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AQUATICECOLOGY

AbundanceBiomassComparisonMethod

RMWarwick, PlymouthMarineLaboratory,Plymouth,UK

r 2008ElsevierB.V.Allrightsreserved.

Introduction

The`abundancebiomasscomparison' (ABC)methodisameansofdetectingtheeffectsofanthropogenicperturbationsonassemblages oforganismsthatisunderpinnedbythe r-and K-selectiontheory(see r-Strategist/K-Strategists).Understableconditionsofinfrequent disturbancethecompetitivedominantsintheclimaxcommunityare K-selectedorconservativespecieswithalargebodysizeandlong lifespan,andareusuallyoflowabundancesothattheyarenotdominantnumericallybutaredominantintermsofbiomass. Frequentlydisturbedassemblagesarekeptatanearlysuccessionalstageandcomprise r-selectedoropportunisticspeciescharacterized bysmallbodysize,shortlifespanandhighabundance.TheABCmethodexploitsthefactthatwhenanassemblageisperturbedthe conservativespeciesarelessfavoredincomparisonwiththeopportunists,andthedistributionofbiomassamongspeciesbehaves differentlyfromthedistributionofnumbersofindividualsamongspecies.

TheMethod

TheABCmethodasoriginallyformulatedinvolvestheplottingofseparate k-dominancecurves(see k-DominanceCurves)for speciesabundancesandspeciesbiomassesonthesamegraphandcomparingtheformsofthetwocurvesrelativetoeach other.Thespeciesarerankedinorderofimportanceintermsofabundanceorbiomassonthe x-axisonalogarithmicscale, withpercentagedominanceonthe y-axisonacumulativescale.Ofcoursethespeciesorderingisunlikelytobethesamefor abundanceandbiomass.Inundisturbedassemblagesafewlargespeciesaredominantintermsofbiomassbutnotabundance,resultingintheelevationofthebiomasscurverelativetotheabundancecurvethroughoutitslength(Fig.1a). Perturbedassemblages,however,haveafewspecieswithveryhighabundancebutsmallbodysize sothattheydonot dominatethebiomassandtheabundancecurveliesabovethebiomasscurve(Fig.1c).Undermoderateperturbationthe largecompetitivedominantsareeliminatedbutthereisnopopulationexplosionofsmallopportunists,sothattheinequality insizebetweenthenumericalandbiomassdominantsisreducedandthebiomassandabundancecurvesareclosely coincidentandmaycrossovereachotheroneormoretimes(Fig.1b).

Thecontentionisthatthesethreeconditions(unperturbed,moderatelyperturbed,orgrosslyperturbed)shouldbe recognizablewithoutreferencecontrolsamplesintimeorspace,thetwocurvesactingasaninternalcontrolagainsteach otherandprovidingasnapshotoftheconditionoftheassemblageatanyonetimeorplace.Ofcourse,conﬁrmatory comparisonswithspatialortemporalreferencesamplesarestillhighlydesirable.Aprerequisiteofthe methodisadequate samplesizeorreplicationbecausethelargebiomassdominantsareoftenrareandliabletoa highersamplingerrorthanthe numericaldominants.

TheevaluationofABCcurvesinvolvestheirvisualinspection,andcanbecumbersomeifmanysites,times,orreplicatesare involved.Insuchcasesitisconvenienttoreduceeachplottoasinglesummarystatistic.Iftheabundance(A)valuesaresubtracted fromthebiomass(B)valuesforeachspeciesrankintheABCcurve,thesumofthe B A valuesacrosstherankswillbestrongly positiveintheunperturbedcase(Fig.1a),nearzerointhecasewherethecurvesarecloselycoincident(Fig.1b),andstrongly negativewherethecurvesaretransposed(Fig.1c).Thesummationneedstobestandardizedtoacommonscalesothatcomparisonscanbemadebetweensampleswithdifferingnumbersofspecies(S),themostwidelyusedformbeingthe W (forWarwick)statistic:

Forreplicatedsamples,the W statisticalsoprovidesanobviousrouteforhypothesistesting,usingstandardunivariateANOVA.

Applications

Forthemostpart,ABCcurveshavebeenusedtoindicatepollutionordisturbanceeffectsonmarineandestuarinemacrobenthic assemblages,whicharethemaintargetfordetectionandmonitoringprogramsinthesehabitats.Forexample,ABC curvesforthe

Species rank (log scale)

Fig.1 Hypothetical k-dominancecurvesforspeciesbiomassandabundance,showingunperturbed,moderatelyperturbed,andgrosslyperturbed conditions.

macrobenthosinLochLinnhe,Scotlandinresponsetoorganicpollutionbetween1963and1973aregivenin Fig.2.Thetimecourseof pollutionfromapulpmill,andchangesinspeciesdiversity(H0 ),areshowntopleft.Moderatepollutionstartedin1966,andby1968 speciesdiversitywasreduced.Priorto1968theABCcurveshadtheunpollutedconﬁguration.From1968to1970theABCplots indicatedmoderatepollution.In1970therewasanincreaseinpollutantloadingsandafurtherreductioninspeciesdiversity,reachinga minimumin1972,andtheABCplotsfor1971and1972showthegrosslypollutedconﬁguration.In1972pollutiondecreasedandby 1973diversityhadincreased,andtheABCplotsagainindicatedtheunpollutedcondition.Thus,theABCplotsprovideagoodsnapshot ofthepollutionstatusofthebenthiccommunityinanyoneyear,withoutreferencetothehistorical comparativedatawhichwouldbe necessaryifasinglespeciesdiversitymeasurebasedontheabundancedistributionwasusedastheonlycriterion.

MoststudiessuggestthattheABCcurvesrespondtoanthropogenicperturbationsbutarenotaffectedbylong-termnaturalstresses, sincetheorganismslivinginsuchenvironmentshaveevolvedadaptationstotheprevailingecologicalconditions.UnperturbedABC plotsmaybefound,forexample,inestuarieswheretheorganismsaresubjectedtolowand fluctuatingsalinities,providedthereareno anthropogenicdisturbances.ABCplotsindicatedthatmacrobenthiccommunitiesnearanoilrefineryinTrinidadweregrosslyto moderatelystressed,whilethoseclosetotheTrinidadPitchLake(oneofthelargestnaturaloilseepsin theworld)werenot.Thereis littleevidence,however,thatthemethodcandistinguishbetweendifferenttypesofanthropogenicdisturbances.Responsestoorganic pollutionandtophysicaldisturbancecausedbydemersaltrawl fisheries,forexample,appeartobesimilar.

Themethodhasbeenlesswellexploredwithrespecttoothercomponentsofthebiota.However,ithasbeenusedsuccessfully toindicateenvironmentalimpactsonmarinephytoplankton,thecryptofaunaandmollusksofrockyshores,invertebratesof freshwaterstreams,and ﬁshassemblagesinbothmarineandfreshwater.

ProblemsandTheirSolutions

Veryoften k-dominancecurvesapproachacumulativefrequencyof100%foralargepartoftheirlength,andinhighlydominated assemblagesthismaybeafterthe firsttwoorthreetop-rankedspecies.Thus,itmaybedifficulttodistinguishbetweentheformsof thesecurves.Thesolutiontothisproblemistotransformthe y-axissothatthecumulativevaluesareclosertolinearity,an appropriatetransformationbeingthemodifiedlogistictransformation: yi ¼ log ½ð1 þ yiÞ=ð

ApotentiallymoreseriousproblemwiththecumulativenatureofABCcurvesisthattheirformisoverdependentonthesinglemost dominantspecies.Theunpredictablepresenceoflargenumbersofaspecieswithsmallbiomass,perhapsaninﬂuxofthejuvenilesof onespecies,maygiveafalseimpressionofdisturbance.Withgenuinedisturbance,onemightexpectpatternsofABCcurvestobe unaffectedbysuccessiveremovaloftheoneortwomostdominantspeciesintermsofabundanceorbiomass,andasolutionistheuse ofpartialdominancecurves,whichcomputethedominanceofthesecond-rankedspecies overtheremainder(ignoringthe ﬁrst-ranked species),thesamewiththethirdmostdominant,etc.Thus,if ai istheabsolute(orpercentage)abundanceofthe ithspecies,when rankedindecreasingabundanceorder,thepartialdominancecurveisaplotof pi againstlog i (i ¼ 1,2, , S 1),where

Earliervaluescanthereforeneveraffectlaterpointsonthecurve.Thepartialdominancecurves(ABC)forundisturbedmacrobenthic communitiestypicallylooklike Figs.3g and 3h,withthebiomasscurve(thinline)abovethe abundancecurve(thickline)throughout itslength.Theabundancecurveismuchsmootherthanthebiomasscurve,showingaslightandsteadydeclinebeforethe inevitable ﬁnalrise.Underpollutedconditionsthereisstillachangeinpositionofpartialdominancecurvesforabundance and biomass,withtheabundancecurvenowabovethebiomasscurveinplaces,andtheabundancecurvebecomingmuchmorevariable. Thisimpliesthatpollutioneffectsarenotjustseeninchangestoafewdominantspeciesbut areaphenomenonwhichpervadesthe

Fig.2 LochLinnhemacrofauna:Shannondiversity(H0 )andABCplotsoverthe11years,1963to1973.Abundance,thickline;biomass, thinline.

completesuiteofspeciesinthecommunity.ThetimeseriesofmacrobenthosdatafromLochLinnhe(Fig.3)showsthatinthemost pollutedyears,1971and1972,theabundancecurveisabovethebiomasscurveformostofitslength(andtheabundancecurveisvery atypicallyerratic),thecurvescrossoverinthemoderatelypollutedyears1968and1970andhave anunpollutedconﬁgurationpriorto thepollutionimpactin1966and1967.Althoughthesecurvesarenotsosmooth,andtherefore notsovisuallyappealing,asthe originalABCcurves,theymayprovideausefulalternativeaidtointerpretationandarecertainlymorerobusttorandom ﬂuctuationsin theabundanceofasmall-sized,numericallydominantspecies.

Inmostcaseswherethepresenceoflargenumbersofsmall-bodiedmacrobenthicspeciesinunperturbedsituationshasgivena falseimpressionofdisturbance,thosespecieshavenotbeenpolychaetes.PriortotheAmocoCadizoilspilloffthenorthcoastof Francein1978,smallampeliscidamphipods(Crustacea)werepresentatthePierreNoirestationinrelativelyhighabundance,and theirdisappearanceafterthespillconfoundedtheABCplots.Theerraticpresenceoflargenumbersofsmallamphipods(Corophium)ormollusks(Hydrobia)alsoconfoundedtheseplotsintheWaddenSea.Thesesmallnonpolychaetousspeciesarenot indicativeofpollutedconditions.AtaxonomicbreakdownoftheABCresponsehasshownthatitresultsfrom(1)ashiftinthe proportionsofdifferentphylapresentincommunities,somephylahavinglarger-bodiedspeciesthanothers,and(2)ashiftinthe relativedistributionsofabundanceandbiomassamongspecieswithinthePolychaetabutnotwithinanyoftheothermajorphyla (Mollusca,Crustacea,Echinodermata).Theshiftwithinpolychaetesreﬂectsthesubstitutionoflarger-bodiedbysmaller-bodied species,andnotachangeintheaveragesizeofindividualswithinaspecies.Inmostinstancesthephyleticchangesreinforcethe trendinspeciessubstitutionswithinthepolychaetes,toproducetheoverallABCresponse,butinsomecasestheymaywork

Fig.3 LochLinnhemacrofaunainselectedyears1966–68and1970–72.(a–f)ABCcurves(logistictransform).(g–l)Partialdominancecurves forabundance(thickline)andbiomass(thinline)forthesameyears.

againsteachother.Indicationsofpollutionordisturbanceformarinemacrobenthosdetectedbythismethodshouldthereforebe viewedwithcautionifthespeciesresponsiblefortheperturbedconﬁgurationsarenotpolychaetes,andtherobustnessoftheplots shouldbetestedusingpartialdominancecurves.

Finally,apracticalratherthanaconceptualproblemwiththemethodisthatitreliesonapainstakingandtime-consuming(and hencecostly)analysisofsamplesinwhichallthespeciesmustbeseparated,counted,andweighed.Severalgroupsofmarineorganisms aretaxonomicallydifficult,forexample(inthemacrobenthos),severalfamiliesofpolychaetesandamphipods;asmuch timecanbe spentinseparatingafewofthesedifficultgroupsintospeciesastheentireremainderofthesample,eveninNorthernEurope where taxonomickeysforidentificationaremostreadilyavailable.Manytaxareallyrequiretheskillsofspecialiststoseparatetheminto species,andthisisespeciallytrueinpartsoftheworldwherefaunaispoorlydescribed.Identificationtosomehighertaxonomiclevel, forexample,familyratherthanspecies,isconsiderablyeasierandquicker,andtheABCmethodhasprovedtobeencouraginglyrobust toanalysisatthefamilylevelforbothmacrobenthosand fish;verylittleinformationappearstobelost.

Seealso: AquaticEcology: TheEstuarineQualityParadoxConcept. ConservationEcology: k-DominanceCurves;EcologicalHealth Indicators. Ecosystems: Estuaries. GeneralEcology: Abundance;Biomass;Dominance

Blanchard,F.,LeLoc'h,F.,Hily,C.,Boucher,J.,2004.Fishingeffectsondiversity,size,andcommunitystructureofthebenthicinvertebrateand ﬁshmegafaunaontheBayof BiscaycoastofFrance.MarineEcologyProgressSeries280,249–260. Clarke,K.R.,1990.Comparisonsofdominancecurves.JournalofExperimentalMarineBiologyandEcology138,143–157.

