This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. 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 This content downloaded from 166.2.95.220 on Fri, 01 May 2015 17:13:29 UTC All use subject to JSTOR Terms and Conditions 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- This content downloaded from 166.2.95.220 on Fri, 01 May 2015 17:13:29 UTC All use subject to JSTOR Terms and Conditions 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 This content downloaded from 166.2.95.220 on Fri, 01 May 2015 17:13:29 UTC All use subject to JSTOR Terms and Conditions 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- This content downloaded from 166.2.95.220 on Fri, 01 May 2015 17:13:29 UTC All use subject to JSTOR Terms and Conditions 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) This content downloaded from 166.2.95.220 on Fri, 01 May 2015 17:13:29 UTC All use subject to JSTOR Terms and Conditions 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 This content downloaded from 166.2.95.220 on Fri, 01 May 2015 17:13:29 UTC 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- This content downloaded from 166.2.95.220 on Fri, 01 May 2015 17:13:29 UTC All use subject to JSTOR Terms and Conditions 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. LITERATURE CITED Asbury,C. E., E A. Vertucci,M. D. Matson,and G. E. Likens. 1989. Acidificationof Adirondack lakes. Environmental Science and Technology23:362-365. Baker,J. P., D. P. Bernard,M. J. Sale, and S. W. Christensen. 1990. Acidic deposition: state of science and technology, Report 13. 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