using Area, geospatial Florida techniques subsidence Estimating soil and carbon loss in the EvergladesAgricultural Agriculture, Ecosystems and Environment

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Agriculture, Ecosystems and Environment

j ou rn a l h o m epa g e :w w w . e l s e v i e r . c o m / l o c a t e / a g e e

Estimating soil subsidence and carbon loss in the Everglades Agricultural Area, Florida using geospatial techniques

Sumanjit Aich

^a

, Christopher W. McVoy

^b

, Thomas W. Dreschel

^c,∗

, Fabiola Santamaria

^d

aPhotoScienceGeospatialSolutions,St.Petersburg,FL,UnitedStates

bEvergladesNationalPark,FL,UnitedStates

cSouthFloridaWaterManagementDistrict,WestPalmBeach,FL,UnitedStates

dAtkinsNorthAmerica,WestPalmBeach,FL,UnitedStates

a r t i c l e i n f o

Articlehistory:

Received1November2012

Receivedinrevisedform25March2013 Accepted26March2013

Available online 30 April 2013

Keywords:

Everglades Peat Oxidation Subsidence Carbon Carbondioxide

a b s t r a c t

Climatechangeduetoelevatedcarbondioxidelevelsintheatmospherepresentsalong-termthreatto thebiosphere.Thecontributionofsoiloxidationtoglobalcarbondioxidelevelsisofgrowingconcern.

Untilthepastcentury,foroverfivemillennia,theEvergladeshasbeenaccretingpeatsoilsandactingasa carbonsink.Anthropogenicdrainageofthenorthernmostone-fourthoftheEverglades,oneofthelargest depositsoforganicsoilsinNorthAmerica,beganinthe1880s.Subsequently,thepeatsoilsofthatarea begansubsidingandreleasingcarbondioxide(CO2)intotheatmosphere.Wequantifiedsubsidenceand CO2evolutionattheregionalscalebycalculatingthechangesinpeatvolumeusingsurfacemapsrecon- structedfromhistoricalandcurrentdata.Theestimatedpeatvolumewasoriginallyabout7×10⁹m³and iscurrentlyabout3×10⁹m³.TheaveragesubsidencewasabouttwometersandtheCO2emittedwas 4.9×10⁸metrictons.AssumingaconstantCO2emissionrateduringthecenturysincedrainage,thenthe regionalscaleCO2fluxratefromthisstudy(0.2gCO2m⁻²yr⁻¹)wassimilartoshort-term,small-scale measurementsmadeundercontrolledconditions(0.2–6gCO2m⁻²yr⁻¹).

Published by Elsevier B.V.

1. Introduction

Climatechangepresentsalong-termthreattothebiosphereas globalcarbondioxidelevelsrise.Thecontributiontoglobalcar- bondioxidelevelsbytheoxidationofsoilsisofgrowingconcern.

NorthAmericanwetlandsandtheirassociatedorganicsoilsarean importantcomponentoftheglobalcarboncycle,containinganesti- mated2.2×10¹¹metrictonsofcarbon(Bridghametal.,2006).The largestcontiguousbodyoforganicsoils(peats)withintheconti- nentalUnitedStatesoccursintheFloridaEverglades(Zelaznyand Carlisle,1974),forminganareaoriginallygreaterthan8900km² (McVoyetal.,2011).Peatdepositionbeganapproximately5000 yearsago(McDowelletal.,1969;GleasonandStone,1994),reach- ingamaximumthicknessof3–3.7mbythe1900s(Baldwinand Hawker,1915),correspondingtoanaverageaccumulationrateof about0.07cmyr⁻¹.Thedepositthinnedwithdistancesouthward fromLakeOkeechobeetoaminimumof0.3–0.5mthick(Baldwin andHawker, 1915;McVoyet al.,2011).Prior toanthropogenic alterations,theEvergladeswasabroad,shallowfreshwatermarsh

∗Correspondingauthorat:SouthFloridaWaterManagementDistrict,MSC4352, WestPalmBeach,Florida,UnitedStates.Tel.:+15616826686;fax:+15616825529.

E-mailaddresses:tdresche@sfwmd.gov,tdreschel@gmail.com(T.W.Dreschel).

systemoriginatingatthesouthernedgeofLakeOkeechobeeand flowing more than150km south todischarge intotheAtlantic Ocean,BiscayneandFloridaBays,andtheGulfofMexico(Fig.1).

Thecombinationofthickorganicsoilsandsubtropicalclimate attractedinterestindrainingtheEvergladesforagriculturaluseat leastasearlyasthe1840s(Smith,1848).

Thiseffortbeganinearnestinthe1880s,wheninitiallower- ingofLakeOkeechobeewaterlevelsreducedlakeinflowsintothe Everglades(McVoyetal.,2011).Drainageeffortsfurtherintensi- fiedduringthe1910sand1920s,whendikesfullyisolatedLake OkeechobeefromtheEvergladesandwhenfourmajorcanalswere dredgedthroughtheEverglades(LightandDineen,1994).

TheloweringofLakeOkeechobee,dikeconstruction,andunre- strainedcanaldrainagetogetherstronglyaffectedtheEverglades.

Watertables were loweredfrom aboveground surface to well belowgroundsurface(Sklaretal.,2002),alteringvegetation,and exposingtheorganicsoilstosubsidence.Theoccurrenceofdra- maticsoil subsidenceis well-documented(Claytonet al.,1942;

Neller, 1944; Stephens and Johnson, 1951; Shih et al., 1978, 1979a,b,c, 1997; Cox et al., 1988; Snyder and Davidson, 1994;

Ingebritsenetal.,1999;Snyder,2005).Thenorthernmostportion of theEverglades,approximately2540km² (23%), wasformally designatedinthe1950sastheEvergladesAgriculturalArea(EAA).