Dauer,D.M.,Luckenbach,M.W.,Rodi,A.J.,1993.Abundancebiomasscomparison(ABCmethod) – Effectsofanestuarinegradient,anoxichypoxiceventsandcontaminated sediments.MarineBiology116,507–518.

Ismael,A.A.,Dorgham,M.M.,2003.EcologicalindicesasatoolforassessingpollutioninEl-DekhailaHarbour(Alexandria,Egypt).Oceanologia45,121–131. Jouffre,D.,Inejih,C.A.,2005.Assessingtheimpactof ﬁsheriesondemersal ﬁshassemblagesoftheMauritaniancontinental;shelf,1987–1999,usingdominancecurves.ICES JournalofMarineScience62,380–383. Lasiak,T.,1999.Theputativeimpactofexploitationonrockyinfratidalmacrofaunalassemblages:Amultipleareacomparison.JournaloftheMarineBiologicalAssociationof theUnitedKingdom79,23–34.

Magurran,A.E.,2004.MeasuringBiologicalDiversity.Oxford:Blackwell. Penczak,T.,Kruk,A.,1999.Applicabilityoftheabundance/biomasscomparisonmethodfordetectinghumanimpactson ﬁshpopulationsinthePilicaRiver,Poland.Fisheries Research39,229–240.

Warwick,R.M.,1986.Anewmethodfordetectingpollutioneffectsonmarinemacrobenthiccommunities.MarineBiology92,557–562.

Warwick,R.M.,Clarke,K.R.,1994.RelearningtheABC:Taxonomicchangesandabundance/biomassrelationshipsindisturbedbenthiccommunities.MarineBiology118, 739–744.

Warwick,R.M.,Pearson,T.H.,Ruswahyuni,1987.Detectionofpollutioneffectsonmarinemacrobenthos:Furtherevaluationofthespeciesabundance/biomassmethod.Marine Biology95,193–200.

Yemane,D.,Field,J.G.,Leslie,R.W.,2005.Exploringtheeffectsof ﬁshingon ﬁshassemblagesusingabundancebiomasscomparison(ABC)curves.ICESJournalofMarine Science62,374–379.

AcidificationinAquaticSystems

MorganaTagliarolo,Ifremer,UMSRLEEISA(CNRSUGIfremer),Cayenne,France

Glossary

Buffer AcompoundthatlimitslargechangesinpHinasolution.Thebuffersolutionconsistsinamixtureofweakacidand bases.ThebufferingabilityisdefinedasthequantityofstrongacidorbasethatmustbeaddedtochangethepHof1Lof solutionby1pHunit.

Chemicalequilibrium Stateofareversiblechemicalreactionwhentherateoftheforwardandbackwardreactionisequal. Consequently,theconcentrationsofbothreactantandproductarestables.Thisequilibriumcanbedescribedbyaconstant (K).

Homeostasis Abiologicalprocessmaintainingastableconditioninanorganismeveniffacedwithexternalchanges.An exampleistheabilityofthehumanbodytomaintainarelativelyconstantbodytemperatureindependentofexternal temperatures.

Hypercapnia DisequilibriuminbodyfluidswithanabnormalincreaseofcarbondioxideandpHdrop.

Oligotrophic Anenvironmentpoorinorganicandinorganicnutrients.

Resilient Anecosystemororganismisdefinedasresilientwhenitisabletorespondandrecoverfromadversesituations.

Introduction

Acidificationinanaquaticsystemisatermdescribingsignificantchangestothechemistryoffreshwater,marine,andbrackish systems,mostlycausedbythedissolutionofatmosphericcarbondioxide(CO2).TheCO2 fromtheatmospherecombineswith otherdissolvedinorganiccarbonalreadypresentinthewatercausingseveralcomplexchemicalchanges.Thecharacterizationofthe physicochemicalpropertiesofthecarbonatesysteminnaturalwatersisnotstraightforwardsinceitcanbedescribedbyalarge numberoftermsandunits.However,pHisthemorecommonparameteremployedfordescribingtheacidificationphenomenain ecology.

WaterpHisanexpressionoftheconcentrationofhydrogenions(Hþ)onalogarithmicscale,whereaneutralpHof7.00is representedbypurewaterat25 C.SurfacewatersintheopenoceanareslightlyalkalinewithrelativelysmallpHvariability(average valuesrangingbetween7.9and8.3).Variabilityrangesarewiderincoastalandfreshwaterecosystemswherecomplexbiogeochemicaldynamicsplayimportantrolesonthephysicalandchemicalconditionsofthosewaters.InshallowcoastalareaspHcan varydrasticallyoverdailycyclesandsmallspatialscales( 0.3–0.5units).InnaturalfreshwaterecosystemspHhasanevenwider range(between <2and12)dependingontheregionandonthewaterbodycharacteristics.

PartofatmosphericCO2 iscontinuouslyabsorbedbytheaquaticsystemswhereitreactswiththewatermoleculestoformweak acids.TheseacidsmostlydissociateintoHþ causingpHreductions.Freshwaterandseawatercontainavarietyofacid–basespecies abletoreactwiththeadditionalHþ ions.Thepredominantionsarecarbonateandbicarbonate,butothermoleculescanalso interact.Thisabilityofnaturalwaterstoneutralizeprotonsisdescribedbyitstotalalkalinity(Fig.1).Thecarbonatechemistryis significantlyaffectedbyacidificationandtheformationofnumerouscarbonate-containingmineralssuchasaragonite,calcite,and magnesiumcalciteisdisrupted.

TheabsorptionofCO2 andthefateofhydrogenionsinwaterarethereforedependentonvariouschemicaltransformationsand equilibriumconstants.Theequilibriumconstantsareinturndependentonsalinity,temperature,andpressure.Forthisreason,fresh andseawaterareconsiderablydifferentinthedistributionofthecarbonicacidfractions.Freshandbrackishwaterhasalower bufferingcapacityandthusexperienceshigherpHfluctuationscomparedwithopenoceanwaters.

Althoughtheacidificationprocesshasmostlybeenstudiedinthemarineenvironment,declinesinpHcanalsoconsiderably affectfreshwaterecosystems.Marineandfreshwatersystemsmaybeacidifiedeitherfromnaturalorman-madeprocesses,but,while naturalprocessesareslow(geologicaltimeframes),anthropogenicactivitiesareacceleratingandamplifyingthesechanges.

TheacidificationofaquaticsystemsismostlyaresultofthecontinuousCO2 releaseintotheatmospherebyfossilfuel combustion(coal,oil,andnaturalgas),deforestation,andcementproduction.Sincethebeginningoftheindustrialera,thepH ofoceansurfacewaterhasdecreasedby0.1pHunits,correspondingtoa26%increaseinacidity.Moreover,pHofinternaland coastalareascanalsobealteredbyotheranthropogenicinputssuchasnitrogen,ammonia,andsulfur.Consequently,freshwater acidificationcanbeafasterandlargerphenomenonthanoceanacidificationinvolvingnotablepHdrops(upto2.5)during episodicevents.

TheanthropogenicCO2 inducedchangesinwatercarbonchemistryhavesomedirecteffectsonphotosynthesis,calcification, andacid–basebalanceofaquaticorganisms.TheincreasedavailabilityofCO2 canpotentiallyenhancephotosynthesiswhenlight andnutrientsareavailable.However,theabilityofmicroandmacroalgaetoutilizethisexcessofCO2 appearstovarywidelyacross taxa.Anumberofphysiologicalprocessescanbealteredinphotosyntheticorganismsandthefinalresponseisoftenacompromise betweentheantagonisticeffectofCO2 onphotosynthesisandrespirationmetabolism.LowpHoftencausesanincreaseofthe overallenergeticcosts,whichinturnleadtoanaugmentedrespirationrate.Therefore,thebenefitsofanenhancedphotosynthetic activityaregenerallyrelativelyminorrelativetothenegativeeffectsofacidificationonrespiration.Moreover,acidificationhasbeen showntopromoteseveralmetabolicpathwaysleadingtotheproductionandaccumulationoftoxiccompoundsinphytoplankton cells.

AdecreaseinpHandincarbonateionsgenerallycausesadeclineinthecalcificationratesproducingcalciumcarbonate(CaCO3) forshellsandskeletons(Fig.2).Calcifyingorganismsareextremelydiverseandincludemanytaxonomicgroupsandecological niches.Forinstance,calcificationisperformedbymanyphotosyntheticprimaryproducers,zooplankton,mollusks,andcrustaceans.Althoughthecalcificationprocesscanbeexplainedbyasimplechemicalequation,thebiologicalmechanismsaremore complexandcanvarybetweenspecies.Thecalcificationprocessrequiresanenergyinvestmentfortheorganismsandmodifications ofthechemistryoftheexternalaquaticenvironmentcancauseimportantperturbationsofcalcificationrates.Althoughcalcifying organismsaremostlynegativelyimpactedbyacidification,thegrowthrateofsomespecieshasbeenreportedasinsensitivetothis stressorpositivelyimpacted.

Shellcalcificationinmollusksisperformedintheextrapallialspacethatisisolatedfromthesurroundingambientwater(Fig.2). Manyspecieshavebeenshowntoproducetheirowncarbonateionsinthisspacewithoutanyinteractionwithseawaterions.The formationofcalciumcarbonatestructuresinthisspaceisthusnotdirectlyinhibitedbydecreasingcarbonateionsconcentrationin theexternalseawater.Theimpactofoceanacidificationonshellgrowthisthereforearesultofseveralinterlinkedphysiological processesaffectingmetabolismandinternalbodypH.Furthermore,theantagonisticprocessesofcalcificationanddissolutionsmay alsorepresentanimportantenergeticcostfortheorganisms.Forthesereasons,marinemolluskshavebeenshowntorespondvery differentlytoacidification.Inamoreacidocean,onaverage,shellsareexpectedtobesmallerandhaveamodifiedmineralogical structuretoovercometheenergeticcosts.

Similarlytomollusks,scleractiniancoralsdonotprecipitatetheircarbonateskeletondirectlyfromseawater,buttheyproduceit inanextracellularmediumwheretheanimalcanactivelymanipulatethepH.Cold-watercoralsareparticularlygoodinregulating

Fig.1 Generalschemeonhowatmosphericcarbondioxidecaninterferewiththeaquaticchemistry.

Fig.2 Crosssectionofamolluskshellshowingthecalcificationprocessintheextrapallialspace.ModifiedfromMcConnaughey,T.A.,Gillikin,D.V.(2008).Carbon isotopesinmolluskshellcarbonates. Geo-MarineLetters 28,287–299. 10.1007/S00367-008-0116-4

theirinternalpHandcancontinuetocalcifyeveninacidicandundersaturatedwaters.Thisimportantadaptivemechanismenables thesedeep-seacalcifierstooccupyuniqueniches.Incontrastwiththesehighlyadaptedorganisms,othertaxasuchasthecalcitic foraminifera,arenotabletocontroltheirinternalpHanddependentirelyonseawaterconditions.

Theshellsofmarineorganismsaregenerallymadefromeithercalciteoraragonitecalciumcarbonate.Aragoniteismoresoluble thancalciteandcandissolveeasierinundersaturatedwaters.Changesinwaterchemistrythereforeaffectcalcifyingorganismsin differentwaysdependingontheirshellscompositions.Forexample,oneofthemoreabundantandimportantplanktonicshelled grouplivinginpolarandsubpolarwaters(pteropods)areexpectedtobeparticularlyaffectedbyacidificationsincetheyareunable tomaintaintheirshellinundersaturatedwaterswithrespecttoaragonite.Ontheotherhand,arcticbivalveshavebeenshowntobe generallyresilienttodecreasingpH.

MetabolicandbiochemicalprocesseswithinaquaticorganismsareinfluencedbywaterpHsincebiologicalmembranesare generallyhighlypermeabletofreeionsandpotentiallyaffectedbyacid–basedistresses.TheinternalpHofmostheterotopic organismsislowerthanthesurroundingseawater.Activemechanismsofiontransportareconstantlyworkinginlivingcellsto maintainpHfluctuationsinbodyfluidswithinatolerablerange.Forthisreason,metabolicratesarelinkedtothehomeostasisof theinternalpHandtothepHgradientbetweenthebodyandtheexternalenvironment.Underacidificationconditionssome metabolicfunctionsaredepressed,oxygentransportefficiencyisreducedandacid–baseregulationrequireshigherenergeticcosts. Theeffectivityofiontransportsystemsdiffersbetweenaquaticorganismsdependingontheirstructureandcomplexity.Some crustaceanandechinodermsseemtobeabletocompensateforacidificationbyincreasingbicarbonateconcentrationintheirbody butlittleisknownaboutthesustainabilityofthisresponseduringlongtermexposures.

AlthoughthereisageneralagreementabouttheeffectsofincreasedCO2 concentrationsontheoceanandfreshwaterchemistry, themagnitudeandseverityofthepotentialimpactondifferentorganismsandecosystemsisstilllargelyunknown.Thelarge variabilityintheresponsesoforganismstoacidificationismostlyduetothehighbioticandabioticpatchinessofnaturalhabitats andthemultipleinteractionswithotherexistingstressesliketemperaturechanges.Thisarticlewillgiveanoverviewonthedifferent mechanismsandresponsesoftheocean,coastal,andfreshwaterhabitatsandprovidesomewiderconceptontheecologicaland economicimpactofthisphenomena.

