Mercury and the Everglades. A Synthesis and Model for Complex Ecosystem Restoration: Volume III – Temporal Trends of Mercury in the Everglades, Synthesis and Management Implications

176 Pages·2020·5.626 MB·English

by  Curtis D. Pollman, Donald M. Axelrad, Darren G. Rumbold

Curtis D. Pollman
Donald M. Axelrad
Darren G. Rumbold  Editors
Mercury and
the Everglades.
A Synthesis and
Model for Complex
Ecosystem Restoration
Volume III – Temporal Trends of Mercury
in the Everglades, Synthesis and
Management Implications
Mercury and the Everglades. A Synthesis and Model
for Complex Ecosystem Restoration
(cid:129) (cid:129)
Curtis D. Pollman Donald M. Axelrad
Darren G. Rumbold
Editors
Mercury and the Everglades.
A Synthesis and Model
for Complex Ecosystem
Restoration
–
Volume III Temporal Trends of Mercury in
the Everglades, Synthesis and Management
Implications
Editors
CurtisD.Pollman DonaldM.Axelrad
AquaLuxLucis,Inc InstituteofPublicHealth
Gainesville,FL,USA FloridaA&MUniversity
Tallahassee,FL,USA
DarrenG.Rumbold
CoastalWatershedInstitute
FloridaGulfCoastUniversity
FortMyers,FL,USA
ISBN978-3-030-55634-1 ISBN978-3-030-55635-8 (eBook)
https://doi.org/10.1007/978-3-030-55635-8
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“TheEarlyIbisGetstheCrayfish”byBrianGarrett
Preface
Ifyouwanttograbtheworldandrunit
Iseethatyouwillnotsucceed.
Theworldisaspiritualvessel,whichcan’tbecontrolled—LaoTzu1
Thisbookistheculminationofmorethana4-yearefforttosynthesizeessentially
30 years of research into the mercury (Hg) problem in the Everglades. For Hg to
enter food webs in aquatic ecosystems such as the Everglades, two things are
required: first, of course, is a source of Hg; second is that the Hg in the aquatic
ecosystemneedstobe either presentasorconvertedtothemethylatedform.Once
Hg is methylated, it can enter the base of aquatic food web and then increasingly
bioaccumulate by trophic transfer. This biogeochemical cycling of Hg is complex
and involves multiple processes and variables. This cycling with respect to methyl
Hg(MeHg)productioncanalsobeprofoundlyinfluencedbyanthropogenicdistur-
bances.Formanyotherwisepristineaquaticecosystems,disturbancemanifestingin
elevatedbiotaHgconcentrationsrelatesprimarilytochangesinatmosphericinputs
of Hg. For the Everglades, however, such disturbance includes not only anthropo-
genicimpactsonatmosphericHgcyclinganddeposition,butalsodirectimpactson
thehydrologyandwaterchemistryofthesystem.
Thisbookisultimatelydevotedtounderstandingthenatureandevolutionofthe
Hg problem in the Everglades and the attendant role played by anthropogenic
disturbance. Our hope is that synthesizing much of the vast amount of extant
research in the Everglades itself can lead to informed approaches and policies by
key decision makers toward mitigating the problem. These objectives accordingly
are embodied in the sequential exposition of the three volumes comprising this
synthesis.
VolumeIpresentedanoverviewoftheEvergladesfromaphysicalandecological
perspective and how anthropogenically driven disturbance has changed the system
1Lao Tzu (n.d.) Tao Te Ching. Translated by Charles Muller (2005) Barnes and Noble Books,
NewYork,NY.
vii
viii Preface
spatially, hydrologically, and ecologically over the past approximately 140 years.