Drainageandadegreeofagriculturehadalreadybeenpresentthere 0167-8809/$–seefrontmatter.Published by Elsevier B.V.

http://dx.doi.org/10.1016/j.agee.2013.03.017

Fig.1.LocationoftheEvergladesAgriculturalArea(EAA;solidline,heavy)within thehistoricalEverglades(dottedline),showingclosecorrespondenceoftheEAAto thepredrainagesawgrassplainsandcustardappleswamplandscapes.

fora number ofdecades.Aconcrete markerattheAgricultural ResearchandEducationCenterinBelleGladeshowsthatthe2.6m oforganicsoilpresentin1924(alreadyreducedfrompredrainage conditions)hadbeenreducedtolessthan1.25mby1978(Stephens etal.,1984).FieldmeasurementsofsubsidencewithintheEAAindi- catelossofelevationata rateofabout3cmyr⁻¹ (Neller,1944;

StephensandSpeir,1969;Snyder,2005).

TheEAAencompassedessentiallyalloftheoriginalSawgrass Plains landscape (Fig. 1), an areadominated by sawgrass, Cla- dium jamaicense, as well as the much smaller Custard Apple Swamp,dominatedbypondapple,Annonaglabra.Bothlandscapes wereassociatedwithsimilarlysizedareasofcorrespondingsoils (Jonesetal.,1948;McVoyetal.,2011).Thecustardapplemucks, foundborderingthesoutheastshorelineofLakeOkeechobee,was describedin1915as“black,finelydividedandwell-decomposed vegetablematter,...fairlyuniformintexture,structure,andcolor toadepthof40–75in.[1–1.9m]” (Baldwin andHawker,1915).

Thetotalsoilthicknessofthecustardapplemuckswas2.8–3.8m andtheaverageorganicmattercontentabout60%(Baldwinand Hawker,1915).Atthetimeofobservation,1915,thewatertable hadalreadybeenlowered0.5–1mbelowthesurface,soitispos- siblethatthissoilwasoriginallylessdecomposedthaninBaldwin andHawker’sdescription,thatis,morebrownthanblackandmore fibrousthanfinelydivided.Thesoilmineralcontentmayalsohave increasedsomewhatbutprobablyalwayswasrelativelyhigh.The custardapplemucksoilsarepresentlyclassifiedastheTorryseries ofTypicHaplosapristsandin1988comprised7%oftheEAA.

Thesawgrass peatsthat underlaidabout 90%of thepresent EAAoriginallywerehighlyorganic,containing90%organicmat- ter(BaldwinandHawker,1915).Whileadegreeofdrainagehad alreadyoccurredby1915,surfacewaterwasstillpresentinsome areasandthesoilappearstostillhaveretainedmuchofitsoriginal character.Referredtoas“Brownfibrouspeat,”itwasdescribedas

“uniformincompositionandtextureformanysquaremiles”and as“typicallyconsist[ing]ofbrownfibroustodark-brownsemifi- brous,slightlydecomposedorganicmatter”(BaldwinandHawker, 1915).Whenlatermappedagaininthe1940s,thesawgrasspeats haddecomposedfurthertodevelopasurfacelayerof“black,finely fibrous,welldecomposed organicmaterial,6–18in.[15–45cm]”

thick(Jonesetal.,1948;McVoyetal.,2011).Althoughnotpart oftheformalsystemofsoiltaxonomy,thenamegivenbyJones etal.(1948),“Evergladespeats,”haspersistedingeneraluse.By the1970sthesawgrasspeatshadfurthersubsidedandwereclassi- fiedastheMontverde(sawgrass)muckseriesofTypicMedifibrists (Volk,1973).Underthepresentclassificationsystem(SoilSurvey Staff,1998,1999),allthesoilsoftheEAAareclassifiedasSaprists, themostdecomposedsuborderofHistosols.Fourseriesarerecog- nized,inorderofdecreasingsoilthickness:theTerraCeiaseries, aTypicHaplosaprist,andthreeLithicHaplosaprists,thePahokee, LauderhillandDaniaseries.In1988,lessthan10%oftheEAAfell withinthethickestseries(TerraCeia,>130cmthick).Asnotedby Riceetal.(2005),subsidenceiscausingthesoilsoftheEAAtocon- tinuallytransitionfromthickertothinnersoilseries,andtheynote thatthesesoilsmayeventuallybeclassifiedasmineralsoils.

Thisstudyestimatesthevolumeandmassofpeatsoilthathas beenlostoverthelast125yearswithintheEAAasaresultofanthro- pogeniclandusechangefromEvergladeswetlands intodrained agriculturalfields.Thetimeperiodofinterestwasdefinedasimme- diatelypriortotheonsetofanthropogenicdrainage(i.e.,<1880) throughthepresent,whichrequiredtheuseofimperfecthistor- icaldatasetsdevelopedfromlandandsoilsurveysconductedin theearly1900s(McVoyetal.,2011).Tooffsetlimitationsinthe historicalaswellascurrentdatasets,weusedtwoindependent methodsandsetsofdatatoestimatethevolumesofpeatlost.The firstmethodwasbasedonhistoricalandcurrentestimatesofpeat thickness,inbothcasesmeasuredwithsoundingrodsinsertedto theunderlyingmineralsubstrate.Thesecondmethodwasbasedon estimatesofthehistoricalandcurrentsurfacetopography.Weused aGIStoorganizethehistoricaldata,tointerpolate,andtocalculate differences.

WeadditionallyestimatedatmosphericreleasesofCO2fromthe EAAassociatedwiththelandusechangefromEvergladeswetlands toagriculture.Thisestimaterequireddifferentiationofthesoilvol- umechanges(subsidence)intophysicalandbiochemicalprocesses.

Subsidenceiscausedbyacombinationofphysicalconsolidation duetolossofbuoyancy,physicalshrinkageduetoincreasedmatric potential,biochemicalmineralizationoforganicCtoCO2 dueto microbialoxidation(Volk,1973;Schothorst,1977),and,ifthesoil becomesdryforextendedperiods,occasionallyoutrightburning (Davis,1943a,b;Allison etal., 1944;Robertson, 1953;Simpson, 1920;Loveless,1959;Mayo,1940).Weusedhistoricalandcurrent dataforbulkdensity,fractionoforganicmatter,andcarboncontent toestimatetheportionofsubsidencethatproducedatmospheric CO2.Byproratingtheestimatedreleasesoverthecenturyofsub- sidenceandbytakingintoaccountsoilnomenclaturechanges,we wereabletocompareourestimatedratesofCO₂releasewithliter- aturevaluesmeasuredinlabandfieldcolumnstudiesofEverglades soils(e.g.,Kniplingetal.,1970;Volk,1973;Geschetal.,2007).