OceanAcidification

OceansplayanimportantroleinmitigatingatmosphericCO2 emissionsbyabsorbingaround25%–30%oftheCO2 addedtothe atmospherebyanthropogenicactivities.Thisprocesscausesadecreaseinwater’spH,intheconcentrationofcarbonateionsandin thecarbonatesaturationstate.Thesechanges,togetherwithotherstressorssuchastemperature,aresignificantlyaffectingmarine communities.Numerousresearchprojectshavefocusedontheeffectofoceanacidificationonmarineorganisms. Negativeeffectshavebeenrecordedonmosttaxonomicgroups,andearlydevelopmentalstagesaresuspectedtobemost susceptible.Multigenerationalstudiesonmarinecopepodsshowedthatfecunditycoulddecreaseupto29%underlowerpH scenarios.Moreover,thelarvalnaupliistageofthesetaxaexhibitgreatersensitivityandmortalityrateswithincreasingCO2 comparedtotheotherlifestages.HigherconcentrationsofseawaterCO2 havebeenshowntocausemalformationsanddelaysin thelarvaldevelopmentofseveralcalcifiers.Seaurchinlarvaelosesymmetryandtheirskeletonishighlydeformedandcorroded withdecreasedpH(Fig.3).Besideinvertebrates,fishembryosandyounglarvaearealsomoresensitivethanadultstotheeffectsof oceanacidification.Eggsurvival,hatchingsize,andgrowthratearegenerallyretardeddespitesomepositiveeffectsthathavebeen

Fig.3 Seaurchinlarvaerearedincontrolandacidifiedtreatments.LowerpHcausesasymmetricalanddeformedskeletaldevelopment.

seenonfewspecies.Evenifthegrowthrateisnotaffected,otherperturbationssuchasimpairedolfactorydiscriminationand homingabilitycanstillreducefitnessinthenaturalenvironment.

Theconsequencesofoceanacidificationonthebiologyofadultmarinefishhavebeenextensivelystudied.Resultsshowa multitudeofdiverseresponsesfromthedeathoftheindividual,tonoeffectatall.Despitethis,mostadultfishescanefficiently compensateforahypercapnicacid–basedisturbance,althoughneurosensory,growth,metabolism,andmitochondrialfunctions areoftendisrupted.Therefore,thepHcompensationabilitydoesnotnecessarilymeanthattheconsequencesofoceanacidification arereduced.Nonetheless,themechanismsunderneaththisprocessarestilllargelyunknownandpredictingspeciesresponses remainsdifficult.

Anotherunexpectedconsequenceofoceanacidificationintheopenoceanisthesignificantdecreaseinoceansoundabsorption. Lowfrequencyrangeispredictedtochangesignificantlyinthenearfuture,mostlyduetopHchanges.Underwaterambientnoiseis thuspredictedtoincrease,causingpotentialproblemstohuman(scientific,commercial,andnavalacousticapplications)and naturalactivities.Disturbanceinanimal’scommunicationislikelytointerferewiththebiologyandbehaviorofwhalesandother marinemammals.

Acidificationisalsoaffectingthedeepoceanalthoughtheresponsesaredelayedbythetransporttimeofwatermassesbetween thesurfaceandthedeepareas.Inthoseareas,theacidificationcanhappenthroughawatermassmovementorfromthe decompositionoforganicmatter “raining” intothedeepoceanfromthesurface.Deep-seaecosystemshavethepotentialtobe significantlyaffectedbypHchangessincetheirinhabitantsareadaptedtoliveinextremelystableconditions.Thedeep-seaisacold anddarkenvironmentwheremetabolicactivitiesandgasexchangesaregenerallyreducedcomparedtoshallowareas.Historically exceptionalchangesinoceansbottompHhavebeendocumentedtocausesignificantspeciesextinctionsandscientistsare concernedthatthiscouldhappenagaininthenearfuture.

ThepHofmanydeep(below500m)areasisprojectedtodecreaseby0.2unitsby2100.Carbonate-basedorganismssuchas cold-watercoralsandspongereefsareunderseriousthreats.Animalslivinginthedeepoceanaregenerallycharacterizedbyslow growthandlimitedrecoveryabilities.Acidificationhasbeenshowntonegativelyaffecttissuefunctioningandmembranepumpsof deepglassspongereefscompromisingtheirfeedingefficiency.Asimilarreductioninfeedingrateshasbeenshowninseveralcoldwatercorals.ThecalcificationrateofthesecoralsisnegativelyaffectedbypHdropsinlaboratoryexperiments,butsomespecieshave beenshowntobemoreresilientthanothers.ThegeneralideaisthatthedecreaseinpHandaragonitesaturationwillcausethe dissolutionofcoralsskeleton.However,anumberofstudiessuggestedthatdeep-seacoralshavesomephysiologicalmechanismsto compensateforundersaturationandseveralnaturalscleractinianreefshavebeenfoundinareaswithlowaragonitesaturation levels.

AcidificationintheCoastalArea

Intertidalareas,estuaries,coralreefs,sandy,androckyshoresarejustafewexamplesofhowdiverseandvariablecoastalareasare. Theseecosystemsaresubmittedtocontinuousfluctuationsduetotides,riversinputs,waves,currents,andanthropogenicactivities. Becauseofthisvariabilityinphysical–chemicalcharacteristics,thecarbonatesystemsandpHexperiencesignificantfluctuations. TheintensevariabilityandtheeffectofmultipledriversonwaterpHimpliesthatitismuchmoredifficulttodetectacidification trendsintheseareascomparedtotheopenocean.

Unlikeopenoceanareas,coastalecosystemsareoftendominatedbybenthiccommunitiesandthemetabolicactivityofthese organismscandriveimportantdielandseasonalpHoscillations.Coastalcommunitiescansupportintensemetabolicprocesses, includinghighprimaryproduction,respiration,andcalcificationrates.Allthoseactivitiesdirectlyinfluencethewaterchemical propertiessuchasCO2 concentration,alkalinity,andpH.Forinstance,macrophytesgrowinginshallowcoastalzoneshavethe capacitytomodifyseawaterpHandcausedielchangesdrivenbyprimaryproductivity.Bivalveshavebeenshowntobecapableof raisinglocalpHby0.3–0.5unitsduringthedaytime.ThepHofintertidalrockpoolscanvarybetweendayandnightduetothe antagonisticeffectofrespirationandphotosynthesisonthewaterchemistry.

ThepresenceofalltheselocalbiologicalprocessescreatesalargediversityofpHnichesincoastalhabitats,offeringareaswith improvedconditionsforcalcification.Forexample,calcifierslivingclosetomacrophytescanadapttoacidificationbyincreasing theircalcificationratesduringdaytimewhenCO2 isdrawndownbyprimaryproductivity.Despitethefactthatthislocaladaptation canbevaluableforadultindividuals,itispossiblethatthepelagiclarvalstage,typicalofmanycoastalcalcifiers,maybecriticalfor thespeciessurvivalinamoreacidicocean.

AnotherimportantphenomenaaffectingcoastalpHisupwelling.Upwellingisaprocessinwhichdeep,cold,andnutrientrich waterupwellsfromthebottomoftheoceantothesurface.Thiswatermovementiswinddrivenandconstantlyorperiodically affectsseveralcoastalareasaroundtheword.Inupwellingregions,thehighconcentrationinnutrientspromotesbiological activities,butCO2 concentrationsaregenerallyhighandwatersareundersaturatedwithrespecttoaragonite.Theimpactof acidificationcausedbyupwellinghasbeendemonstratedinestuarineareaswhereoysterlarvaeproductionwasgreatlyreduced duringstrongupwellingperiods.

OrganismslivingincoastalareasaregenerallytoleranttochangesinpHduetothehighnaturalvariabilityoftheirhabitats. However,theeffectofanthropogenicCO2 emissionsisnotyetfullyunderstoodforthoseorganisms.Despitetheirhighertolerance toavariableenvironment,animalscanstillbeaffectedbyoceanacidification,butinamoresubtleway.Thephysiologyand developmentofintertidalsnailsareaffectedunderacidifiedconditionscausingadecreaseinfitnessandsurvival.Juvenileoysters showareductionintheirenergystorageandaweakershell.TheimmuneresponseofintertidalmusselsisdisruptedbyhigherCO2 conditions.

Oneofthemoreemblematicecosystemaffectedbyacidificationworldwidearecoralreefs.Thereisnowabundantevidencethat acidificationandtemperaturechangesareleadingtocoralbleachingandmortality.Mostshallowwatercoralsdependonan obligaterelationshipwithaphotosymbiontmicroalgae.Thealgaeprovidesnutrientsandenergytothecoralfacilitatinghisgrowth andcalcificationeveninoligotrophicwaters.Thelossofsymbiontsincoralsisgenerallycalled “coralbleaching” duetothe impressivecolorchangeincoraltissues.Whenthecoralbleaches,itisnotdeadbutitbecomesmorevulnerabletostressandshows highermortalityrates.Oneofthemoreimportantcausesofcoralbleachingisthermalstress,butacidificationisalsoknowntofavor thisphenomenon.ThesymbioticrelationshipbetweencoralorganismsandphotosyntheticsymbiontsisoftendisruptedbypH changessincethesymbioticinteractionislimitedtoaverysmallinternalpHrange.

Thenegativeimpactofacidificationoncoralsisnotlimitedtobleaching,butitalsocausessignificantproductivityreductions andhigherratesofnetdissolutionofthecalcifiedtissues.Experimentsrestoringpreindustrialalkalinityconditionsinrestrictedcoral reefareasshowedthatnetcommunitycalcificationishighlyaffectedbyoceanchemistry.Coralcalcificationhasbeenshowntobe reducedofabout6%–7%sincethebeginningoftheindustrialera.Inadditiontocoral,severalotherorganismslivinginthese habitatshavebeenshowntobeparticularlyvulnerabletopHchanges.Coralreefinvertebraterecruitmentandabundanceare significantlylowerinelevatedCO2 areas.Theseorganismsarethekeyfoodsourceofseveralreeffishandtheirdeclinewillprobably entrainacascadeeffectalongthefoodchain.

AcidificationinFreshwaterEcosystems

Infreshwaterecosystems,theacidificationprocessduetoatmosphericCO2 emissionsisoftenamplifiedbytheeffectofother anthropogenicforcessuchasacidrainandminingdrainage.CO2 concentrationinlargerlakestendstobeinequilibriumwiththe atmosphere,butthisisgenerallynotthecaseforloticsystems.Biologicalactivities,rainfalls,andstreamorderplayanimportant roleinregulatingpHoffreshwatersystems.AssumingonlyanatmosphericCO2 increaseofabout550 matm,thepHinGreatLakes isestimatedtodeclineatasimilarratecomparedtotheopenocean(about0.19/0.09units),butriversandsmalllakesarelikelyto havedifferentresponses.MiningactivitycancauseanevenhigherimpactwithpHdropsbelow4incertainsystems.Thereleaseof oxidationproductsfromthesoiltothewateriscalledacidminedrainage.Thisdrainagecausesimportantacidificationsinhundreds oflakesandriverssituatedinproximitytotheminingareas.

Theeffectsofacidrainareperhapsmoreflagrant.Sulfurandnitrogenoxidesproducedbyvolcanoes,industries,andother anthropicactivitiesreactwithatmosphericcompoundstocreatesulfuricandnitricacid.Theacidsdepositonthegroundanddrain intolakesandstreams.NorthAmericaandEuropewereparticularlyaffectedbythisphenomenonaftertheindustrialrevolutionand newdevelopingindustrialcountriessuchasChinaarenowsubjectedtothesameissue.AcidicatmosphericdepositioncancausepH dropsbelow5oreven4insomefreshwatersystems.Theeffectsontheaquaticecosystemsareoftendramaticandtheuseof bufferingcompounds(limestone)hasbeenemployedinsensitiveareastolimitthedamage.

Theimpactofacidificationonfreshwaterbiotahasbeenoftenunderestimated.Onlyrecently,someresearchersfocusedonthe effectofacidificationatthespecies/organismlevel.Asdocumentedinmarinesystems,freshwatermacrophytesandphytoplankton canbepositivelyaffectedbyhigherCO2 concentrations,buttheresponseisstronglydependentonnutrientavailabilityinthe system.StudiesperformedonlakeshistoricallyaffectedbyacidicatmosphericdepositionsshowedthatsomefreshwaterzooplanktontaxaarehighlyadaptabletoacidicconditionsandcansurviveandproliferateinawidepHrange.Followinganacidification event,communitydiversityandstructureisstronglyaffectedbutthereissomeevidencethatecosystemscanrecoverwhenlakepH levelsriseagainabove6.

Anotherimportantfreshwatercommunityimpactedbyacidificationisthebacterioplankton.Althoughdiversityandrichness werefoundtobegenerallysimilarbetweenacidicandmoreneutrallakes,thecommunitystructurewaschanged.Waterchemistry andacidstresshaveadirectinfluenceonabundancesandorganizationofbacterialassemblages.

Infreshwatersystems,mollusks,crustaceans,andmanyaquaticinsectsplaytheimportantroleofbreakingdowntheorganic materialtoregeneratenutrientsandserveasafoodsourceforseveralspeciesoffishandbirds.Studiesperformedonseveral EuropeanlakessuggestthatpHlessthan5mightbecriticalforinsectcommunities.VerylittleisknownontheeffectofpHdeclines onfreshwatermollusks,buttheyareusuallyabsentfromsystemswithaciditybelow5probablyduetoanexcessivelyhigh constructionandmaintenancecostoftheircalcareousstructures.Thisisparticularlytrueforfreshwaterbivalves’ shellsprimarily madeupofaragonite,sincethistypeofshellismoresolublethanthecalciteoneofmostmarineorganisms.Thedecreaseincalcium availabilityinacidifiedwatershasbeenshowntosignificantlyaffectfreshwatercrustaceansthatareparticularlysensitiveduring theirrapidpostmoltcalcificationoftheexoskeleton.Instreams,crabs,shrimps,andcrayfishdisappearedfromanthropogenic acidifiedareas.Specieslossamongdetritivorousinsectsandcrustaceansresultinalossofthelitterbreakdownprocesswith importantconsequencesfortheentireecosystem.

