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Resilience to shading influenced by differential allocation of biomass in Thalassia testudinum

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Title:
Resilience to shading influenced by differential allocation of biomass in Thalassia testudinum
Series Title:
Barry, S. C., Jacoby, C. A. and Frazer, T. K. (2018), Resilience to shading influenced by differential allocation of biomass in Thalassia testudinum. Limnol. Oceanogr. 00:00-00. doi:10.1002/lno.10810
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Barry, Savanna
Jacoby, Charles
Frazer, Thomas
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Limnology and Oceanography
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English
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Journal Article

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Abstract:
Seagrasses are marine plants with fully developed leaves, roots, and rhizomes and a high degree of phenotypic plasticity. In the shallow waters along Florida's central Gulf of Mexico coast, leaf morphology of the dominant seagrass, Thalassia testudinum, varies along a spatial gradient in concentrations of total phosphorus (TP) in the water column. We examined ratios of aboveground to belowground biomass (AG : BG) for T. testudinum along this gradient to determine if they varied consistently with TP. Ratios were positively correlated with TP, indicating T. testudinum allocated more carbon to leaf biomass relative to belowground biomass as TP increased. To determine if this variation in AG : BG influenced resilience to shading, we carried out an 8‐week, comparative shading experiment in three T. testudinum meadows that spanned the range of recorded ratios. The experiment showed that seagrasses employing a range of AG : BG strategies persisted for 5 weeks with ambient light reduced by ∼ 93%. Thalassia testudinum with intermediate AG : BG exhibited the least severe impacts and strongest recovery when compared to T. testudinum with either high or low AG : BG. Seagrasses with high AG : BG ratios showed the most severe responses and weakest recovery. These results suggest T. testudinum allocates biomass such that growth and survival are maximized under the local, long‐term nutrient regime, which affects the direction and magnitude of a response to a short‐term reduction in light availability. In addition, we suggest that AG : BG is an important metric to monitor in T. testudinum meadows because of its potential to identify areas of high and low resilience.
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Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Savanna Barry.

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Resiliencetoshadinginfluencedbydifferentialallocationofbiomass in ThalassiatestudinumSavannaC.Barry ,1,2*CharlesA.Jacoby,3ThomasK.Frazer2,41NatureCoastBiologicalStation,UniversityofFlorida,CedarKey,Florida2FisheriesandAquaticSciencesProgram,UniversityofFlorida,Gainesville,Florida3SoilandWaterSciencesDepartment,UniversityofFlorida,Gainesville,Florida4SchoolofNaturalResourcesandEnvironment,UniversityofFlorida,Gainesville,FloridaAbstractSeagrassesaremarineplantswithfullydevelopedleaves,roots,andrhizomesandahighdegreeofphenotypicplasticity.IntheshallowwatersalongFlorida’scentralGulfofMexicocoast,leafmorphologyofthe dominantseagrass, Thalassiatestudinum ,variesalongaspatialgradientinconcentrationsoftotalphosphorus (TP)inthewatercolumn.Weexaminedratiosofabovegroundtobelowgroundbiomass(AG:BG)for T.testudinum alongthisgradienttodetermineiftheyvariedconsistentlywithTP.Ratioswerepositivelycorrelated withTP,indicating T.testudinum allocatedmorecarbontoleafbiomassrelativetobelowgroundbiomassas TPincreased.TodetermineifthisvariationinAG:BGinuencedresiliencetoshading,wecarriedoutan8week,comparativeshadingexperimentinthree T.testudinum meadowsthatspannedtherangeofrecorded ratios.TheexperimentshowedthatseagrassesemployingarangeofAG:BGstrategiespersistedfor5weeks withambientlightreducedby 93%. Thalassiatestudinum withintermediateAG:BGexhibitedtheleast severeimpactsandstrongestrecoverywhencomparedto T.testudinum witheitherhighorlowAG:BG.SeagrasseswithhighAG:BGratiosshowedthemostsevereresponsesandweakestrecovery.Theseresultssuggest T.testudinum allocatesbiomasssuchthatgrowthandsurvivalaremaximizedunderthelocal,long-term nutrientregime,whichaffectsthedirectionandmagnitudeofaresponsetoashort-termreductioninlight availability.Inaddition,wesuggestthatAG:BGisanimportantmetrictomonitorin T.testudinum meadowsbecauseofitspotentialtoidentifyareasofhighandlowresilience. Seagrassesarevascularplantsthatcompetewellagainst phytoplanktonandotherbenthicmacrophytesinoligotrophicenvironmentsbecausetheycanacquirenutrientsfrom sedimentarysourcesviaextensiverootsystems(Duarte1995; TouchetteandBurkholder2000 a ).Alargevolumeofwork alsosuggeststhatbelowgroundtissuescontributetothe resilienceofseagrassesbecausetheystorecarbohydratesand nutrientsthatfuelphysiologicalprocessesduringperiodsof stress(Zieman1975;Ziemanetal.1984;Burkeetal.1996; LeeandDunton1996;TouchetteandBurkholder2000 a b ; Alcoverroetal.2001).Forexample,estuarineseagrasses, suchas Thalassiatestudinum ,canoffsetperiodsofreduced photosynthesisduetoshort-termreductionsinlightby mobilizingsugarsandstarchesstoredinrhizomes(Leeand Dunton1996,1997).However,productionandmaintenance oflargeamountsofrootandrhizomebiomasscanbemetabolicallyexpensive(FourqureanandZieman1991;Hemminga 1998)andincreasetheriskofexposuretotoxicsuldes (Pedersenetal.2004).Largeinvestmentsinbelowgroundbiomassarelessnecessaryif T.testudinum leavescanobtainsufcientnutrientsdirectlyfromthewatercolumn(Leeand Dunton1999 a ;TouchetteandBurkholder2000 a ;Grasetal. 