Thecarbon release estimatesmadehereprovide contextfor ongoing proposals to return surface water to portions of the EAA. Potential plansinclude thecreationof treatmentmarshes toreduceundesirednutrientconcentrationsinwaterdestinedfor theremainingEverglades,thecreationofshallowflowwayswith

Fig.2.Mapofestimatedcontoursofpeatthickness(ft)intheEAA,ca.1915,show- ingmeasurementsofpeatthicknessfromtownshipsurveys(dots)andfromcanal surveys(pluses).

someresemblancetopredrainagesawgrassmarshes,orthecre- ationofdeeperreservoirstostorewaterforEvergladesrestoration.

Whilecarbonaccumulationratesassociatedwiththesepotential landusechangeshavenotbeenquantified,ataminimum,itcanbe assumedthatrefloodingofthesoilswouldreduceoreliminatethe soilsubsidenceandCO₂releasesthathavebeenoccurringduring theprevious125years.

2. Methods

TheEvergladesAgriculturalAreaor EAA,locatedin southern Florida,USAistheformernorthernportionoftheEvergladesthat wassetasideintheearly1900sanddrainedforagriculturalpro- duction.Sugarcane(Saccharumspp.)isgrowninmostoftheEAA withalargeportionofthenation’swintervegetablecropsgrown thereaswell(Geschetal.,2007).Soilsubsidence hasbeenpro- nouncedinthisareaandisofconcernbothduetosoillossandto thecontributiontoatmosphericcarbondioxide.Weutilizeddata fromlandsurveysconductedinthelate19thandearly20thcen- turies,a bedrockelevationmap,anda recentspace-basedradar topographicsurvey.ContourmapsshowninFigs.2–6reflectthe Englishunitsusedinthehistoricfieldmeasurements;thesewere convertedtoSIunitsforthefinalcalculationsofpeatvolumeand carbonloss.Thecalculationswereconductedintheseunitsforthe twomethodsdiscussedandthenconvertedtoSIunitsforthefinal calculationsofpeatvolumeandcarbonloss.

2.1. Method1(peatthickness)

Weestimatedthevolumeofpeatlostbetweenapproximately 1915and2003asthedifferencebetweenamapofhistoricpeat thickness(Fig.2)andamapofcurrentpeatthickness(Fig.3),both createdaspartofthisstudy.

2.1.1. Historicpeatthickness,ca.1915(Fig.2)

Theearliestmeasurementsofpeat thicknesswithintheEAA areaweremadeaspartofanagriculturalsurvey(Kreamer,1892).

Fig.3.Mapofestimatedcontoursofpeatthickness(ft)intheEAA,2003,showing datapoints( ).

FromSnyder(2005).

DatacoveringalargerportionoftheEAAdidnotbecomeavailable untilthe1911–1916periodwhenpeatthicknessesweremeasured alongproposedcanalroutes(EnseyandElliot,1911;FEEC,1913) andduringlandsurveyscarriedoutfortheTrusteesoftheInter- nalImprovementFund,StateofFlorida(Elliot,1911a,b,c,1912a,b;

Frederick,1914a,b,c,d;Horne,1914a,b,c;Franklin,1914a,b;Hardin, 1915a,b,c,d,e,f,g,1916a,b,c,d,e,f,g,h,i,j,k,l,m; field notes for these surveysextractedfromtheLandBoundary InformationSystem,

Fig.4.Contourmapofbedrockelevation(ft,NGVD29)fortheEAA.

FromParkeretal.(1955).

Fig.5. Contourmapofhistoricpeatsurfaceelevation(ft,NGVD29)fortheEAA,ca.

1880.

FromSaidetal.(2006).

FDEP,1984).Landsurveystypicallydonotincludesoildepthmea- surementsbutbecauseoftheinterestinconvertingtheEverglades toagriculture,thesesurveysdid.Asthelargemajorityofthesemea- surementsweremadebetween1911and1916,anaveragedateof 1915wasassigned.Assurveys,allofthesemeasurementswere georeferenced(e.g.,thedistancefromthecornersofatownshipor

Fig.6. Contourmapofcurrentpeatsurfaceelevation(ft,NGVD29)fortheEAA,ca.

2005.

ModifiedfromHoltetal.(2006).

fromtheendofacanal),thuswewereabletocreateaGISdatabase (LoandYeung,2002)ofdepthmeasurements.

Mostofthepeatdepthsinthelandsurveyswererecordedas

“9ft”(2.74m)or“10ft”(3.05m),withsomeindicatedas“>10ft”, suggestingthatthelandsurveyorsuseda10ft-longsoundingrod thatinthosecasesdidnotreachbedrock.Atotalof34(7%)ofthe soundingswerereportedas“>10ft”;forlackofadditionalquantifi- cation,werecordedtheseas10ft.Surveyorsfromadifferentagency (FEEC,1913)usedlongersoundingrods;theirmeasurementsofup to14ft(4.27m)alongthenorthernportionofsomecanalswere includedinourGISdatabase.

Estimation ofa thickness surface required additionalspatial informationinareas.NeartheeasternedgeofthehistoricEver- glades,wheremeasurementsfromthelate1800stoearly1900s wereabsentorscarce,weincludedpeatthicknessesmeasuredby soundingrodinthe1940sbyJonesetal.,1948recognizingthat thesemeasurementsunderestimate1915thicknesses.Theknown boundariesoftheEverglades(McVoyetal.,2011),recognizedboth asavegetationalboundaryandastheboundarybetweenpeatand sandsoils,wereusedtodefinethelocationsofzeropeatthickness.

Ahistoricalmap(rastergrid)ofpeatthicknesswascalculated from456agricultural,landandcanalsurveypointsacrosstheEAA.

OrdinarykrigingwithaGaussiansemi-variogram(Bonham-Carter, 1994;ESRI,2004)wasusedtocreatethemap.Fig.2showsthe estimatedpeatthicknesspresentca.1915withinthepresentEver- gladesAgriculturalArea.

2.1.2. Currentpeatthickness,ca.2003(Fig.3)

Current(2003)peat thicknesswasestimatedfrommeasure- mentstakenat15locationsthroughouttheEAA(Snyder,2005).