TheeffectoflowpHonfreshwaterinvertebratescanbeexacerbatedbyincreasedmetaltoxicity.Mercuryconcentrationsaremore elevatedincrayfishfromlowerpHlakescomparedtolessacidifiedareas.Aluminumappearstobehighlyabsorbedbyinvertebrates atlowerpH.Nonetheless,leadtoxicityisoftenspecies-specificandlittleinformationisavailableonpotentialbiomagnification alongthefoodchain.

Theeffectofacidifiedwateronhighertrophiclevelssuchasfishandamphibianshasreceivedincreasedinterestinthepastfew years.Underacidifiedconditions,thegrowthratehasbeenshowntodecreaseinseveraltaxa,mostlyduetoahigherenergeticcostof theacid–baseregulationsysteminthefluidsandtissues.Thedevelopmentofpinksalmonduringitsfreshwaterphasehasbeen showntobeaffectedbyincreasedCO2 levels.Thegrowthrateandearlyembryonicdevelopmentofthisspeciesisimpairedby elevatedfreshwaterCO2 partialpressure.Moreover,thecapacitytodetectolfactorycuesandavoidpredatorsissignificantlyreduced underprojectedincreasesofCO2.But,thisisjustanexampleofhowacidificationcaninducebehavioralchangesinfreshwaterfish. Altereddialmovement,behavioralchangesandmodifiedfeedingpatternshasbeenobservedinseveralspeciessuggestingthat acidificationlinkedbehavioralchangescouldincreasethevulnerabilityoftheseorganismstopredationandfoodcompetition.

Amphibiansareexperiencingageneraldecreaseinabundanceduetoseveralanthropogenicactivities,oneofthemostimportant beingacidification.Commonspeciesoffrogshavebeenreportedtobegraduallydecreasingandeventuallydisappearingfrom poorlybufferedandacidifiedpondsandlakes.LowerpHandcalciumconcentrationsignificantlylimitedfrogreproductionand development.Indwarfnewts,acidificationdidnotinhibitfemalesfromlayingeggsbuttheembryoswereexposedtohigher mortalitiesunderlowpH.Asimilarresponsehasbeenseenontoadswithupto100%mortalityineggsmaintainedatapHbelow5.

Allthosechangesinfreshwaterecosystemcompositionandproductivityhavedirectconsequencesonthehigherconsumerssuch asbirds.Acidifiedstreamshavebeenseentohostyoungerandlesssite-faithfulbreedingpopulationsofbirdssincetheyprovidea lowerqualityhabitat.Furthermore,potentiallytoxicmetalsconcentrationandadecreasedavailabilityofcalcium,canalso negativelyaffectbirds.Piscivorousbirdscansufferfromstrongmetalbioconcentrationintheirpreyandareductionofcalcium contentinthefoodcouldaffectbonesandeggshellsformation.

EcologicalImpact

Thenegativeeffectsofaquaticacidificationonsinglespecieshavereceivedthemostattentioninrecentyears.However,the consequencesofaquaticacidificationarealsoextremelyimportantatacommunityandecosystemlevel.Allorganismsare embeddedinacomplexnetworkofinteractionbetweendifferentspeciesandpopulations.Theimpactofacidificationonaquatic ecosystemsisthereforeinextricablylinkedtotheimpactofstressorsatdifferenttropiclevels.

Predator–preyinteractionscanbeextremelyimportantinregulatingandstructuringpopulationsandcommunities.Ocean acidificationcanleadtoimportantindirectchangesinthestructure,flows,andcompositionofthefoodweb.Observationsfrom naturalCO2 enrichedareasshowthatcommunitiesdiffersubstantiallyfromnearbyareaswithhigherseawaterpH.Lowerdiversity

andhigherfoodwebspecializationaretypicallyfoundinacidifiedareas,suggestingthatacidificationcouldinduceadramatic communityshiftinvulnerablesystems.

Phytoplanktonconstitutesthefoundationofaquaticfoodwebsandregulatesmultiplebiogeochemicalprocesses.Acidification hasbeenshowntohaveagreaterimpactthanseawaterwarmingorreducednutrientsupplyonphytoplanktoncommunities.About halfoftheglobalfunctionaldiversityofmarinephytoplanktoncommunitieshasbeenestimatedtobealteredby2100andthisis mostlycausedbyoceanacidification.FreshwaterphytoplanktoncommunitiesarealsosignificantlyimpactedbyelevatedCO2 concentrationsincelessdiversepopulationsandsmallercellswerecollectedinalteredsystems.ToleranceforlowpHvaried significantlybetweenspeciesandsomeauthorssuggestedthattoxiccyanobacteriumstrainscouldbemoretolerantandableto proliferateunderweakacidificationscenarios.

Oceanacidificationcanalsoprofoundlyaffectsettlementandbenthiccommunities.Althoughawiderangeofspeciesareableto settleandsurviveunderacidifiedconditions,diversityisgenerallyreducedandfewtaxabecomedominant.Thisispartlyduetothe factthatmanybenthicinvertebratesdependoncalcifiedstructuresthatareparticularlysensitivetoacidification.Thenegative impactsonthebenthiccommunitycanhaveacascadeeffectonthetrophicchainandseveralcasesofflatfish,sharks,andrays declininghavebeenreported.Regardlessofthepreciseconformationofthelocalfoodweb,itislikelythatbottom-upchangescan beimportant.Moreover,otherrelativelyunexploredproblemssuchaschangesinbacteria,pathogens,andparasitescouldalsoalter thefunctioningoftheecosystem.

Assumingthatcalcifiedstructurescanprovideprotection,functionassubstrateandgiveawiderangeofotherbenefittoother organisms,achangeintheirabundanceanddistributioncansignificantlyaffectlocalcommunities.Coralreefs,oysters,mussels, corallinealgae,andmanyotherorganismsareconsideredasimportantecosystemengineersprovidinganumberofecosystem servicesandcreatingfavorablehabitatsbymodifyingtheenvironmentalphysicalandbiologicalconditions.Negativeimpactson theseorganismscancauseacascadingeffectoncompetitorsandconsumers.Dependingonspeciestoleranceandadaptabilityto acidificationandclimatechanges,theeffectontheecosystemcanbemoreorlessimportantandkeystonespeciesmaybereplaced bymoreresistantones.

Anotherimportantecosystemalterationrelatedtooceanacidificationisthemodificationofcarbonandnutrientsbiogeochemicaldynamics.Variationsinnutrientratioshaveimportanteffectsonphytoplanktonandmicrobialcommunitiesandthiscould leadtoadegradedfoodqualityforheterotrophicconsumers.Changesinwaterchemistrycouldalsobedirectlyresponsiblefor modificationsinelementsavailabilityandorganicmatterdegradationandcomposition.Thesechangescandirectlyaffectthebase ofthefoodwebsandconsequentlytheentireecosystem.

SocioeconomicalImpact

Despitetheclearnegativeimpactofacidificationfortheaquaticcommunitiesandecosystemfunctioning,thisphenomenonalso hasanimportanteconomiccost.Thelossofeconomicallyimportantspeciessuchasfishandmollusksisaccompaniedbyadecrease inaquacultureproductivityandthelossoftouristicandvaluablehabitatssuchascoralreefs.Moreover,anynegativeeffectof aquaticacidificationisexpectedtoenhancethealreadypresentstressorssuchasoverfishingandglobalwarming.

Oneofthesocioeconomicalchallengesconnectedtooceanacidificationistheeffectthatthisphenomenonhasonseafood. DecreasedpHhasbeenshowntodecreaseappearanceandtasteofcommerciallyimportantspecieslikeshrimps.Fishandmollusks arepredictedtobesmallerundertheeffectsofacidification.Somecommerciallyindemandspeciesmightdecreasecausingashiftin marketablegoods,suchasfromfishtojellyfishandalgaethatarepredictedtobemoreresilienttoacidification.

TheUSoysterindustryhasbeendecliningsince2005withanaveragelossof$111millionperyearandoneofthecausesisthe presenceofacidifiedwatersinthearea.Theglobaleconomiccostofmollusklossfromoceanacidificationhasbeenestimatedat about$6billionannually.InEurope,theannualeconomiclossesduetooceanacidificationareestimatedtobeover$1billionin 2100.Theglobalvalueofcoralreefbasedtourismwasestimatedtobe$11.5billionin2010andtheindustryisinrapidgrowth.The consequencesofcoralreefshabitatlossareexpectedtocostabout$49–69billionbytheendofthiscentury.

Economistsestimatedthatthecostofthetotalimpactofoceanacidificationisinthesameorderofmagnitudeasclimatechange despiteahighlevelofuncertaintystillbeingpresent,duetofactthataquaticacidificationhasonlyrecentlybeguntoreceive attentionininternationaldiscussions.Oneofthemaineconomicissues,isthatoceanacidificationislikelytohaveagreaternegative impactonpoorerfishingandaquaculturecommunitiessuchasthesmalldevelopingislandstates.Theseareasareparticularly vulnerabletoclimatechangeandoceanacidificationimpacts,andtheyhavefewerpossibilitiesforalternativelivelihoods.

Forislandnations,coralreefslosscanalsoconstituteasubstantialsocioeconomiccollapse.Marinenaturaltouristicattractions suchasdiving,snorkeling,sightseeing,andrecreationalfishingwillsignificantlysufferfromoceanacidificationeffects.Moreover, coralreefsoffershorelineprotectionandsupportfisheriesforabudgetestimatedat$30billionayear.

ResponseStrategiestoAquaticAcidification

Limitingtheeffectsofaquaticacidificationisnowcriticalconsideringthehighrisksofimpactonnaturalandhumansystems.An effectiveresponsetoaquaticacidificationislikelytorequirealarge-scaleinvestmentplan.Duetoastrongconnectionbetween

aquaticacidificationandtheotherclimatechangestressors,regional,national,andglobalstrategiesneedtoconsiderandmanage thoseissuestogether.

TheprimarypossibleactiontomitigateaquaticacidificationistoreduceCO2 emissionssincenolarge-scalesystemisyet availabletoremoveCO2 fromtheatmosphere.Nonetheless,evenifallCO2 emissionsweretoendnow,theCO2 alreadyreleased intotheatmospherewillcontinueacidifyingtheoceanforcenturies.Itisindeedimportanttoestablishsomemitigationand adaptationresponses.

Oceanandfreshwateracidificationwillnotleadtothedisappearanceofallaquaticorganisms.Somespecieswillbeableto toleratethenewconditionsbutecosystemsdiversityandabundancearelikelytochange.Thedisappearanceofeconomicallyand culturallyimportantspeciesmayleadtoextremesocialforcing.

Theimpactofaquaticacidificationonjuvenilefish,foodwebs,andcoastalhabitatsarelikelytoentrainimportantreductionsin fisheryresources.Improvedmanagementsandreductionoffishingpressurearejustsomeexamplesonhowwecanprotectand rebuildfishstocks.Otherpossibleactionscouldinvolvethedevelopmentofprotectionandrestorationplansforparticularly degradedandimportantareas.

Inaquaculture,theimpactofacidificationcanbelimitedbyselectingmoreresilientspeciesandemployingselectivebreeding. Theentireculturesystemscouldalsobemonitoredandcontrolledtomaintainoptimalwaterconditionsforthegrowthofthe selectedspecies.Moreover,monitoringandresponseplanscanbeorganizedtowarnandprotectaquaculturesystemsfrom acidificationandotherepisodicstressorevents.

Nevertheless,alltheseactionstomitigatetheeffectsofacidificationrequirehighfinancialcostsandpolicycommitmentand cannotfacetheextremescenariosofglobalCO2 emissions.Itisthereforefundamentaltotakeactioninreducingcarbonemissions toleaveagreaternumberofeffectivesafeguardingoptionstoprotectthemarineandfreshwatersystemsandtheirservicesto humans.

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IshimatsuA,HayashiM,andKikkawaT(2008)Fishesinhigh-CO2,acidifiedoceans. MarineEcologyProgressSeries 373:295–302. MoiseenkoTI(2005)Effectsofacidificationonaquaticecosystems. RussianJournalofEcology 36:93–102. https://doi.org/10.1007/s11184-005-0017-y

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ConstructedWetlandsforWastewaterTreatment☆

JanVymazal,CzechUniversityofLifeSciencesPrague,Praha,CzechRepublic

Constructedwetlandtreatmentsystemsareengineeredsystemsthathavebeendesignedandconstructedtoutilizethenatural processesinvolvingwetlandvegetation,soils,andtheirassociatedmicrobialassemblagestoassistintreatingwastewater.Constructedwetlandsmustcontainwetlandvegetation,thetreatmentsystemwithnoplantsarenotconsideredconstructedwetlands. Theyaredesignedtotakeanadvantageofmanyofthesameprocessesthatoccurinnaturalwetlands,butdosowithinamore controlledenvironment.Someofthesesystemshavebeendesignedandoperatedwiththesolepurposeoftreatingwastewater, whileothershavebeenimplementedwithmultiple-useobjectivesinmind,suchasusingtreatedwastewatereffluentasawater sourceforthecreationandrestorationofwetlandhabitatforwildlifeuseandenvironmentalenhancement.Synonymoustermsto “constructed” includeman-made,engineered,andartificialwetlands.

