Simulated Effects of Nutrient Enrichment by Canada Geese on Macrophytes and Algae in Ohio Lake

2004·5.6 MB·English

by  Pamela B Keiper

KIRTLANDIA
The Cleveland Museum of Natural History
March 2004 Number 54:33-42
SIMULATED EFFECTS OF NUTRIENT ENRICHMENT BY
CANADA GEESE ON MACROPHYTES AND ALGAE IN OHIO LAKES
PAMELABRUTSCHEKEIPER*andFERENCA.DESZALAY
DepartmentofBiologicalSciences
KentStateUniversity,Kent,Ohio44242-0001
ABSTRACT
Wetestedthehypothesisthatphosphorusenrichmentinlakesbygoosedroppingscanenhance
growthofalgaeandmacrophytesatgoosedensitiesatorabovethoseoccurringinnature.Seasonal
changesinCanadagoosepopulationsweredeterminedonfivenortheasternOhiolakestoestimatethe
amountofnutrientsaddedasgoosedroppings.Numbersofgeesewerehighestduringwintermigra-
tion, but someresidentgeese remainedyear-round. In alaboratoryexperiment, effectsofvarious
goosedensities(0-2.6x105geese/ha)weresimulatedbyaddingdifferentamountsofdroppingsweek-
lyto 1-Lbottlescontaininglakewater,algaeandCeratophyllumdemersum.Afterfourweeks,dis-
solvedtotalphosphorus concentrations inbottlesreceiving 1 dropping/week(-6000pg P/L) was
higherthaninbottleswithothertreatments(16-75pgP/L).Phytoplanktonchlorophyllalevelsand
periphytonbiomasswerehighestinthe 1 dropping/weektreatment. Ceratophyllumdemersumbio-
massincreasedby4.5 g/plantinthe0.1 dropping/weektreatment,butplantssenescedin 1 drop-
ping/weektreatment.Wecalculatedhowmuchphosphoruswasaddedingoosedroppingsandhow
muchwastakenupbyCeratophyllumdemersumandperiphytonbiomassineachtreatment,andwe
estimatedthatplantstookupmostoftheavailablephosphorusin0,0.0001,and0.001droppings/week
treatments.However,therewasasurplusofphosphorusin0.01,0.1,and1droppings/weektreatments,
whichsimulated2.6x10\2.6x 104,and2.6x 105geese/ha,respectively.Theseresultsindicatethat
goosedroppingscanincreasemacrophytegrowthandpromoteaswitchfromclearlakesdominated
bymacrophytestoturbidlakesdominatedbyalgae,butthiswillprobablyonlyoccuratgoosedensi-
ties(26,000-260,000geese/ha)thatarenotlikelytooccurinlakesinOhio.
Introduction achievingthisgoalcanbedifficult.Forexample,contamina-
Lakesareimportantsourcesfordrinkingwaterandrecre- tionbytoxinsandalgalbloomsarecriticalissues inlakes
ational activities such as swimming, boating, fishing, and usedfordrinkingwater,andhighratesofnutrientinputsin
waterfowlhunting.Althoughtheneedtodevelopsustainable many areas make itdifficultto maintain macrophyte beds
management strategies for multiple use areas is growing, usedasrefugiaforjuvenilefishinlakesusedforrecreation
^Correspondingauthor;presentaddress:TheClevelandMuseumofNaturalHistory,1WadeOvalDrive,Cleveland,Ohio44106-1767;email;[email protected]
34 BRUTSCHEKEIPERANDDESZALAY No.54
(Mossetal., 1980;BronmarkandWeisner, 1992). enrichmentbygoosedroppingscouldenhancegrowthof
Interactions amongnutrientlevels andgrowthofalgae periphyton, phytoplankton, and Ceratophyllum demer-
andmacrophytesarecomplex(Vollenwieder, 1968;Correll, sum,andweexaminedifthiswouldoccuratgoosedensi-
1998). Forexample,rootedmacrophytescanobtain nutri- tiesoccurringinnature.
