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Ecological Applications 6(2), 1996, pp. 449-457
EVALUATION OF EPISODIC ACIDIFICATION AND
AMPHIBIAN DECLINES IN THE ROCKY MOUNTAINS'
FRANK
A.
VERTUCCI
ENSR Consultingand Engineering,4413 WestLa Porte Avenue,Fort Collins, Colorado 80521 USA, and
Departmentof Biology, Colorado State University,
Fort Collins, Colorado 80523 USA
PAUL STEPHEN
CORN2
National Biological Service, MidcontinentEcological Science Center,
4512 McMurryAvenue,Fort Collins, Colorado 80525-3400 USA
Abstract. We definecriteriafordocumentingepisodic acidificationof amphibianbreeding habitatsand examinewhetherepisodic acidificationis responsibleforobserveddeclines
of amphibianpopulations in the Rocky Mountains. Anthropogenicepisodic acidification,
caused by atmosphericdepositionof sulfateand nitrate,occurs when the concentrationof
acid anions increases relativeto the concentrationof base cations, resultingin a decrease
in acid-neutralizingcapacity (ANC). However,because several naturalprocesses can also
depress ANC, monitoringpH and ANC alone cannotprovide evidence thatepisodic acidificationof amphibianhabitats is anthropogenic.We examined published data on water
chemistryfromcentralColorado and southernWyomingforevidence of episodic acidification, and we also comparedoriginalwaterchemistrydata to observationsof amphibian
breeding phenology at three sites in northernColorado. There is limited evidence that
anthropogenicepisodic acidificationmay occur in high-elevationhabitats in the Rocky
Mountains,but thereis no evidence thatepisodic acidificationhas led to acidic conditions
(ANC <0) or that amphibianembryos are presentduringthe initial phase of snowmelt
when episodic acidificationmightoccur. The declines of some amphibianspecies in the
Rocky Mountainsare more likelydue eitherto naturalor anthropogenicfactorsotherthan
acidic deposition.
Key words: acid-neutralizingcapacity; acidification;Ambystomatigrinum;amphibians; Bufo
boreas; Colorado; decliningamphibians; episodic acidification;Pseudacris triseriata;Rana sylvatica;
snowmelt;Wyoming.
INTRODUCTION
Declines of common, widespread species of amphibians have been documentedrecently,particularly
in thewesternUnitedStates (Corn and Fogleman 1984,
Hayes and Jennings1986, Bradford1989, Corn et al.
1989, Carey 1993, Fellers and Drost 1993, Kagarise
Shermanand Morton 1993, Blaustein 1994, Blaustein
et al. 1994b). Many of thesedeclines are due to obvious
humanalterationof breedinghabitats(Hayes and Jennings 1986, Bradford1989, Corn 1994, Pechmannand
Wilbur 1994) and otherdeclines are likelyattributable
to naturaland randomprocesses (Corn and Fogleman
1984, Pechmann et al. 1991, Pechmann and Wilbur
1994). However,whena commonspecies declines synchronouslyover a wide area withoutapparenthabitat
degradation,for example boreal toads (Bufo boreas)
since about 1975 in the southernRocky Mountains
(Corn et al. 1989, Carey 1993, Stuartand Painter1994),
randomevents are an unlikelyexplanation and other
anthropogeniccauses mustbe investigated.
Acidificationof amphibianhabitatshas been invesI Manuscriptreceived 26 September1994; revised 4 April
1995; accepted 6 April 1995; finalversion received 15 May
1995.
2 Authorto
whom reprintrequests should be addressed.
tigatedas one possible cause of widespreadamphibian
declines. Natterjacktoads (B. calamita) in Britainmay
have declined due to long-term,persistent(chronic)
acidificationof breedinghabitats(Beebee et al. 1990),
butno examples of amphibiandeclines in NorthAmerica have been attributedto acidificationdespite numerousstudiesof theeffectsof acid conditionson amphibian embryos (see reviews by Freda et al. 1991,
Pierce 1985). Corn and Vertucci(1992) and Bradford
et al. (1992, 1994) concluded that therewas no evidence for chronic acidificationof amphibianhabitats
in the mountainsof westernNorthAmerica.