2003).Thus,variationinsedimentandwatercolumnpoolsof nutrientscouldinuenceinvestmentinbelowgroundstructures(Hemminga1998;LeeandDunton1999 b ;Romeroetal. 2006;Leeetal.2007),withtheabilitytoalteraboveground tobelowgroundbiomassratios(AG:BG)inresponsetonutrientavailabilitybeinganimportantecophysiologicaladaptation.Overall,priorworkpresentsaparadoxicalrolefor belowgroundtissuesthatareconsideredbothan“assetanda *Correspondence:savanna.barry@u.edu AdditionalSupportingInformationmaybefoundintheonlineversion ofthisarticle. ThisisanopenaccessarticleunderthetermsoftheCreativeCommons Attribution-NonCommercial-NoDerivsLicense,whichpermitsuseand distributioninanymedium,providedtheoriginalworkisproperlycited, theuseisnon-commercialandnomodicationsoradaptationsare made.1 LIMNOLOGYandOCEANOGRAPHY Limnol.Oceanogr. 00,2018,00–00VC2018TheAuthorsLimnologyandOceanographypublishedbyWileyPeriodicals,Inc. onbehalfofAssociationfortheSciencesofLimnologyandOceanography doi:10.1002/lno.10810

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burden,”anditoffersnoinsightsintochangesinresilience withvariationinAG:BG(Hemminga1998). Theseeminglyparadoxicalroleofrootsandrhizomes mayarisefromthefactthatmanyshadingstudiesdonot accountformorphologicalplasticityofseagrasses,whichis extensivefor T.testudinum (LeeandDunton1999 b ;Hackney andDurako2004;Brickeretal.2011;Barryetal.2017).In addition,manystudiesdonotplaceresultsinthecontextof long-termvariationsinwaterqualityorthelightenvironment experiencedbyseagrasses.Forexample,Yaakubetal.(2014) foundthat Halophilaovalis growinginturbidconditionswas lessresilienttoadditionalshort-termshading,whereasMaxwelletal.(2014)foundtheoppositeresultfor Zosteramuelleri thosegrowinginlessturbidconditionswerelessresilientto shading.Incombination,theseresultssuggestthatresilience toacutelightreductionmaybeinuencedbythehistoryof lightavailabilityandtheeffectagivenhistorycreatesmaydifferamongspecies.Therefore,thereisaneedtoinvestigate resiliencetodisturbancesacrossarangeofrelativelystable environmentalconditionsandthevarietyofAG:BGmorphologiesexhibitedbyaquaticplantsexposedtosuchgradients(Sculthorpe1967;Perezetal.1994;Barrat-Segretain 2001;Peraltaetal.2002,2005;Puijalonetal.2005;Maxwell etal.2014;Thormaretal.2016;Barryetal.2017). TheshallowwatersalongthecentralGulfcoastofpeninsularFloridaarecharacterizedbyanaturalsouthtonorth increaseintotalphosphorus(TP)concentrationsinthewater column,causingarelatedgradientinwatercolumnproductivity,asmeasuredbychlorophyll a (Chl a )concentrations (Frazeretal.1998;Jacobyetal.2012). Thalassiatestudinum shootsexhibitpersistentandnotablemorphologicalvariation inconcertwiththespatialTPgradient,suchthatleafarea shoot2 1increasesbyanorderofmagnitudeandshootheight increasesbyafactorof 5alongthegradient(Barryetal. 2017).ThisregionofFloridarepresentsanidealnaturallaboratoryforinvestigatingdifferencesinresilience,i.e.,theabilitytoresistandrecoverfromadisturbance,acrossdifferent seagrassmorphologiesobservedalonganutrientgradient.We hypothesizedthattheobserveddifferencesinleafmorphology alongthisgradientinTP(Table1)wouldbeaccompaniedby patternsinbelowgroundbiomass,tocreatevariationin AG:BG.WefurtherhypothesizedthatseagrasseswithdifferentAG:BGmorphologieswouldexhibitdifferentdegreesof resiliencetoshading.Weinvestigatedthesetwohypotheses bycollectingcorestodocumentabovegroundandbelowgroundbiomassofseagrassbeforeandattheconclusionofa 5-weekperiodofshadingin T testudinum meadowsspanning theTPgradient.Inaddition,growthrates,numberofleaves, leafwidths,leaflengths,andleafareashoot2 1weremeasured weeklyduringthe5-weekshadingand3-weekrecoveryperiodstodocumentandcomparethenatureoftheresponsesof T.testudinum shootswithdifferingallocationtoaboveground andbelowgroundbiomass.MethodsandmaterialsStudysystem TheshallowwatersalongthecentralGulfcoastofpeninsularFloridaprovideafavorableenvironmentforthedevelopmentofseagrassmeadows,andtheycurrentlysupport oneofthelargestcontiguousmeadowsinNorthAmerica (Haleetal.2004;Mattsonetal.2007).Extensivemonitoring ofwaterquality(15–18yr,monthlysampling)andseagrasses (4yr,bi-annualsampling)alongthiscoastrevealedpersistentspatialgradientsinboththeconcentrationofTPinsurfacewatersandleafmorphologyof T testudinum ,the dominantseagrass(Frazeretal.1998;Jacobyetal.2012; Barryetal.2017).Inaddition,previousworkhasshownthat T.testudinum andotherseagrassesreceivesufcientlightto supporttheirmetabolisminallthecoastalsystems(Choice etal.2014). Samplingstationsforthepresentworkspannedthefull extentofthespatialgradientinTPandleafmorphology, withstationslocatedinthecoastalwatersadjacenttothe WeekiWachee,Chassahowitzka,Homosassa,Crystal,and Waccasassarivers(Fig.1;Table1).Waterdepthsinallsystemswere1–2m,andmeansalinitieswere24 & Cores Toquantifyabovegroundtobelowgroundbiomassratios (AG:BG),weextractedthree10-cmdiameter 3 20-cmdeep coresfrommonospecic T.testudinum meadowsattwostationsineachofthevesystemsalongtheTPgradient ( n 5 30cores)inAugust2013(Fig.1).Monospecicstands of T.testudinum wereselectedtoavoidpotentiallyconfoundingeffectsofinterspecicrootcompetition(Duarte etal.1998).Coreswereplacedoniceimmediatelyaftercollectionandstoredfrozenuntiltheywereprocessedinthe laboratory. Table1.Mean 6 SDfortotalwatercolumnphosphorus(TP), T.testudinum leaflengths,and T.testudinum leafwidthsfor eachofveestuarinesystemsalongtheTPgradient.Valuesfor TParethemeansofmonthlysamplescollectedfromJanuary 2008throughDecember2013.Valuesforleafmorphologiesare meansofsamplescollectedfromthesamestationsastheTP datainMay/JuneandAugust/Septemberof2010through 2013.Estuarinesystemsareorganizedfromsouthtonorth. System Mean TP ( l gL2 1)SD Mean leaflength (mm)SD Mean leafwidth (mm)SDWeekiWachee7.2 6 2.875.4 6 46.33.0 6 0.6 Chassahowitzka8.3 6 3.989.4 6 56.13.3 6 0.5 Homosassa10.8 6 6.7152.8 6 102.64.4 6 1.2 Crystal15.6 6 7.2195.6 6 105.54.9 6 0.9 Waccasassa24.2 6 16.6301.6 6 171.84.8 6 0.8 ModiedfromBarryetal.(2017).Barryetal ResiliencetoshadingvarieswithAG:BGratio 2