ents from sediments, but phytoplankton and non-rooted
macrophytes competefordissolved nutrients in the water Methods
column (Barko and James, 1998). Ceratophyllum demer- Studysitedescription
sumL.isacommonnon-rootedmacrophytefoundthrough- All surveyed lakes (East and West Twin Lakes, Lake
out temperate regions ofthe world (Sculthorpe, 1967, p. Hodgson,LakePippen,andLakeRockwell)arelocatedin
167), andabsorbs most ofits nutrients with foliaruptake northeasternOhio(PortageCo.).EastandWestTwinLakes
(Denny, 1972). This species can reduce phytoplankton are4kmnorthofthecityofKent.ThewatershedoftheTwin
abundancewithshading(Goulder, 1969)andbytherelease Lakes is mostly residential, and the shorelines are largely
of allelopathic chemicals (Nakai et al., 1999). However, mowedlawns(Cookeetal., 1977).Bothlakesareusedfor
highturbidityfromphytoplanktonorresuspensionofinor- recreational purposes, including swimming, fishing, and
ganic sedimentsfrombioticfactors(e.g.,feedingcarp)or boating(Cookeetal., 1977). LakesHodgson,Pippen, and
abiotic factors (e.g., wave action) will inhibitmacrophyte Rockwell are surrounded by deciduous and coniferous
growthandpromotedominancebyphytoplankton(Spence, woodlandwithscatteredresidentialhomes.LakeHodgsonis
1982;Nagidetal.,2001;Wetzel,2001,p.626). about5kmeastofKentandisadrinkingwaterreservoirfor
Recently, expanding populations of Canada geese thecityofRavenna.LakesPippenandRockwellareabout4
(Branta canadensis L.) have become an important man- kmnortheastofKent.Botharedrinkingwaterreservoirsfor
agement concern in Ohio’s lakes. Tens ofthousands of thecityofAkron. Morphologicalandhydrologicaldataof
Canada geese pass through northern Ohio (Ohio thestudylakesarelistedinTable 1.
DepartmentofNaturalResources,2002)astheytravelto
andfromtheirwintering grounds inthe southernUnited Goosepopulationsurvey
States and breeding grounds in Canada. Since the late SeasonalchangesinCanadagoosenumbersonEastand
1960swhenCanadagoosepopulationswereattheirlow- WestTwinLakes, LakeHodgson, LakePippen, andLake
estlevels,populationshavesteadilyincreasedthroughout Rockwellweredeterminedbycountingthenumberofgeese
NorthAmerica(Ankney,1996;NationalAudubonSociety, using each lake once every 2-4 weeks from February to
2002). Historically, mostCanadageeseonly occurred in October 1999. Surveys wereconducted withbinoculars at
Ohioduringthemigratoryperiod(Bookhoutetal., 1989). eachsurveysite(20-30minutespersite)between12:00and
Todayhowever,manygeesespendtheentireyearinOhio, 17:00.Allgeesethatwereswimmingonthelake,andgraz-
andthesearecalled“resident”geese. ing, sleeping, or loafing near the edges ofthe lake were
Waterfowlcancontributesubstantialamountsofnitro- counted.Fewgeesewereobservedflyingduringthesurvey,
genandphosphorustolakesandwetlandswiththeirdrop- andthereforeindividualgeesewereprobablynotcountedon
pings(Mannyetal.,1975;BazelyandJefferies, 1985;Gere morethanonelakeduringasurvey.
andAndrikovics, 1992;Marionetal., 1994;Mannyetal.,
1994; Mitchell and Wass, 1995; Moore et al., 1998). Nutrientadditionexperiment
Canada geese graze on aquatic and terrestrial vegetation In2001,alaboratoryexperimentwasusedtodetermine
andareoftenfoundinurbanareas.Geesereleaseanaver- thegrowthresponseofC.demersumandalgaeacrossawide
ageof28droppingsperday(Mannyetal.,1975),anddrop- rangeofgoosedensities.EastTwinLakewaschosenasthe
pings are a source ofnitrogen and phosphorus in lentic model lakebecause extensive limnological andecological
habitats(Mannyetal.,1994;Mooreetal.,1998;Kitchellet work has been conducted there in the past (Cooke et al.,
al., 1999). Therefore, large populations ofCanada geese 1977; Fleischmann, 1999; Lombardo, 2002). On 17 June
have the potential to contribute significant amounts of 2001, C. demersumplantsandfreshgoosedroppingswere
nutrients to lakes (Conover and Chasko, 1985; Ankney, collectedatEastTwinLake.Droppingswerestoredfrozen
1996). However, the limnological impact of increasing untilusedintheexperiment.Plantsthatwereapproximately
populationsofgeeseinlakesinOhioisnotclear. 15-20cminlengthwerechosenfortheexperiment,andall
In this study, five lakes in northeast Ohio were sur- macroinvertebrateswereremoved.Plantswereblottedwith
veyed to determine seasonal changes in populations of apapertowelfor10secondsandtheirinitialwetweightwas
Canadageese,andgoosedensitieswereusedtoestimate measured. Wide-mouthNalgene, bottles (1-L) werefilled
theamountofnutrientshypotheticallyaddedbygeeseto withfiltered(212pmmesh)EastTwinLakewater,andone
oneoftheselakes (EastTwinLake). Laboratoryexperi- plantwasrandomlyassignedtoeachbottle.Periphytonden-
ments were used to test the hypothesis that phosphorus sitywassampledbysuspendingoneglassmicroscopeslide
2004 NUTRIENTENRICHMENTBYGEESE 35
Table1.Physicalcharacteristicsofthefivelakessurveyedin1999.