Short-term
decreases in the acid-neutralizingcapacity(ANC) of surfacewatersduringhydrologicalevents
like precipitationor snowmelthave been referredto as
episodic acidification(Wigingtonet al. 1990, 1992),
which can resultin a short-term
loss of all acid-neutralizingcapacity (ANC <0 &molc/L),
at which time
effectson aquatic biota may occur due to increased
concentrationsof hydrogenions and metals like aluminum(Baker et al. 1990). Despite the large volume
of researchon acidic deposition,episodic acidification
events and the effectsof acidificationon species other
thanfishare under-represented
in the literature(Schindler 1992). However, Harte and Hoffman(1989) hypothesizedthatepisodic acidificationhad caused a de-
449
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450
F. A. VERTUCCI AND P. S. CORN
cline of tiger salamanders (Ambystomatigrinum)in
Colorado.
Contraryto the situationin easternNorthAmerica,
wherespringand summerrainstormsare the dominant
hydrologicevents thatinfluencethe chemistryof amphibianbreedinghabitats(Freda et al. 1991), episodic
acidificationevents in high-elevationamphibianhabitats in the West occur largely during snowmelt.We
have suggestedthatlife historytraitsof most amphibian species made themunlikelyto be exposed to potentialepisodes of acidificationduringsnowmelt(Corn
and Vertucci1992). A fullassessmentof episodic acidificationas a cause of amphibiandeclinesrequirescareful documentationof water chemistrychanges associated with the timingof actual breedingor presence
of sensitiveamphibianlife stages.
Identifying
episodic acidification
Ecological Applications
Vol. 6, No. 2
which can generateacidityand NO3- therebyincreasing Ca relative to Cb (Galloway et al. 1987); and organic-acid pulses fromsoil layers,whereANC decline
is not associated witheitherCb decrease or an increase
in Ca (Denning et al. 1991).
The amountof episodic acidificationpotentiallydue
to acidic depositionis the amountof ANC decline in
excess of thedecline in base-cationconcentrations(dilution) or increase in organic acids. To quantifythe
actual amount of episodic acidificationdue to acidic
deposition,the acid anions (i.e., NO3- and SO42-) associated with the ANC decline must be fromthe atmosphericdeposition of nitricand sulfuricacids and
notothernaturalsources. Sea salt effectsare evidenced
by a depression of the Na+ to Cl- ratios in surface
waters and are likely only in coastal areas. Organicacid episodes can be distinguishedby highlycolored
eventwaterswithhighdissolved organiccarbon(DOC)
concentrationsor large apparent anion deficits. Increased pCO2 due to microbialactivitywill onlyreduce
pH and not affectANC. Increased Ca resultingfrom
is difficultto distinguishfromdeposition
nitrification
sourcesof nitratebutwill be characterizedby veryhigh
levels of surface-waternitrateconcentrationsrelative
to sulfate.
In this paper,we examine threesets of data for evidence of episodic acidificationof amphibianbreeding
habitatsin the Rocky Mountains,describe the phenology of amphibian breeding relative to potential episodic acidification,and describe how to documentepisodic anthropogenicacidification.
Episodic acidificationevents may be "natural" or
associated with the effectsof atmosphericdeposition
of strongacids. Definingor quantifyingacidification
as pH decline is not advocated due to both the difficultiesin measuringpH (Metcalf 1984, Turk1988, Asburyet al. 1989) and the nonconservativebehaviorof
pH with respect to CO2 (Stumm and Morgan 1981).
The possible mechanismsresponsibleforepisodic acidificationcan be illustratedusing a charge-balancedefinitionof ANC, which is applicable to most surface
waters. If organic-acid and aluminumconcentrations
are low, then ANC is equivalent to carbonate-bicarbonate alkalinity,which can be definedby Alkalinity
= Cb - Ca, where Cb is the sum of the concentrations
METHODS
of charges on the base cations Ca2+, Mg2+, Na+, K+,
and NH4+, and Ca is the sum of the concentrationsof
Data sources and studysites
charges on the acid anions NO3-, S042-, F-, and Cl-.
We examined publisheddata fromtwo locations for
Unanalyzed heavy-metalcations could influencethis
charge-balancedefinitionof alkalinity,and anthropo- evidence of anthropogenicepisodic acidification:the
genic depositionof trace metals does occur in pristine West Glacier Lake outlet,Albany County,Wyoming,
ecosystems(Nriagu and Pacyna 1988). However,con- in theMedicine Bow Mountains(Vertucci1988, Reuss
centrationsof trace metals are too low to materially et al. 1993); and the ponds in the Mexican Cut Nature
affectchargebalance, so these are routinelydisregard- Preserve, Gunnison County, Colorado, in the Elk
ed in calculatingalkalinity(e.g., Wigingtonet al. 1990, Mountains(Blanchard et al. 1987, Harte and Hoffman
Vertucciand Eilers 1993).
