Validation of the ecosystem services of created wetlands

Validation of the ecosystem services of created wetlands: Two decades of plant succession, nutrient retention, and carbon sequestration in experimental riverine marshes

William J. Mitsch

^a,b,

*, Li Zhang

^a,b

, Evan Waletzko

^a,b

, Blanca Bernal

^c

aEvergladesWetlandResearchPark,FloridaGulfCoastUniversity,4940BayshoreDrive,Naples,FL34112,USA

bTheOhioStateUniversity,Columbus,OH43202,USA

cBiogeochemistryLab,SmithsonianEnvironmentalResearchCenter,Edgewater,MD21037,USA

ARTICLE INFO Articlehistory:

Received25August2014

Receivedinrevisedform27September2014 Accepted27September2014

Availableonline18November2014 Keywords:

Waterquality Carbonsequestration Wetlands

Nitrogen Phosphorus

OlentangyRiverWetlandResearchPark

ABSTRACT

Wetlands provide manyecosystem services tosociety, most notably the provision of habitatfor important plantsandanimals,theimprovementof waterquality,and thesequestrationofcarbon.

Nitrogen and phosphorus budgets, vegetation structure and function, and carbon fluxes and accumulation aredescribed forapair of1-hacreated riverinewetlands in centralOhio USAover 20years(1994–2013)ofprimarysuccession.Theprimaryinflowtotheseexperimentalwetlandswas fromwaterpumpedfromtheadjacentfourth-orderOlentangyRiver.Thepumpingratemaintainedfor mostoftheyears(anexceptionwasatwo-yearcomparisonofpulsingandnon-pulsinghydrologyinthe twowetlandsin2004–2005)wasaccordingtoapre-determinedformulabasedonriverstage.The pumpedinflowtothewetlandsaveraged38.71.5myr¹withprecipitationaveraging1.1myr¹forthe sameyears.Surfaceoutflowaveraged27.11.4myr¹and subsurfaceseepagewasestimatedtobe 13.20.2myr¹overthatperiod.BothoutflowsreturnedwatertotheOlentangyRiverviasurfaceand subsurfacepathwaysrespectively.

Wetlandplantrichnessincreasedfrom13speciesinitiallyplantedinoneofthewetlandsto116species overallafter17years,withmostofthatrichness(99species)occurringinthefirst5years.Theplanted wetland hadhigher communitydiversityeveryyearexceptoneover20years whilethenaturally colonizingwetlandwasmoreproductiveespeciallyinthefirst7yearsandoverallstillhad2000kgmore organicmatterinputbyANPPafter17yearsthandidtheplantedwetland.

Nutrientmassinflowstothewetlandsaveraged5.610.30g-Pm²yr¹(n=30wetlandyears)fortotal phosphorus,2.170.27g-Pm²yr¹(n=30wetlandyears)forsolublereactivephosphorus,1223g- Nm²yr¹(n=14wetlandyears)fortotalnitrogen,and1005g-Nm²yr¹(n=32wetlandyears)for nitrate–nitrogen. Retention rates were 2.400.23g-Pm²yr¹ for total phosphorus,0.870.10g- Pm²yr¹forsolublereactivephosphorus,38.82.2g-Nm²yr¹fortotalnitrogen,and15.62.7g- Nm²yr¹fornitrate–nitrogen.Totalphosphorusretentionwashigherintheplantedwetlandcompared tothenaturalcolonizingwetland(44.34.4%vs.38.85.3%respectively;p=0.059)whiletotalnitrogen retentionwassignificantlyhigherinthenaturallycolonizingwetlandcomparedtotheplantedwetland (32.12.0% vs. 28.42.6% respectively; p=0.000085). Investigation of trends of water quality improvement showed that,overall, nutrient retention decreasedfrom thebeginning of thestudy throughthe17thyearin2010.Morerecenttrendsshowedtendenciesforwaterqualityimprovementin9 outof11ofthenutrientparametersandtimeperiodsinvestigatedbetween2003and2010.

Thewetlandswereeffectivecarbonsinks,withratesofcarbonsequestration(219–267g-Cm²yr¹, higherthanthosemeasuredinareferencenaturalflow-throughwetland.Bothcarbonsequestrationand methaneemissionshavebeenconsistentlyhigherinthenaturallycolonizingwetland,theorizedasdueto thegreaterproductivityofthiswetlandoverseveralyearsinthemiddleofthisstudy.

ã2014ElsevierB.V.Allrightsreserved.

* Correspondingauthor.Tel.:+16149466715;fax:+12397327043.

E-mailaddress:wmitsch@fgcu.edu(W.J. Mitsch).

http://dx.doi.org/10.1016/j.ecoleng.2014.09.108 0925-8574/ã2014ElsevierB.V.Allrightsreserved.

ContentslistsavailableatScienceDirect

Ecological Engineering

j o u r n a l h o m e p a g e : w w w . e l s ev i er . c o m / l o c a te / ec o l e n g

1.Introduction

Wetlands provide a multitude of ecosystem services and continuetobecitedasthemostvaluablepartsofourlandscape in recent ecosystem service assessments (deGroot et al., 2012;

McInnes,2013;Costanzaetal.,2014;MitschandGosselink,2015).

Ourresearchteamhasbeeninvestigatingtheecosystemservicesof inland wetlands for 20 years in a multi-year, whole-ecosystem experimentintwo1-hacreatedfreshwaterriverineflow-through marshesatOlentangyRiverWetlandResearchParkatTheOhio StateUniversity,Columbus,Ohio,USA.Mitschetal.,1998,2005a,b, 2012describedecologicalservicesandfunctionofthesewetlands atyears3,5,10,and15,includingthesuccessionandecosystem functionofthesewetlandlaboratories,Thispaperinvestigatesthe changeinvegetationsuccessionandnutrientmassretentionover

anevenlongerperiodandupdatescarbonfluxesinthewetlands.