Therootsofthisdisturbancelayinincreasingdevelopmentinsouth Florida which
broadenedovertimeandledtomoreconcertedeffortstobothconvertnaturalhabitat
to agricultural and urban land uses and manage the resultant remnant Everglades
hydrologically.ThesheermagnitudeoftheseeffortswasintimatedbyShelton2,who
in1957wroteaboutthe“largestlow-levelpumpingstationeverconstructed”aspart
ofan“enormousfloodcontrolandwaterconservationproject”hetoutedas“oneof
the greatest examples in America of what man’s intelligence and vision can do in
convertingtheerraticforcesofnatureintosolidassetsforthevegetableandanimal
kingdom.” While disturbance is an important theme in understanding the mercury
problemintheEverglades,theproblemwouldbemootwithoutsourcesofmercury
enteringthesystem.AtmosphericdepositionofHgistheoverwhelmingpathwayfor
Hg entering the Everglades,3 and Volume I thus includes chapters on atmospheric
cycling of Hg and how key aspects of its cycling serve to exacerbate atmospheric
inputstotheEverglades.VolumeIalsoincludeschaptersonthechangingtemporal
relationshipbetweenlocalandlargerscalesourcescontributingtotheEvergladesHg
problem and the implications of this current relationship for mitigation through
controllingemissionsources.
Volume II has two parts, with the first part providing greater context regarding
how direct disturbance in the Everglades has or likely has contributed to the Hg
problem.ThefoundationforthisassessmentliesinunderstandinghowHgentering
theEvergladesistransformed and entersthe aquatic food web, which inturn links
trophictransferofHginterrestrialwildlifetoHginputstothemarsh.Thisbiogeo-
chemical cycling is critical with respect to how strongly Hg in the Everglades
bioaccumulates—the importance of which cannot be overstated. As mentioned
earlier, Hg must be present as methyl Hg (MeHg) for it to enter aquatic food
webs; since external inputs of Hg to the Everglades are greatly dominated by
inorganicHgratherthanMeHg,thisHginturnmustbemethylatedforittobecome
problematic. As we saw in Volume II, two water chemistry variables—sulfate and
dissolvedorganiccarbon—emergeasplayingkeyrolesinthemethylationofHgin
theEverglades.Inaddition,bothofthesevariables—which,alongwithHg,George
AikenoftheU.S.GeologicalSurveyidentifiedasthe“biogeochemicalaxisofevil”
responsible for the global Hg problem—also are profoundly influenced by
2SheltonWR(1957)LandoftheEverglades.TropicalSouthernFlorida.DepartmentofAgriculture,
StateofFlorida,Tallahassee,FL.
3AtkesonTD,AxelradD,PollmanCD,KeelerG(2003)Integratingatmosphericmercurydeposi-
tionandaquaticcyclingintheFloridaEverglades:anapproachforconductingatotalmaximum
daily load analysis for an atmospherically derived pollutant. Integrated Summary. Final report
submittedtoUSEPA.FloridaDepartmentofEnvironmentalProtection,Tallahassee,FL.
StoberQJ,ThorntonK,JonesR,RichardsJ,IveyC,WelchR,MaddenM,TrexlerJ,GaiserE,
ScheidtD,RathbunS(2001)SouthFloridaEcosystemAssessment:PhaseI/II—Evergladesstressor
interactions: hydropatterns, eutrophication, habitat alteration, and mercury contamination (sum-
mary). EPA 904-R-01-002. USEPA Region 4 Science & Ecosystem Support Division, Water
ManagementDivision,andOfficeofResearchandDevelopment.
Preface ix
disturbancewithintheEverglades.ThesecondpartofVolumeIIinturnpresentsa
series of chapters elucidating how Hg bioaccumulates in the Everglades, the mag-
nitudeofthatbioaccumulation,andtheresultantecologicalandhumanhealthrisks.
Asthelastofthreevolumescomprisingthissynthesis,thisvolumeisdevotedto
the overarching question of how the Hg problem can be mitigated or managed.
Similar to theother two volumes, Volume III is organized into two parts. The first
partincludesthefollowingchapters:
Chapter1:TrendsinAtmosphericDepositionofMercury
Chapter2:TemporalChangesinMercuryConcentrationsinEvergladesBiota
Chapter3:LegacyMercury
Chapter4:SimulatingMercuryCyclingintheFloridaEverglades:ACaseStudy
Chapter5:TemporalChangesintheMercurySignalintheEverglades:ASynthesis
The first two chapters focus on trends observed for two key components of the
Everglades Hg problem—trends in atmospheric deposition and trends in aquatic
biota and terrestrial wildlife Hg concentrations. The analysis evaluated various
datasets for possible long-term and short-term trends. Long-term trends were
assessed in Hg accumulation rates in sediment cores (ca.1800–2010) and Hg
concentrations in the scapular feathers of wading birds, including museum speci-
mens(ca.1910–2010).Analysesofshort-termtrendsincludethoseinwetdeposition
ofatmosphericHg(ca.1984–2016)andbiota,includingprimarilyfish(ca.1980to
2016).Chapter3focusesontheimportantmanagementquestionofHgoriginating
from historical or previous inputs (so-called legacy Hg) and residing in different
environmental pools (e.g., Everglades sediments) that can still contribute to Hg
biogeochemicalcyclingandtheimplicationsthatlegacyHgposesforremediation.