East West LakeHodgson2 LakePippin1 LakeRockwell1
TwinLake' TwinLake1
Latitude 41°12TSf 41°12'N 41°08'N 41°11'N 41°11'N
Longitude 81°20'W 81°21'W srn'w 81°19'W 81°20'W
Watershedarea(ha) 334.5(bothTwinLakes) 376.5 410 53,612.8
Altitudeabovesealevel(m) 318 319 330 321 321
Lakesurfacearea(ha) 26.9 34.0 77.5 49.0 250.9
Maximumdepth(m) 12.0 11.5 20.7 23.7 9.1
Meandepth(m) 5.0 4.3 7.8 11.9 3.1
Volume(m!) 13.5x105 15.0x10s n/a 2.7x106 7.6x10s
Waterrenewaltime(years) 0.54 1.49 n/a 1.0 0.05
1Cooke,1977
2Olive,1961
5K.Coy,AkronCityWaterSupply,personalcommunication
on the side ofthe containerwith dental floss. Periphyton ofEastTwinLakeis26.9ha,ourtreatmentswereequivalent
wereallowedtocolonizetheslidesduringatwo-weekcon- toapproximately:0;26;2.6x 102;2.6x 103;2.6x 104;and
ditioningperiod.Anymacroinvertebratesthatwereobserved 2.6x105geese/ha(Table2).
inthebottlesduringthisperiodwereremoved. Aslurryofgoosedroppingswasmadefresheachdayitwas
Physical characteristics ofsome goose droppings were neededbymixing10randomlyselectingdroppingswith1Lof
determined. Droppings were thawed and wet weight was distilledwater.Dilutionsofslurrythatcorrespondedtodiffer-
measured.Dryweightwasmeasuredafterdryingthedrop- entgoosedroppingtreatmentswereaddedonceweekly;con-
pingsat105°Cfor24hours.Moisturecontentofdroppings trolsreceivedanequalvolumeofdistilledwatereachweek.
wascalculatedbysubtractingdryweightfromwetweight. Eachtreatmentwasreplicatedfivetimes(30bottlestotal),and
Ashfree dry weight (AFDW) was determined afterdrop- theexperimentranforfourweeks.Allbottleswereheldinan
pingswereignitedinamufflefurnaceat550°Cfor60min- environmentalchamberat 15°Cwitha 14hlight/10hdark
utes.OrganiccontentwasdeterminedbysubtractingAFDW cycle, and fight levels were 22-28 microeinsteins/m2/sec at
fromdryweight.Totalphosphoruscontent(TP)ofindivid- 400-700nm.Distilledwaterwasaddedtoeachbottleevery
ual droppings was estimated by multiplying dropping dry 4—7daystocompensateforlossesfromevaporation.
weightby1.5percentasdescribedinMannyetal.(1994). Attheendofthetwo-weekconditioningperiodandatthe
Afterthetwo-weekconditioningperiod,sixgoosedrop- endofthefourweekexperiment,wesampledthewatercol-
pingtreatmentswereestablished: 1)control:nogoosedrop- umnbyslowlypipetting20mlofwaterfromeachbottlewith-
pings added; 2) 0.0001 droppings added/week; 3) 0.001 outdisturbingany soliddebris (e.g. undissolveddroppings,
droppings added/week; 4) 0.01 droppings added/week; 5) senescentplants)inthecontainer.WatercolumnTPwasmeas-
0.1 droppingsadded/week;and6) 1 droppingadded/week. uredusingstandardmethods(Clescerietal., 1998,p.6-139).