1989, Wissinger and Whiteman 1992). West Glacier
Naturaldeclines in ANC may occurduringsnowmelt Lake is permanent,and the Mexican Cut ponds are a
as dilute melt waters increase their contributionto mixtureof small temporaryand semi-permanent
ponds.
streamflowand pre-episodesurface-watercation con- In addition to these published studies, from 1987
centrationsare diluted. Episodes of acidificationare through1990 we monitoredamphibianbreedingactivproducedby thedilutionof base cations if thedecrease ityand waterchemistryat threesites in LarimerCounin Cb is greaterthanany decrease in Ca. Dilutioncannot ty, Colorado, between the Never Summer Mountains
by itselfcause acidic episodes whereANC <0 pmol,/L. and the Medicine Bow Mountains: Matthews Pond
Other natural episodic acidification events can be (2800 m elevation,40?35'44" N, 105?50'42" W), Lily
caused by: a "salt effect"in coastal areas wherecations Pond (2900 m, 40?34'27" N, 105?50'36" W), and Trap
deposited in saline precipitationexchange with ad- Lake (3050 m, 40?33'36" N, 105?48'36" W). Lily and
sorbedprotonsin thesoil, reducingsurface-water
ANC Matthewsponds are snowmelt-filled
vernalponds that
duringperiods of high flow(Wrightet al. 1988, Heath are dry in most years by late summer,and Trap Lake
et al. 1992); increasedCO2 due to microbialrespiration, is a permanentdrainage lake.
where increased pCO2 decreases pH but leaves alkaDuring our observations,two of theLarimerCounty
linityunchanged(Kratz et al. 1987); soil nitrification, sites had breeding amphibians. Chorus frogs (Pseu-
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May 1996
EPISODIC
ACIDIFICATION
dacris triseriata)bredat MatthewsPond and Lily Pond
and wood frogs (Rana sylvatica) bred at Matthews
Pond. Two sites have lost amphibianpopulations: B.
boreas and P. triseriataat Trap Lake, and B. boreas
and R. sylvaticaat Lily Pond (Corn et al. 1989). Also,
P. triseriatahas undergonea long-termdecline at Lily
Pond. From 1961 to 1973, numbersof breedingmales
fluctuatedannuallyfrom-500 to 1500 individuals,but
since 1986, the annual estimateof breedingmales has
varied between45 and 140 animals (Corn et al. 1989;
P. S. Corn, unpublisheddata).
Waterchemistry
AND AMPHIBIANS
120
H
451
7.0
episodicacidification
period
1002
80
.o
605
-6.8
pH
-6.6
6.48
O40 -
ANC -6.2
20 -
6.0
0
We collectedwatersamples weeklyduringsnowmelt
I
I
I5.8
0
25 May 1Jul 1 Aug 1 Sep 1 Oct
and the amphibianbreedingseasons at Lily Pond and
MatthewsPond during1987-1990 and Trap Lake durFIG. 1. Episodic acidificationin 1988 at West Glacier
ing 1988-1990. Each sample was taken in a 250-mL, Lake Outlet,Wyoming.
opaque, high-densitypolyethylenebottle from the
breedinghabitatin thelittoralzone. Unfilteredsamples
snowmelt indicated a subtle episodic acidification
were kept refrigerated
beforetheywere delivered for
event(Vertucci1988). Duringthewinterand fall,baseanalysis. Water samples collected in 1987 were anacation concentrationsand ANC at the lake outletwere
lyzed by theSoils TestingLaboratoryat Colorado State
relativelyhigh,about 100 and 65 Kmolc/L,
respectively.
University.All subsequentsamples were analyzed by
As initialsnowmelt-generated
outletflowfromday 144
the WaterAnalysis Laboratoryat the U.S. Forest Serof the year (24 May) throughday 150 (30 May), the
vice Rocky Mountain Forest and Range Experiment
concentrationof base cations initiallydecreased <10
Station in Fort Collins (Corn and Vertucci 1992). At
The ANC dropped -30 ,.molc/Land the acid
both laboratories,pH was measured electrometrically V&molc/L.
anions increased 20 Kmolc/L,resultingin an apparent
with a glass electrode,and ANC was determinedby
anthropogenicepisodic acidificationof -20 pmolcIL.