Chapter4presentsananalysisoflikelydriverscontributingtothetemporaldynam-
ics in largemouth bass Hg concentrations using the deterministic Everglades Mer-
cury Cycling Model (E-MCM). Because they use a priori functional, mechanistic
relationships in a dynamic modeling framework, models such as E-MCM are
consideredtobemorestronglyindicativeofcauseandeffectthanempiricalmodels
(e.g., linear regression models). While the E-MCM has the ability to be applied
acrosstheEvergladesasaseriesofspatiallydiscretecells,itisquitedataintensive.
Asaresult,themodelwasappliedtosite3A-15withinWaterConservationArea3A
because the site was identified during the early days of the South Florida Mercury
Science Program (see Preface to Volume I for a discussion of the evolution of the
SFMSP)asaHg“hot-spot”ofconsiderableinterest;thisinterestledtoanumberof
intensive studies led by the U.S. Geological Survey and others4 that produced
4USGS (2018) USGS SOFIA Data Exchange. https://sofia.usgs.gov/projects/index.php?project_
url¼evergl_merc
Aiken GR, Gilmour CC, Krabbenhoft DP, Orem W (2011) Dissolved organic matter in the
Florida Everglades: implications for ecosystem restoration. Crit Rev Environ Sci Technol 41
(S1):217–248.
KrabbenhoftDP(1996)MercurystudiesintheFloridaEverglades.U.S.GeologicalSurveyFact
SheetFS-166–196.
x Preface
sufficientbiogeochemicaldataforparameterizingandcalibratingtheE-MCM.The
part then concludes with a chapter synthesizing key results from the first four
chapters.
Severalkeypointsemergefromthefirstpartchapters.Firstistheparadigmthat
changes in atmospheric deposition of Hg have contributed to changes in biota Hg
concentrations and the trends in atmospheric deposition had a local emissions
component. Second is that the magnitude of biota Hg concentration changes—
bothlongtermandshortterm—exceedsinferredchangesinatmosphericdeposition.
Theimplicationofthisresultisthatsomeothervariablechangingovertimeisacting
inadditiontochangesinatmosphericdepositiontodrivebiotaHgtemporaldynam-
ics. The third key point is that trends in biota Hg vary across the Everglades
landscape;thisfactisimportantinassessingtheroleofchangesinwaterchemistry
ascausativeandtheattendantimplicationsofsuchchangesfromamercurymitiga-
tionperspective.AfourthkeypointrelatestotheimpactoflegacyHgontherateand
magnitude of recovery of the Everglades from atmospheric Hg input mitigation.
Legacy Hg has two spatial components—large or global scale and local scale.
Understanding the very different dynamics these two components expectedly will
impose on the Everglades over time has profound implications with respect to Hg
mitigationsuccess.
The second part of this volume speaks to the question of trying to mitigate the
Everglades problem from a quantitative and philosophical perspective. It contains
twochaptersthattouchuponthisquestion:
Chapter6:StructuralEquationModelforMercuryCyclingintheEverglades
Chapter7:MercuryMitigationManagementStrategiesandLikelyOutcomes
The first chapter of this final part uses structural equation modeling (SEM) to
identify thebiogeochemical variablesthat moststronglycontributetothemethyla-
tionofHgandtrophictransferofMeHgtoGambusia5withintheEvergladesmarsh.
SEMoffersseveral advantagesoverother modeling approachesfortheEverglades
Hgproblem.First,unliketheE-MCM,themodelcanbeappliedacrosstheentirety
of the Everglades Protection Area (EvPA) with existing data. Second, similar to
mechanisticmodeling,SEMreliesonaprioriconstructsorpathways.Fittingofthe
SEM establishes whether the model pathways are statistically significant and sup-
portable;SEMisthusconsideredtobemoreinformativewithrespecttocauseand
Gilmour CC, Riedel GS, Ederington MC, Bell JT, Benoit JM, Gill GA, Stordal MC (1998)
Methylmercury concentrations and production rates across a trophic gradient in the northern
Everglades.Biogeochemistry40:327–345.