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Inthelaboratory,coreswereplacedona1-mmmesh screenandrinsedthoroughlywithfreshwatertoremove sediments.Live T.testudinum biomasswasseparatedinto root,rhizome,andleaffractions,whichsubsequentlywere driedforatleast72hat65 8 C.Rootsandrhizomeswereconsideredliveiftheywereattachedtolivingshortshootsorif theywerefullyintact,lightcolored,andrm.Thedrymass ofleaftissuedividedbythesumofthedrymassofliveroot andrhizometissuesyieldedAG:BG.TrendsinAG:BG wereidentiedusingleast-squaresregressionagainstthe averageTPconcentrationinthewatercolumnforthe24 monthsprecedingthecollectionofthecores(Frazeretal. 1998;Jacobyetal.2012).The24-monthmovingaveragewas selectedbecause T.testudinum isalong-livedspeciesthathas beenshowntointegrateenvironmentalconditionsover suchtimescales(Fourqureanetal.2005)andadjustitsleaf morphologyonthescaleofapproximately2yr(vanTussenbroek1996). Shadingexperiment ToexploretheinuenceofvariationinAG:BGonresiliencetoshading,weperformedaninsitu,disturbancerecoveryexperiment,with T.testudinum growthandleaf morphologymonitoredduring5weeksofshadingand3 weeksofrecoveryfromJune2014toAugust2014.The experimentwasdesignedtocomparetherelativeresponses toshadingby T.testudinum withdifferingAG:BGmorphologies,withresponsesdenedbytemporaltrajectoriesforvariousmetricsduringtheperiodsofstressandrecovery.Six 1.5-m2plotswereestablishedatninestations(Fig.1)along theTPgradient( n 5 54plots).Threestationswerelocated ineachofthreesystemstocapturetheTPgradient,i.e.,low TP(WeekiWachee[WEE]),intermediateTP(Homosassa [HOM]),andhighTP(Waccasassa[WAC]).Ateachstation, threeplotswereassignedhaphazardlytotheshadingtreatmentandtheremainingthreeweredesignatedascontrol plots.Plasticframescoveredwithfabricscreen(75%reductioninambientlight)wereanchoredabovetheappropriate plots,andidenticalframeswithoutscreenwereplacedat controlplots.Frameswithscreenwereopenonallfoursides tominimizerestrictionofwaterow.Onlyshootswithin theinnermost1.0m2ofthe1.5-m2areaweresampledto reducetheinuenceofsunlightenteringthroughtheopen sidesofthescreens.A75%lightreductionwasselected becauseitisbelowthecompensationirradiancefor T.testudinum inthisregion(Callejaetal.2006),andlow-light eventsofsimilarmagnitudehavebeenrecordedfollowing rainfallevents(Frazeretal.2001).Weaimedtocreateconditionsunderwhich T.testudinum wouldmobilizecarbon reservestoavoidanegativecarbonbalance(Tomaskoand Dawes1989;LeeandDunton1996,1997),butnotperish duetotoxiceffectsofsuldeaccumulatinginthesediment (Callejaetal.2006).Wedidnotseverrhizomesalongthe perimeteroftheplotsbecausepreviousworkhasshownseveringrhizomesin T.testudinum hasfeweffectsonkey responsevariablesandtheprocesscanintroduceundesirable artifacts(Ibarra-Obandoetal.2005).Onaweeklybasis, screenswerescrubbedtoremovefoulingandafterward,we quantiedphotosyntheticallyactiveradiation( l Em2 2s2 1) withinshadeandcontrolplotsusingadataloggerconnected totwoquantumlightsensors(Li-CorInstruments,Lincoln, Nebraska)thatsimultaneouslymeasuredsurfaceirradiance (abovethewater)andbottomirradianceinexperimental plots. Aspartoftheexperiment,wetrackedchangesinAG:BG andshootdensities.Coresofseagrassbiomass(10-cmdiameter 3 20-cmdeep)werecollectedfromthecenterofallplots attheconclusionofshading(week5).Coreswereprocessed likethosecollectedtocharacterizeAG:BG.Shootdensityat theconclusionoftheexperimentwascalculatedbyaveragingshootcountsfromtwotossesofa0.0625-m2quadratin eachplot. Fig.1.SamplingstationsalongthecentralGulfcoastofpeninsularFlorida. Graycirclesindicateseagrassbiomasscoringstationsandredtrianglesrepresentexperimentalshadingstations.Watercolumnphosphorusconcentration(TP)ispersistentlylowinthesouthandincreasestowardthenorthof thestudyregion.Barryetal ResiliencetoshadingvarieswithAG:BGratio 3

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Inaddition,wemonitoredshootgrowth(mm2shoot2 1d2 1),numberofleaves,leafwidth(mm),leaflength(mm), andleafarea(mm2shoot2 1)insideexperimentalplotsfor theentire8-weekperiod(5weeksshading,3weeksrecovery).Onceaweek,sevenshootsineachplotweremarked usingstandardleafmarkingtechniquesthatinvolvedpunchingaholejustaboveeachshoot’sbasalmeristem(Zieman 1974)andaggingthemforlatercollectionbygentlyconnectingadrinkingstrawtothebaseoftheshootwitha smallcabletie.Shootsmarkedinthepreviousweek(6–10d ofgrowth)werecollectedandfrozenuntilprocessinginthe laboratory.Duetoinclementweather,wewerenotableto collectshootsfromHOM(intermediateAG:BG)during week4oftheexperiment. Inthelaboratory, T.testudinum leaveswerescrapedthoroughlytoremoveepiphyticmaterial,andwidthsandnumbersofleaveswererecordedforeachshoot.Foreachleaf withahole,oldgrowth(materialabovethehole)andnew growth(materialbelowthehole)wereseparatedbycutting