Wecalculatedthenumberofdroppings (X)thatwouldbe Phosphoruscontentofsoliddebriswasnotmeasured.Atthe
depositeddailytoEastTwinLake(volume= 13.5x10sL) endoftheexperiment,periphytonbiomassonthemicroscope
togettheequivalentconcentrationforeachtreatmentas: slideswasmeasured,andphytoplanktonresponsewasdeter-
minedbymeasuringchlorophyllaconcentrationsinthewater
X=[(no.ofdroppingsperL)* 13.5x108]/7 columnusingstandardmethods(Clescerietal., 1998,p. 10-
18). Final wetweight ofeach C. demersum was measured
Mannyetal.(1975)estimatedthatoneCanadagoosereleas- afterblottingtheplantfor10secondsonapapertowel.Mean
esanaverageof28droppingsperday,andwecalculatedthe changeinC.demersumbiomasswasdeterminedbysubtract-
numberofgeese(Y)thatwouldproducethenumberofdrop- inginitialwetweightfromfinalwetweight.
pingsdepositeddailytoEastTwinLakeforeachtreatmentas: PhosphorusuptakebyC. demersumandperiphytonwas
estimatedtodeterminewhethertheirgrowthwaslimitedby
Y=X/28 the amount ofphosphorus available in the different treat-
ments.Available phosphorus was calculatedby addingTP
These calculations show thatourtreatments simulatedthe measuredinthewatercolumnattheendoftheconditioning
expected amount of droppings added to East Twin by periodtoTPaddedindroppingsoverthefourweekexperi-
approximately:0;69;690;69,000;690,000;and6,900,000 ment.Periphytonbiomassineachcontainerwasestimatedby
geese,respectively.Furthermore,giventhatthesurfacearea calculatingtheamountofperiphytonexpectedtooccuronthe
1
36 BRUTSCHEKEIPERANDDESZALAY No.54
Table2.EffectsofgeeseatEastTwinLakesimulatedinthenutrientadditionexperiment.
Simulatedconditions3
Treatment1 TPadded(jig)2 Geese Geese/nv’ Geese/ha TP/lake(kg)
0 0 0 0 0 0
0.0001 11.04 6.9x102 5.2x10-4 26 14.9
0.001 1.104x102 6.9x103 5.2x10-' 2.6x102 1.49x102
0.01 1.104x105 6.9x104 5.2x10-2 2.6x103 1.49x103
0.1 1.104x104 6.9x105 5.2x10-‘ 2.6x101 1.49x104
1 1.104x10s 6.9x106 5.2 2.6x10s 1.49x10s
1Numberofdroppingsaddedperweekto1-Lbottles
2AmountofTPaddedasdroppingsoverthefour-weekexperiment
3Numberofgeese(Geese),numberofgeeseperm'ofwater(Geese/m1),numberofgeeseperha(Geese/ha),andTPaddedoverfourweeks(TP/lake)atEastTwinLake
simulatedbyeachtreatment
inside ofthe 1-Lbottle (surface area: 0.055 nr) given the levelsduringApril-Octoberwhenresidentgeeseremained
meanperiphytondensitymeasuredonmicroscopeslidesat inthearea(Figure 1).Goosepopulationswereanorderof
endoftheexperimentforeachtreatment.Publisheduptake magnitude higher during the migration period (mean
rates(McCormicketal„2001)forperiphyton(1000pgTP/g geese/1ake = 136) than during the rest ofthe year (mean
periphytondryweight/day)wereusedtoestimatephosphorus geese/lake=11).TheTwinLakesandLakeRockwellhad
uptake by periphyton over the four week experiment. thehighestgoosepopulationsinFebruary-March(400-500
Lombardo(2002,p. 125-134)reportedthatuptakeratesfor geese),butnumbersinalllakeswere<70geeseinthesum-
C. demersum ranged from 1.7-3.2 pg TP/g plant wet mer and fall. Physical characteristics and estimatedphos-
weight/day.BecauseC.demersumbiomassincreasedduring phorus contentofgoose droppings collected atEastTwin
thefourweekexperiment,expectedphosphorusuptakewas LakearepresentedinTable3.
