BY lll

THE EFFECTS OF GRAZINGBY LlITORINlDGASTROPODSONlllE STRUCTUREuFALGALCOMMUNITIESIN

NEWFOUNDLAND TIDEPOOLS

BY

©MARK D. HAWRYLUK B.Se.Hon.

Athesis submittedtothe Schoolof GraduateStudies inpartial fullilmento fth ercquircments

for thedegree of Masterof Science

Departmentof Biology Memorial Universityof Newfou ndland

September 1992 Re vised September199 5

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ABSTRACT

GrazingbytheJiltorinid gastropods,Liuonna littoreaandLsaxailis.

significantlyaffectedthe struct ureofalgalcommunities in 10 interti dalpools onthe AvalonPeninsula ofinsular Newfoundland.Pools were sampledapproximately monthly between Feb ruary andNovemberof1987.Diversity of the algalcommunities was highestat intermediatelevels ofgrazing and lowestwhen a givenspecies ofalgae formedamonoculturcinthe absenceofgrazing.Diversity was alsolowinpools with verylarge grazer populationswhichovergrazed themacroph ytes. Thediversi tywas relatively stableinmoderately and heavily grazedpoolsbut fluctua ted widelyin lightl y grazedpools.

TIle greatesteffectthatthegrazershad onthealgaewas 10increase theamount of bare substrateandcalcareousalgae byremoving thefllemenrous and blade-fo rming algae.Lntorinatiuo reaandLittorinasaxotilisprevented blade-formin g algaefrom establishing by graz;:;gsettlingpropagulcsbUIonlyL.tutoreowas able to slgmficantly reducetheabundance of the adultthallusof filamentousalgaewhich scruedbefore the gra7.c r~becameactivein the spring.

The populationofLsaxaulisincrea sedtoa peakin midsum merasindividuals movedfrom nearbyupperintertidal emergentsubstrataand offspring were producedin the pools.LargepopulationsofI.tinoreain two of thepools decli ned during the course of the study. Thecauseofthis decli neisnotknown .

ACKNOWLEDGEMENTS

I would liketothankmysupervisor, Dr.DonSteele.aort mysupervisory committee. Dr.A.Whiuick and Mr.R.Hooper(1)1"theirsUPJlClr1andencouragement throughout thisstudy.Asslstancewiththefield workwasgenerou slygivenbyJill Bugden, JillHambrook,and SUZ3nneNurse.1bcWilliamsfamilyofBayBullsiU\ll Mr.D.LeGI1IWgraciouslyprovided me access10 my"tullysues acrosstheir I:md.1 wouldalsolike tothankDr.P.Schwinghamcr whosecommentsgreatlyimprovedthis thes is.

Financial support fo rthis studycame from NSfiRCgramsawardedtn Dr.Don Steeleand from aMemorial University Graduate StudentFellowsh ip.

iii

CONTENTS

AUSTRACf. ACKNOWLEDGEMENTS CONTENTS.

LISTOF TABLES LISTOF HGURES , 'NTROOUC.TJON MATERIAl~~AND METHODS

Site Descriptions.

Bauhnc..•. .. ... ... . .. ... •..

811yBulls .

PortugalCove..

SamplingTechniques.. . StatisticalAnalysis. RESULTS.

TIleGrazerCommunity Patterns ofAlgalAbundance•.

Bladc- forrnln gAlgae FilamentousAlgae Enc rustingAlgae. Calcareousalgae.

Bare Rock . . . ... . .

EffectsofLlttcnnidGrazersonSpecies Diversity Physical CharacteristicsofTide- eels . DISCUSSION

Summary REFERENCES APPENDIX A

iv

.. . .. .iii tv

vi

f

"

"

9 10 10 13 16 16 17 17 23 26 28 29 31 33 35 46 48 53

LIST OF TABLFS

Table1.Physical characteristicsof individual tidepcols.. . . ..R

Table2.Speciesofmacrophylicalgaein eachmorphologicalgroup ..12 Table3.Meangrazing index and numberofgralc rsper 11I1±I sd groupedby

grazingintensity , 14

Table4.Analysisofvariance of%coverofmorphologicalgroupsandLim:rock betweenlevels ofgrazingintensity... ... . . •.. .. ... .. .19

LIST OF FIGURES

Figure I.Location ofstudysites andindivid ualpools.All maps arc oriented

withtrueNorth at thetopofthe page. . ..7

figure2.Mean abundanceofnucrtnds oneachsampling date at Bay Bulls

PoolIand BaulincPool1 ,.. ,.18

Figure3.Mean abundanceofalgaeineachmorphologicalgroupandbare rockat ellengrazing intensitylevelovertheentirestudyperiod ...20 figure4.Meanabundanceofblade-fo nningalgae at eachgrazingintensity