throughtheholewitharazorblade,andthelengthsand widthsoftheresultingpiecesweremeasuredtothenearest mm.Allunmarkedleaveswereconsiderednewgrowth.Surfaceareaofnewmaterialdividedbythenumberofdays betweenmarkingandharvestingrepresentedgrowthrates for T.testudinum shoots(mm2shoot2 1d2 1).Thewidthof thesecondyoungestleaf(theyoungerofthetwoleavesadjacenttothecentral,youngestleaf)ofeachshootwasusedto evaluatechangesinleafwidthovertime.Totallength(old length 1 newlength)ofallleavesonashootwereaveraged toproduceestimatesofmeanleaflength.Thesurfaceareas ofoldandnewleavesweresummedtorepresentthetotal leafareashoot2 1ateachtimepoint. Lightdatacollectedduringtherst5weeksoftheexperiment(shadingperiod)wereaveragedacrossweekstoyield estimatesoftheactualpercentagereductioninambientirradiance.Averagepercentagesofambientlightwereanalyzed withaKruskal-Wallistesttodetermineifsignicantdifferencesinlightreductionexistedamongthesystemsthat werestudied. EstimatesofAG:BGandshootdensitycollectedinweek 5wereanalyzedwithtwo-wayanalysesofvariance (ANOVAs),whereintreatmentandsystemweretreatedas xedmaineffects.WhenANOVAsrevealedsignicantdifferences,multiplepairwisecomparisonswereperformedusing Tukey’shonestlysquareddifferences(HSD)test. Bottomirradiance,shootgrowthrate,numberofleaves, leafwidth,leaflength,andleafareashoot2 1wereinitially analyzedusingrepeatedmeasuresmultivariateanalysisof variance(MANOVA)intheRpackage{car}(FoxandWeisberg2011;RCoreDevelopmentTeam2014),withsuccessive weekstreatedasordered,dependentvariables.Whenoverall MANOVAsindicatedsignicantmaineffects,datawerefurtherexploredusinghierarchicalmodeling(i.e.,growthcurve analysis,Mirman2014)usingtheRpackage{lme4}(Bates etal.2014;RCoreDevelopmentTeam2014).Thisanalysis enabledustocomparetheshapeandmagnitudeofchanges inthelightenvironmentandseagrassresponsesinshading vs.controlplotsovertime.ThiscombinedMANOVA-growth curveanalysisstrategyisrecommendedforrepeatedmeasuresdata(TabachinkandFidell1983;Davis2002;Quinn andKeough2002).Weappliedsecond-order,orthogonal, polynomialmodelstothedatabecauseweexpectedthe removalofshadingduringweek5wouldcauseasingle inectionpointintheresponses.Allcandidatemodels, therefore,containedalinearandaquadratictermfortime. Fixedeffectsandinteractionswereaddedsuccessively,and theeffectontwasevaluatedbyexaminingAkaike’sinformationcriterion(AIC)valuesandperforminglog-likelihood ratiotestsagainstcandidatemodelswithfewerpredictors. Randomeffectsofsamplingstationonlinearandquadratic termsfortimewereincludedinallmodels.Modeltwas takentobeimprovedsignicantlywhenlog-likelihoodratio testsreturneda v2valuewith p < 0.05andAICwasminimized.Themodelthatmaximizedtandminimizedcomplexitywaschosenasthebestmodel.ControlsatHOM (intermediateAG:BG,noshading)representedthebase condition,andparameterswereestimatedfortheeffectsof treatment(shadeorcontrol)andsystem(WEE 5 low AG:BG,WAC 5 highAG:BG).Signicanceofindividual parametersintheselectedmodelwasassessedusingthenormalapproximation,computedwithRpackage{lmerTest} (Kuznetsovaetal.2014;RCoreDevelopmentTeam2014) andaSatterthwaiteestimationofdegreesoffreedom. Assumptionsforallanalyseswerecheckedanddatatransformationwasappliedwhereappropriate.ResultsAG:BGratios Abovegroundtobelowgroundbiomassratios(AG:BG) weresignicantlycorrelatedwithTPaveragedovertheprevious24months( p 5 0.001, R25 0.75,df 5 8).Thus, T.testudinum allocatedmorebiomasstoleavesthantorootsand rhizomesasphosphorusbecamemoreavailableinthewater column(Fig.2). Giventhisnding,theWeekiWachee(WEE),Homosassa (HOM),andWaccasassa(WAC)systemswereselectedtorepresentlow(0.41),intermediate(0.88),andhigh(1.60) AG:BGratios,respectively.Theshadingexperimentwas conductedinthesethreeestuarinesystems. Shadingexperiment Overallmeanreductioninambientirradiancewas 93% duringthe5-weekshadingperiod,amoreintensereduction thanourtargeted75%.AKruskal-Wallistestshowedthat therewerenosignicantdifferencesinambientlightreductionamongthesystems( p 5 0.733,df 5 2),indicatingseagrassesacrossthestudyregionexperiencedsimilarrelative reductionsinavailablelight.TheoverallMANOVAfor Barryetal ResiliencetoshadingvarieswithAG:BGratio 4

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bottomirradiancefurthershowedthatthereductioninirradiancedidnotdiffersignicantlyamongthesystems(Table 2).TheeffectoftreatmentwasmarginalintheoverallMANOVA,likelybecausethelast3weeksofirradiancedatawere nearlyidenticalbetweenshadeandcontrolplots.Inaddition,theirradianceMANOVAlikelyhadlowerpower becausefewerreplicatemeasurementswerecollectedthan forleafmorphologyparameters.Clearly,an 93%reduction inambientlightwasbiologicallymeaningfulandweproceededwithhierarchicalmodelingforirradiancedespitethe lackofastatisticallysignicanttreatmenteffect.Afully parameterized,secondorder,polynomialwasselectedasthe bestttingmodel(Table3).Therewasasignicanteffectof timeandasignicanttime-by-treatmentinteraction(SupportingInformationTableS1),bothdrivenbytheremoval ofthescreensinweek5.Shadingbecamemoreintensefrom weeks1to5(Fig.3),likelyduetoincreasingfoulingofthe meshthatwasnotremovedbyscrubbing.Irradiancesin shadedplotsconvergedwithvaluesforcontrolplotsupon removalofscreens,and,asexpected,theavailabilityoflight waseffectivelyidenticalforallplotsinweeks6–8(Fig.3). Thalassiatestudinum shootspersistedthroughouttheentire experimentinallplots,despitetheinitialandincreasing reductionsinlightavailabilityduringthe5-weekshading period. Datafromexperimentalplotsinweek5indicatedthat AG:BGwassignicantlydifferentamongsystems( p < 0.001, F2,475 