estimatedforinitialandfinalC.demersumbiomasswithboth
low(1.7pgTP/gplantwetweight/day)andhigh(3.2pgTP/g Nutrientadditionexperiment
plantwetweight/day)uptakerates.Therangeoftotalphos- InitialTotalPhosphorus(TP)inthewatercolumnwasnot
phorusuptakeinthebottleswascalculatedbyaddingperi- significantly different among treatments (P > 0.05) and
phytonuptaketotheminimumC. demersumuptake(initial rangedfrom43-80pgP/L(Table4).FinalTPwasnotdif-
biomass; lowuptakerate) andthemaximumC. demersum ferentamongtreatments(F=90.11,P<0.001)exceptthat
uptake(finalbiomass;highuptakerate).Wecalculatedthis levelsinthe1dropping/weektreatment(5984pgP/L)were
usingtheaverageinitialandfinalbiomassoveralltreatments. significantlyhigherthanallothertreatmentswhichranged
Wecouldnotestimatephosphorusuptakebyphytoplankton from16-75pgP/L(Table4).
becausewedidnotdirectlymeasuretheirbiomass,therefore Initial C. demersum biomass ranged from 3-5 g/plant
ourestimateduptakeratesaresomewhatconservative. (Figure2)andwasnotsignificantlydifferentamongtreat-
ments(P>0.05).Ceratophyllumdemersumgrewwellinall
Statisticalanalyses treatments,exceptmanyplantssenescedinthehighesttreat-
Datawere analyzedusing SigmaStatVersion 2.0 (SPSS ment(1dropping/week)bytheendoftheexperiment.There
Inc.Headquarters,Chicago,Illinois)withanalphaerrorsetat wereno significantdifferencesin final C. demersumbio-
0.05forallanalyses.Datawerefirsttestedfornormalityand massamongtreatments (P>0.05). However, therewasa
equalvarianceandwerelog (x+1)transformedwhenneces- generalpatternthatbiomassincreasedslightlyfrom0to0.
10
sary.Datacollectedattheendoftheconditioningperiodand droppings/weektreatmentsandthen sharplydeclinedat 1
attheendoftheexperimentwereanalyzedseparately.Total dropping/week,asplantsdiedoff(Figure2).ChangeofC.
phosphorus,chlorophylla, C. demersumbiomass,andperi- demersum biomass was different among treatments (F =
phytonbiomassmeanswerecomparedamonggoosedropping 5.885,P<0.002). Duringthefourweekexperiment,bio-
treatmentswithone-wayANOVAsfollowedbyTukey’spost- massofC. demersumdecreased 1.4g/plantinthe 1 drop-
hoctestwhensignificantdifferencesweredetected. ping/weektreatment, which was differentthan in the 0.1
dropping/week treatment where biomass increased 4.5
Results g/plant.ChangesinC.demersumbiomasswerenotsignifi-
Canadagoosepopulations cantlydifferentamonganyothertreatments.
Goosepopulationsonmostlakeswerehighestduringthe Periphytonbiomasswashigherinthe 1 dropping/week
migrationperiodinFebruary-Marchanddeclinedtolower treatmentthanallothertreatments (F -4.898,P<0.005)
1
2004 NUTRIENTENRICHMENTBYGEESE 37
Figure1.CanadagoosepopulationsonfivenortheastOhiolakesin 1999.
(Figure 2), and chlorophyll a levels were higher in the 1 addition of nutrients in their droppings. Ceratophyllum
dropping/week treatment than the 0.0001 dropping/week demersumandalgaegrewslowlyincontroltreatmentsthat
treatment(F= 11.716,P<0.001)(Figure2).Thewaterin had little availablephosphorus. There was aninsignificant
thecontroltreatmentbottleswasalmostclear.Waterinthe1 trendofhighergrowthwithincreasingphosphorusaddedin
dropping/weektreatmentbottleswasturbidwithadarkyel- droppings.ThehighestC.demersumandalgaegrowthwere
lowish-browncolor,andthesurfacewascoveredwithdebris foundinthe0.01 and 1 dropping/weektreatments,respec-
fromdroppingsanddeadC.demersum.Waterinthe0-0. tively.Thesetreatmentsalsohadthemostsurplusphospho-
dropping/week treatment bottles was light yellow with rus. Muchofthephosphorusnottakenupbyplantgrowth
almostnofloatingdebris. remained in the bottles as debris from undissolved drop-
Table5reportstheamountofphosphorusthatweestimat- pings,especiallyinthe 1 dropping/weektreatmentbottles.