Atpresent,therearemanydifferenttypesofconstructedwetlands(Fig.1).Constructedwetlands(CWs)forwastewatertreatment maybeclassifiedaccordingtotheflowregimeintosurfaceflow(SForfreewatersurface FWS)andsubsurfaceflow(SSF)systems. TheFWSCWscouldbefurthercategorizedaccordingtothelifeformofthedominatingmacrophyteintosystemswithfreefloating, floating-leaved,emergentandsubmergedmacrophytes.WithintheSSFCWsitispossibletodistinguishbetweensystemswith horizontal(subsurface)flow(HForHSSFCWs)andvertical(subsurface)flow(VForVSSFCWs).Asmanywastewatersaredifficult totreatinasinglestagesystem,hybridsystemsthatconsistofvarioustypesofconstructedwetlandsstagedinserieshavebeen introduced.IntheEuropeansense,hybridCWsareusuallyformedbyacombinationofHFandVFsystems.However,anytypesof CWscouldbecombinedinordertoachievebettertreatmentperformance,especiallyfortotalnitrogen.

ThefirstexperimentsaimedatthepossibilityofwastewatertreatmentbywetlandplantswereundertakenbyDr.Seidelin Germanyin1952attheMaxPlanckInstituteinPlön.However,Seidel’sconcepttoapplymacrophytestosewagetreatmentwas difficulttounderstandforsewageengineersandtherefore,itwasnosurprisethatthefirstfull-scalefreewatersurface(FWS) constructedwetland(CW)werebuiltoutsideGermany,intheNetherlands,inthelate1960s.However,thefirstsubsurfaceflow constructedwetlandwasbuiltinGermanyin1974.

Fig.1 Constructedwetlandsclassification.

☆ChangeHistory: April2018.Vymazalupdatedthetextandfiguresfromthe1stedition,allTablesarenew.

Constructed Wetlands

FreeWaterSurfaceCWs

Atypicalfreewatersurfaceconstructedwetlandconsistsofashallowbasinconstructedofsoilorothermediumtosupporttheroots ofvegetation(whenrootingmacrophytesareused)andawatercontrolstructurethatmaintainsashallowdepthofwater(Fig.2). Flowisdirectedintoacellalongalinecomprisingtheinlet,upstreamembankment,andisintendedtoproceedallportionsofthe wetlandtooneormoreoutletstructures.Theshallowwaterdepth,lowflowvelocity,andpresenceoftheplantstalksandlitter regulatewaterflowand,especiallyinlong,narrowchannels,ensureplug-flowconditions.FWSCWscanbeclassifiedaccordingto thetypeofvegetationused(Fig.2).

FWSCWsfunctionasland-intensivebiologicaltreatmentsystems.Inflowwatercontainingparticulateanddissolvedpollutants slowsandspreadsthroughalargeareaofshallowwater.Particulates,typicallymeasuredastotalsuspendedsolids,tendtosettleand aretrappedduetoloweredflowvelocitiesandshelteringfromwind.Mostofthesolidsareusuallyfilteredandsettledwithinthefirst fewmetersbeyondtheinlet.WhilesettleableorganicsarerapidlyremovedinFWSCWsbyquiescentconditions,attachedand suspendedmicrobialgrowthisresponsibleforremovalofsolubleBOD(BiochemicalOxygenDemand,i.e.,organicmatter).The majoroxygensourceforthesereactionsarealgaeandcynobacteriagrowinginthewater.

NitrogenismosteffectivelyremovedinFWSsystemsbynitrification/denitrification.Ammoniaisoxidizedbynitrifyingbacteria inaerobiczones,andnitrateisconvertedtofreenitrogenornitrousoxideintheanoxiczonesnearthebottombydenitrifying bacteria.VolatilizationislikelyasbothplanktonandperiphytonalgaegrowinFWSCWsandhigherpHvaluesduringthedaymay befavorableforammonialoss.FWSCWsprovidesustainableremovalofphosphorus,butatrelativelyslowrates.Phosphorus removalinFWSsystemsoccursfromadsorption,absorption,complexationandprecipitation.However,precipitationwithAl,Fe andCaions islimitedbylittlecontactbetweenwatercolumnandthesoil.MacrophyteuptakeasaremovalmechanisminFWS CWsisrestrictedbythefactthatvegetationisnotregularlyharvested.Also,theamountofNandPsequesteredinaboveground biomassisusuallyquitelowascomparedtoinflowloading(usually <10%).TheonlyexceptionareCWswithfreefloating macrophyteswhereharvestinginnecessaryforaproperfunctionofthesystem.FWSCWshavebeenbuilttotreatvarioustypesof wastewater(Table1)aroundtheworldincludingdomesticandmunicipalwastewater,minedrainage,urban,airport,highwayand agriculturaldrainage,landfillleachateandvarietyofindustrialandagriculturalwastewaters.

ThesystemwithemergentvegetationisthemostcommonlyusedtypeofFWSCWswith Phragmitesaustralis (commonreed), Typha spp.(cattail), Scirpus spp.(bulrush), Juncus spp.(rush)and Eleocharis spp.(spikerush)beingthemostfrequentlyusedspecies. InothertypesofFWSCWsfollowingspeciesarecommonlyused:

-freefloating: Eichhorniacrassipes (waterhyacinth,tropicsandsubtropics), Pistiastratiotes (waterlettuce,subtropicsandtropics), Lemnaceae(duckweeds,worldwide)

-floating-leaved: Nupharlutea (spatterdock), Nelumbonucifera (Indianlotus)

-submerged: Ceratophyllumdemersum (coontail), Najasguadalupensis (southernwaternymph), Trapanatans (waterchestnut, sometimesclassifiedasfreefloating), Myriophyllumheterophyllum (variable-leafwatermilfoil)

-floatingmats: Phragmitesaustralis, Cyperuspapyrus (Papyrus), Alternantheraphiloxeroides (Alligatorweed), Hydrocotyleumbellata (Pennywort)

RemovaloforganicsandsuspendedsolidsinallFWSCWsisveryhighwhileremovalofnutrientsisonlymoderate.FWSCWs providelimitedcontactwithsoil(ifpresent)soadsorptionandprecipitationprocessesareverylimitedandthereforephosphorus removalmostlyproceedsviasoilaccretion.FWSCWsprovidebothaerobicandanaerobiczonesbutneithernitrificationor denitrificationprocessesarecomplete.FWSCWsalsoprovidehighremovalofentericbacteria(e.g.,fecalcoliforms,fecal streptococci, Clostridiumperfringens),usuallyintherangebetweenonetotwoordersofmagnitude.

ConstructedWetlandsWithSubsurfaceFlow

HorizontalFlowCWs

ThemostwidelyusedconceptofSSFCWsisthatwithhorizontalsubsurfaceflow(HForHSSFCWs, Fig.3).Thedesigntypically consistsofarectangularbedplantedwiththemacrophytesandlinedwithanimpermeablemembrane.Mechanicallypretreated wastewaterisfedinattheinletandpassesslowlythroughthefiltrationmediumunderthesurfaceofthebedinamoreorless horizontalpathuntilitreachedtheoutletzonewhereitiscollectedbeforedischargevialevelcontrolarrangementattheoutlet. Duringthepassageofwastewaterthroughthereedbedthewastewatermakescontactwithanetworkofaerobic,anoxicand anaerobiczones.

HSSFCWsrequiregoodmechanicalpretreatmentwithsuspendedsolidsbeingthemajortarget.Excessivesuspendedsolidsmay causefiltrationbedcloggingandsubsequentsurfaceflow.Smallsystemsfordomesticsewagewithlowflowsusuallyuseathreechamberseptictank.PretreatmentinsystemsdesignedformunicipalsewagemostlycomprisescreensandImhofftank(sedimentationofparticles,bothinorganicandorganic).Whenstormwaterrunoffisalsotreated(combinedsewersystem),agritchamber (removalofinorganicparticles)isincluded.Varioustypesofwastewatermayrequiredifferenttypesofpretreatment.Forexample, landfillleachatetreatmentsystemsusuallyincludeaeratedlagoons,systemsforthetreatmentofconcentratedwastewatersfrom agriculturaloperationscommonlyincludefacultativelagoons.

Fig.2 Freewatersurfaceconstructedwetlands.(A)Withemergentvegetation,(B)withfloatingmatsofemergentvegetation,(C)withfreefloatingvegetation, (D)withsubmergedvegetation,(E)withfloating-leavedvegetation(Vymazal,2008).

Table1 ExamplesoftheuseofFWSCWsfortreatmentofvarioustypesofwastewater VegetationTypeofwastewaterLocation

FFMunicipalUnitedStates,Cameroon,Poland,Thailand,China,Taiwan FLMunicipalUnitedStates,China

SMunicipalUnitedStates,Sweden,China

AgriculturalrunoffSweden,UnitedStates UrbanrunoffCanada,UnitedStates

EMunicipalAllcontinents

UrbanstormwaterAustralia,UnitedStates,UnitedKingdom AgriculturalrunoffAustralia,NewZealand,UnitedStates,Sweden,Denmark,Italy,Korea,Taiwan,China,Norway,Finland,Spain AirportrunoffSweden,Canada

FeedlotoperationsIreland,Canada,UnitedStates

MinedrainageUnitedKingdom,UnitedStates,Spain,SouthAfrica,Canada,Australia,Germany,Ireland

RefineryUnitedStates,China,Hungary

PulpandpaperUnitedStates,China

AquacultureUnitedStates,Taiwan LandfillleachateSweden,Norway,Canada,Poland,UnitedStates FoodprocessingGreece,Kenya,UnitedStates,Canada,Thailand,Italy,NewZealand TanneryTurkey

WoodwasteCanada

FMMunicipalUnitedStates,China,Turkey,Uganda,SriLanka,Colombia,Italy,Taiwan RoadrunoffBelgium,UnitedStates,NewZealand AgriculturalrunoffChina

MinedrainageCanada

FeedlotoperationsUnitedStates

FF,freefloating; FL,floating-leaved; S,submerged; E,emergent; FM,floatingmats.

Fig.3 Constructedwetlandswithhorizontalsubsurfaceflow.1 distributionzonefilledwithlargestones,2 impermeableliner,3 filtrationmedium(gravel, crushedrock),4 vegetation,5 waterlevelinthebed,6 collectionzonefilledwithlargestones,7 collectiondrainagepipe,8 outletstructurefor maintainingofwaterlevelinthebed.Thearrowsindicateonlyageneralflowpattern.FromVymazal,J.(ed.)(2001). Transformationsofnutrientsinnaturaland constructedwetlands.Leiden,TheNetherlands:BackhuysPublishers.

Filtrationbedsarefilledwithporousmaterialwhichallowsforgoodhydraulicconductivityinordertokeepthewaterlevel belowthesurfaceaswellassupportsgrowthofmacrophytes.Themostcommonlyusedfiltrationmaterialsarewashedpeagravel andcrushedrock.Thefractionsizevariesamongcountriesbutingeneral,sizebetween5and20mmisthemostcommon.Itis recommendedtouseonlyonefractionasvariousfractionsdifferinhydraulicconductivityandshortcircuitingmayoccur.The inflowdistributionandoutflowcollectionzonesarefilledwithlargestones(ca.50–200mm).

Thefollowingequation,firstproposedbyKickuth,iswidelyusedforsizingofHSSFsystemsfordomesticsewagetreatment: Ah ¼ Qd ln Cin ln Cout ðÞ=KBOD , where Ah ¼ surfaceflowofbed(m2), Qd ¼ averageflow(m3 day 1), Cin ¼ influentBOD5 (mgL 1), Cout ¼ effluentBOD5 (mgL 1), KBOD ¼ rateconstant(mday 1).

Therewerealotofdiscussiononthe KBOD value.Formerlyproposedvalueof0.19mday 1 byKickuthresultedintoosmallarea ofthebed(about2m2 peronePE,populationequivalent)andconsequentlylowertreatmenteffect.Thefieldmeasurementsin operationalsystemsindicatedthatthevalueof KBOD isusuallylower(0.07–0.1mday 1).Thedatafrom66DanishHSSFCWs identified K-valuesfortotal-N:0.033mday 1 andtotal-P:0.025mday 1

Thedepthofvegetatedbedswithhorizontalsubsurfaceflowwasinitiallybasedontherequirementthatrootsandrhizomesof thevegetationshouldpenetratethefulldepthofthebedinordertoeliminatetotallyanaerobiczones.Astherootsofthemost frequentlyusedplant,commonreed(Phragmitesaustralis)arecapableofsuccessfulpenetrationtothedepthofabout0.6mandstart toweakenbeyondthatpoint,therecommendedbeddepthwas0.6m.Althoughithasbeenproventhatoxygentransportedfrom abovegroundorgansdiffusesonlytothethinsubstratelayeradjacenttotherootsandrhizomesthetypicaldepthoffiltrationbedis usually0.6–0.8m.

Constructedwetlandswithhorizontalsubsurfaceflowareusuallysealedinordertopreventuncontrolledwaterseepageinto groundwater.MostofthesystemsuseaplasticlinerormembranesuchasHDPE,LDPEorPVC0.5–2mmthick.Wherelocalsubsoil haslowhydraulicconductivity(approx.10 8 ms 1 orless),itisnotnecessarytouseplasticliners.Inordertopreventlinerdamage byfiltrationmaterialparticles,geotextileiscommonlyusedtocoverplasticliner.Also,geotextilecouldbealsousedbeneaththe liner.