These data are crucial for accurate representation of wetland ecosystem services, particularly for those referred to by the MillenniumEcosystemAssessment(2005)asregulatingservices suchaswaterpurificationandclimateregulation.

Bothwetlandbasinshadidenticalinflowsofriverwaterand hydroperiodsfrom1994to2013butoneofofthewetlandswas plantedwithnativemacrophtyesin1994whiletheotherwasleft asanunplanted control.Thatset-upessentiallytested theself- designcapabilitiesofnaturewithandwithouthumaninterven- tion. These experimental wetlands have allowed simultaneous long-termstudyof threedifferentquestions relatedtowetland development:(1)howimportantiswetlandplantintroductionon ecosystemfunction?(2)Howlongdoesit takehydric soilsand otherwetlandfeaturestodevelopatasitewherenohydricsoils

Fig.1.OlentangyRiverWetlandResearchParkattheOhioStateUniversity.Thetwoexperimentalwetlandsinthecenterofthesitemaparethesubjectofthispaper.

previously existed? (3) What are the long-term patterns of biogeochemicalchangesofflow-throughwetlandsastheydevelop fromopenpondsofwatertovegetated,hydric-soilmarshes?

2.Methods

2.1.Experimentaldesign

Two 1-ha experimental wetlands were created on the floodplain of the Olentangy River in 1993 and 1994 at what eventually became the 20-ha Wilma H. SchiermeierOlentangy River WetlandResearchPark (Fig.1), a complexofcreated and naturalfreshwaterriverinewetlandslocatedonthecampusofThe Ohio State University in Columbus. The wetlands were used extensivelyforwetlandresearchfrom1991to2012andsomeof thatresearchisreportedhereand intheaccompanyingspecial issue(seeMitschetal.,2014a).Thetwokidney-shapedwetlands havebeencomparedsincewaterwasfirstaddedonin1994(year 1),withresearchresultsregardingwaterquality,plantcommunity structureandfunction,soildevelopment,sedimentation,andgas exchange(seeMitschetal.,1998,2005a,b,2012forsummariesat variousyears).Theseurbanwetlandsarelocatedattheeastern- mostedgeoftheCentralPlainsportionoftheEasternTemperate Forestecologicalregion(biome)ofNorthAmericaandclosetothe intersectionofthreelevelIIecoregionsinthatbiome:MixedWood Plains,CentralPlains,andAppalachianForests.Theoriginalalluvial soiltypeatthesitebelongstotheRossseries—adeep,dark,and well-drained silt loam, silty clay loam, or loam that forms on floodplainssoilsintheexperimental wetlandbasinswerenon- hydricbeforethewetlandswerecreated.Waterfromtheadjacent OlentangyRiverhasbeenpumped,exceptforshortoutagescaused bypoweroutagesorpumprepairsinceMarch4,1994,accordingto aformulabasedonriverstage.TheOlentangyRiverisafourth- orderstreamintheagriculture-dominatedSciotoRiverWatershed ofcentralOhio.TheScioto,inturn,isoneofthemajortributariesof theOhioRiver.In2003,extramuralfundingfromOhioandFederal agencies was obtained for a pulsing study whereby artificial

“floods” were introduced to the wetland basins (Mitsch et al., 2005b;Hernandez and Mitsch, 2006;Altorand Mitsch,2008);

each wetland was administered with the same hydrologic conditions,sothe“plantingexperiment”wasnotviolated.Water depthsin themajorportionsofthewetlandaregenerally20to 40cmintheshallowareaswheremostoftheemergentmacro- phytesgrowand50 to80cmin thedeepwaterareasthat were constructed in the wetland toallow overwintering of fish (for mosquitocontrol)andlong-termsedimentstorage.

2.1.1.Planting

The western basin (Wetland 1) was planted with13 native speciesofmacrophytesinMay1994andisthereforelabeledthe

“plantedwetland”whereastheeasternbasinwasallowedtobe colonizednaturally(Mitschetal., 1998)andsoisreferredtohereas the“natuallycolonizing”or“unplanted”wetland.Over2400plant propagules(mostlyrootstockandrhizomes;Table1)representing 13speciestypicalofMidwesternUSAmarsheswereplantedinone wetland(Wetland1=W1)ononeSaturdaywiththehelpof200 volunteers in early May 1994. Wetland 2 (W2) remained unplanted.Nootherplantswereintroducedtothewetlandbasins over the 20 years reported here. Extensive colonization of additional plants was allowed to occur in both basins for the entiretwodecades.

2.2.Hydrologybudgets

Dailywater budgetsof thetwo experimentalwetlands were estimated for 19 years (1994–2012) per wetlandwater budget

methods described by Mitsch and Gosselink (2007). Wetland inflowswere calculated bytwice-daily (morningandevening) readingsof bothinstantaneous andtotalintegrated volumeof pumpingratesfromflowmonitorsforeachwetland.Whendata fromonlyonewetlandinflowwereavailable,themissingflow ratewasassumedtobethesameastheavailableflowrate.The protocolfortheexperimentalwetlandshasbeen,sincethestart, todeliverthesameflowtoeachwetlandatalltimes.Wetland outflowswereestimatedbyregressionmodelswithtwice-daily water level readings located in outflow of the wetlands and calibrated outflow rates. Precipitation and estimated evapo- transpirationdatawerecollectedfromtheweatherstationatThe OhioStateUniversity.Seepagerateswerecalibratedasafunction of water level by measuring changes of water level readings when the wetlands had no inflows or outflows. Daily water budgets weretranslatedtoannualwaterbudgetspresentedin thispaperforthe19years.

2.3.Vegetationcoveranddiversity

The macrophyte-dominant community cover was estimated eachyearinmiddletolateAugustfromcoloraerialphotography followed byground-truthverifications. Themaps for eachyear werenormalizedtoabasinmapofastandardsize,usingArcView 3.1.A10-mgridsystemmarkedwithpermanent,numberedwhite polesfacilitatedidentificationofthelocationsofplantcommuni- tiesineachwetlandduringgroundtruthingandaerialphotogra- phy.Wewereabletodevelopvegetationmapsforthe19ofthe20 yearsfrom1994to2013.