Gran titration(Gran 1952). Major anions and cations
The continuingdecline in ANC after30 May coincided
were measured following methods recommendedfor
with declines in both acid anions and base cations.
studiesof acid deposition(U.S. EnvironmentalProtecThus, the continueddecline in ANC was due to the
tion Agency 1987).
increasingdominance of dilute snowmeltwaters,not
Data on water chemistryfrom the Mexican Cut
anthropogenicacidification.Only withnear-dailysamponds used by Harte and Hoffman(1989) were taken
pling duringthe initial snowmeltperiod were we able
as describedin Blanchardet al. (1987). Data on water
to detect any evidence of episodic acidificationconchemistryfromtheMexican Cut ponds (obtainedfrom
sistentwithan anthropogenicsourceof thestrongacids
Wissingerand Whiteman1992), and WestGlacierLake
present in deposition and delivered to the lake via
outlet (obtained fromVertucci 1988 and Reuss et al.
snowmelt(Vertucci1988). Althoughthesnowpackwas
1993), were also analyzed by the Forest Service labslightlyacidic each year,no acidic episodes (ANC <0
oratoryin Fort Collins.
imolc/L) were recorded at the lake outlet duringthe
subsequent3 yr of monitoring(Reuss et al. 1993).
RESULTS AND DISCUSSION
Episodic acidificationof
West Glacier Lake outlet
Episodic acidificationin the central
RockyMountains of Colorado
Monitoringof waterchemistryat West Glacier Lake
outletbegan withthefirstmeltwater-generated
flowon
24 May (Fig. 1). Liquid water present in the outlet
flumebeforethisdate was deriveddirectlyfrommelting of the snowpack above the flume.Such meltwater
is acidic but is not representativeof the lake outletor
any potentialamphibianhabitat.While snowmelt,as
sampledfromlysimetersadjacentto thelake, is acidic,
Ca > Cb and ANC =-20
[imolc/L(Vertucciand Conrad 1994), thelake outletwaterderivedfroma mixture
of snowmeltand wateraftercontactwiththewatershed
soils and rock is not acidic (ANC >0 wmolc/L;Fig. 1).
Daily sampling of outlet chemistryduring early
Complete major cation and anion data are available
fromPond L- 12 at Mexican Cut duringspringand summerof 1984 and 1985 (Blanchardet al. 1987) and 1991
(Wissingerand Whiteman 1992). Except for the first
samples in 1984 and 1985, there is no evidence of
acidic conditionsin thishabitator of anyepisodic acidificationassociated withstrongacid anions (Blanchard
et al. 1987; Fig. 2). The patternof ANC, Cb, and Ca
was similareach year and clearly shows thatthe seasonal patternof ANC is directlyrelatedto changes in
Cb (dilution) and is relativelyunaffectedby Ca. According to Blanchard et al. (1987), the early acidic
samples in 1984 and 1985 were collected fromabove
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F. A. VERTUCCI AND P. S. CORN
452
80- Apr IMay
60-
Jun
1984
Jul
Aug I Sep I Oct 6.8
16.4
- - -
40
cXb
20-
-
pH
6.0
5.6
ANC
5.2
080-
O
E 60=L
6.8
1985
pH6
pH
6.4~
~~~~~~~1
4__
ANC
~20
5
.. a.6
0
80
W
60o-1\
5 .2
6.8
1
1991
6.4
,
*
40=5
~~~~~~pH
ANCb60
20,
ANC
5.6
0 -- - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - -5.2
100 120 140 160 180 200 220 240 260 280 300
Day ofYear
FIG. 2. Episodic acidificationat Pond L-12 at Mexican
Cut, Colorado. The data are fromBlanchardet al. (1987) and
Wissingerand Whiteman(1992).
thefrozenpond surface.Such samples clearlyrepresent
thechemistryof local snowmelt,not thebreedinghabitat of A. tigrinum.For example, samples collected
above the frozenpond surfaceon 7 May 1985 had pH
27 p~mol,/L,Ca = 33 pKmolc/L,
and ANC
=5, Cb
(calculated from C1 - Ca) = -6stmol/L. Samples
collected frombeneaththe ice on 7 May 1985, where
A. tigrinumcould have been breeding,showed pH
= a75 mol/L, and
C( a
5.92, ANC =( 33 mole CHL,
53 rapmol/L,
and these samples neitherwere,acidicnor
evidenced anthropogenicepisodic acidification(Blanchard et al. 1987).