Themacrophytesgrowinginconstructedwetlandshaveseveralpropertiesinrelationtothetreatmentprocessesthatmakethem anessentialcomponentofthedesign.ThemostimportanteffectsofthemacrophytesinHSSFCWsinrelationtowastewater treatmentprocessesarethephysicaleffectstheplanttissuesgiverisetosuchasinsulationofthebedsurfaceduringtheperiodof coldweatherorprovisionofsurfaceareaforattachedmicroorganismsinthefiltrationbed.Themetabolismofthemacrophytes (plantuptake,oxygenreleasefromtheroots)isofminorimportanceinHSSFCWs.ThemacrophytesforHSSFCWsshould(a)be tolerantofarelativelyhighorganicloadinthewastewater,(b)havehighbelowgroundandabovegroundbiomass,and(c)should growquicklyinordertocoverthefiltrationbedsurfacesoonafterplanting.

Themostfrequentlyusedplantaroundtheworldis Phragmitesaustralis,especiallyinEurope,Australia,AfricaandAsia.Insome countries, Phragmites isusedexclusively;forexample,intheUnitedKingdom,whereHSSFCWsarecalledReedBedTreatment Systems.Ontheotherhand,inNewZealandormanyareasoftheUnitesStates, Phragmites isconsideredanexoticandinvasive speciesbynaturalresourceagenciesandasaresult,useofthisspecieshasbeenlimited.Othercommonlyusedspeciesare Phalaris arundinacea (reedcanarygrass), Glyceriamaxima (sweetmannagrass)orvariouscattails(Typhalatifolia,T.angustifolia)inEurope, Cyperuspapryrus (papyrus)inAfrica, Typhadomingensis inSouthAmericaand Scirpus spp.(bulrush)inNorthAmerica.Weeds(plants thatwerenotintentionallyplanted)occurmostlywithinvegetatedbedmarginsanddonothaveanydetrimentalimpacton treatmentperformance.

DissolvedoxygensupplyisverylimitedinfiltrationbedsofHSSFCWsand,therefore,anoxicandanaerobicprocessesusually prevail.Aerobicdegradationisrestrictedtonarrowzonesadjacenttorootsandrhizomeswhereoxygenleakstotherhizosphere. Removaloforganics(BOD5,COD)isusuallyhighandexceeds85%incaseofsewage.Suspendedsolidsthatarenotremovedin pretreatmentsystemareeffectivelyremovedbyfiltrationandsettlement.Mostsuspendedsolidsarefilteredoutandsettledwithin thefirstfewmetersbeyondtheinletzone.SuspendedsolidsareremovedinHSSFCWSwereeffectively,commonly >90%.The accumulationoftrappedsolidsisamajorthreatforgoodperformanceofHSSFsystemsasthesolidsmayclogthebed.Therefore,the effectivepretreatmentisnecessaryforHSSFsystems.However,ithasbeenshownthatifHSSFCWsareappropriatelyloaded,thatis, <10gBOD5 m 2 day 1 , <20gCODm 2 day 1 , <10gTSSm 2 day 1,thepartialcloggingintheinflowzoneoccursonlyafter about15years.Inaddition,partialclogginghasnoeffectontreatmentperformanceofthesystem.

ThemajorremovalmechanismofnitrogeninHSSFconstructedwetlandsisnitrification/denitrification.Fieldmeasurements haveshownthattheoxygenationoftherhizosphereofHSSFconstructedwetlandsisinsufficientand,therefore,incomplete nitrification(i.e.,oxidationofammoniatonitrate)isthemajorcauseoflimitednitrogenremoval.Ingeneral,nitrificationwhich isperformedbystrictlyaerobicbacteriaismostlyrestrictedtoareasadjacenttorootsandrhizomeswhereoxygenleakstothe filtrationmedia.Ontheotherhandprevailinganoxicandanaerobicconditionsoffersuitableconditionsfordenitrificationbutthe supplyofnitrateislimitedasthemajorportionofnitrogeninsewageisintheformofammonia.Adsorptionandplantuptakeplay amuchlessimportantroleinnitrogenremovalinHSSFCWs.Volatilizationisnoteffectiveasthereisnofreewatersurfaceand adsorptionisgreatlylimitedbythefactthatfiltrationmedia(gravel,rock)donotprovidesuitablesorptionsites.Plantharvesting contributestoanoverallnitrogenremovalonlymarginally(usually <10%oftheinflowload)withNstandingabovegroundstocks intherangeof20–60gNm 2.Incaseofsewage,removalofammoniaandtotalnitrogenusuallydoesnotexceed50%.

Phosphorusisremovedprimarilybyadsorptionandprecipitation,however,mediausedforHSFwetlands(e.g.,peagravel, crushedstones)usuallydonotcontaingreatquantitiesofFe,AlorCaandtherefore,removalofphosphorusisgenerallylow(<40% insewage).Removalofphosphoruscouldbeenhancedbytheuseoffiltrationmediawithhighsorptioncapacitybutthesorption capacityisalwayssaturableandtherefore,themediamustbereplacedaftersaturationinordertomaintainhighPremoval.Removal viaharvestingaccountsusuallyfor <5%oftheinflowloadwiththePstandingstockintheabovegroundbiomassintherangeof 3–6gPgm 2.Theremovalofmicrobiologicalpollutionisveryseldomtheprimarytargetforconstructedtreatmentwetlands. However,HSSFCWsareknowntoactasexcellentbiofiltersthroughacomplexofphysical,chemicalandbiologicalfactorswhichall participateinthereductionofthenumberofbacteriaofanthropogenicorigin.

HSSFCWsareusedformanytypesofwastewateraroundtheworld(Table2).Indeed,themostcommonuseisformunicipal anddomesticsewage,howeverindustrialandagriculturalwastewatershavebeensuccessfullytreatedaswell.Besidesthat, applicationsforvarioustypesofstormwaterrunoff(e.g.,urban,highway,agricultural,golfcourses,nurseries,airports)andlandfill leachatehavebeenputinoperation.

Table2

ExamplesoftheuseofHFCWsforvarioustypesofwastewaters WastewaterLocation

MunicipalWorldwide PetrochemicalUnitedStates,Taiwan,China,Sudan,Oman PulpandpaperUnitedStates,Kenya,India TanneryPortugal,Tanzania TextileAustralia,Slovenia,Tanzania AbbatoirAustralia,Mexico,Ecuador,Uruguay,NewZealand FoodprocessingItaly,Spain,Slovenia,UnitedStates,France,Lithuania,Turkey Distillery,wineryItaly,Spain,UnitedStates,India,SouthAfrica,Mexico,UnitedKingdom FeedlotoperationsAustralia,China,UnitedStates,Canada,Thailand,Lithuania FishfarmsUnitedStates,Germany,Canada DairyItaly,Lithuania,Germany,UnitedStates,UnitedKingdom,NewZealand HighwayrunoffUnitedKingdom,Italy,UnitedStates AirportrunoffUnitedKingdom,Switzerland,UnitedStates,Germany AgriculturalrunoffChina,NewZealand LandfillleachateUnitedStates,Portugal,Norway,Poland,Slovenia

VerticalFlowCWs

Constructedwetlandswithverticalsubsurfaceflow(VForVSSFCWs)usuallycompriseaflatbedofcoarsesandorgravelplanted withmacrophytes(Fig.4).ThemostimportantfactorsinthedesignofaVFCWsare:(1)toproduceabedmatrixthatallowsthe passageofthewastewaterthroughthebedbeforethenextdosearriveswhilstatthesametimeholdingtheliquidbacklongenough toallowthecontactwiththebacteriagrowingonthemediaandachievetherequiredtreatment.(2)Toprovidesufficientsurface areatoallowtheoxygentransfertotakeplaceandsufficientbacteriatogrow.VFCWswereproposedduringthe1960sbutdidnot spreadasquicklyasHSSFCWs,probablybecauseofhigheroperationandmaintenancerequirements.However,theincreased demandfornitrogenandespeciallyammonia’nitrogenremovalinthe1990srevivedthistypeofconstructedwetlands.

AllVFsystemsaredosedintermittently,however,thereisnoclearrecommendationhowmanybatchesperdayareoptimal.Itis essentialtoachievequickwatercoverofthesurfaceinordertotrapairintheintersticesinthebed.InmostVFsystemsthereal distributoristhelayerofcarefullyselectedsandwhichfirstallowsfloodingofthesurfaceandthengradualseepagedownthrough thedepthofthemedia.ThevastmajorityoftheVFsystemsemployanetworkofpipeswithsmallholesacrossthesurfaceareaofthe bed.Thedistributionpipescouldbeinsulatedbya0.2mlayerofcoarsewoodchipsorseashellsonthesurfaceofthefilter.Itisalso possibletodistributewastewaterfromopen-endedpipesontothebed.Theareaoftheimmediatevicinityofthedischargeshouldbe protectedbysomepavingortilestopreventthewashawayofthesandorgravel.

ThedataonmaximumHLRvarywidelyintheliterature.However,itseemsthatVFCWscanoperateintherangeof 100–1200mmday 1 andusuallynocloggingproblemsoccurbelow800mmday 1 ofpretreatedwastewater.Theorganicloading rateshouldnotexceed25gCODm 2 day 1 inordertopreventclogging.TheareausedforVFCWsvariesbetween0.9and 5m2 PE 1 (PE ¼ populationequivalent)butmostsystemsaredesignwiththespecificarea2–3m2 PE 1 stemsareusuallydesigned asasingleunit,largersystemsmayhaveseveralbedswhicharefedwithwastewaterinrotation.Also,somelargersystemshavetwo stagesofverticalbedsinoperation.

ThesizeofVFCWsisusuallybasedonthehydraulicloadingrate(HLR).ThemaximumHLRthatcanbeachievedwithout surfacefloodingwillbeaffectedbymanyvariablesbutismoststronglyrelatedtomediasizeanddistribution,rateofbiofilmgrowth andhencetheBOD5/organicmassloadingrateandsuspendedsolidsloadingrate.TheDanishguidelinesrecommendspecificarea of3.2m2 perperson,maximumorganicloadingof18.8gBOD5 m 2 day 1 andhydraulicloadingof47mmday 1.Filtration mediumissandwithad10 between0.25and1.2mm,ad60 between1and4mm,anduniformitycoefficient(U ¼ d60/d10)should be <3.5.Thecontentsofclayandsilt(particles <0.125mm)mustbe <0.5%.Austrianguidelinesrecommendthespecificareaof 4m2 perpersonandmaximumloadingof20gCODm 2 day 1.Thefiltrationlayersshouldconsistof5–10cmofgravel(4/8or 8/16mm)ontop,followedby50cmofwashedsand(0–4mm),5–10cmtransitionlayer(gravel4/8mm)and20cmdrainage layeratthebottom(gravel8/16or16/32mm).Thisset-upguaranteesoutflowconcentrations <90mgL 1 COD,25mgL 1 BOD5 and10mgL 1 N-NH4 atwatertemperature > 12 C.InFrance,theVFsystemsconsistoftwobedsinserieswithonly1.2m2 per personinthefirststageand0.8m2 perpersoninthesecondstage.Thepretreatmentconsistsonlyofprescreening,maximum organicloadis100gCODm2 day 1 andmaximumhydraulicloadingis90cmday 1

PlantsplayaveryimportantroleinVFCWs.Theystabilizesurfaceofthebed,theirrootsandrhizomespositivelyaffectthe hydraulicconductivityofthefilterandmovementofabovegroundstemshelpstopreventclogging.Theabovegroundbiomass providesinsulationofthebedandbelowgroundorgansprovidesubstrateforattachedbacteriagrowth.Theoxygentransfertothe rhizosphereislimitedbutcreatesmicrozoneswhereaerobicbacteriacanbepresent.ThevastmajorityofVFCWsisinoperationin Europeandthemostcommonlyusedplantis Phragmitesaustralis (commonreed). VFCWsareveryeffectiveinremovaloforganics,suspendedsolidsandammonia.Theintermittentfeedingallowsforregular emptyingthefiltrationbedwhichresultsingoodoxygenationofthebedallowingfornitrification.Therefore,VFCWSareusedin

Fig.4 Layoutofaverticalflowconstructedwetlandsystemforasinglehousehold.Rawsewageispretreatedina2m3 sedimentationtank.Settledsewageis pulse-loadedontothesurfaceofthebedbyalevel-controlledpump.Treatedeffluentiscollectedinasystemofdrainagepipes,andhalfoftheeffluentisrecirculated backtothepumpingwell(ortothesedimentationtank).FromBrix,H.(2005).Theuseofverticalflowconstructedwetlandsforon-sitetreatmentofdomestic wastewater:NewDanishguidelines. EcologicalEngineering 25,491–500.

Table3 ExamplesoftheuseofVFCWsforvarioustypesofwastewater WastewaterLocation Municipal,domesticFrance,Belgium,Austria,Poland,CzechRepublic,Italy LandfillleachateNorway,NewZealand AbbatoirCanada RefineryPakistan AirportrunoffUnitedStates,Canada Textile UnitedKingdom,Portugal,Japan AquacultureUnitedStates,China,Canada,Vietnam Winery Germany OlivemillTurkey,Greece ChemicalindustryPortugal SteelindustryChina

casewhenammoniaisthetargetofthetreatment.Ontheotherhand,duetooxicconditionsinthebed,thedenitrificationislimited andoftenmissinginthesystem.VFCWsarecommonlyusedfortreatmentofdomesticandmunicipalsewagebuttheuseforother typesofwastewateriscommon(Table3).