143.01,Tables4,5).Tukey’sHSDpost-hoccomparisonsindicatedthatAG:BGgenerallyincreasedfromsouthto north,asexpected,butWEEandHOMcontrolplotswerenot statisticallydifferentinweek5.Littleevidencewasfoundfor differencesinAG:BGbetweenshadeandcontroltreatments byweek5( p 5 0.073, F1,475 3.37;Table4).However,there wasahighlysignicantinteractionterm( p < 0.001, F2,475 9.40),indicatingthattheresponsetotreatmentdifferedbysystem.Tukey’sHSDpairwisecomparisonsrevealed thatAG:BGinshadevs.controltreatmentswassignicantly differentatWAC( p 5 0.001,AG:BGshade < AG:BGcontrol), butnotatHOMorWEE( p 5 0.684and p 5 0.999,respectively).OverallmeanvaluesforAG:BGobtainedinweek5of theexperimentwerelowerthanthoseobtainedbeforethe experiment(Fig.2;Table5),likelybecausethecollectionof markedshootsthroughouttheexperimentreducedtheabovegroundbiomassacrossallplots.Despitethisdifferencein absolutevalues,thegeneralpatternofatleastadoublingin AG:BGvaluesbetweenanyestuarinesystemandthenextsystemtothenorthwaspreservedinexperimentalplots. Shootdensitydifferedsignicantlyacrosssystems( p < 0.001, F1,485 36.51),butitdidnotexhibitanysignicantdeclinesin responsetoshadingwithinasystem( p 5 0.508, F1,485 0.444; Table4).Tukey’sHSDpost-hoccomparisonsshowedthatshoot densitywasloweroverallinWAC(Table5). RepeatedmeasuresMANOVAsyieldedsignicantresults forbothtreatmentandsystemforgrowthrate,numberof leaves,leafwidth,andleafareashoot2 1(Table2).Forleaf length,systemwasfoundtobeasignicantmaineffect (Table2).Modelselectionproceduresidentiedtherst ordermodelasthebesttforleafwidth,leaflength,and leafareashoot2 1andthesecondorderpolynomialmodelas thebesttforgrowthrateandnumberofleaves(Table3). Dailygrowthratesdeclinedacrossallplotsduringthe8weekstudy,withasignicanttime-by-systeminteraction termhighlightingthefactthatthiseffectwasslightlymore pronouncedforWAC(Fig.4a).ShootsatWACandWEEhad signicantlyhigherandloweroverallgrowthratesthanHOM shoots,respectively(Fig.4a).Thisresultisreectedinthe abovegroundmorphologyofshootsineachsystem,anditis consistentwithpreviousworkthatshowedshootgrowthrate washighlycorrelatedwithTPinthisregion(Barryetal.2017; Table1).Asaresult,responsestoshadingbetweensystems wereevaluatedbycomparingtherelativeseverityofresponses inshadedplotstocontrolswithineachsystembecauseinitial abovegroundmorphologiesandpotentialforgrowthdiffered acrosstheregion.Forexample,aresponsetoshadinginone systemwasconsideredmoreseverethanresponsesinother systemswhenthechangeinagivenmetricrelativetothe appropriatecontrolwasmorepronouncedandshowedless reboundduringtheperiodofrecovery. ShootsinshadedplotsatbothWEEandWACshoweda signicantdeclineingrowthratescomparedtocontrolplots, whereasshootsatHOMdidnot,resultinginthesignicant Fig.2.Linearregressionofabovegroundtobelowgroundbiomassratio (AG:BG)againsttotalwatercolumnphosphorusconcentration( l gL2 1TP,24-monthmovingaverage).Eachpointrepresentsthemeanvalue calculatedfrom10stationswithineachsystemalongtheTPgradient. DarkerbluesindicatemorenorthernlatitudesandhigherTP.Errorbars representstandarderrors.Barryetal ResiliencetoshadingvarieswithAG:BGratio 5

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Table2.OverallrepeatedmeasuresMANOVAresultsforirradianceand T.testudinum growthandmorphology.Signicancelevels denotedbyasterisks:*** 5 < 0.001;** 5 < 0.01;* 5 < 0.05;. 5 < 0.1;blank 5 > 0.1;colons 5 interactionsbetweenfactors;df 5 degreesoffreedom;Approx. F 5 approximate F -ratio. dfErrordfPillaiÂ’straceApprox. FpIrradiance Intercept121.00499.720.002** Treatment120.8612.100.074. System120.745.780.138 Treatment:system120.010.020.913 Shootgrowthrate Intercept1321.0010,604.35 < 0.001*** Treatment1320.5741.75 < 0.001*** System1320.93455.00 < 0.001*** Treatment:system1320.218.640.006** Time7260.9138.13 < 0.001*** Treatment:time7260.717.91 < 0.001*** System:time7260.564.730.002** Treatment:system:time7260.463.130.016* Numberofleaves Intercept1320.995252.99 < 0.001*** Treatment1320.5945.32 < 0.001*** System1320.176.780.014* Treatment:system1320.124.420.044* Time7260.9035.21 < 0.001*** Treatment:time7260.544.410.002** System:time7260.210.970.471 Treatment:system:time7260.372.170.072. Leafwidth Intercept1320.994932.69 < 0.001*** Treatment1320.2410.260.003** System1320.79122.32 < 0.001*** Treatment:system1320.051.550.223 Time7260.463.120.016* Treatment:time7260.513.820.006** System:time7260.301.560.193 Treatment:system:time7260.291.520.205 Leaflength Intercept1321.0029,135.20 < 0.001*** Treatment1320.000.100.733 System1320.95612.00 < 0.001*** Treatment:system1320.010.200.625 Time7260.372.200.066. Treatment:time7260.200.900.515 System:time7260.200.900.487 Treatment:system:time7260.291.500.217 Leafarea Intercept1321.0035,149.18 < 0.001*** Treatment1320.249.950.003** System1320.93439.04 < 0.001*** Treatment:system1320.072.270.142 Time7260.667.23 < 0.001***Barryetal ResiliencetoshadingvarieswithAG:BGratio 6

