it is importantto validatebiological indicatorsexperimentally.
We suggestthatintegratingexperimentalstudies with
routinefieldbiomonitoringwill supportthe establishment of causal relationshipsbetween anthropogenic
disturbancesand communityresponses.
Results of the microcosmstudysupportthe hypothesis thateffectsof heavy metals were greateron communitiescollected fromthe downstreamsite. We suggest thatdifferencesin metal effectswere a resultof
naturaldifferencesin communitycompositionbetween
sites. Communitiesfromthe upstreamsite were dominated by early successional species (Achnanthesminutissimaand Fragilaria vaucheriae), which were relativelytolerantto metals.In contrast,downstreamcommunitieswere dominatedby colonial and filamentous
species (Melosira varians and Diatoma vulgare),which
were highlysensitiveto metals.Althoughsome diatom
species collected fromthe downstreamsite were relatively tolerantto metals,M. varians and D. vulgare
were eliminatedin the 0.5x treatments.
Differencesin
sensitivityto contaminantsbetween locations, which
Conclusions
have also been reportedforbenthicmacroinvertebrate
communities(Kiffneyand Clements 1996), have imDespite the commonuse of diatomsas indicatorsof
portantpractical implications. If communitiesfrom waterquality,many of the relationshipsbetween spe-
August 1998
DIATOM RESPONSES
cies abundance and specificwaterqualityvariables remain weak and untested.Cattaneo et al. (1993) concluded thatthe predictivepower of diatomcommunity
models was limited because environmentalvariables
includedin analyses are inappropriateforthe sampling
scale and taxonomiclevel chosen. The use of diatoms
as indicatorsof water quality is also hamperedby a
lack of informationon how diatom assemblages respond across largerspatial scales (Kutka and Richards
1996). In our study,multipleregressionanalysis indicated thatZn was an importantpredictorforperiphytoncommunitymeasures;however,elevationand other variables significantly
correlatedwithelevation(pH,
and alkalinity)were also important
conductivity,
in explaining communitycomposition.Differencesin life
historycharacteristics
and morphologicalgrowthforms
of dominanttaxa between locations will influenceresponses to contaminants.We suggest that it may be
usefulto studyperiphyton
community
responsesto pollutantsin relationto life historytraits,ecological strategies, and morphological growthforms.Conducting
researchwithinthis frameworkmay improvethe predictiveabilityof periphytonmodels and providea theoretical basis for understandingthe relationshipsbetween observed communitypatternsand the evolutionaryprocesses thatdeterminea species responseto
disturbance.
ACKNOWLEDGMENTS
Ron Schuett,Mark Pearson, Diana Hammerdorfer,
and numerousundergraduatestudentsprovidedfieldand laboratory
assistance for this research. Special thanksto Dave Beeson
and Paul Krugrensfor taxonomic assistance. Commentsby
Rex Lowe, Jan Stevenson, and two anonymous reviewers
improvedthis manuscriptand are greatlyappreciated.Metal
analyses were conductedat theColorado Division of Wildlife
withthe assistance of Steve Brinkmanand Pat Davies. Funding for this research was provided by the U.S. Geological
Survey (award number14-08-0001-G2099) and the National
Instituteof EnvironmentalHealth Sciences, SuperfundBasic
Research Program(award numberP42 ES05949).
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