Tenreplicatesweretakenateachlocation(Snyder,2005).Soilsub- sidencehastendedtolevelthisareasubstantially(Snyder,2005) sothatasmalldatasetmaybeappropriatetouse.Amapofcurrent peatthickness(Fig.3)wascalculatedusingordinarykrigingwitha Gaussiansemi-variogram(Bonham-Carter,1994;ESRI,2004).

2.2. Method2(topography)

Asecondandindependentestimateofthevolumeofpeatlost wasmadeonthebasisofpredrainageandcurrentsurfacetopog- raphy. These two topographic surfaces wereconverted topeat thicknessesbysubtractingthebedrocksurfacefromParkeretal.

(1955)(Fig.4).Thetworesultantpeatthicknessesweresubtracted, yieldingthevolumeofpeatlost.

2.2.1. Bedrocksurface(predrainageandcurrent)(Fig.4)

A100ftby100ft(30.5mby30.5m) DigitalElevationModel (DEM)ofthebedrocksurfaceunderlyingtheEAAwasestimated from the only known source of suchinformation, a set of 1ft (30cm)contoursshowninParkeretal.(1955).Weassumedthat thebedrocksurfacehasbeenconstantbetween1880andthecur- renttime.Parker’smapisaslightmodificationofanearliermap (Jonesetal.,1948),entitled“Approximatecontoursontherocksur- faceundertheorganicsoilsintheEvergladesRegion.”Jonesetal.

(1948)indicatedthat:(1)thesecontoursweredrawnfromdata obtainedwhilerunningapproximately440miles(710km)ofsur- veylinescoveringtheEverglades;(2)landsurfaceelevationswere surveyedto0.1foot(3cm)alongthelines;and(3)thicknessesof thepeatsoil(i.e.,distancefromsurfacetobedrock)weremeasured at660ft(200m)intervals.Thesenumberssuggestthatapproxi- mately3500measurementpointsthroughouttheEvergladesmight havebeenusedbyJonesetal.(1948)todrawthecontours.Jones etal.(1948)describedthebedrocksurfaceas“veryuneven”,i.e., locallyvariable,andindicatedthattheymappedonlyits“general configuration.”Shih etal.(1979a,b,c) confirmthelocalvariabil- ity, showing a magnitude of about 0.5m along line transects

300–1000mlong.Atthescaleofthe2540km²EAA,thecontours obtainedfromParkeretal.(1955)andshowninFig.4,likelyare a good regional representation of theaverage bedrock surface.

WeconvertedtheParkeretal.(1955)contoursfrom‘PuntaRassa datum’toNGVD29bysubtracting1.44ft(0.44m)(Parkeretal., 1955;U.S.ArmyCorpsofEngineers,1978).

2.2.2. Predrainagepeatsurface,ca.1880(Fig.5)

Saidetal.(2006)estimatedthepredrainagetopographyofthe Evergladesbycombiningknownelevationsofthebordersofthe EvergladeswithcontourshapeswithintheEvergladesestimated frompeatlanddirectionality.Thepredrainageeasternandwest- ernborderelevationswereknownfrommodernmeasurements, basedontheassumptionthatthemineralsoilsalongthoseborders had not been altered by drainage. The elevation of the north- ernborder ofthepredrainage Everglades,thatis, thesouthern, peat-based shoreline of Lake Okeechobee, seasonally overflow- ingintotheEverglades,wasknownfrompredrainagestages of LakeOkeechobee.Contourshapesforthepeat-filledpredrainage Evergladeswereestimatedbasedontheassumptionthatcontours wereperpendiculartopredrainagedirectionsofsurfacewaterflow.

Directionsof flowwereassumed tohavebeenparalleltoland- scapedirectionality,asreportedbyanumberofpredrainageand earlypost-drainageobservers(McVoyetal.,2011).Theestimates ofpredrainagetopographycomparedwellwiththegroundsur- faceelevationsderivedfromearlypost-drainagesurveysalongthe lengthsofmuckcanals(FEEC,1913).Forthisstudyweusedthe EAAportionofthe100ftby100ft(30.5mby30.5m)DEMgrid estimatedbySaidetal.(2006),assigningtoitthedateof1880, representingtheendofpredrainageconditions.

2.2.3. Currentpeatsurface,ca.2000(Fig.6)

Acorresponding100ftby100ft(30.5mby30.5m)DEMgrid ofthecurrent peatsurface(Fig.6)wascreatedfromtheSouth FloridaDigitalElevationModel(SFDEM).Thiswasproducedfrom datacollectedduringan11-daySpaceShuttleRadarTopography Mission(SRTM)inFebruaryof2000,whichutilizedaspeciallymod- ifiedradarsystemaboardtheSpaceShuttleEndeavor(Holtetal., 2006).Holtetal.(2006)processedtheSRTMdatausingadditional, laterdatacollectedinthesurroundingregions.Weconvertedthe datareportedbyHoltetal.(2006)fromNAVD88verticaldatum toNGVD29 byapplyingauniformoffsetof 1.39ft(0.42m).The decisiontoapplyauniformoffsetwasbasedonresultsfromthe CORPSCONprogram(U.S. ArmyCorpsof Engineers,2009)when appliedtoa 2 by 2 mile(3.22km)grid of 251points covering theEAA.Theresultsindicatedanoffsetof1.39±0.05ft.Thesmall standarddeviation,0.05ft,suggestedthatapplyingasingleoffset forthewholeEAAwasanappropriateapproximation.