Episodic acidificationof Front Range
amphibianhabitats
Intervalsof -=1I00m of elevationseparatedthethree
ponds we surveyed.Consequently,in mostyearssnowmelt lagged by 1 wk at each step fromMatthews
Pond to Lily Pond to Trap Lake. We did not sample
MatthewsPond early enough to see initial snowmelt
in any year (Table 1). However,at Lily Pond (Table 2)
and TrapLake (Table 3) our initialwatersamples were
takenbeforetheappearanceof surfacewaterand breeding amphibians.There is no evidence of eitheranthropogenic episodic acidificationor acidic conditionsat
Ecological Applications
Vol. 6, No. 2
any site in any year (Tables 1-3). MinimumANC values are associated withlow base-cationconcentrations
(Cb), not elevated concentrationsof acid anions (Ca).
All threeof these sites showed discrepanciesbetween
ANC and (Cb - Ca) attributableto organic acids. Dissolved organiccarbon (DOC) was measuredoccasionally and was highestat MatthewsPond (>20 mg/L),
intermediateat Lily Pond (8-15 mg/L),and lowest at
Trap Lake (5-8 mg/L). There is no evidence thatorganic acids produced acidificationepisodes at these
sites. The discrepancyobservedbetweenANC and (Cb
- Ca) was greatestat MatthewsPond, the high-DOC
habitat,consistentwith an anion charge of 4 Kmolc/L
per milligramof DOC.
As with snowmelt,we did not observe Matthews
Pond beforefrogsbegan breedingexceptin 1988, when
breedingby R. sylvatica and P. triseriatabegan after
8 May (Table I). Both frogs,however,had prolonged
breedingseasons with most eggs laid afterall of the
surface ice and much of the snow in the surrounding
foresthad melted.Rana sylvaticais well knownas an
explosive breederelsewherein its range (Seale 1982),
but in all 4 yr at MatthewsPond, eggs were deposited
fairlyuniformlyover periods of :2 wk. We have less
informationabout actual depositionof eggs by female
P. triseriata,which distributetheirclutchesin several
to detect.At one site
small egg masses thatare difficult
at a lower elevation in Colorado, however,breeding
activityby male P. triseriataoccurredover 6 wk, and
gravid females were presentthroughout(Corn 1980).
At Lily Pond, no P. triseriatawere observed until
ice covered <50% of thepond's surface(Table 2). Trap
Lake historicallyhad a large population of B. boreas
(Corn et al. 1989), but we observed toads at this site
only in 1987. That year,we founda few males and one
spoiled egg mass on 11 June.We did not collect serial
watersamples at Trap Lake in 1987, but snowmeltthat
year was the earliest of the period 1987-1990. Bufo
boreas breeds latest in the year of all the amphibians
in themountainsof Colorado (Corn and Vertucci1992).
Our analysis of waterchemistryduringsnowmeltat
Matthews Pond, Lily Pond, and Trap Lake failed to
reveal evidence of anthropogenicepisodic acidification.But, as we concluded fromtheWest Glacier Lake
data, daily samples duringthe earliest snowmeltmay
be necessary to detect an episode of anthropogenic
acidification,and our samples at these threesites were
notfrequentor earlyenoughto satisfythatrequirement.
However,all threespecies of anuransbred afteropen
waterappearedon theponds,longpast initialsnowmelt
and any potentialacidificationepisode. Regardless of
thetimingofbreeding,mortality
of amphibianembryos
thatresultedfromexposureto low pH was unlikely.In
laboratoryexposures of embryos to acid conditions,
the pHs lethal to 50% of B. boreas, P. triseriata,and
R. sylvatica individuals obtained from our research
sites were 4.5, 4.8, and 4.3, and the lowest pHs with
no significantmortalitywere 4.9, 5.2, and 4.6, respec-
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May 1996
EPISODIC
ACIDIFICATION
AND AMPHIBIANS
453
1. Breeding activityof Pseudacris triseriata(PSTR) and Rana sylvatica (RASY), ice and snow conditions,and
water chemistryat MatthewsPond, LarimerCounty,Colorado, in 1987-1990.