AmacrophyteCommunityDiversityIndex(CDI;Mitschetal., 2005a)wasusedtoquantifyvegetationdiversityinthewetland basins.Theindexuses relativeareasofmacrophytecommunity coverfromthemapsderivedaboveandthemathematicsofthe Shannon–Weaverdiversityindex,withareaofeach community instead of number of individuals of each species used. It is expressedas:

CDI¼X^N

i¼1

SðCilnðCiÞÞ

whereCi=percentcoverofwetlandcommunity“i”(0to1)inthe wetlandbasin,N=numberofwetlandcommunities.

Table1

PlantsintroducedtoexperimentalWetland1atOlentangyRiverWetlandResearch Parkin1994(fromMitschetal.1998).

Edgesandmiddle(marsh) Schoenoplectustabernaemontani^a Scirpusfluviatilis^b

Deepwater Nelumbolutea^b Nymphaeaodorata Potamogetonpectinatus^b

Mudflatgradient(currentlyforestededge) Acoruscalamus^a

Cephalanthusoccidentalis Juncuseffusus^a Pontederiacordata Sagittarialatifolia^b Saururuscernuus Sparganiumeurycarpum^b Spartinapectinata^a

aPresentinplantedwetland(W1)in2012.

bAbundantinplantedwetland(W1)in2012.

2.4.Vegetationproductivity

Aboveground biomass was measured each August as an estimateof aboveground net primary productivity (ANPP) for 1997–2010 by the direct aboveground harvesting of 16 1-m² plotsineachwetlandalongsamplingboardwalks.Forthe1994– 1996 data, ANPP was estimated from fewer plots. The aboveground biomass in each plot was clipped at the soil surface and weighed immediately. Subsamples were takento the lab and dried to a constant weight to estimate dry:wet ratios.

2.5.Nutrientanalsysesandbudgets

Weeklywatersamplesweretakenattheinflow,themiddle, andthe outflowsof thewetlandsfor 17 years(1994–2010) for nutrientconcentrations(totalphosphorus [TP],solublereactive phosphorus[SRP],andnitrate+nitritenitrogen[NO3-N])aswas determinedby standard methods (USEPA,1983; APHA, 1989).

TotalKjeldahlmeasurementsandthustotalnitrogenwereadded to the suite of nutrient species measured in 2004 to enable estimatesoftotalnitrogenfluxes.Samplesweresplitintofiltered (0.45

m

^{m) and un}filtered samples, frozen until analysis, and analyzedfortotalphosphorus,solublereactivephosphorus,and nitrate–nitrogen(USEPA,1983;APHA,1989).Bothtotalphospho- rus and soluble reactive phosphorus methods employ the ascorbic acid and a molybdate color reagent method with a Lachat QuikChem IV automated system. Total phosphorus samplesare first digestedbyadding0.5ml of5.6NH2SO4 and 0.2g(NH4)2S2O8to25mlofsampleandexposingthesamplesto aheatedandpressurizedenvironmentfor20mininanautoclave.

Nitrate+nitrite were analyzed on a Lachat QuikChem IV automatedsystemwith thecadmiumreduction method. Early samplesfromApril1994toJuly1995wererunbysimilarmethods bytheHeidelbergCollegeWaterQualityLaboratoryusingaTraacs 800 autoanalyzer. The accuracy of the nutrient analysis was checkedevery10–20 sampleswitha knownstandard,andthe sampleswereredoneiftheaccuracywasoffby5%.Waterquality concentrationsintheinflow,outflow,andmiddleweremultiplied byestimatedinflows,surfaceoutflows,andseepagerespectively toestimateannualnutrientbudgets.

2.6.Carbonandnitrogenaccumulation

Soilcoreswereextractedfromtheexperimentalwetlandsin May2004(127cores10yearsafterwetlandcreation)andinMay 2009(44cores15yearsafterwetlandcreation)usinga10-mgrid systeminstalledin1993.Coreswere7–10cmindiameter,andtheir lengthvarieddependingonthedepthofthesedimentaccumulat- edovertheunderlyingnonhydricsoil(10–35cm).Extractedcores weredividedin thefieldintoincrementsand packedinsealed containersthatwerestoredat4Cuntilanalysis.Soilsampleswere ovendriedtoconstantweighttodeterminebulkdensity,groundto a2-mmparticlesize,andhomogenized.TotalCcontentofthesoil samplesforthetwoperiodsisdescribedbyAndersonandMitsch (2006)andBernalandMitsch(2013a).TheCandNaccumulation sincewetlandcreationin1994wascalculatedbyestimatingthe totalsoilpoolofthehydriclayer(i.e.,fromthesoilsurfacetothe underlying non-wetland soil over which the wetland soil was developed)ineachpointofthegrid.Theaccumulationrateswere determinedbydividingthepoolsbytheageofthewetlandatthe timeofsampling(10or15years).

2.7.Dataanalysis

Waterfluxesandwaterqualitydatawererecordedseasonally onExcelspreadsheetsforthatyearandsummarizedattheendof each year. Inflow rates were compared annually for the two wetlandsandfoundtobestatisticallysimilar(p<0.05).Student’s paired t-tests were used to compare the two experimental wetlandsonannualandlongerperiodsforwaterquality,nutrient removalandprimaryproductivity.Trendsofnutrientsforwetlands wereexaminedbycurveestimateswithANOVAand t-tests.All statisticalanalysiswasconductedusingSPSSPCversion(SPSSInc.).

A95%significantlevel(p<0.05)wasusedfordataanalysisinmost casesalthougha90%significancelevel(p<0.10)wasusedforsome waterqualityindices.