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treatment-by-systeminteraction.Therewasasignicant interactionoftreatmentwiththequadratictimeterm,indicatingthatshootsinshadedplotsshowedasignicantcurvilinearresponsenotobservedforshootsincontrolplots (Fig.4a).Thisnonlinearresponsebeganwhenscreenswere removedinweek5.Themostpersistentresponsetoshading wasobservedatWAC,withshadedshootsstillgrowingmore slowlythancontrolsafter3weeksofrecovery,whereas shootsinshadedplotsintheothertwosystemswereeither growingasfast(WEE)orfaster(HOM)thancontrolshoots inweek8(Fig.4a). Numberofleavesshoot2 1wasinitiallysimilarinallplots, andthismetricdeclinedsignicantlyacrossallsystemsand treatments,withthedeclinebeingmorepronouncedfor Table2. Continued dfErrordfPillai’straceApprox. FpTreatment:time7260.463.130.015* System:time7260.291.540.199 Treatment:system:time7260.463.180.014* Table3.Growthcurveanalysismodelselectionresultsforirradianceand T.testudinum growthandmorphology.Signicancelevels denotedbyasterisks:*** 5 < 0.001;** 5 < 0.001;* 5 < 0.01;. 5 < 0.05;blank 5 > 0.1;  5 selectedmodel;df 5 degreesoffreedom;AIC 5 Akaike'sinformationcriterion;LogLik 5 negativelog-likelihood; v25 chi-squaredstatisticfornegativelog-likelihoodratio test. ModeldfAICLogLik X2X2df pIrradiance Base10477.99 2 229 Allxed13426.22 2 200.157.773 < 0.001*** Allxedwithalllineartimeinteractions20405.54 2 182.834.687 < 0.001*** Allxedwithallinteractions25401.34 2 175314.2050.014*,  Shootgrowthrate Base10 2 287.68153.84 Allxed13 2 378.23202.1196.543 < 0.001*** Allxedwithalllineartimeinteractions20 2 472.99256.50108.777 < 0.001*** Allxedwithallinteractions25 2 495.40272.7032.405 < 0.001***,  Numberofleaves Base10587.04 2 283.52 Allxed13474.48 2 224.24118.563 < 0.001*** Allxedwithalllineartimeinteractions20429.93 2 194.9658.557 < 0.001*** Allxedwithallinteractions25419.86 2 184.9320.0650.001**,  Leafwidth Base10595.99 2 288.00 Allxed13515.06 2 244.5386.933 < 0.001*** Allxedwithalllineartimeinteractions20490.28 2 225.1438.787 < 0.001***,  Allxedwithallinteractions25496.67 2 223.333.6150.607 Leaflength Base10 2 793.86406.93 Allxed13 2 835.77430.8847.903 < 0.001*** Allxedwithalllineartimeinteractions20 2 866.71453.3644.957 < 0.001***,  Allxedwithallinteractions25 2 860.85455.424.1350.531 Leafarea Base10 2 442.36231.18 Allxed13 2 482.88254.4446.523 < 0.001*** Allxedwithalllineartimeinteractions20 2 556.85298.4387.977 < 0.001***,  Allxedwithallinteractions25 2550.41300.213.5650.614Barryetal ResiliencetoshadingvarieswithAG:BGratio 7

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shadedshoots(Fig.4b;SupportingInformationTableS1). Numberofleavesinshadedplotsshowedasignicantquadraticresponse,whereascontrolplotsrespondedinamore linearfashion(Fig.4b).Therewasasignicantsystem-bytreatmentinteractionterm,indicatingdifferencesinthe magnitudeoftheresponseofshootsacrosstheregion(SupportingInformationTableS1). Meanleafwidthswereinitiallysimilarattheoutsetofthe experimentatWACandHOM,butloweratWEE(Fig.4c;SupportingInformationTableS1).Leafwidthincreasedovertime inHOMcontrolplots,remainedfairlyconstantovertimein WEEandWACcontrolplots,anddecreasedovertimeinall shadedplots(Fig.4c),whichgeneratedsignicanteffectsfor time,timebysystem,andtimebytreatment(SupportingInformationTableS1).TheoverallreductioninleafwidthwassignicantforshootsatbothWACandHOM.Themostsevere relativereductioninleafwidthwasobservedinWACshaded plots(Fig.4c).Thelackofasignicantquadraticresponseindicatedthatleafwidthinshadedplotsgenerallycontinuedona negativetrajectoryduringthe3-weekrecoveryperiod,although leafwidthinHOMshadedplotsincreasedinweek8(Fig.4c). Meanleaflengthwasinitiallydifferentacrosssystemsin allplots,withshootsatWACandWEEhavingsignicantly longerandshortermeanleaflengthsthanHOMshoots, respectively(Fig.4d;SupportingInformationTableS1).Relativetocontrolsinthesamesystem,meanleaflength increasedprimarilyinHOMshadedplots,withleaflengths remainingsimilarinallplotsthroughouttheexperimentat WEEandWACasconrmedbysignicanttreatment-bysysteminteractionsinthegrowthcurveanalyses(Fig.4d; SupportingInformationTableS1). Fig.3.Naturallogofbottomirradianceovertimeforshadeandcontrolplotsineachsystem.Linesrepresent2ndorderpolynomialmodel predictions.Darkerbluesrepresentmorenorthernlatitudesandhigher TP.Errorbarsrepresentstandarderrors. Table4.Two-wayANOVAresultsforabovegroundtobelowgroundbiomassratio(AG:BG)andshootdensities(shootsm2 2)in experimentalplotsattheconclusionoftheshadingperiod(week5).Signicancelevelsdenotedbyasterisks:*** 5 < 0.001;** 5 < 0.001;* 5 < 0.01;. 5 < 0.05;blank 5 > 0.1;colons 5 interactionsbetweenfactors;df 5 degreesoffreedom. dfSumofsquaresMeansquare F value pAG:BG System28.104.05143.01 < 0.001*** Treatment10.100.103.370.073. System:treatment20.530.279.40 < 0.001*** Residuals471.330.03 Shootdensity System22.201.1036.51 < 0.001*** Treatment10.010.010.440.508 System:treatment20.050.030.880.420 Residuals481.450.03 Table5.Mean 6 SDforabovegroundtobelowgroundbiomassratio(AG:BG)andmeanshootdensity(shootsm2 2)inexperimentalplotsattheconclusionoftheshadingperiod(week5).Systemsarelistedfromsouthtonorth.SignicantdifferencesamongsystemsandtreatmentsfoundwithTukey'sHSDtestareindicatedbylettergroupings. SystemTreatmentAG:BGSDShootdensitySDWeekiWacheeControl0.090.03ab656199a Shade0.090.02a796102a HomosassaControl0.160.06ab757282a Shade0.210.07b678151a WaccasassaControl1.300.58c296125b Shade0.540.22d244518bBarryetal ResiliencetoshadingvarieswithAG:BGratio 8

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Finally,leafareashoot2 1differedacrosssystemsatthe outsetoftheexperiment(Fig.4e;SupportingInformation TableS1),withWAChavingsignicantlylargershootsthan HOM,which,inturn,hadlargershootsthanWEE.Leafarea shoot2 1inshadedplotsinallthreesystemsdivergedfrom controlvalues,whichremainedfairlyconstantforthedurationoftheexperiment.Leafareashoot2 1declinedsignicantlyinshadedplotsinbothWEEandWAC,withthe effectbeingmoresevereinWACplots(Fig.4e).Conversely, leafareashoot2 1increasedsignicantlyinHOMshaded Fig.4.Responsesovertimefor( a )log10growthrate(mm2shoot2 1d2 1),( b )numberofleaves,( c )leafwidth(mm),( d )log10leaflength(mm), and( e )log10leafarea(mm2shoot2 1)inshadeandcontrolplotsineachsystem.Linesrepresent2ndorderpolynomialmodeltsinpanels( a )and ( b ),and1storderpolynomialmodeltsinpanels( c e );darkerbluesrepresentmorenorthernlatitudesandhigherTP.Errorbarsrepresentstandard errors.Barryetal ResiliencetoshadingvarieswithAG:BGratio 9