TABLE
Date
Amphibianbreedingactivity
1987 season:
29 April
PSI'R calling; 2 RASY egg masses
6 May
PSTR calling; 25 new RASY egg masses
14 May
PSTR calling; 11 new RASY egg masses
21 May
PSTR, RASY calling
27 May
PSTR, RASY calling
1988 season:
8 May
none
PSTR calling; 2 RASY egg masses
15 May
23 May
PSTR calling; 7 new RASY egg masses
30 May
PSTR calling; 9 new RASY egg masses
5 June
PSTR, RASY calling
12 June
none
1989 season:
1 RASY egg mass
3 May
11 May
PSTR calling; 12 new RASY egg masses
18 May
PSTR calling; 9 new RASY egg masses
25 May
PSTR calling; 1 new RASY egg mass
1 June
none
1990 season:
23 May
PSTR calling; 8 RASY egg masses
30 May
PSTR, RASY calling
6 June
PSTR calling; 7 new RASY egg masses
13 June
PSTR, RASY calling
Ice condition
Snowpack
surrounding
pond
pH
Acidneutral- Acid
Base
izing anions, cations,
capacity Ca
Cb
([mol,I
L HCO3)
([molc/ (p.molc/
L)
L)
<10% ice cover
open water
complete
patchy
patchy
gone
7.0
7.2
6.9
7.0
7.1
119
159
150
146
166
49
49
32
37
35
112
252
265
247
250
90% ice
open water
complete
complete
patchy
patchy
gone
6.0
6.1
6.1
6.0
*6.1
159
112
108
133
176
33
43
54
48
-46
210
207
247
279
30% ice
open water
complete
patchy
patchy
gone
6.2
6.3
6.2
6.6
6.2
144
133
164
149
186
52
63
76
80
78
289
271
274
330
357
open water
complete
patchy
patchy
gone
6.2
6.5
6.3
6.1
123
135
161
169
62
60
66
57
215
236
320
315
tively(Corn et al. 1989, Corn and Vertucci1992). The
lowest pH we recordedat MatthewsPond, Lily Pond,
or Trap Lake was 5.8 on 11 May 1989 at Lily Pond
(Table 2), apparentlybeforeP. triseriatahad begun to
breed.All otherpHs recordedat thesethreeponds were
-6.0. The pH of the outflowfromWest Glacier Lake
was <6.0 duringtwo shortearly snowmeltepisodes in
1988 and 1989 (Reuss et al. 1993).
It is uncertainwhetherA. tigrinumembryosare exposed to acidic conditions.Harte and Hoffman(1989)
inferredthatA. tigrinumembryoswere in association
with acidic pond waters at Mexican Cut Pond L-12,
but no direct observationswere made of salamander
breeding activity.Wissinger and Whiteman (1992),
however,documentedA. tigrinumbreedingactivityat
Mexican Cut from1989 to 1991, and found thatembryos were absent during early snowmelt (embryos
werenotpresentuntilmid to late June,long afterinitial
snowmelt)and chemical conditionswere neveracidic.
Analysisof snowmeltwaterchemistryfromsouthern
Wyomingand northernand centralColorado does not
indicate a regional threatof episodic acidificationfor
amphibians,but Corn and Vertucci(1992) suggested
that a localized "hot spot" could exist in the Park
Range in Colorado. Turk (1995) found thatthis area,
northof Rabbit Ears Pass in Routtand Jacksoncounties, has the highestconcentrationin snow of S042,
N03-, and H+ in theRocky Mountains.ANC of surface
345
watersin this area is low, so potentialexists now for
biological damage from episodic acidification(Turk
1995). However, because Turk's measurementswere
taken at elevations above those occupied by amphibians, the magnitudeof threatto amphibian breeding
habitatsis unknown.
Episodic acidificationand documented
amphibiandeclines
Recent declines of B. boreas in the southernRocky
Mountainsresultedin a petitionto the U.S. Fish and
WildlifeService to listthesepopulationsas endangered
(U.S. Fish and WildlifeService 1994). In 1994, Bufo
boreas was listed as endangeredin Colorado by the
Colorado Division of Wildlife.This decline cannotbe
attributedto episodic acidification.The breedingphenology of thisspecies does not coincide withpotential
episodes of acidification(Corn and Vertucci1992; Fig.
3); therefore,directmortalityof embryosfromexposure to low pH cannot occur.