3.Results 3.1.Hydrology

Thewaterstageofthetwoexperimentalwetlandsfor19years (1994–2012)ispresentedinFig.2.Waterlevelsoscillated30cm

Fig.2. Waterelevationsabovesealevel(hydroperiods)oftheexperimentalwetlandsattheOlentangyRiverWetlandResearchPark,centralOhio,for1994through2012.

aboveandbelowthenormroutinelyforthe19years,followingthe general hydrology of the watershed because of our pumping protocol.Pulsesindicateriverpulses andlow periodsindicated baseflowriverconditions.Theoverallannualwaterbudgetsforthe wetlandsfor1994–2012areillustratedinTable2.Annualinflow, throughthese19years,was38.71.5myr¹(toestimatevolume toeachwetland,multiplyby10,000m²,thesizeofeachwetland basin).Ofthetotalinflowincludingprecipitationapproximately 70% (27.11.4myr¹) of the water flowed out at the surface outflowweiratthesouthendofthewetlandandanestimated34%

(13.20.2myr¹) infiltrated to the groundwater from this perched wetland. All of these water fluxes were estimated independentlyfor the19years,sonot unexpectedly, therewas anunbalanceerrorof8%onthehighside,i.e.,outflowswerehigher by8%overestimatedinflows).Webelievethattheerrorisinthe lackofaccuracyinbothinflowandoutflowmeasurements.The surfaceoutflowandgroundwaterseepagemaybeafewpercent higherthanactualratesduetoinaccuracyofweirequationsathigh flows;thepumpedinflowswereprobablyslightlyunder-estimat- ed,asshort-termpulseswerenotalwayscaughtinthetwo-per- daymanualmeasurements.

3.2.Vegetationrichness,successionandcommunitydiversity

Ofthe13speciesoriginallyintroducedtotheplantedwetlandin 1994,nine werestill presentin that wetlandin 2010, the17th growingseasonafterplanting(Table1).Thenumber ofspecies increasedrapidlyineachwetlandbasinwith87–96speciesinthe unplantedandplantedwetlandsrespectively5yearsafterplanting

(Table3).Thespeciescountincreasedby10intheunplantedor

“naturallycolonizing”wetlandoverthenextdecadeto97andby5 intheplantedwetlandoverthenextdecadeto101(Table3).Based onmeasurementstwoyearslater(2010)richnessappearstohave leveled off. Onlytwo of the original 13species planted in the planted wetland (W1) were found in the naturally colonizing wetlandbasin(W2)throughoutthestudy.

Vegetation community maps of the wetlands over 20 years (1994–2013;Fig.3)illustrate12differentvegetationcommunities, includingopenwaterandalgalmats.Communitiesweregenerally named for the dominant taxa in the community. Patterns of vegetationdiversityinthetwowetlandbasinsoverthatperiod, estimated by our community diversity index (CDI), shows an interesting long-term effect of planting. Except for one year (2009=year16)duringthatperiod,theplantedwetland(W1)had consistentlyhigherspatialdiversityofplantcommunitiesthanthe naturally colonizing wetland (W2). Community diversity (CDI) appearstobedecreasingoverthelast2yearsofthisstudyinboth wetlands(Fig.4),causedbyacontinuedincreaseincoveragebythe clonal dominateTyphaspp. (see Figs.3 and 5).Typhaspp., not amongtheplantsintroducedtotheplantedwetland,hasshownan interesting contrastingpattern in the two wetlands since 1994 (Fig.5).In 1999,fiveyears afterWetland1 was planted,Typha coveragepeakedat56%inthenaturallycolonizingWetland2while therewasonlyapeakofonly9%Typhacoverageintheplanted wetland.TheTyphapeakwasfollowedbymuskrateatoutsinboth ofthewetlandbasinsandmostoftheemergentvegetationwas gonein2001(seeFig.3;alsoMitschetal.,2005a).Typhacoverage wasreducedto5to10%coverageinbothwetlands.Elevenyears afterthefirstpeak,Typhacoverageagainreached55%coveragein the naturally colonizing wetland. During this period, Typha coverageintheplantedwetlandappearstoparalleltheincrease in thenaturallycolonizing wetlandand reacheda peak of 45%

Typhacoveragein2009.

3.3.Vegetationproductivity

Abovegroundnetprimaryproductivity(ANPP)ofmacrophytes inthetwoexpermentalwetlandbasinsfor1994–2010(17years) wassignificantlydifferentinpairedt-testcomparisonsin7ofthe 17yearsanalyzed,withANPPinthenaturallycolonizingwetland higherthanthanANPP intheplantedwetland5ofthe7times (Fig.6a).After17years(1994–2010),therewasstillanestimated 2000kg/hamoreabovegroundplantaccumulationinthenaturally colonizingwetlandthanintheplantedwetland(Fig.6b).Mostof the increase in accumulation in plant biomass occurredin the 1998–2001 period when Typha dominance was dramatically higherinthenaturallycolonizingwetland(seealsoFigs.3and5).

3.4.Annualnutrientbudgetsandnutrientretention

Nutrientinflowsandsubsurfaceand surfaceoutflowsforthe experimentalwetlandsforvariousperiodsoverthetwodecades Table2

Annualwaterbudget(m/year)fortheexperimentalwetlandsattheOlentangyRiver WetlandResearchParkfrom1994to2012.