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plots(Fig.4e),resultinginasignicanttreatment-by-system interaction(SupportingInformationTableS1).Again,the lackofaquadraticresponseshowedthatshadedWEEand WACshootscontinuedtoshowdeclinesinleafareashoot2 1, whereasshadedHOMshootscontinuedtohavehigherleaf areathancontrolshootsthroughouttheentireexperiment (Fig.4e).DiscussionWepresentevidencethatbuildsonpreviousworktosuggestthatAG:BGfor T.testudinum variesinconjunction withconcentrationsofphosphorusinthewatercolumn. Thalassiatestudinum ,likemanyseagrassspecies,canacquire nutrientsfromsedimentporewaterviaroottissuesorfrom thewatercolumnviauptakethroughitsleaves(Touchette andBurkholder2000 a ;Grasetal.2003).Nutrientratiosin T testudinum leavesindicatethatphosphoruslimitationis presentatthesouthern(lowTP)endofthegradientand decreasesinseveritynorthward(highTP,Barryetal.2017). Therefore,theobservedalterationsinAG:BGratiolikely occurredinrelationtoalleviationofphosphoruslimitation alongthegradient.Asphosphorusbecomesmoreavailable inthewatercolumn,theneedtoacquirenutrientsfromsedimentporewatersshoulddecline,andpressuretogather morelightshouldincreaseduetoincreasedChl a concentrationsthatcorrelatewithincreasednutrientsinthewatercolumn(Frazeretal.2002;Hoyeretal.2002;Jacobyetal. 2012).Ourobservationsareconsistentwiththehypothesis thatseagrassesalterallocationofbiomassinresponseto environmentalfactors,suchaslightlevelsandnutrient availabilityinsedimentaryandwatercolumnpools(Perez etal.1994;Hemminga1998;LeeandDunton1999 a b ,2000; Romeroetal.2006;Leeetal.2007). InlightofthepronouncednaturalgradientinAG:BGin ourstudysystem,wedesignedanexperimentalmanipulationtoexamineifseagrassmeadowsindifferentlocations alongthegradientwouldshowsignicantvariationinresiliencetoshading.TheAG:BGgradientobservedincores takenin2013persistedthroughouttheshadingexperiment conductedin2014,althoughtheexperimentalmanipulation likelyhadaneffectontheabsoluteAG:BGvaluesthrough theremovalofabovegroundbiomass(shoots)formeasurement.Theoveralldeclineingrowthrateandnumberof leavesobservedinallexperimentalplotslikelywasrelatedto seasonalsenescencebecauseourexperimentoccurredin mid-summertolate-summer,whenseagrassstandingstock andproductionhavebeenshowntodeclineinsimilarsystems(Zieman1975;BarberandBehrens1985).Seagrasses withallthreeAG:BGmorphologiespersistedthroughout theexperiment,illustratingagainthegeneralabilityof T. testudinum tosurviveshort-termandmedium-termshading disturbance(CzernyandDunton1995;KraemerandHanisak 2000;MajorandDunton2002;Callejaetal.2006;Lamote andDunton2006).Ourintentwastocomparetheformof theresponsetostressandthetrajectoryofrecoveryamong differingAG:BGmorphologiesratherthandenethe lengthoftimerequiredforrecovery;however,thisbodyof evidenceregardingresiliencetoshadingin T.testudinum leadsustoexpectthatshadedplotsinallthreeestuarinesystemswouldmakeafullrecoverygivensufcienttime. Importantly,seagrasseswithdifferentinitialAG:BG exhibitedresponsesthatdifferedindirectionandmagnitude whenconfrontedwithrelativelysevere( 93%),short-term (5-week)reductionsinambientirradiance.InhighAG:BG meadows(WAC),shadingcausedsignicantreductionin AG:BG,shootgrowthrate,numberofleaves,leafwidth, andleafareashoot2 1.SeagrasseswithlowAG:BG(WEE) showedfewerandlesssevereresponsestoshading,withsignicantreductionsinshootgrowthrate,numberofleaves, andleafareashoot2 1.SeagrasseswithintermediateAG:BG (HOM)showedthefewestandleastseverenegative responsestoshading,withsignicantreductionsinleaf widthandnumberofleaves,anincreaseinleaflengthand leafareashoot2 1,andnochangeinAG:BG.Furthermore, thoseseagrassesinintermediateAG:BGmeadows(HOM) showedrapidrecoveryfollowingremovalofshading, approachingorsurpassingcontrolvaluesbyweek8formost parameters.InlowAG:BGbeds(WEE),seagrassespartially recoveredafterscreenswereremoved,withgrowthratesin shadedplotsequaltocontrolsupontheconclusionofthe experiment.However,numberofleavesandleafarea shoot2 1hadnotrecoveredtocontrollevelbytheendofthe experimentinlowAG:BGplots.SeagrassesinhighAG:BG meadows(WAC)showedtheweakestrecovery,eithercontinuingadownwardtrajectoryoronlyshowingpartialrecoveryrelativetocontrolsbyweek8.Shootdensitydidnot showasignicantresponsetoshading,perhapsbecausethe durationofshadingwasnotlongenoughtoelicitadetectablelossofshoots(McMahonetal.2013).Overall,resilience toshadingwaslowestforseagrasseswithhighAG:BG, intermediateforlowAG:BG,andhighestforintermediate AG:BG.Wepresentaconceptualdiagramtoillustratethe generaldifferencesinresponseacrosstheAG:BGgradient (Fig.5). ThephysiologyofseagrassescanhelpexplainthevariationinresponsestoshadingamongthethreeAG:BGmorphologies.HighAG:BGseagrasseslikelywereleastresilient toshadingbecauseofthelargeamountoffast-respiringleaf tissue(FourqureanandZieman1991)relativetobelowground“storage”tissues.Whenlightlevelswerereduced belowthecompensationirradiance,highAG:BGseagrasses mayhaveexhaustedcarbohydratereservesfasterthanlowor intermediateAG:BGseagrassesbecauseleaveshavemuch higherrespiratorydemandsthanrootsorrhizomes.Thesignicantreductioningrowthrate,numberofleaves,leaf width,leafareashoot2 1,andAG:BGwerelikelyresponses aimedatreducingrespiratorydemandbysheddingleaves Barryetal ResiliencetoshadingvarieswithAG:BGratio 10