HurleyJP, Krabbenhoft DP, Cleckner LB, OlsonML, AikenGR, Rawlik RS (1998) System
controlsontheaqueousdistributionofmercuryinthenorthernFloridaEverglades.Biogeochem-
istry40:293–310.
5GambusiaisquiteusefulasanindicatorspeciesbecauseitisbothubiquitousintheEverglades
marshandhasarelativelyshortlifespan(~1year).Moreover,Gambusiaindividualsarelesswidely
rangingthanlargerfishoccupyinghighertrophiclevelniches.Asaresult,GambusiaHgconcen-
trationdynamicsshouldmorecloselyreflectbothsmallerspatialandshortertemporaldynamicsin
Hgbiogeochemicalcycling.
Preface xi
effect than typical regression models. Moreover, unlike conventional regression
modeling,SEMallowsforindirectrelationshipsbetweenvariableswheretheeffect
of a particular variable A on a response variable C is mediated by the direct
interaction of A with a third variable B that in turn interacts directly with C. With
respect to Hg biogeochemical cycling, this ability to model indirect effects is
importantbecause,forexample,theeffectsofdissolvedorganiccarbonandsulfate
onGambusiaHgconcentrationsaremediatedthroughtheeffects ofboth variables
onMeHgconcentrations.
TheSEMinChap.6alsolaysthepredicateforconductingscenariomodelingfor
mitigating the Everglades Hg problem. From a management perspective, it is
extremely useful for such modeling to include a quantitative assessment of the
uncertainty inherent in the predictions. One such approach that increasingly is
gaining broader use is the application of marginal analyses. Marginal analyses are
used to isolate the effects of single predictive parameter included as part of a
multivariate model by holding the other predictive parameters at fixed levels and
allowingthepredictiveparameterofinteresttovaryacrossaspecifiedrangeduring
thepredictiveanalysis.Thisallowsfortheconstructionofconfidenceintervalsinthe
predicted values of the response variable (e.g., Gambusia Hg concentrations) as a
function of the predictive variable of interest (e.g., sulfate). Chapter 7 thus uses
marginal analyses coupled with a “reduced form” generalized linear model
abstracted from the SEM developed in Chap. 6 to evaluate the effects of reducing
sulfate on predicted Gambusia Hg concentrations in each of the major hydrologic
unitswithintheEvPA.Therationaleforrecommendingandthusfocusingonsulfate
alone rather than including other variables as the foundation of a strategy for
mitigatingtheEvergladesHgproblemisdelineatedasthechapterbeginsandreflects
broad support for the causative role of sulfate across multiple lines of evidence
coupledwiththefactthatsulfateconcentrationsaregreatlyelevatedacrossmuchof
theEvPA.
Intheverylate1970s/early1980swhenmonitoringoffishtissueHgconcentra-
tionswasfirstinitiated,anongoinganddireHgproblemintheEvergladesbecame
immediatelymanifest.FishtissueHgconcentrationsintheEvergladeswereamong
the highest recorded in otherwise pristine systems worldwide (which by their very
natureareparticularlysensitivetotheinsultsofatmosphericinputsofHg)coupled
withprofoundimpactstoterrestrialwildlifeincludingwadingbirdsandtheendan-
gered Floridapanther.The storysince then, however,providesaglimmer ofhope.
Sincetheearly1980swhentheywereatpeakrecordedlevels,biotaHgconcentra-
tions throughout much—albeit not all—of theEvPA declined by upwards to 75%.
The variability in spatial and temporal dynamics of biota Hg concentrations in the
Everglades further indicates that substantial reductions in biota Hg concentrations
canoccurrapidlyandthatvariationsinbiogeochemistryareamajorcauseofthese
dynamics.Thisfactbringsusbacktothethemeofdisturbance.Keyamongthewater
chemistry variables controlling Hg methylation is sulfate, the occurrence of which
within the Everglades is overwhelmingly a product of disturbance. If we have the
will,thisdisturbancecanbereversed,andthebenefitsofreducedHgconcentrations
intheEvergladesshouldbereapedquickly.