Year Inflow Surface outflow

Seepage Precipitation Evapotranspiration 1994 41.50.3 33.30.5 12.20.1 1.0 0.9

1995 37.90.3 37.21.9 14.00.0 1.4 0.8 1996 21.60.2 16.20.1 13.70.1 1.7 0.9 1997 34.50.0 25.51.7 13.50.1 1.1 0.9 1998 36.80.1 32.62.5 13.30.1 0.7 0.8 1999 48.00.3 32.70.2 15.30.0 0.5 0.9 2000 32.10.1 29.60.1 15.70.0 1.1 0.9 2001 37.60.5 27.10.1 15.60.2 1.1 0.8 2002 25.40.1 22.11.4 13.40.2 1.4 0.8 2003 24.31.3 19.61.5 13.50.8 1.3 0.8 2004 43.40.0 21.61.8 11.90.1 1.0 0.9 2005 36.00.0 13.21.8 13.50.4 1.1 0.9 2006 33.30.6 13.10.5 11.10.3 1.0 0.8 2007 56.30.0 33.90.1 13.50.0 1.4 0.8 2008 45.40.5 22.31.7 13.60.2 0.9 0.8 2009 41.90.1 25.91.3 14.80.2 0.7 0.8 2010 37.61.6 33.35.4 15.70.1 0.8 0.8 2011 43.20.6 33.65.3 14.50.2 1.3 0.8 2012 58.20.0 41.77.0 14.50.3 0.8 0.9

38.71.5 27.11.4 13.20.2 1.10.1 0.90.0

Table3

Vegetationspeciesrichnessinthetwo1-haexperimentalwetlandsattheOlentangyRiverWetlandResearchParkfrom1996to2010.Bothwetlandswerecreatedin1994;the

“plantedwetland”(W1)wasplantedwith2500individualplantsrepresenting13nativewetlandspeciesinMay1994whileW2remainedanunplantedcontrol(datafrom MitschandGosselink,2015).

Wetlandage(#ofgrowingseasonscompleted) Year Numberof species

Numberofwetland species

Numberofplanted species

Numberofwoody species

Numberofinvasive species

W1 W2 Total W1 W2 Total W1 W2 W1 W2 W1 W2

3 1996 67 56 72 43 31 44 9 1 5 7 1 1

5 1998 96 87 99 56 46 57 9 2 15 15 4 4

15 2008 101 97 116 55 52 61 9 2 18 21 7 9

17 2010 99 97 118 51 49 63 9 2 18 21 7 10

arepresentedinFig.7for(a)totalphosphorus,(b)totalnitrogen, (c) soluble reactive phosphorus, and (d) nitrate–nitrogen. Total annualphosphorusretention(Fig.7a)bythewetlandbasinsfor30 wetlandyears (=15 years of measurementsat 2 wetlands)was 2.400.23g-Pm²yr¹,averaging42.7%retentionofphosphorus bymassovertheentireperiodofnutrientanalysis.Solublereactive phosphorusinflow(Fig.7c)representedlessthanhalfofthetotal phosphorusinflowtothewetlandbasins.OverallretentionofSRP for30 wetlandyearswas 0.870.10g-Pm²yr¹,anaverageof 40.3%ofthesurfaceinflowandabout36%ofthetotalphosphorus retention.

Totalnitrogenretention(Fig.7b)bythewetlandbasinsfor14 wetland-years (TKN analysis was only started in 2004) was 38.82.2g-Nm²yr¹, an average of 31.9% retention by mass.

Nitratenitrogenretention(Fig.7d)was15.62.7g-Nm²yr¹for 32 wetland years, or an average of 15.5%of the inflownitrate Fig.3. VegetationcommunitymapsfortwoexperimentalwetlandsattheOlentangyRiverWetlandResearchPark,1994–2013.Vegetationcommunitiesaredefinedinthe maplegend.

0.0 0.4 0.8 1.2 1.6 2.0

1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 Wetland 2 (naturally colonizing) Wetland 1

(planted)

Community Diversity Index

Fig.4.Communitydiversityindex(CDI)ofvegetationintheplantedandnaturally colonizingexperimentalwetlands,1994–2013.

nitrogen.For the 14 wetland-years used for the total nitrogen budgetinthelast7years ofmeasurements(14wetlandyears), nitrate nitrogenretentionwas 69% higher at 26.42.2g-Nm² yr¹.Thisnitrateretentionrepresented68%ofthetotalnitrogen retentionforthose14wetlandyears.

3.5.Comparisonofnutrientretentioninplantedandunplantedbasins

Whentheannualretentionoftotalphosphoruswascompared betweenthetwobasinsfor30wetlandyears,itwassignificantly higher (p<0.10) in the planted wetland than in the natural colonizingwetland(44.34.4%vs.38.85.3%; p=0.059).When theannualretentionoftotalnitrogenwascomparedbetweenthe two basins for 14 wetland years, it was significantly higher (p<0.05)inthenaturallycolonizingwetlandthanintheplanted wetland (32.12.0% vs. 28.42.6% respectively; p=0.000085).

Neithersoluble reactivephosphorusnornitrate–nitrogenreten- tionwassignificantlydifferentbetweenthetwowetlandbasins.

3.6.Trendsinnutrientremoval

Trends of nutrient retention over different periods were investigated,includingthelongestperiod1994–2010,bycomparing thepercentchangeinnutrientconcentrationsfromtheinflowtothe surfaceoutflow(Table4;Fig.8).Strongsignificantpositiveslopesof nutrientreduction(i.e.,wetlandeffectivenessinremovingnutrients isdecreasingovertime)were seenfortotalphosphorus,soluble reactivephosphorus,andnitrate-nitrogenfortheentireperiodof 1994–2010.Whenthelast6years(2005–2010)and5years(2006– 2010)wereexamined,theslopesofallthreeoftheseparameters reversed and were negative (i.e., the wetland effectiveness in removingnutrientswasincreasingovertime).Howeverthetrends forthese3nutrientmeasurementswerenotstrongenoughtobe significantexceptfortotalphosphorusin2006–2010(Table4).

3.7.Carbonandnutrientaccumulationinthesoils

Sedimentation rateshave been investigatedby a number of researchersovertheyearsintheexperimentalwetlandsandare summarizedinacompanionpaperbyMitschetal.(2014b).Twoof thosestudiesduringthesetwodecadeswereundertakenspecifically toestimatethenetaccumulationofcarbonandnitrogeninthesoils ofthesewetlands(Table5).Carbonsequestrationforthefirst10 years of the experimental wetlands ranged from 181 to 193g- Cm²yr¹.Whenre-estimated from soil cores taken 5yearslater,the carbonsequestrationincreasedto 219–267g-Cm²yr¹.Nitrogen 0

10 20 30 40 50 60

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 Wetland 1-planted wetland Wetland 2- naturally colonizing wetland

% c o v er of w etland b y Typha spp .