2.3. Calculationofcarbonemissions

Theabovecalculationsquantifythedrainage-inducedlossof volumeintheorganicsoilsoftheEAA.Thevolumelossreflects a combinationof physical consolidation, biochemical oxidation, andpossiblyerosivelosses.Weassumedthatlossestowaterero- sionwerenegligibleduetotheextremelylowsurfaceslopeand totheabsenceofsurfacewaterduringmostofthepost-drainage period.WinderosionfromEAAsoilshasrarelybeenreported;addi- tionally,mosterodedparticleswouldhavebeendepositedondry landwheretheywouldultimatelyhaveoxidizedandbeenreleased asCO2,apathwayequivalenttoinsituoxidation.Carbonemis- sionswereestimatedhereasthefractionofvolumelossnotdue tophysicalconsolidation. FollowingSchothorst(1977),weused changesinthesoilbulkdensityandorganicmattertodistinguish

Fig.7.Estimatedvaluesofsoilparametersforpredrainageandcurrentsoilprofiles intheEvergladesAgriculturalArea.

consolidationfromoxidationusingthefollowinggeneralformula- tionforanorganicsoilwithklayers:

Closs=A

k(bIkfocIkOMIkzIk)−A

k(bFkfocFkOMFkzFk) (1) whereC_loss(MgC)isthemetrictonsofCtransferredfromthesoil totheatmospherebyoxidationfromanareaA(m²),b isbulk density(Mgm⁻³),focisthefractionoforganiccarboninthesoil organicmatter(kgCkgOM⁻¹),OMisthefractionofsoilorganic matter(kgOMkgsoil⁻¹),andzisthethicknessofthelayer(m).The subscriptsIandFrefertotheinitial(i.e.,predrainage)andfinal(i.e., current)timeperiods.

ThesummationsignsinEq.(1)reflectthepossibilityofuptok distinctlayersofsoilbeingpresentwithinthefullsoilprofile.The availablecurrentandpredrainagesoilprofileinformationforthe EAAsuggeststhattheseorganicsoilswereverticallyhighlyuni- form.Wethereforerepresentedtheinitialpredrainageprofileasa singlelayerandthecurrentprofileastwodistinctlayers(Fig.7).

Divisionofthecurrent,post-subsidenceEAAsoilprofileintotwo layerswasbasedonsoiltillage:anupperlayerfrom0to30cm thickcorrespondingtothetilledrootzone,andalowerlayerfrom

−30cmdowntobedrock.

ThepredrainageparametersneededforEq.(1) werethesoil bulk density,bI, and thesoil organicmattercontent,OM_I.No predrainagemeasurementsofsoilbulkdensitywerefound.We estimatedbI byselectingfrommeasurementsmadeinthecur- rently remaining soils (Table 1), adjusted for location and the effectsofdrainage.WechosebI=0.10Mgm⁻³,thelowestofthe Saundersetal.(2008)values.Wechosethelowestvaluebecauseall areassampledbySaundersetal.haveexperiencedincreasedpost- drainagedryingandcanthereforebeexpectedtohaveincreased slightlyindensity.Weavoidedtheevenlowervaluesof0.06–0.08

Table1

PropertiesofpeatsoilsoftheEverglades[AgriculturalArea].BD=bulkdensity(Mgm⁻³);OM=organicmattercontent(kgkg⁻¹);OC/OM=organiccarboncontent/OM.

BD OM OC/OM Source

Predrainage(oranalogs) 0.12 Saundersetal.(2008):3B1,Sawgrass,0–30cm

0.13 Saundersetal.(2008):3B1,Sawgrass,30–74cm

0.10 Saundersetal.(2008),SRS-3,Sawgrass,0–30cm

0.12 Saundersetal.(2008),SRS-3,Sawgrass,30–100cm

0.07 Corstanjeetal.(2006),WCA1,0–10cm

0.08 Corstanjeetal.(2006),WCA1,10–20cm

0.06 USEPA-REMAP(2007),WCA1,0–10cm(median)

0.10–0.15 USEPA-REMAP(2007),WCA3B,0–10cm

0.92 Miller(1918);Sawgrass1,0–152cm

0.91 Miller(1918);Sawgrass1,152–305cm

0.93 Miller(1918);Sawgrass2,0–63cm

0.90 Miller(1918);Sawgrass2,63–208cm

0.92 BaldwinandHawker(1915),0–30cm

0.88 BaldwinandHawker(1915),>30cm

0.93 USEPA-REMAP(2007),WCA1,0–10cm

0.47 Corstanjeetal.(2006),WCA1,0–10cm 0.49 Corstanjeetal.(2006),WCA1,10–20cm

Current,top30cm 0.36 WeaverandSpeir(1960),Everglades,0–8cm

0.36 WeaverandSpeir(1960),Everglades,8–15cm

0.34 WeaverandSpeir(1960),Everglades,15–23cm

0.29 WeaverandSpeir(1960),Everglades,23–30cm

0.26 ZelaznyandCarlisle(1974),Montverde,0–13cm

0.22 ZelaznyandCarlisle(1974),Montverde,13–36cm

0.33 ZelaznyandCarlisle(1974),Pahokee,0–18cm

Estimated 0.3 Smithetal.(2001),RotenbergerWMA,2–10cm

0.45 WrightandInglett(2009),Daniamuck,0–30cm

0.94 VolkandSchnitzer(1973),TerraCeia,0–20cm

0.93 VolkandSchnitzer(1973),Pahokee,0–25cm

0.75 JanardhananandDaroub(2010),Lauderhill,0–20cm

0.83 JanardhananandDaroub(2010),Pahokee,0–20cm

Estimated 0.9 Smithetal.(2001),RotenbergerWMA,2–10cm

0.6 Snyder(2005)(fromTalismanProperty,10in.profile)

0.50 VolkandSchnitzer(1973),TerraCeia,0–20cm 0.53 VolkandSchnitzer(1973),Pahokee,0–25cm

Current,>30cm 0.14 WeaverandSpeir(1960),Everglades,30–43cm

0.13 WeaverandSpeir(1960),Everglades,51–58cm

0.12 WeaverandSpeir(1960),Everglades,66–74cm

0.15 WeaverandSpeir(1960),Everglades,81–99cm

0.08 ZelaznyandCarlisle(1974),Montverde,36–81cm

0.18 ZelaznyandCarlisle(1974),Pahokee,18–86cm

0.49 WrightandInglett(2009),Daniamuck,30–45cm

0.93 VolkandSchnitzer(1973),TerraCeia,20–122cm

0.94 VolkandSchnitzer(1973),Pahokee,25–107cm

0.56 VolkandSchnitzer(1973),TerraCeia,20–122cm 0.48 VolkandSchnitzer(1973),Pahokee,25–71cm 0.55 VolkandSchnitzer(1973),Pahokee,71–107cm 0.51 BhattiandBauer(2002),usedbyWrightandInglett

fromCorstanje etal. (2006)and USEPA-REMAP (2007)because theseweremeasuredonlyintheupper0–10cm,whichisoflower densitythantheoverallprofile.