HybridConstructedWetlands

Inhybridconstructedwetlands(CWs),theadvantagesofvarioussystemscanbecombinedtocomplementeachother.Hybrid constructedwetlandswerefirstintroducedbySeidelinGermanyasearlyasinthe1960s.Thedesignconsistedoftwostagesof severalparallelverticalflow(VF)bedsfollowedbytwoorthreehorizontalflow(HF)bedsinseries.TheVFstageswereusually plantedwith Phragmitesaustralis,whereastheHFstagescontainedanumberofotheremergentmacrophytes,including Iris, Schoenoplectus (Scirpus),Sparganium,Carex,Typha and Acorus.TheVFbedswereloadedwithpretreatedwastewaterfor1–2days, andwerethenallowedtodryoutfor4–8days.Inthissystem,nitrification,thatis,oxidationofammoniatonitrate,takesplacein theVFstageanddenitrificationofnitrate,thatis,reductionofnitratedoN2OandN2,proceedsintheHFstage.Theoxidationof ammoniainintermittentlyloadedVFstageisveryhighbuttheconcentrationoforganicsinthesecondstagemaynotbehigh enoughtosupportfulldenitrification.Intheearly1980s,severalhybridsystemsofSeidel’stypewerebuiltinFranceandsimilar systemwasbuiltin1987intheUnitedKingdom.Duringthe1990sandtheearly2000s,VF–HFsystemswerebuiltinmany countriesinEurope,forexample,Austria,Slovenia,NorwayorIreland.TheVF–HFhybridconstructedwetlandsweremostly designedtotreatdomesticormunicipalwastewaterwherenitrifiedeffluentswererequiredbuttherewerealsoapplicationforother typesofwastewater(Table4).

Table4 Examplesofhybridconstructedwetlandsusedforvarioustypesofwastewaters

TypeofhybridsystemLocation Wastewater

VF–HF

HF–VF

VF–VF–HF

VF–VF–HF–VF

Belgium,Estonia,Tunisia,Spain,China,Italy,Brazil,FranceSewage

Italy Cheeseproduction

Slovenia Landfillleachate

Spain Winery

Mexico,Poland,France,Italy,Nepal,China,TurkeySewage

Poland Slaughter

France Cheeseproduction

Japan Milkingparlor

VF–VF–VF–HF–VFJapan Starchproduction

VF–HF–VF

HF–VF–HF

VFd–VFu

Japan Pigurine

Morocco,Turkey Sewage

Bangladesh Tannery

Poland,NewZealand Sewage

Korea Greenhouse

Slovenia Mixedindustrial

China Sewage

China Aquaculture

FWSincombinationwithHFandVFChina.Italy,Thailand,Mexico,Taiwan Sewage

Italy Winery

Taiwan Aquaculture

Thailand Fishindustry

Canada Landfillleachate

Kenya Flowerplant

Inthelate1990s,alsoHF–VFconstructedwetlandswereintroduced.ThissystemconsistedofalargeHFbedplacedfirstanda smallVFbedasthesecondstage.Inthissystem,nitrificationtakesplaceintheverticalflowstageattheendoftheprocesssequence. Ifnitrateremovalisneededitisthennecessarytorecirculatetheeffluentbacktothefrontendofthesystemwheredenitrification cantakeplaceinthelessaerobichorizontalflowbedusingtherawfeedasasourceofcarbonneededfordenitrification.TheHF–VF CWshasbeenusedexclusivelyfortreatmentofsewagesofar(Table4).BesidesVF–HFandHF–VFsystemsothercombinationsof subsurfaceflowCWsandalsocombinationsincludingFWSCWswereusedinthe1990sandtheearly2000sfortreatmentof varioustypesofwastewater(Table4).

Conclusions

Constructedwetlandsrepresentanalternativetreatmentsystemtoconventionaltreatmentsystemssuchasactivatedsludgeprocess. Alltypesofconstructedwetlandsexhibithightreatmentefficiencyfororganicsandsuspendedsolids.Theremovalofthese parametersiscomparablewithconventionalsystems.Removalofnitrogendependsonthetypeofconstructedwetlandsand nitrogenspeciesinvolved.AmmoniaisefficientlyremovedinverticalflowCWswhilenitrateisremovedefficientlyinHFCWs. However,combinationofvarioustypesofCWs(usuallyVFandHFCWs)canenhanceremovaloftotalnitrogenandthenthe efficiencyiscomparablewithconventionalsystems.Removalofphosphorusisvariabledependingonthefiltrationmaterial, however,commonlyusedmaterialsdonotsupporthighphosphorusremoval.Theadvantageofconstructedwetlandsascompared toconventionaltreatmentsystemsislowoperationandmaintenancecost,insomecountriesalsotheinvestmentcostis substantiallylowerthanofconventionalsystems,abilitytotreatlowstrengthwastewatersandnorequirementofcontinuous feedingandoperation.

BrixH(2005)Theuseofverticalflowconstructedwetlandsforon-sitetreatmentofdomesticwastewater:NewDanishguidelines. EcologicalEngineering 25:491–500. HammerDA(ed.)(1989) Constructedwetlandsforwastewatertreatment,Chelsea,Michigan:LewisPublishers. Kadlec,R.H.,Wallace,S.D. Treatmentwetlands,2ndedn.BocaRaton,Florida:CRCPress. Kadlec,R.H.,Knight,R.L.,Vymazal,J.,Brix,H.,Cooper,P.F.andHaberl,R.(2000). Constructedwetlandsforpollutioncontrol.Processes,performance,designandoperation.IWA scientificandtechnicalreportno.8.London:IWAPublishing. MoshiriGA(ed.)(1993) Constructedwetlandsforwaterqualityimprovement,BocaRaton,Florida:CRCPress/LewisPublishers. MulamoottilG,McBeanEA,andRoversF(eds.)(1999) Constructedwetlandsforthetreatmentoflandfillleachates.BocaRaton,Florida:CRCPress/LewisPublishers. ReddyKRandSmithWH(eds.)(1987) Aquaticplantsforwatertreatmentandresourcerecovery,Orlando,Florida:MagnoliaPublishing.

ReedSC,MiddlebrooksEJ,andCritesRW(1988) Naturalsystemsforwastemanagementandtreatment.NewYork:McGraw-Hill. SeidelK(1965)NeueWegezurGrundwasseranreicherunginKrefeld,Vol.II.HydrobotanischeReinigungsmethode. GWFWasser/Abwasser 30:831–833. VymazalJ(ed.)(2001) Transformationsofnutrientsinnaturalandconstructedwetlands.Leiden,TheNetherlands:BackhuysPublishers. VymazalJ(2007)Removalofnutrientsinvarioustypesofconstructedwetlands. ScienceoftheTotalEnvironment 380:48–65. VymazalJ(2008)Constructedwetlands,subsurfaceflow.In:JørgensenSEandFathBD(eds.) Encyclopediaofecology,1stedn,pp.749–764.Amsterdam:ElsevierB.V. VymazalJ(2008)Constructedwetlands,surfaceflow.In:JørgensenSEandFathBD(eds.) Encyclopediaofecology,1stedn,pp.765–777.Amsterdam:ElsevierB.V. VymazalJ(2011)Constructedwetlandsforwastewatertreatment:Fivedecadesofexperience. EnvironmentalScienceandTechnology 45(1):61–69. VymazalJ(2014)Constructedwetlandsfortreatmentofindustrialwastewaters:Areview. EcologicalEngineering 73:724–751. VymazalJandKröpfelováL(2008) Wastewatertreatmentinconstructedwetlandswithhorizontalsub-surfaceflow.Dordrecht:Springer. VymazalJ,BrixH,CooperPF,GreenMB,andHaberlR(eds.)(1998) ConstructedwetlandsforwastewatertreatmentinEurope,Leiden,TheNetherlands:BackhuysPublishers.

DeadZones:LowOxygeninCoastalWaters

AndrewAltieri,

UniversityofFlorida,Gainesville,FL,UnitedStates

r 2019ElsevierB.V.Allrightsreserved.

Glossary

Anoxia Absenceofdissolvedoxygen.

Benthos/benthic Thecommunityoforganismsinornear thebottomoftheocean.

Deadzone Oxygen-depletedcoastalwatersthatare hypoxicwithcompromisedbiologicalcommunities.

Eutrophication Excessnutrientsinabodyofwater.

Hypoxia Lowdissolvedoxygen.Theexactthresholdiscontext dependent,butistypicallyregardedasaconcentration o2.8mg/L.

Hysteresis Theexistenceofmultipleecologicalstates underagivensetofenvironmentalconditions,where thresholdsbetweenstatesdifferdependingonpriorstate.

Introduction

Residencetime Theaveragetimeforthewaterinabody ofwatertoberenewed.

Stratiﬁcation Layeringofthewatercolumnintowater masseswithdifferentproperties.

Suspensionfeeders Organismsthatthatfeedby ﬁltering foodfromwater.

Watercolumn Asectionofwaterextendingfromthe surfacetobottom.

Oxygendepletionincoastalwatersisregardedbymanymarinebiologistsandoceanographersasthemostpressingwater pollutionproblemintheworldbecauseofitssevereimpactsandacceleratingspreadworldwide.Inextremecases,thishypoxia createsareasknownas “deadzones” thatarelargelydevoidofmacrofaunabecauseofmortalityandemigration.Concentrationsof dissolvedoxygenarenaturallyvariable,andthisvariationcanleadtoareaswithlowoxygenconcentrations(hypoxia)thatare stressfultomarinelife.However,theduration,severity,andnumberofecosystemswithhypoxiahasincreasedduetoanthropogenicfactorsincludinginputsofexcessnutrientsandorganicmatter,aswellasclimatechange,andtherearenowhundredsof coastalecosystemsaffectedbyhypoxiaworldwide(Fig.1).

Thisarticlebeginsbydescribinganddeﬁningdeadzones,andthenprovidesbackgroundonfactorsthatdrivetheformationof hypoxicwatersincludinganthropogenicinputsandnaturalprocesses.Organismalresponsestolowoxygenarethenpresentedasabasis forunderstandingcommunityandecosystemimpactsofhypoxia.Therelevanceofdeadzonestohumanwelfareareprovidedinthe contextofecosystemservices,andfollowedbyevidencefortheincreasingspreadofdeadzonesworldwide.Finally,thearticleprovides backgroundonemergingareasofhypoxiaresearchandpossiblesolutionstoaddressthethreatofdeadzones.

Globalmapofhypoxicecosystems.Deadzonesaretypicallyassociatedwithareasofintensehumanactivity(or “humanfootprint”)inthe coastalzone.Mostknowndeadzonesarelocatedinthetemperateregions,butrecentresearchsuggeststhenumberofhypoxicecosystemsinthe tropicsaregreatlyunderestimated.FromDiaz,R.J.andRosenberg,R.(2008).Spreadingdeadzonesandconsequencesformarineecosystems. Science 321,926–929.

Fig.1

DeadZonesDeﬁned

Thedepletionofoxygenincoastalwaters,suchasestuaries,continentalshelfareas,andcoastalseas,isnowawidespread phenomenon.Lowoxygentypicallyoccursclosetothesea ﬂoorandindeeperportionsofstratiﬁedwatercolumnswhereratesof oxygenconsumptionarelikelytobehighestandreplenishmentlowest.Deadzonescanrangeinsizefromasmallcanalorchannel tothebasinofinlandseassuchastheBalticSeawhichcommonlycovers 460,000km2 (Fig.2).Deadzonesalsovaryinduration. Somehavebecomepermanentfeatures,asfoundintheBalticSea,whereasothersoccurseasonally,suchastheChesapeakeBay, UnitedStates.Theycanalsovaryinspatialextentandseverityyear-to-yearbasedonfreshwaterinputsandexternalconditionsasin theGulfofMexico.

Fishkills,inwhichdead ﬁshandothermarineorganismsaccumulateonthewater'ssurfaceandonbeaches,areoneofthe mostvisiblesignsofhypoxia.Anotherconspicuousphenomenonisthe “jubilees” inMobileBay,UnitedStateswherehypoxic bottomwatersdrive ﬁshandcrabsintotheshallowswheretheyarecollectedbylocalresidentsinaseafoodbonanza.However, theseeventsrepresentjustahintofhypoxia'simpactsandprevalenceincoastalecosystemssinceoxygenmeasurementsrequire specializedsensors,andtheassociatedbiologicaleffectstypicallyoccuroutofsightbelowtheocean'ssurface.

Oxygendepletionisastressforaerobicorganismsthatcanleadtophysiologicalchanges,behaviorsresponses,andeventually mortality.Hypoxiacan,therefore,haveavarietyofdirectandcascadingeffectsoncommunitystructureandecosystemfunction.In themostextremecases,hypoxicecosystemsbecomelargelydevoidoflivingmacrofauna,hencethename “deadzone.” Thisterm originatedinthepopularpresswhichreportedonhypoxicareasthat fishersreferredtoas “deadwater” forlackofany finfishor shellfishtocatch.Inthedecadessincetheterm “deadzone” was firstcoined,ithasbecomemoregenerallyappliedtoanyhypoxic ecosystem,andthetermisusedassuchinthisarticle.Whilethereissomedisagreementaboutwhethertheterm “deadzone” is appropriatesincemicrobesandafewothertolerantfunctionalgroupscanpersistinseverelyhypoxiawaters,itdoesserveasa usefulnontechnicalshorthandthatconveystheseriousnessofthephenomenonforthelivingsystemsonwhichhumansareso dependent.