Fig.5.PatternofTyphacoverageintheplantedandnaturallycolonizingexperimentalwetlands,1994–2013.

Cumulative productivity, Mg/ha

0 10 20 30 40 50 60

1994 1996 1998 2000 2002 2004 2006 2008 2010 0

2000 4000 6000 8000 10000

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Wetland 1 Wetland 2

kg-dry wt ha-1 yr-1 * ANPP different in two

wetlands (α = 0.05)

*

*

* *

*

*

*

a)

b)

Fig. 6.Productivity of macrophytes in the planted and naturally colonizing experimentalwetlands,1994through2010asillustratedby(a)abovegroundnet primaryproductivityofemergentmacrophytes;and(b)accumulatedmacrophyte productivityovertheperiod1994–2010.

accumulationrateswereestimatedwiththesame10yearand15 yearcores.Aswithcarbon,ratesofaccumulationincreasedwith time,withanestimatedrateof16and17g-Nm²yr¹forthefirst10 yearsincreasingfrom18to21g-Nm²yr¹forthefirst15years.

Thesenitrogenaccumulateratesrangefrom41to54%ofthetotal nitrogenretentionestimatedinthenutrientbudgetsabove.

4.Discussion

4.1.Successionofcreatedwetlandsinhigh-nutrientlandscapes

Thepatternseeninthistwo-decadestudysuggests,inthelong run, that the vegetation cover for wetlands created in the U.S.

Midwest,unlesscreatedinunusualsettingsoflownutrients,will ultimatelybedominatedbyTyphaspp.iftheyremainwetenough tokeepoutwoodyvegetation.TheMidwesternUSAisdominated byhighlyindustrializedagriculturethathasledtomajorincreases innutrients,especiallynitrogenandphosphorusinthelandscape andaquaticecosystemsfordecadesormore.Thishighnutrient statushasledtowhatOdum (1987)calledthe“Typhazationof America” where the expected plant community in freshwater marshesthroughoutmuchofeasternNorthAmericaisnowTypha.

Keddy(2010)agreeswiththisconceptandputTyphaasthecore plantspeciesinhiscentrifugalorganizationmodelwithvarious disturbances(icescour,beaverdams,etc.)asthemaincausefor otherplantcommunitiestodevelopinfreshwatermarshes.Fig.9 illustratesourinterpretationofhiscentrifugalmodelspecifically fortheseexperimentalwetlandsandforfreshwatermarshesinthe Midwestern USA in general. Typhais now and will befor the foreseeable future the core habitat or community. In the experimental wetlands in our study, Typha dominated the naturallycolonizingwetland(unplantedbasin)bythesixthyear ofthestudy(1999)thoughnotinitiallyintheplantedwetland.This dominationledtoadivergencebetweenthetwobasinsinwetland structureandfunctionthatwereportedonearlier(Mitschetal., 2005a). We also suggested at that time that perhaps planting wouldhaveapositiveeffectoncurtailingthe“invasion”ofTypha spp. in created and restored wetlands (Mitsch et al., 2005a,b,).

Typhawas heldatbay byseveral plantedspecies, mostnotably Sparganium eurycarpum (bur reed) in the planted wetland. A muskrat“eatout” disturbancein 2000 causedboth wetlands to resetwithlow Typhadominationaswouldbepredicted bythe

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0

INFLOW SEEPAGE SURFACE

OUTFLOW

RETENTION Total Nitrogen Budget

g-P m-2 yr-1g-N m-2 yr-1

a)

b)

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

INFLOW SEEPAGE SURFACE

OUTFLOW

RETENTION Total Phosphorus Budget

g-P m-2 yr-1 g-N m-2 yr-1

c)

d)

0.0 20.0 40.0 60.0 80.0 100.0 120.0

INFLOW SEEPAGE SURFACE

OUTFLOW

RETENTION Nitrate-Nitrogen Budget

0.00 1.00 2.00 3.00

INFLOW SEEPAGE SURFACE

OUTFLOW

RETENTION Soluble Reactive Phosphorus Budget

Fig.7.Nutrientbudgetsforbothexperimentalwetlands,1994through2010;(a)totalphosphorus;(b)totalnitrogen;(c)solublereactivephosphorus;and(d)nitrate–

nitrogen.Fluxesareshownforinflows,verticalseepagetogroundwater,surfaceoutflowthroughweir,andnetretention=inflowseepagesurfaceoutflow.

Table4

Trendsforannual%changeofnutrientsfrominflowtosurfaceoutflowfor1994–

2010,2003–2010,2004–2010and2006–2010atthetwo1-haexperimentwetlands attheOlentangyRiverWetlandResearchPark.Apositiveslopemeansthatnutrient changefrominflowtooutflowis becomingmorepositive, i.e.,thewetlandis retainingfewernutrientsfromyeartoyear.Anegativeslopemeansthatnutrient changeisbecomingmorenegativefromyeartoyear,i.e.,thewetlandsareretaining morenutrientsfromyeartoyear.

Period Parameters Slope,%changeperyear n F p

1994–2010 TotalP 2.536 32 19.552 0.000^**

SRP 2.814 30 41.666 0.000^**

NO3+NO2 0.790 30 8.884 0.006^**

2003–2010 TotalP 3.250 16 6.730 0.021^**

NO3+NO2 1.506 16 6.956 0.020^**

2004–2010 TotalP 0.982 14 0.673 0.428

SRP 0.446 14 0.084 0.777

NO3+NO2 0.125 14 0.179 0.679

2005–2010 TotalP 0.943 12 0.349 0.568

SRP 2.929 12 2.947 0.117

NO3+NO2 0.414 12 1.943 0.193

2006–2010 TotalP 3.850 10 4.790 0.060^*

SRP 0.900 10 0.158 0.701

NO3+NO2 0.450 10 1.123 0.320

n=number of annual data points; F=Fratio from ANOVA; p=probability at significantconfidencelevels.