edwasavailableinthebottles,andtheamountofphosphorus Therefore,additionalphosphoruswouldbecomeavailableif
takenupbyC.demersumplantsandperiphytonineachtreat- thismaterialdecomposed.Thesedatasuggestthatgrowthof
ment.Estimatedtotalphosphorusuptakebyperiphytonand C.demersumandperiphytonatthe0-0.001droppings/week
C. demersumwasaboveavailableTPinthe0,0.0001,and treatmentswerelimitedbyavailablephosphoruslevels,but
0.001droppings/weektreatments,buttherewasasurplusof morephosphoruswasaddedathighertreatmentlevelsthan
TPinthe0.01,0.1,and1droppings/weektreatments. couldbetakenupbyC.demersumandalgae.
Shallowlakestendtobeeitherturbidwithabundantphyto-
Discussion planktonorclearwithabundantmacrophytes(Schefferetal.,
Thisresearchdemonstratesthatgeesecanaffectperiphy- 1993).Habitatsthatareextremelyeutrophic(> 1000pgP/L)
ton, phytoplankton, and macrophytes in lakes through the suchaswastewatertreatmentplantsaregenerallydominatedby
1
38 BRUTSCHEKEIPERANDDESZALAY No.54
Table3.Mean(±onestandarderror)physicalcharacter-
isticsofonegoosedropping.
N1 Mean±SE
Wetweight(g) 72 11.9±0.9
Dryweight(g) 72 1.8±0.1
Ashfreedryweight(g) 24 1.07±0.1
Percentmoisturecontent 72 84.5±1.8
Organiccontent(g) 24 0.70±0.08
Pcontent(g)2 72 0.0276±0.002
1Indicatesthenumberofdroppingsusedtodeterminethemeasurement
2EstimatedfromdatapresentedinMannyetal.(1994)
Table4.Mean(±onestandarderror)totalphosphorus
(TP)concentrations(pgP/L)measuredinthewatercol-
umnof1-Lbottlesofnutrientadditionexperiment.Each
treatmentwasreplicatedfivetimes.
Treatment1 InitialTP2 FinalTP’
Mean±SE Mean±SE
(N=5) (N=5)
0 47.21±6.52 16.11±1.78a
0.0001 56.50±7.40 42.11±9.35a
0.001 70.43±15.01 34.51±9.26a
0.01 43.34±5.39 38.83±8.30a
0.1 50.31±4.24 74.78±12.38a
79.72±12.75 5983.90±295.85b
1Treatmentsarethenumberofdroppingsaddedperweek.
2TPattheendoftheconditioningperiod
3TPattheendoftheexperiment.Valueswithdifferentlettersaresignificant
lydifferent.
350
phytoplanktoncomprisedmostlyofcyanobacteria(Jeppesenet
al., 1994).Becausephytoplanktongettheirnutrientsfromthe 300
watercolumn,algalbiomassislowunderlownutrientcondi-
250
tions(Moelleretal., 1988).Therefore,oligotrophiclakeswith
lownutrientlevels (< 10pg P/L) areusually dominatedby =CO 200
macrophytesthatobtainnutrientsfromsediments(Schefferet -QOC.
al., 1993).Lakescanexistineitherstateatintermediatenutri- o 150
entlevels(Ballsetal., 1989;BekliogluandMoss, 1996),but O
Irvineetal.(1989)suggestedthatoncephytoplanktonaredom- 100
inant,macrophytescanbepermanentlysuppressedduetolight
50
attenuation. In the nutrient addition experiment, algae were
mostabundantatconcentrationsof-6000pgP/L(i.e.,thefinal
TPin1dropping/weektreatment),butthehighestgrowthofC.
demersumoccurredat75pgP/L(i.e.,thefinalTPin0.1drop- Treatment
ping/week treatment). Therefore, the switch from a macro-
phytetoanalgaedominatedsystemoccurredbetweenthe0. Figure2.AmountsofC.demersum,periphyton,andchloro-
andthe 1.0dropping/weektreatments,whichcorrespondsto phyllainthenutrientadditionexperiment.Treatmentindicates
densitiesof26,000-260,000geese/ha. thenumberofdroppingsaddedperweek.A,mean(±onestan-
SeveralfactorsmayhavecausedthedramaticdeclineofC. darderror)wetweight(g)ofC.demersumatendofcondition-
demersum atthe 1 dropping/weektreatment. Forexample, ingperiod(initial)andatendofexperiment(final).B,final
decreased light on C. demersum leaves caused by phyto- mean(±onestandarderror)periphytonbiomass(g/m2)collect-
planktonorperiphytonandelevatedconcentrationsoftoxic edonmicroscopeslides.C,finalmean(±onestandarderror)
chlorophyllaconcentration(pgChia!L)inthewatercolumn.