Anumberofpredrainageornear-predrainagedatasetsofsoil organicmattercontent,OMI,wereavailable(Table1).Thespatially andnumericallymostextensivewasthatofBaldwinandHawker (1915),withmorethan400coressamples.Weusedtheaverageof theirupperandlowerprofilevalues,OMI=0.90kgkg⁻¹.

Thethicknessofthepredrainagepeatprofile,z_I,wascalculated simplyasV_I/A,thatis,forthepurposesofEq.(1),thepredrainage EAApeatdepositwasassumedtobeofequalthicknessthroughout.

TheavailabledataforcurrentsoilsintheEAAappearstobe morelimitedthanmightbeexpected(Table1).Weassumedthat asthepredrainagesoilprofileoxidized,itsoriginalmineralcontent wasdepositedontotheupperportionofthecurrentlyremaining soil,andredistributedbytillagethroughouttheupper30cm.As themineralfractionaccumulated,thesoilorganicmattercontent oftheupperprofile,OM_F1,wouldhavecorrespondinglydecreased.

Theupperprofilebulkdensity,bF1,wouldhaveincreasedwith increasingmineralcontent,andadditionallyasaresultoftillage

anddryingofthesoil.ForbF1,weconsideredtheTable1valueof 0.45Mgm⁻³ fromWrightandInglett(2009)tobeanoutlierand chose0.32Mgm⁻³asamidrangeamongtheremainingvalues.For OMF1,weexcludedtheTable1valuesof0.93and0.94fromVolkand Schnitzer(1973)becausethesevalueswereactuallygreaterthan thepredrainagesoilorganicmattercontents.Withthesevalues excluded,wechoseamidrangevalueof0.80kgkg⁻¹.

Forthecurrentsoilprofilebelow30cm,weassumedthatthis lower,untilledportionof theprofilewouldretainsomeresem- blancetotheoriginalpredrainagesoilcharacteristicsbutalsothat the profileswould have experienced lowered water tables and periodicdryingofessentiallytheentireprofile,leadingtoslight decreasesinorganicmatterandincreasesinbulkdensity.

Forthelowerprofilesoilbulkdensity,bF2,wecomparedto thepredrainagesoilbulkdensity,bI=0.10Mgm⁻³,excludedwhat seemed to be an outlier value in Table 1 of 0.49Mgm⁻³, and choseahighermidrangevalueof0.14Mgm⁻³ fromTable1.For thelower profilesoilorganicmattercontent, OM_F2,few litera- turevalueswereavailableandtheywereoflittleguidanceasthey wereactuallygreaterthanthepredrainagevalueof0.90kgkg⁻¹

(Table 1). We chosea value of 0.86kgkg⁻¹,slightly lower than BaldwinandHawker’s(1915)measuredpredrainage,lowerprofile valueof0.88kgkg⁻¹.

Weassumedasimplifiedgeometryforthecurrentlyremaining peatprofile,thatis,bothlayersuniformlythick,withtheupper layer,z_F1,equalto0.3mandthelowerlayer,z_F2,equalto(V_F/A)

−0.3.

Weassumedthatthefractionofcarboninthesoilorganicmat- ter,foc,wasconstantovertimeanddepth.Giventhesimilarityto valuesreportedfortheEAA(Table1),weadoptedthevalueused byBhattiandBauer(2002)offoc=0.51foralldepthsandbothtime periods.

ThesoilparametersshowninFig.7reflectourbestestimatesof predrainageandcurrentvalues,basedonsynthesisoftheavailable data(Table1),thepredrainageandthecurrentdistributionofsoils andlandscapes(McVoyetal.,2011),andthehistoryofEverglades drainage(McVoyetal.,2011).

Carbonloss(MgC)calculatedinEq.(1)wasconvertedtocarbon dioxidereleasedtotheatmosphereusingthestoichiometricfac- torof44/12andtheassumptionthatallthepeatcarbonlostwas immediatelyoreventuallyconvertedtocarbondioxide.

3. Results

We estimate the volume of peat lost over approximately a century of drainage as 4.5×10⁹m³ (method 1; Table 2) and 4.9×10⁹m³ (method 2). Using Eq. (1) to distinguish physical changesfromactualoxidationgivesanestimatedpeatmasslossof 2.50×10⁸metrictons(bothmethods)andareleaseofCO₂tothe atmosphereof4.85×10⁸metric tons(method 1)and 4.92×10⁸ metrictons(method2).Withasimplifyingassumptionofacon- stantoxidationrate,justifiedbyrelativelyconstanttemperatures and levels of drainage, this corresponds to emission rates of 0.22gCO₂m⁻²h⁻¹formethod1and0.16gCO₂m⁻²h⁻¹formethod 2.Thegenerallysmalldifferencesbetweenthetwomethods,along withtheuncertaintiesinherentinallofthedatasets,suggestthat theresultsfrombothmethodsarebestcombinedandconsidered asasingleestimate.

Asaresultofanthropogenicdrainage,approximatelytwo-thirds ofthepeatvolumeoriginallypresentwithinthepresentEverglades AgriculturalAreahasbeenlostduringthelastcentury,decreasing fromabout7×10⁹m³ toabout2.5×10⁹m³ (Table2).Thislost volumecorrespondstoabout2.5×10⁸metrictonsofpeat.

TheaverageverticalsubsidenceovertheentireEAAhasbeen almost2m(6ft;Table2).However,subsidencehasnotbeenuni- form.TheslopeofthepredrainageEvergladeslandsurfaceshowed astrongsouthwardtrend,rangingfromabout6.4m(21ft)above NGVD29nearLakeOkeechobee,downtoabout4.3m(14ft)atthe southernendoftheEAA(Fig.5).Thebedrockslopewasmorevari- able(Fig.4),butgenerallydidnotdecreasesouthward,indicating asouthwardlythinningpredrainagewedgeofpeat,thickestnear LakeOkeechobee.Post-drainagesubsidencehasalsofollowedthis generallynorth–southtrend,withagreaterdegreehavingoccurred northward(compareFigs.5and6),wherethepeatwasoriginally thicker(Fig.2;seealsoFigs.4and5).Animportantconsequence ofthis differentialsubsidenceisthattheEAAlandsurface,once southwardlysloped,is now essentiallylevel (Fig.6).Under the assumptionthatthepeatvolumesandmassesshowninTable2 werelostspecificallytopeatoxidation(ratherthansoiltransport bywindorwater,whichalsomayultimatelyleadtothetransported carbonbeingoxidized),theselossescorrespondtoreleaseofabout 5×10⁸metrictonsofCO2.