Indirecteffectsmay resultfromexposure to acidity
at less thanlethal concentrations.In amphibiansthese
are usually manifestedas reduced larval growthrates
(Pierce and Wooten 1992), but alterationin food supplies and stressthatmightcompromiseimmunesystem
functionare two otherpossible results.Reduced larval
growthmay have a variety of consequences, all of
which reduce fitness.Pierce and Montgomery(1989)
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F. A. VERTUCCI AND P. S. CORN
454
EcologicalApplications
Vol. 6, No. 2
TABLE 2. Breeding activityof Pseudacris triseriata,ice and snow conditions,and water chemistryat Lily Pond, Larimer
County,Colorado, in 1987-1990.
Amphibian
breeding
activity
Date
Ice
condition
Snowpack
surrounding
pond
pH
Acidneutralizing
capacity
(pmol,/L
HC03)
Acid
anions,
Base
cations,
C.
Cb
([.mol,/L) ([.mol,/L)
1987 season:
29 April
6 May
14 May
21 May
27 May
none
none
PSTR calling
PSTR calling
PSTR calling
100% ice
90% ice
< 50% ice
open water
complete
complete
patchy
patchy
patchy
6.8
6.7
6.8
7.0
7.0
90
79
119
112
132
34
28
36
37
34
148
138
208
176
186
1988 season:
15 May
23 May
30 May
5 June
12 June
20 June
27 June
none
none
PSTR
PSTR
PSTR
PSTR
none
100% ice
25% ice
open water
complete
complete
patchy
patchy
patchy
gone
6.1
6.0
7.1
6.3
6.4
6.9
7.0
107
84
111
122
141
168
185
47
30
...
32
28
26
21
170
161
189
197
215
246
263
1989 season:
3 May
11 May
18 May
25 May
1 June
8 June
none
none
PSTR
PSTR
PSTR
PSTR
calling
calling
calling
calling
100% ice
30% ice
10% ice
open water
complete
complete
patchy
patchy
gone
6.3
5.8
6.4
6.8
6.6
6.6
126
65
114
117
148
150
62
33
28
30
32
31
227
163
165
197
214
212
1990 season:
23 May
30 May
6 June
13 June
20 June
none
PSTR
PSTR
PSTR
PSTR
calling
calling
calling
calling
100% ice
50% ice
open water
complete
complete
patchy
patchy
gone
6.2
6.5
6.6
6.3
6.6
130
111
124
143
180
41
30
30
20
29
172
144
203
205
240
TABLE
3.
calling
calling
calling
calling
Ice and snow conditionsand waterchemistryat Trap Lake, LarimerCounty,Colorado, in 1988-1990.
Date
1988 season:
15 May
23 May
30 May
5 June
12 June
20 June
27 June
Ice
condition
Snowpack
surrounding
pond
pH
Acidneutralizing
capacity
(VLmol,/L Acid anions, Base cations,
HCO3)
Ca (Vtmol,/L) Cb (Vtmol,/L)
100% ice
100% ice
10% ice
open water
complete
complete
patchy
patchy
patchy
gone
6.3
6.3
6.3
6.6
6.1
7.4
7.2
110
111
91
122
103
203
229
50
46
37
33
19
32
31
175
198
166
191
220
263
283
1989 season:
3 May
11 May
18 May
25 May
1 June
8 June
100% ice
100% ice
90% ice
open water
complete
complete
complete
patchy
patchy
gone
6.6
6.4
6.3
7.0
7.0
7.2
329
156
72
134
176
225
118
65
81
63
57
58
443
238
181
216
235
288
1990 season:
30 May
6 June
13 June
100% ice
open water
complete
patchy
patchy
6.6
6.8
6.8
129
141
179
63
*193
238
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All use subject to JSTOR Terms and Conditions
55
53
May 1996
FIG. 3.
EPISODIC ACIDIFICATION
AND AMPHIBIANS
455
Northshorelineof Lost Lake, Rocky MountainNational Park,Colorado, on 6 June 1994, duringthe latterstages
of snowmelt.
andat themarginof thelake
byBufoboreasoccurredin theshallowpools in theforeground
Egg deposition
of theremaining
fromabout30 Mayto 8 June.Notetheabsenceof snowon thenorthshoreand thepatchiness
snowpack.
exposed tadpolesof Woodhouse'stoad (B. *voodhousei)
and the Gulf Coast toad (B. valliceps) to pH 4 for3 d.