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andreducingnewinvestmentinmetabolicallyexpensive leaftissue.Similar,thoughlesspronounced,responsesin lowAG:BGmeadowsindicatedthattheseseagrassesalso werestressedbyshading,likelybecausetheyhadlessphotosynthetictissuetosupportarelativelylargepoolofheterotrophicbelowgroundbiomass.Thisresultsupportsthe hypothesissetforthbyHemminga(1998)thatseagrasses withverylowAG:BGarevulnerabletoenteringanegative carbonbalancewhenlightisreduced.GrowthratesforseagrasseswithlowAG:BGrecoveredfromshadingmore quicklythanothermetrics.Previousworkhasshownthat largebelowgroundreservespromotefastrecoveryin Fig.5.Conceptualdiagramillustrating( a )thegeneralpatternininitial T.testudinum abovegroundtobelowgroundbiomassratio(AG:BG)alonga spatialphosphorusgradientand( b )thegeneralizedshadingresponseofseagrassesforeachAG:BGcondition.Drawingnottoscale. T.testudinum vectordrawing:TracySaxby,IANImageLibrary(http://ian.umces.edu/imagelibrary).Barryetal ResiliencetoshadingvarieswithAG:BGratio 11

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perturbedseagrassmeadows(Ziemanetal.1984;Moranand Bjorndal2005,2007).Ingeneral,itappearsthatmaintaining apositivecarbonbalancewhenlightisreducedismoredifcultforseagrasseswithalargeamountofleaftissue(high AG:BG,WAC)thanforseagrasseswithalargeamountof belowgroundtissue(lowAG:BG,WEE),thoughseagrasses ofbothtypesseemtobemorevulnerabletonegative impactsthanseagrasseswithamoreevendistributionoftissue(intermediateAG:BG,HOM). Interestingly,seagrasseswithintermediateAG:BG (HOM)showedfewnegativeresponsestoshadingandactuallyincreasedleaflengthandleafareashoot2 1.Other researchhasshownthatseagrassessometimesincreaseallocationtoleaftissueinlowlightconditions,presumably benetingfromgarneringmorelightthroughleavesthat reachhigherinthewatercolumnorhavemorephotosyntheticarea(Bulthuis1983;LeeandDunton1997;Olesen etal.2002;Maxwelletal.2014).Ourworksuggeststhatthis responsemightbeadvantageousonlytoseagrassesthathave arelativelybalancedAG:BG,whichprovidesthemwith adequatecarbohydratereservestosupportleafmetabolism andadequateleaftissuetosupportbelowgroundrespiration. TheincreaseinleaflengthandleafareainHOMinresponse toshadingledtoanincreaseinmeanAG:BGinshaded plots,althoughtheincreasewasnotlargeenoughtobestatisticallysignicant(Table5).Theincreaseinleaflengthand leafareaissuggestiveoflightstress,andthesechangesindicate thatthemorphologyofseagrassesinHOMshadeplotswas becomingmoresimilartothemorphologyobservedinmeadowsexposedtohigherTP(higherAG:BG).Thus,furthershadingeventsintheseplotscouldelicitmoreseverenegative responsesbecausemorphologicalresponsestotherstshading eventpushedtheseseagrassestowardalessresilientstate.Overall,theresponsetosuccessiveshadingeventsbyseagrasses withdifferentinitialAG:BGremainsunderstudied. Pastshadingstudieshaveproducedarelativelygood understandingoftheprimaryresponseofseagrassestolight reduction,buttheyalsohaveresultedinseveralinconsistencieswithrespecttospecicresponsesofdifferentmetricsof seagrasscondition(McMahonetal.2013).Theseinconsistenciesmaybedue,inpart,todifferencesintheduration andseverityofexperimentallightreduction(McMahon etal.2013),butourdatasuggestthatdifferencesinAG:BG alsocouldproducedifferencesinthedirectionandmagnitudeofresponseforagivenseagrassmetricwithinaspecies. Forexample,inourstudy,seagrasseswithintermediateinitialAG:BGrespondedtoshadingbyincreasingleafarea shoot2 1,whereasthosewithhighinitialAG:BGshoweda markeddecreaseinleafareashoot2 1.Intheregionwestudied,AG:BG,watercolumnphosphorusconcentrationsand theavailabilityoflightarelinked,whichincombination withtheresultsoftheshadingexperiment,indicatesthat informationaboutbackgroundconcentrationsofnutrients andhistoricallightregimesisimportantforpredictingthe responsesofseagrassestoshading.Forexample,seagrassesin meadowsexposedtohigherconcentrationsofnutrientsmay bemorevulnerabletoextirpation,giventheirallocationofbiomassandthefactthatcompetitiveadvantageshiftstoward phytoplanktonandalgaethatcancreateshade(Duarte1995; Valielaetal.1997).Furthermore,theinuenceofbackground environmentalgradientsneedstobeinvestigatedformorespeciesofseagrasses,giventhatotherresearchershaveshownthat lighthistoryyieldedoppositeresponsestofurtherlightreductionintwodifferentspeciesofseagrass(Maxwelletal.2014; Yaakubetal.2014)andthefewstudiesthathaveinvestigated theeffectsofrepeatedshadingeventshaveyieldedmixed resultsforalimitedvarietyofspecies(LongstaffandDennison 1999;Biberetal.2009) Intermsofmanagingcoastalresources,seagrassesare knowntothriveinoligotrophicenvironments,anditisgenerallyacceptedthatnutrientinputsandconcomitantdegradationsinthelightenvironmentshouldbelimitedwherever possible.However,limitedresourcesmayprecludetheequal protectionofallseagrassmeadows,anditisoftenusefulor desirabletoprioritizeareasforrestorationorprotection.The presentworksuggeststhatAG:BGcouldbeausefulmetric forprioritizingmanagementgoalsbyhelpingtodistinguish meadowsthatmaybeatriskofextirpationiflightisreduced (highAG:BG)fromthosethatshouldhavehigherresilience (mediumAG:BG).Inaddition,AG:BGcouldbemonitored inseagrassbedsofinteresttoprovideavaluableearlywarningoflightstressifratiosbeginshiftinghigher.ReferencesAlcoverro,T.,M.Manzanera,andJ.Romero.2001.Annual metaboliccarbonbalanceoftheseagrass Posidoniaoceanica :Theimportanceofcarbohydratereserves.Mar.Ecol. 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