Anhistoricalapproachnecessarilyincludesuncertainties,par- ticularly if the historical data was collected for very different purposes.Toestimatetheeffectofparameteruncertaintiesonour

results, weconducteda simple perturbationanalysisofEq. (1), varyingeachoftheparametersincombinationsthatwouldresult inthehighestandlowestvaluesofcarbonlost.Highandlowvalues ofeachparameterwerechosenfromtherangeofvaluesshownin Table1,combinedwithbestprofessionalknowledgeofEverglades soilsaswellasofphysicalconstraints(e.g.,currentorganicmatter valuescannotexceedpredrainagevalues;Table3).Theresultant calculationssuggestthattheestimateof2.5×10⁸metrictonsof peatlosscouldbeaslowaszero(calculatedvalueactually<0)oras highas8.9×10⁸metrictons,andthattheestimateof5×10⁸metric tonsofCO2 lostcouldbeaslowaszero(calculatedvalueactu- ally<0)orashighas17×10⁸metrictons.

4. Discussion

Reconstructionofhistoricalecologicalconditions,particularly inanareaoforganicsoilswheretheoriginaldatahasbeenlost (literally“oxidizedaway”)ischallengingandcertainlysubjectto uncertainties.Wediscussherespecificsourcesofuncertaintyinour estimatesofcarbonlost,alongwithasimpleerroranalysisofEq.(1), andemergentinsightsintoapparentconstraintsonthephysically plausibleparameterspaceforEq.(1).

Thesourcesofparameteruncertaintycanbedividedintouncer- taintiesassociatedwiththegeometryofthepeatdeposit,which herereducesto uncertainty related totheoriginal and current thicknesses,anduncertaintiesrelatedtosoilproperties,specifically thebulkdensityand thesoilorganicmattercontent.Additional sourcesaretheconceptualizationofthepeatdepositin Eq.(1), specificallytheassumptionthatthedepositcouldbeadequately represented as having a single, uniform thickness, z, and the assumptionthat theprofilewasand still is(aside fromtillage) verticallyquiteuniform.

The assumption of uniform peat thickness contradicts the knownnorthtosouththinningoftheoriginaldeposit.However, since Eq. (1) is linear and becausethe deposit wassufficiently thickthatthepostdrainagewatertablehasgenerallybeenabove bedrock,thesoil processesnorth ofa lineofaveragethickness wouldbecounterbalancedbytheprocessessouthofthesameline, suggestingthatanaveragethicknesscanbereasonablyexpected tocapturethedepositbehavior.

Theassumptionofverticaluniformityofthepeatprofilehas strongsupportintheavailablecurrentandpredrainagedata.In 1915,undernearlypredrainageconditions,BaldwinandHawker (1915)extractedmorethan400soilcoresalongthefulllengthof theNorthNewRiverCanalandseveralmilestoeitherside.Valuesof lossonignition(%organicmatter)forthecoresextractedwithinthe areathatlaterbecametheEAAallfellwithinaverynarrowrange:

90±2%,withverylittledifferencebetweentheupperandlower profile(Table1).Althoughnopredrainageorearlypost-drainage dataontheverticaldistributionofbulkdensityappearstoexist, presentdaydetailedprofilesofbulkdensityobtainedbySaunders etal.(2008)frompeatsoilcoressectionedat1cmintervalsshowed stronguniformitywithdepth.Whilethesecoreswerenotfrom theEAA,theywerefromcomparableareaswithoriginallysimilar hydrologyandthesamesawgrassvegetation.

Uncertaintiesregardingtheoriginalandcurrentaveragethick- nessofthepeatdepositaresomewhathardertoassess.Neither thecurrent northepredrainagedatasetsare ideal.Theoriginal thicknessislikelytobeanunderestimateforatleastthreerea- sons.Theearliestmeasurementsofpeatthicknessweremademore thanthree decadesaftertheonset ofdrainagein 1882(McVoy etal.,2011).Atthattime,botanistsreportedplantspecieschar- acteristicofadrainedsawgrassmarsh:willow(Salixcaroliniana), elderberry(Sambucuscanadensis),carelessweed(Amaranthusaus- tralis) and dog fennel (Eupatorium capillifolium)(Harper, 1927;

Table2

PeatandcarbonlossesfromtheEAA.

Method1(thickness) Method2(elevation) Otherstudies

Timeperiod 1915–2003(88years) 1880–2000(120years)

Originalpeatvolume(m³) 6.5×10⁹ 8.3×10⁹

Peatvolumeremaining,ca.2003(m³) 2.0×10⁹ 3.4×10⁹ 4.7×10⁹(in1940s)^a

Peatvolumelostby2000/2003,(m³) 4.5×10⁹ 4.9×10⁹

Ave.subsidence,m(ft) 1.6(5.2) 1.7(5.6) 88yr:1.3(4.2)^b;120yr:1.8(5.8)^b

Peatmasslost(metrictons) 2.50×10⁸ 2.50×10⁸

MassCO2lost(metrictons) 4.85×10⁸ 4.92×10⁸

Averageemissionrate(gCO2m⁻²h⁻¹) 0.22 0.17 0.2–6.1^c

aPeatvolumeestimatedasremaininginearly1940s.FromDavis(1943a,b)forareaalreadyinagriculturaluseandDavis(1946)forsawgrasspeatareanotinagricultural use.

bSubsidencecalculatedfromliteratureratesof0.015myr⁻¹(Snyder,2005).

c EmissionratesfromEAAsoilsundertypicalFloridatemperatures(10–30^◦C);soilcolumnsandfieldstudies(Kniplingetal.,1970;Volk,1973;Geschetal.,2007).