2004 NUTRIENTENRICHMENTBYGEESE 39
Table5.Estimatedphosphorus(pg)availableandtakenupbyC.demersumandperiphytoninthefour-weeknutrientaddi-
tionexperiment.
Treatment' TPavailable3 C.demersum1 Periphyton4 Totaluptake5
Initial Final
low high low high minimum maximum
0 47.21 198.40 373.45 307.71 579.23 18.47 216.87 597.70
0.0001 67.54 207.54 390.66 312.29 587.84 22.28 229.82 610.12
0.001 180.00 200.40 377.22 281.68 530.22 19.35 219.75 549.57
0.01 1147.00 212.01 399.08 330.03 621.23 14.37 226.38 635.59
0.1 11,090.00 195.45 367.90 408.77 769.44 21.11 216.56 790.55
1 110,480.00 197.35 371.48 129.73 244.19 65.09 262.44 309.29
1Numberofdroppingsaddedperweek
3Totalavailabledissolvedphosphorus(pg/L)=meanTPinwatercolumn+TPindroppingsaddedto5replicatesofeachtreatmentduringthe4weekexperiment
3C.demersumuptakeestimatedusinginitialandfinalmeanplantbiomassandlow(1.7pgTP/gplantwetweight/day)andhigh(3.2pgTP/gplantwetweight/day)uptake
ratesreportedbyLombardo(2002,p.125)
4Periphytonuptakerates(1000pgTP/gperiphytondryweight/day)reportedinMcCormicketal.(2001)
5CombineduptakebyC.demersumandperiphyton.MinimumuptakeestimatedforinitialC.demersumbiomassandthelowuptakerate.Maximumuptakeestimatedfor
finalC.demersumbiomassandthehighuptakerate.
compoundsfromblue-greenalgaeordecompositionofdrop- we surveyed. Although the highest treatment (1
pings could have negatively affected C. demersum plants. dropping/week) caused asharp increase in phytoplankton,
Anotherpotentialmechanismisthatfloatinganddeposited goosedensities (260,000geese/ha) simulatedbythistreat-
debris from droppings directly shaded the C. demersum mentareunlikelytooccurinOhio.Forexample,peaknum-
plants. Shadingby inorganic sedimentshas been shownto bersofgeeseobservedinnorthernOhioin2001 wereonly
decrease growth of submerged macrophytes and increase 55.000 geese (Ohio Department of Natural Resources,
algalgrowth(Houghetal.,1989).Moreover,highnumbersof 2002).Furthermore,thenumberofgeesecorrespondingto
waterfowlcanremovealargepercentofvegetationinlakes 260.000geese/hacouldnotphysicallyfitonthesurfaceofa
andwetlands(GirouxandBedard, 1987;Woolhead, 1994), lakeatonetime.Therefore,thegoosedensitiessimulatedat
whichwouldotherwiseshadeoutphytoplanktoninthewater the0.1 and 1.0droppings/weektreatmentarenotlikelyto
column.Furtherresearchcouldexamineifgoosegrazing,in occurinlakesinOhio.However,itisimportanttonotethat
conjunctionwiththeadditionofnutrientsanddebrisintheir others(e.g.,Kitchelletal., 1999)havereportedthatnutrient
droppings,canactasasuiteofco-occurringfactorsthatpro- additionsfromhighnumbersofwaterfowlcancausealgal
moteconditionsleadingtoturbidalgae-dominatedlakes. bloomsinsomenaturalaquatichabitats.Theeffectsofnutri-
ent addition by geese will interact with other abiotic and
Managementimplications bioticfactorssuchasnutrientenrichmentfromothersources,
Numbersofgeeseobservedatthefivelakeswesurveyed bioticstresses,orco-occurringchangesinpH,temperature,
(Figure1)werebelowthosewehaveshowntopromotealgal orlightlevels.Forexample,lakeswhereherbivoresreduce
bloomsorsuppressmacrophytegrowth.Forexample,peak macrophytebeds mayhave algalbloomsatlowernutrient
numbersofmigratorygeeseobservedatEastandWestTwin addition levels than lakes with abundant macrophytes.