Tadpoles stoppedgrowingduringthisperiod,but after
an additional 7 d at pH 7, wet mass did not differ
significantly
fromthatof tadpoles held at pH 7 forthe
entire10 d. Reduced growthhas seldom been observed
outside the laboratory.In fact,Wissingerand Whiteman (1992) found that 2nd-yrlarval A. tigrinumat
Mexican Cut were largerin low-ANC ponds compared
to high-ANC ponds. Carey (1993) observed adult B.
boreas with symptomsof redleg disease, a bacterial
infection.She hypothesizedthata regional anthropogenic stress thatsuppressedimmunesystemfunction
was responsibleforthe declines of B. boreas in Colorado. This hypothesishas notbeen evaluated.Chronic
acidificationcould alter the food supply of tadpoles,
because itchangesthespecies compositionand reduces
the diversityof phytoplankton(Findlay and Kasian
1986, Geelen and Leuven 1986, Stokes 1986, Charles
of a short
et al. 1989). The effectson phytoplankton
pulse of acidityat thebeginningof thegrowingseason
assemblages in lakes
are notknown,butphytoplankton
in the westernU.S. show no changes attributableto
acidification(Baron et al. 1986, Charles et al. 1989,
Toetz and Windell 1993). Indirecteffectson embryB. boreas fromepisodic
onic, larval, or transformed
acidificationare unlikelyforthesame reasonthatdirect
mortalityof embryosis unlikely:potentiallysensitive
life stages of toads are not presentwhen an acid pulse
mightoccur. The decline of B. boreas in the southern
Rocky Mountainsis probablydue to some factorother
than acid deposition; for example, direct or indirect
effectsof increased ultravioletradiation(Blaustein et
al. 1994a), pesticidesor contaminantsfromatmospheror heavymetalsnotmeasuredin thisstudy.
ic transport,
Leopard frogs(Rana pipiens) have declined in Colorado and Wyomingby a magnitudesimilarto thatof
B. boreas (Corn and Fogleman 1984, Corn et al. 1989,
Corn 1994). This decline is also notlikelyto have been
caused by episodic acidification,because R. pipiens
generallyoccupy lakes thathave high ANC and that
are at lower elevations lacking extensive snowpack
(Corn and Vertucci1992).
CONCLUSIONS
MonitoringpH and ANC alone cannot provide evidence of anthropogenicepisodic acidificationof amphibian habitats. Besides pH and ANC, Freda et al.
(1991) recommendedmeasuringtotal Al, DOC, Ca ,
Na+, Mg2+, K+, S042 and NO.,-. To apply our criteria
fordetectinganthropogenicepisodic acidification,other importantacid anions should be measured(Cl- and
Fl-). Also, the sampling frequencyrecommendedby
Freda et al. (1991), threesamples duringthe breeding
to detect and characterizeepiseason, is insufficient
sodic acidification.Water samples must also be collected fromthe habitatwhereamphibiansare present.
There is some evidence thatanthropogenicepisodic
acidificationmay occur in high-elevationhabitats in
the Rocky Mountains, but there is no evidence that
episodic acidificationhas led to acidic conditions(ANC
<0) or thatamphibianembryosare presentduringthe
initial phase of snowmeltwhen episodic acidification
mightoccur.Our surveysof habitatswhereamphibians
have declined in the Rocky Mountainregion show no
evidence of chronicepisodic acidificationas the cause
of observeddeclines. Similarly,amphibianpopulations
in sensitivehabitatsin the Sierra Nevada in California
have not been found to be affectedby currentlevels
of acidic deposition (Bradfordet al. 1992, 1994, So-
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456
F. A. VERTUCCI AND P. S. CORN
iseth 1992). Declines of amphibians in the western
United States may be due to many natural and anthropogenic factors, but acidic deposition does not appear
to be one of them.
ACKNOWLEDGMENTS
Funding forthis studywas provided by the U.S. Fish and
Wildlife Service as part of the National Acid Precipitation
Assessment and Global Change Programsand the U.S.D.A.
Forest Service, WesternAtmosphericDeposition Research
Unit of the Rocky Mountain Forest and Range Experiment
Station. We thankR. B. Bury,H. H. Whiteman,D. W. Schindler,S. A. Wissinger,and two anonymousreviewersforcommentingon and suggestingimprovementsto the manuscript.
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