*p<0.05.

** p<0.10.

Keddy(2010)model.Butinthesecondcycleof2000–2012,both wetlands increased in Typha coverage. The planted vegetation provided competitionfor Typhain thefirst fewyears after the wetlandswerecreatedbutinthesecondpulse15orsoyearsafter thewetlandswerecreated,therewaslittlecompetitionforTypha leftfromoriginalplantings.

In general, theplanted wetlandmaintaineda higherspatial macrophytediversitythroughout mostof the20 years and the

naturallycolonizingwetlandappearedtobemoresusceptibleto disturbances suchas muskratherbivory and hydrologic pulses thandidthemorediverseplantedwetland.OurmodelinFig.9 suggeststhatsinceTyphaisnowestablishedinbothwetlands,it willdivergefromthatcommunityonlythroughtheinterventionof organisms(muskrats,beavers,orgeeseasprimaryexamples)or humanaction(changingtheinflowofwateroralteringtheregional groundwaterlevelthroughdamremoval).Infactalargedamwas removedfromtheOlentangyRiver3kmdownstreamfromthese wetlandsinSeptember2012(Zhangetal.,2014)anditisnotclearif that dam removal has had any effect on these experimental wetlandsintermsofthegroundwaterelevation.

The continual introduction of species, whether introduced throughfloodingandotherabioticandbioticpathways,appeared to have a much longer-lasting effect in development of these ecosystemsthanthefewspeciesofplantsthatwereintroducedto one of the wetlands in thebeginning. Studies described above suggestthatplantingdidhaveashort-term(16yearsorless)effect onmaintainingplantdiversityandperhapsalonger-termeffecton functions such as nutrient retention, methane emissions (see NahlikandMitsch,2010),andcarbonaccumulationinthesoil(see alsoBernalandMitsch,2013a).Buttheeffectofplantingseemsto bedisappearingafterthetwodecadesastheplantdiversityisnow decreasingandplantcoverappearstobegoingtowardsaTypha monoculture.Wethinkthataresearchprojectsuchasoursneeds to continue for decades more to see the true picture of the importance of planting. Our original hypothesis (Mitsch et al., 1998)whichstatedthatthetwowetlandswilldivergeinfunction inthemiddleyearsandthenconvergeinstructureandfunction remainsourhypothesis.Wealsonowadmitthatwedonotknow whatthemiddleyearsare,exceptthatnowtheycouldbedecadal inextent.

4.2.Nutrientretentionratesinwetlands

Rules of thumb for sustainable retention of nutrients by wetlandspublished15yearsago(Mitschetal.,2000)suggested thatwetlandscouldconsistentlyretainphosphorusinamountsof 0.5–5g-Pm²yr¹ and nitrogen in amounts of about 10–40g- Nm²yr¹.Nothinginthisstudysuggeststhatthoserecommen- dationsshouldchange.Tomaintainbiologicaldiversityintheplant community, the lower end of these loading rates have been recommendedasreasonablegoals(MitschandGosselink,2015).In thisstudy,thelong-termretentionofphosphorusfor30wetland yearsof datawas 2.400.23g-Pm²yr¹,almostexactlyinthe centeroftheMitschetal.(2000)range.Totalnitrogenretention was38.82.2g-Nm²yr¹for14wetlandyears,arateclosetothe highendoftherangeproposedbyMitschetal.(2000)of10–40g- Nm²yr¹.Anestimated68%ofthetotalnitrogenretentionwas nitrate–nitrogenretention.

Theseretentionratesarecomparabletothosepublishedinthe literature for both temperate and tropical/subtropicalwetlands (Table6).Forexample,Mitschetal.(inpress)describetheaverage retention of phosphorus in the stormwater treatment areas (createdandrestoredwetlands)northoftheFloridaEverglades, withtheoldestrecordsshowinganaverageretentionrateof1.25g- Pm²yr¹.Nutrientretentiononanannualizedbasiswas1.76g- Pm²yr¹ for phosphorus and 4.39g-Nm²yr¹ for dissolved inorganicnitrogen(dissolvednitrateandammoniumnitrogen)in Houghton Lake where treated wastewater has been appliedto peatlandsfor30years(Kadlec,2009).

4.3.Nutrientretentiontrends

Waterqualitytrends,whenevaluatedwithsimplelineartrends overlongperiods,showdecreasingnutrientretentionthatwehave Fig.8.Trendsofchangeinwaterquality(percentchangefrominflowtooutflow)

for(a)totalphosphorus;and(b)nitrate–nitrogen.Trendsareshownfor1994–2010 forbothnutrients,2003–2010fortotalphosphorus,and1994–2010and2005–2010 for nitrate nitrogen. Slopes (m) and significance of lines (F=ANOVA ratio;

p=probabilityat0.05significancelevel).Statisticsforseveralothertimeperiods andparametersaregiveninTable4.

Table5

Carbon, nitrogen, and phosphorus sequestrationin soils ofthe experimental wetlandsattheOlentangyRiverWetlandResearchParksince creationofthe wetlandsin1994.

Annualaccumulationrate (gm²yr¹)

Reference

Planted wetland

Unplanted wetland Carbonaccumulation

1994–

2004

18113 19312 AndersonandMitsch,2006 1994–

2010

21915 26717 BernalandMitsch,2013

Nitrogenaccumulation 1994–

2004

16.21.2 16.61.0 AndersonandMitsch,2006 1994–

2009

18.01.5 21.01.4 Thisstudy

Phosphorusaccumulation 1994–

2004

3.260.25 3.490.25 AndersonandMitsch,2006

described previously (Mitsch et al., 2012). In that paper we attributedthedecreasingwaterqualityimprovementduetothe saturationof nutrientsin soils, detritus,and plants and alsoto moresedimentsbeingdischargedintheoutflowasthewetlands slowlybecomeshallower.Therewereclearlyhighratesofnutrient retention due to densealgal communities that dominated the wetlandbasinsin thefirstyearof two afterthey werecreated (Mitschetal.,1998;WuandMitsch,1998).Inthisstudy,wehad twomoreyearsofdatathanMitschetal.(2012)butwealsotooka closerlookatmorerecenttrendsoverthemostrecent5–8years.