Lakeswouldhavehypotheticallyaddedonly-400gP/day. Furthermore,inputsofgoosedroppingstolakesmaynotbe
IfalloftheirdroppingsweredepositedinEastTwinLake,it constantovertimeorwithinalake.Forexample,if690geese
wouldraisephosphorusconcentrationsinthelakebyonly overwinteredforthreemonthsonafrozenareawithinEast
2.3x 10"9pgP/L.EastTwinLakewatercolumnconcentra- Twin Lake, about 45 kg ofphosphorus could accumulate
tionsmeasuredinthisstudywere43-79pgP/L,andcon- fromgoosedroppings.Thisamountofphosphorusmayhave
centrationsreportedbyCookeetal. (1977)were76-94pg alocalimpactonalgaeandplantsaftericemeltinspring.
P/L.Therefore,inputsfrompeaknumbersofgeeseobserved Moreresearchisneededtofully understandhownutri-
ontheselakeswouldhavenegligibleeffectsonlakenutrient ents from geese affect macrophytes and algae in lakes.
levels. Numbers of geese were slightly higher at Lake Microcosmexperiments, like our nutrient addition experi-
Rockwell.However,thislakeismuchlargerthantheTwin ment,donotalwayspredictbioticresponsesinnaturalhabi-
Lakes, and therefore, expected impacts of the additional tatsbecausetheremaybescale-dependenteffectsoftreat-
nutrientswouldbeminimal. ments(Petersenetal., 1999),andbecausescientistscannot
Ourexperimentsdemonstratethatlowtomoderategoose testforinteractions among all potential variables (Cooper
populations probably do not addenough nutrients to pro- andBarmuta, 1993). Forexample,otherfactorshavebeen
moteaswitchfrommacrophytetoalgaldominanceinlakes showntoaffectwhetherlakesswitchbetweenmacrophyte
40 BRUTSCHEKEIPERANDDESZALAY No.54
andphytoplanktondominatedequilibriaincluding: 1)inver- Clesceri,L.S.,A.E.Greenberg,andA.D.Eaton(eds.). 1998.
tebrate herbivory ofepiphytes and phytoplankton (Timms Standard Methods for the Examination of Water and
andMoss, 1984;BronmarkandWeisner, 1992);2)fishpre- WBaalsttiemwoartee.r,115T6wpe.ntieth edition. United Book Press,
dation of herbivorous zooplankton (Grimm, 1989); 3) Conover, M. R., and G. G. Chasko. 1985. Nuisance Canada
macrophyte production ofalleopathic chemicals that sup- gooseproblemsintheeasternUnitedStates.WildlifeSociety
pressphytoplankton(Wium-Andersonetal„ 1987); and4) Bulletin,13:228-233.
turbidity caused by wave action (Schiemer and Prosser, Cooke,G.D„D.W.Waller,M.R.McComas,andR.T.Heath.
1976;Nagidetal.,2001).However,thedesignofournutri- 1977. Limnological and geochemical characteristics ofthe
entadditionexperimenteliminatedallofthesevariables,and TwinLakeswatershed,Ohio,p.242-270.—InL.SeybandK.
treatmenteffectsmayhavebeendifferentunderothercondi- Randolph(eds.),NorthAmericanProject AStudyofU.S.
Water Bodies. EPA-600/3-78-033. USEPA, Environmental
tions(e.g.,inmacrophytebedswithhighspeciesdiversityor ResearchLaboratory,Corvallis.
atdifferentmacrophytedensities).Moreover,ourexperiment
oshnolryttpeestreidodth(ei.re.e,s4ponwseeekosf).plLanotnsg-atnedramlgiamepaocvtesraofrenluattriiveelnyt Coopm(eeodnrsi,.t)S.o.rFiDrn„ge,sahpnw.dat3Le9.r9A—B.4iB4oam1ro.mnIuinttaoD.r.i1nM9g.93aR.nodFsieeBnlebdneterhxgipcearnMidamceVrn.otisHn.vineRrbetiseoh--
additionmaybedifferent. Futureresearchshouldexamine brates.Routledge,Chapman,andHall,NewYork.
interactions of nutrient addition by geese, algal grazers, Correll,D.L.1998.Theroleofphosphorusintheeutrophication
predatoryfish,andwindactionundernaturalconditions. of receiving waters: a review. Journal of Environmental
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