HypoxiainOtherEcosystems

Thescopeofthisarticleislimitedtohypoxiaincoastalwatersincludingbays,estuaries,coastalandinlandseas,andsomeshelf systems.However,hypoxiaalsooccursinotherbodiesofwaterwithimportantconsequencesfortheecologyofthosesystems.For

Fig.2 Schematicrepresentingthegeneralrelationshipbetweenecosystemsizeanddurationofhypoxia.Therelativecontributionofhuman factorsindrivingoxygendynamicsisrepresentedin red, whereasnaturalfactorsarein green.Thisarticlefocusesonthosesystemsin red.From Rabalais,N.N.,Diaz,R.J.,Levin,L.A.,Turner,R.E.,Gilbert,D.andZhang,J.(2010).Dynamicsanddistributionofnaturalandhuman-caused hypoxia. Biogeosciences 7,585–619.

example,freshwaterlakesandpondscommonlybecomehypoxicbecauseofeutrophicationand/orseasonalstratiﬁcationofthe watercolumn.Therearealsolargepermanentlyhypoxicareasindeeperoceanbasinsknownasoxygenminimumzones(OMZ) thatareanaturalphenomenonwithdiverseadaptedcommunities(Fig.2).Attimes,hypoxicwatersfromtheOMZcanbe upwelledontotheshelfintoshallowcoastalwaterssuchasalongthewesterncoastsoftheUnitedStatesandSouthAfrica.There areimportantdistinctionsbetweenOMZsanddeadzones:OMZsaregenerallyconsiderednaturalandpermanentfeatures (althoughtheirexpansionappearslinkedtolarge-scaleclimateandoceanographicprocesses),whereascoastaldeadzonescanvary indurationandareoftenassociatedwitheutrophicationandotheranthropogenicfactors.

QuantifyingHypoxia

Hypoxiaisgenerallydeﬁnedasanoxygenconcentrationinwaterthatislowrelativetoeithertheoxygensaturationatequilibriumwith atmosphericconcentrationsorthemetabolicneedsofaerobicorganisms.Assuch,dissolvedoxygen levelsaretypicallyreportedaseither percentsaturationorconcentration(mgormLofoxygenperliterofseawater).Percentsaturationisoftenusefulforunderstandingthe driversofhypoxiabecauseitindicatestheseverityofoxygendepletionrelativetothatexpectedatequilibriumconcentrations.Absolute oxygenconcentrations(mg/LormL/L)aremorefrequentlyusedinstudiesofbiologicalresponsesto hypoxia.Saturationand concentrationunitsarenotdirectlyinterchangeablebecausetemperature,barometricpressure,andsalinityaffectthesaturationof oxygeninwaterandmustbeknowntoderivepercentsaturationfromabsoluteoxygenconcentration.

Concentrationsofdissolvedoxygenvarycontinuously,andthresholdsoftolerancetohypoxiavaryamongspecies.Nevertheless,2.8mg/L(equivalentto2.0mL/L)hasbeenwidelyadoptedastheoxygenconcentrationbelowwhichabodyofwateris consideredhypoxic.Belowthatthreshold,negativeeffectsofoxygenlimitationincludingmortalityarelikelytobeobservedacross mosttaxa.Thereareproponentsforelevatingthethresholdsince ﬁshoftenshowsensitivitiestooxygenatthe5–6mg/Lrange,and manymacrofaunaexhibitbehavioralchangeswhenoxygenlevelsdroptothe3–4mg/Lrange.Forcomparison,100%oxygen saturationinwaterat251Cand1atmofpressureis8.2mg/L.

Theoxygendynamicsandecologicaleffectsofoxygenlimitationindeadzonesareoftenmeasuredanddescribedinthewater columnandinthebenthos.Thebenthosinmanydeadzonesiscomprisedofsoftsediments,andthechemicalproperties (includingoxygenconcentrations)andbiologicalactivityofthosesedimentsarecloselylinkedtopropertiesofthewatercolumn. Acomprehensiveunderstandingofdeadzones,therefore,incorporatescharacteristicsofthesediments,includingirrigationby burrowingorganismsandthedepthoftheredoxpotentialdiscontinuitywhichindicatesthedepthwithinthesedimentthat transitionsfromoxictoanoxiclayers.

CausesofDeadZones

Oxygenconcentrationsdeclineindeadzonesbecauseratesofoxygendepletionoutpaceratesofoxygenreplenishment.This sectionofthearticlediscussestheoriginsoflowoxygenwaters.First,thegeneralprocessesthatdepleteoxygenaredescribedwitha distinctionbetweennaturalandanthropogenicfactors.Second,thefactorsthatlimitoxygenreplenishmentareoutlined.Finally,a discussionispresentedastowhysomecoastalwatersareespeciallysusceptibletooxygendepletion.

OxygenDepletion

Decompositionandrespiration

Oxygenconsumptionbyaerobicorganismsisthedominantcauseofoxygendepletionincoastaldeadzones.Therearetwomain processesofbiologicaloxygenconsumptionthatcanleadtolowoxygenconcentrationsincoastalwaters.

Decompositionbymicrobesistheprimaryprocessofoxygenconsumptionthatleadstotheformationofdeadzones.Assuch, factorsthatincreasetheamountoforganicmatteravailablefordecompositionand/orincreaseratesofmicrobialactivityare ultimatelyresponsibleforhypoxia.Manydeadzonesarelinkedtophytoplanktonblooms.Thebiomassofplanktonthatdiesand sinkstothebottomfuelsaerobicmicrobialdecomposition.Macroalgalbloomscanalsogenerateorganicmatterthatfuelshypoxic conditions.Moreover,macroalgaecanfurtherincreaseoxygenstressbysmotheringthebottom.

Allochthonousorganicmatter,suchasdetritus,sediments,andsewage,deliveredtocoastalwaterscanbypasstheroleofalgal bloomsandleadtomicrobialoxygendepletion.Ifoxygenlevelsdropsufﬁcientlytocausemortality,thenthebiomassofdead organismsbecomesasupplementarysourceoforganicmatterthatcanfuelfurtherdecompositioninalocalizedfeedbackloop.

Asecondprocessofoxygendepletionisrespiratoryoxygenconsumptionbyprimaryproducerssuchasphytoplanktonand macroalgae.Oxygenconsumptionbyprimaryproducerswilloutpacetheiroxygenproductioninlightlimitedconditionssuchas thoseproducedbyself-shadingoratnight,leadingtotemporaryorlocalizedoxygendepletion.Oneofthemostconspicuous indicatorsofoxygenconsumptionbyprimaryproducersisdielcyclinginwhichoxygenconcentrationsexhibitasawtoothpattern asasystemoscillatesbetweennetphotosyntheticoxygenproductionduringthedayandnetrespiratoryoxygenconsumption duringthenight.

Anadditionalfactorleadingtooxygendepletionincoastalwatersisoxygenconsumptionbymacrofauna.Thisgenerallydoes notleadtolargeorsustaineddeadzoneformationperse,butcanexacerbatetheeffectsofexistinglowoxygenconditions.For

example, ﬁshthathavebeenshoaledbylowoxygenconditionsintoasemienclosedareacanbecometrappedandrapidlyconsume remainingoxygen,resultingina ﬁshkill.

Nutrients

Manystudieshaveidentiﬁedexcessnutrientsasthefactormostresponsiblefortheformationofdeadzonesworldwide.Asa consequence,hypoxiaisviewedasoneofthemostsigniﬁcantlynegativeimpactsofeutrophication.Sinceprimaryproductivityby phytoplanktonistypicallynutrientlimited,inputsofnutrientscansupporttheexcessphytoplanktonbiomassresponsiblefor hypoxia.Typically,primaryproductivityinmarineecosystemsisnitrogenlimited,andinlowersalinityecosystemsisphosphorus limited.

Thereareseveralpathwaysthatdelivernutrientsandorganicmattertocoastalwaters.Riversandterrestrialrun-offarethe dominantsourcesofnutrientsandorganicmatterformanycoastalecosystems,butatmosphericdeposition,upwelledocean water,andsewagesystemsplaysigniﬁcantrolesinsomecoastalwaters.Theavailabilityofnitrogenandphosphorusisalsobe modiﬁedwithinadeadzonebymicrobialactivityandbioturbatorsinthesediment.Thebalancebetweensequestrationand releaseofnutrientsfromthesedimentisdependentontheoxygenconcentration,withthepotentialforfeedbacksbetweenoxygen, nutrients,andprimaryproducers.

Inputsofnutrientsandorganicmatterthatfueldeadzoneformationincoastalwatersareoftenelevatedabovenaturalratesby humanactivity.Whiletherearenaturalsourcessuchaserosionalprocessesandrun-offfromterrestrialecosystems,anthropogenic sourceshavedramaticallyincreasednutrientsabovenaturalbackgroundlevelsandcontributedtotheprevalenceofhypoxia.For example,overthepastcenturytherewasa20-foldincreaseinnitrogeninputsfromtheMississippiRiverthatfuelstheGulfof Mexicodeadzone.Muchofthesenutrientsandorganicmatteroriginatefromagriculturalsourcesincludingexcessfertilizerfrom fieldsandlivestockwaste.Additionalanthropogenicsourcesincludeeffluentfromsewagetreatmentplantsandhouseholdseptic systems,urbanrun-off,papermills,seafoodprocessingplants,andaquaculturepens.Therelativeimportanceofthesevarious sourcesofnutrientsandorganicmatterislocationspecific,varyingwithland-usepractices,regulations,andhumandensitiesinthe watershed.

Therearetwooveralltrendsthathelpestablishthecontributionofhumanactivitytotheincreaseindeadzoneprevalence.First, hypoxiaismoreprevalentnearmajorpopulationcentersandwatershedswithintensiveinputsofnutrientsandorganicmaterial. Second,thereisatemporaltrendinwhichsewageandindustrialpollutionweredominantdriversofhypoxiathroughthemid20thcenturyfollowedbyashifttowardgreaterrun-off/riverineinputsandatmosphericdepositionofnutrientsThatcoincidedwith intensiﬁcationofagriculturalpracticesandanoverallincreasingrateofnutrientdeliverythatcausedthenumberofnewhypoxic ecosystemstoincreaseatanacceleratedrate.

Warming

Temperaturehasadirecteffectonoxygenconcentrationssincewarmerwaterhasalowercapacityfordissolvedgassesincluding oxygen.Temperaturealsohasindirecteffectsonoxygendepletionsincephytoplanktonbloomsandmicrobialactivityaretemperaturedependent.Thisexplainswhyhypoxiaismostlikelytooccurinthesummer,whentemperatureandlightlevelspeak,and whyclimatechangeissuchaconcernfordeadzones(asdetailedbelow).

LimitedOxygenReplenishment

Replenishmentofoxygeninhypoxicwatersoccursthroughtwoprimarymechanisms:verticalexchangewithsurfacewatersthatare oxygenatedthroughdiffusionandwindmixing,andlateralexchangewiththemoreoxygenatedopenocean.Therefore,hypoxia anddeadzonestypicallyoccurinbodiesofwaterthatareisolatedbyshorelinefeaturesand/orstratiﬁcation.

Stratiﬁcationandverticalmixing

Hypoxiatypicallydevelopsatornearthebottomwheretheorganicmatterthatfuelsdecompositionaccumulates.Stratificationof thewatercolumnisolateshypoxicbottomwatersandpreventssurfacewatersfromre-oxygenatingtheentirewatercolumn. Stratificationreferstolayeringofthewatercolumnintodiscretewatermasseswithdifferentdensitiesduetovariationin temperatureand/orsalinity.Oftenstratificationindeadzonesresultsintwolayers:ashallowlow-densitysurfacelayerthatmaybe welloxygenatedfromdiffusionandwind-mixingofatmosphericoxygen,andasecond,deeper,high-densitylayerwhereorganic mattersettlesandoxygenisdepletedbymicrobialrespiration.

Stratification,andthepotentialforhypoxiatooccur,isstrongestwhenthereareprocessesthatestablishorreinforcethedensity differencebetweensurfaceandbottomwaters.Freshwaterinputsfromriversand/orraincancreateasurfacelensoffreshwater whichislessdensethandeeper,saltierwater.Thesunandwarmaircanalsoheatsurfacewatersandcontributetostratification. Processesthatpromotemixingofsurfaceandbottomwaterssuchasstrongwinds,tidalcurrents,andoceanswellcandisrupt stratification.

Flushingtimeandhorizontalmixing

Oxygenismorelikelytoreachastateofseveredepletioninbodiesofwaterthataresemienclosed.Openoceanwatersadjacentto coastalareasaregenerallywelloxygenatedduetolowerlevelsofproductivityandgreatermixingbywindandwaves.Limited

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AQUATICECOLOGY

AbundanceBiomassComparisonMethod

TheMethod

Applications

ProblemsandTheirSolutions

FurtherReading

AcidificationinAquaticSystems

OceanAcidification

AcidificationintheCoastalArea

AcidificationinFreshwaterEcosystems

EcologicalImpact

SocioeconomicalImpact

ResponseStrategiestoAquaticAcidification

FurtherReading

ConstructedWetlandsforWastewaterTreatment☆

FreeWaterSurfaceCWs

ConstructedWetlandsWithSubsurfaceFlow

HybridConstructedWetlands

Conclusions

FurtherReading

DeadZones:LowOxygeninCoastalWaters

Introduction

DeadZonesDeﬁned

HypoxiainOtherEcosystems

QuantifyingHypoxia

CausesofDeadZones

OxygenDepletion

Nutrients

LimitedOxygenReplenishment