Davis,1943a,b).Thisshifttodrierspecieswascorroboratedbythe landsurveyors’notations of“land dry.”Thebotanists’and land surveyors’observationssuggestthatadegreeofsubsidenceand perhapsoxidationwouldhavereducedthepeatthicknessfromthe originalpredrainagewetlandconditions.Additionally,thesurvey- orsapparentlyusedsoundingrodsof10footlength,butnotedin someofthemorenortherlylocationsthatthepeatwasthicker thanthis, probablyin therangeof 12–14ft.Lackingother site- specificinformation,wenecessarilysetthesemeasuredlengthsat 10ft.Finally,comparisonofcrosssectionsoftheEAAkrigedthick- nesswithEAAcross sectionsdepictedinStephens andJohnson (1951),suggeststhatthekrigingalgorithmsomewhatunderesti- matedtheinteriorpeatthicknessasitattemptedtofittheedgesof thepeatdeposit(zeropeatthickness).Allthreeofthesesourcesof error—unaccountedsubsidence,soundingrodlength,andartifacts ofkriging—willtendtomakethethicknessesmappedinFig.2less thantheactualpredrainagethicknesses.Althoughdifficulttoquan- tify,weestimatethatcollectivelythesethreesourcesoferrordonot causeanunderestimateoftheactualpredrainagepeatthicknessby morethan20–30%.

Theestimationofcurrentpeatthickness(Fig.3)usedinMethod 1isalsolikelytobeasourceofuncertaintyduetothesparseness ofthesupportingdataset.Thatsaid,theuncertaintymaynotbe aslargeasthesparsenessmightsuggest.Theextremeflatnessof thecurrentEAAgroundsurfaceappearstoreducetheuncertainty.

WecomparedFig.3withanolder,muchmoredetailedsoilmap (Coxetal.,1988),whichincludedfiveclassesofsoilthickness(>51;

36–51;20–36;8–20;<8in.).Weassumedthatallthicknessesesti- matedfor2003(i.e.,Fig.3)shouldbelessthanorequaltothose mappedbyCoxetal.in1988,duetoongoingsubsidencebetween 1988and2003.Wefoundonlyminorareas(<5%)thatcontradicted thiscriterion,suggestingthatFig.3mayinfactbeareasonable estimateofcurrentpeatthickness.

Forthetopography-basedcalculationsofpeatloss(Method2), boththepredrainageandthecurrentestimatedtopographycontain potentialsourcesoferror.Surprisingly,thecurrentlandsurface mayincludemoreuncertaintythanthepredrainagesurface.Ifso, thegreateraccuracyofthepredrainagesurfacewouldbedueto

thephysicalconstraintsthatshapedthepredrainagesystem-a peatsurfaceleveledbythepresenceofsurfacewater,withwater depthscontrolledbytheelevationsoftheslightlyhigheruplands borderingtheEverglades.

While it is difficulttoassess theoverall effect ofthickness- related uncertainty,we do notetheclosesimilarityofthepeat volumesestimatedbythetwomethods.Althoughnotconclusive, thesimilaritysuggeststhepossibilitythatbothmayinfactcapture theactualchangeinvolume(andthickness).

TheperturbationanalysisdiscussedintheResultssectionvar- iedtheparametervalues(Table3)ofEq.(1)incombinationsthat wouldproducethelargestandsmallestvaluesofcarbonlost.We estimatedthehighandlowvaluesforeachparameterbasedonval- uesfromtheliterature(Table1),fieldknowledgeofthesoilsand theirdrainagehistory,andlogicalconstraints.Forexample,physi- cally,thepostdrainageorganicmattercontentcannotexceedthe predrainagecontentandthepostdrainagebulkdensitymustbe greaterthanorequaltothepredrainagevalue.Theanalysisclar- ifiedthatEq.(1)ishighlysensitivetotheestimatedvalueofthe predrainagebulkdensity.Theanalysisalsosuggestedthattherange ofphysicallyplausibleparametervaluesmayinfactbenarrow;

eventhequitenarrowrangesshowninTable3resultedinnegative valuesfortheestimatedminimumvalueofcarbonreleased.Thisis ofcoursenotrealistic,suggestingthattheTable3valuesofoneor moreparameters,orcombinationsofparameters,hadbeenvaried toowidely,atleastforthelowextreme.

TheCO₂lossesfoundinthisstudycanbecomparedwithemis- sionratesfoundinotherstudies,providedthatitcanbeassumed thatemissionrateswereconstantoverthecenturyofpeatloss.

Whileitisknownthatrateofsubsidenceofpeatsoilsiscloserto exponentialthanlinear,thisisprimarilyduetotheinitialphysi- calchangesrelatedtodewateringandcompaction.Assumingthat theoxidationratewasapproximatelyconstant(stationarytem- peratureanddepthofdrainedwatertable),wecalculatedaCO₂ emissionrateofabout0.2gCO2m⁻²h⁻¹(Table2).Thisrateissim- ilartobutlowerthanfield-measuredvaluesof0.4–2.7gm⁻²h⁻¹ fortilledanduntilledEAAsoils(Geschetal.,2007)andalsotoval- uesof0.3and0.8gm⁻²h⁻¹measuredinanpeatlandofsimilararea

Table3

ValuesofcurrentandpredrainagesoilparametersfortheEvergladesAgriculturalArea(EAA)usedinthisstudy.

Parameter Timeperiod Depth Symbol Units Low Mid High

Peatthickness Predrainage All zI m 2.33^a 2.63 3.23^a

Peatthickness Current >30cm zF2 m 0.50 0.65 0.80

Bulkdensity Predrainage All bI Mgm⁻³ 0.08 0.10 0.12

Bulkdensity Current 0–30cm bF1 Mgm⁻³ 0.28 0.32 0.36

Bulkdensity Current >30cm bF2 Mgm⁻³ 0.10 0.14 0.18

Organicmatter Predrainage All OMI kgkg⁻¹ 0.88 0.90 0.92

Organicmatter Current 0–30cm OMF1 kgkg⁻¹ 0.75 0.80 0.85

Organicmatter Current >30 OMF2 kgkg⁻¹ 0.84 0.86 0.88

aHighandlowestimatesareintentionallyasymmetrictoreflectthelikelihoodthatpredrainagesourcesunderestimatedpeatthickness.