Wesawpatternsofimprovingwaterqualityfortotalphosphorus andnitrate–nitrogen.Theconceptoftreatmentwetlands“aging” andbeginningtoexportmorenutrientswithtimeisprobablynot yetthecaseinthesewetlands.Weexpectedthenitrate-nitrogen retention to begin improving as denitrification increases with increasedorganiccarbonsubstrate.Forthelast6yearsofourstudy of nutrients in these wetlands (2005–2010), nitrate nitrogen

retentionreductionsweresteadytoslowlyimprovinginbetween 20and30%reductionsinconcentration.Whatisclearisthatthe waterqualitydidnotcontinuetodegradeinthelast5orsoyearsof this17-yearwaterqualitystudy.Theslopesofthelinesfornitrogen andphosphorusshowedwaterqualityimprovementin9outof11 of the parameters and periods investigated between2003 and 2010.

Thereareseveralpossiblereasonsastowhynutrientretention begantoimproveinthesewetlands10–12yearsaftertheywere created,i.e.,thechangeinnutrientconcentrationsfrominflowto surfaceoutflowsstartedtobecomemorenegativewitheachyear.

Thefirstandperhapsmostobviousreasonisthatthesewetlands were created on former agricultural soils (the site was an experimentalfarmfortheuniversityformanyyears)anditsimply took a decade for the nutrients accumulated in the soils to completetheireffluxbacktothewatercolumn.Ina three-year mesocosmexperimentintheFloridaEvergladesMitschetal.(in press)estimatedthatthephosphoruseffluxfromthesoiltothe wateraveraged3.2g-Pm²yr¹over3years,afluxmuchhigher than the inflow of phosphorus in the surface water. So the improvementafter10–12yearsinwaterqualitycouldbeduetothe slowingdownofthissoilefflux.

Itwasalsoexpectedthatthenitrate-nitrogenretentionmight start toimprovein a newly created wetlandas organicmatter accumulatesasthenecessaryfuelfordenitrification.Songet al.

(2014) however, did not see any obvious trends of increasing denitrificationratesovertheperiod2004–2009.Ifanything,the datasummarizedinthatpapershowadecreaseindenitrification overthat5-yearperiod.

4.4.Comparisonofwaterqualitybudgetretentionswithsoil accumulations

Our studies over the 20 years estimated nitrogen and phosphorusintwoindependentways.Thefirstwaywasthrough Typha

peripheral habitat

peripheral habitat peripheral

habitat peripheral

habitat

peripheral habitat peripheral

habitat

ice scour muskrat eatout

geese herbivory dam removal

on river beaver dam

on outlet inflow changes

Fig. 9. Centrifugal vegetation model from Keddy (2010) but illustrating disturbancesspecifictotheexperimentalwetlandsattheOlentangyRiverWetland ResearchPark.

Table6

Nutrientretentionincreatedandnaturalwetlandsreceivinglow-concentrationinflowsofnutrientsfromriveroverflows,agriculturalrunoff,andsecondarilytreated wastewater(fromMitschandGosselink,2015)

Wetlandlocationandtype Wetlandsize

(ha)

Nitrogen (g-Nm²yr¹)

Phosphorus (g-Pm²yr¹)

Reference

OlentangyRiverWetlandsexperimentalwetlands 2 38.82.2 2.400.23 Thisstudy

Generalguidelines 10–40 0.5–5 Mitschetal.,2000

Warmclimate

Evergladesmarsh,S.Florida 8000 – 0.4–0.6^a RichardsonandCraft,1993;Richardsonetal.,1997

FloridaEvergladesstormwatertreatmentareas(STAs) 23,000 – 1.25 Mitschetal.(inpress)

BoneyMarsh,S.Florida 49 4.9 0.36 Moustafaetal.,1996

EvergladesNutrientRemovalProject,S.Florida 1545 10.8 0.94 Moustafa,1999

Restoredmarshes,Mediterraneandelta,Spain 3.5 69 – Cominet al.,1997

Constructedruralwetland,Victoria,Australia 0.045 23 2.8 Raisinetal.,1997 LakeApopkamarshflow-way,centralFlorida 308 304 0.480.59 Dunneetal.,2012,2013

BretonSoundEstuary,LouisianaDelta 110,000 3.5 – Lundbergetal.,2014

Coldclimate

HoughtonLake,Michigan(30years) 100 4.39^b 1.76 Kadlec,2009

Constructedwetlands,NEIllinois PhippsandCrumpton,1994;Mitschetal.,1995

River-fedandhigh-flow 2 11–38^b 1.4–2.9

River-fedandlow-flow 2–3 3–13^b 0.4–1.7

Artificiallyfloodedmeadows,southernSweden 180 43–46 – Leonardsonetal.,1994

Palustrinefreshwaterwetlands,NWWashington ReineltandHorner,1995

Urbanarea 2 – 0.44

Ruralarea 15 – 3.0

Createdin-streamwetland,OH 6 – 2.9 NiswanderandMitsch,1995

Createdriverdiversionwetland,OH 3 32 4.5 FinkandMitsch,2007;Mitschetal.,2008

Agriculturalwetlands,OH 1.2 39^b 6.2 FinkandMitsch,2004

Agriculturalwetlands,IL(3) 0.3–0.8 33^b 0.1 Kovacicetal.,2000

Naturalmarsh,Alberta,Canada 360 – ¹0.43 Whiteetal.,2000

aEstimatedbyphosphorusaccumulationinsediment.

b Nitrate–nitrogenorinorganicnitrogenonly.