levelper month ...•.. . . .. .. 22

figure5.Mean abundanceoffilamentousalgaeateachgrazingintensity level

per mont h .. . ... . . ... ...25

Figure 6.Mean abundanceof encrustingalgae ateach grazing inten sity lev el

per month. ,.27

Figure7.Mean abundance ofcalcareo usalgaeateachgrazing intensityleve l

~~~ W

Figure R.Shannon DiversityIndex (H')oneach samplingvisitforeach

rldcpocl .. ...32

vi

INTRO DUCTION

Althoughlillieworkhas been publis hed oncommunity wucrurc in the Newfoundlandintertidalzone (Pillman 1974.Hooper 19RI.Botton 198.' .Steele19 K.\.

Hoo per andWhiuic k 1984), agreatdealof informationhas been accumulated concerning facto rs affectingthecommunitystructureof ottertemperate rocky shores.

Inver tebrate grazershavebeensho wn tobe ofprimaryimportance indeterminin gthe structure of benthicalgalcommunities.The mostimportantgroll£l'Sin thewestern NorthAtlanticare seaurchins in the lower intertidaland subtidal(Lubchcnco lind Menge1978.Chapman1981) andgastropods. including limpetsand Huortntd s,inthe middleandupperintertidal zone(Me nge 1975.Lcbcbeocc197f1.Branch1981).

Thoroughreviews ofmarineplant-herb ivoreinteractions have been do nebyLubcbe ncn and Gaines (1981) and Hawkins andHartnoll (1983).

Thefocusofthepresentstudy is to determinethe effects of littorin-d gastropods on the algalcommunityinudepocts on the east coast ofNew found land.

Altho ughsimilarin many respects tobenthic communities inNew Engla nd Iidepools, the tidepoo l communitiesin the present studyexperie nce lowerwater temperatures in higherenergyenvs-cn rremswhichmay cause someinterestingdifferences.Lubchcr cn (1978) showedthatgrazing byLutorinatinoreoinfluenced algaldiversityinNew Englandtidepools.In the absence of grazers,a fewspeciesoffast growingephe meral algaebecame very abundant at the expense of competitivel yinferior species.resulting

inalowdiversity.Ontheothe r hand , overgrazing bylarge numbersofL littoeea removedmo st species andalso resuncdinalow diversity.Inpoolswithintermed iate numbers of grazers,competit iveexcl usionwasprevented and diveB ity washigh.Sze (19RO)foundsimilar resultsin hightidepool s attheIsles ofShoals.

Toa large extent,theeffcc t that a grazer has onalgalcom mun itystructu re is depcl,denton thefoodprefere ncesofthe herbivore.Foodpreferenc e ca n be divided intohVOcomponents:edibilityand attractiveness (Watso nlind Norton1985 ).Studies whichmeasurefeedingratesofherbivores ondifferentspeciesof algae examine the edibilityaspectoffe eding preference(for exam ple.Barkerand Chapma n1990.Imrie ('IaI.19R9).Edibilityi5 influencedbynutritio nalvalueandchemicalandstructural untihcrbivoredefenses sucha5 polyphc nols(infucoid s)andtough cell walls(in calcareous algae).Lubcbcrcc(1978 ) didtheclassic attractiveness experimentwhere twoalgalspecies wereaddedtoatankofLliltorraandthenumber of periwinkle s observedoneachspecieswas countedafter agiventimehad elapsed.Thistypeof experimentincorpora tcs aumcti veness ofagive nspeciesofalgaeas ahabitat aswell a.'ia potential food source.Anracuveness isinfluenced by algal morphologyaswellas the factorsinfluencingedibility.Itthereforemakes sense to combin ealgaewith similarmorphological characteristics into groupsto inve stigatetheeffectsofgrazing on[he m.

Inthepresent study.themacroalgae were divided inl01morphologic algro ups:

blade-forming. filame nlous,encrustingandcalcareousalg aebased onthe irgross

morpholog yas itrelatesto resistanceto grazingby littorin idgastropods.Theblade- formingand filamentousalgaeweresimilar in thatbethgroupsweremostlycomprised ofshort lived (ephemeral)species.The highratio ofphotosynthetictissueto structural tissue,high nutritive valueandlack of chemicalor structuraldefensesagainst herbivoryhave been shown tomake thesespeciesattractiveto grazers (Little rlind Littler1980,Hawkins andHartnell1983) asdemonstratedby Luhchenco(1978), Stencckand Watling(1982),Watsonand Norton (1985),andImrie('IIll.(19R9).The filamentousalgaeweresimply definedas thosespecieswhh unon-co rticatcd upright filamentousthallus.Stencckand Watling (1982)demonstrated that the tucnloglossan radu laofLinorinaisparticularlywell adapted 10 grazingfilamentousalgae.Many spec ies ofalgae passthrough a grazersensitive filamentousstageduringcarly developmentbefore taking on amo re grazer resistantfo rm(Stc ncckand Watling 19 82).

The blade-formingalgae are a moreheterogeneous groupcomprisedof ephemeralthinsheetlike or tubularforms(e guonosmma grevilleiamiScylosipl/fJII

!omelllana),corticatedephemerals(egCIlorriariafltwellifnnnis),and oneupright peren nial(FIICI/Sdistichusdistichus).Mostofthe species in this groupwith the exceptionofFlfC/ISare alsohighlyor moderatelypreferred species ofL!it/orca (Lubchenco 1978).

Incontrast, the calcareousandencrustingspecieshave structuraland/or chem ical defenses which makethem less susceptible to damage bygrazing, Infact,

manyof these grazer resistantfonn shave beenshown to benefitdirectlyor indirectly fromgrazing.C/(J/hmmOlphlllll circumsctiptumrequires moderate levels ofgrazing to prevent epiphyticfoulingof the thallus (Steneck1982). Encrusting algaedominate in protected beachesinNewEnglandwheregrazingby L.nuorearemoves all of thc updgh lalgae(BcrtncssetClI.1983).Both species of calcareousalgaein the present study(ClaJlllmnmplwlncin..'//mscriplmnandCora/linaojjicinolis)are toughred algae withcalciumcarbonate inthe cellwalls.C.cisrnmscnp unnwas considered

"calcareous" rathertitan"encrusting" ducto theincreased grazer resistance providedby thecalcificationof its cellwalls.Steneckand Watling(1982)have shownthat the radula of L.liuoreuis poorlydesigned for excavatingcalcareousalgaesince the teeth havenot been chemically hardened ashavetheradularteeth of Notoocmoeo testudincdiswhichpreferentially grazes calcareousalgac.

Thc encrusting algacin thepresent studywererepresentedmostly by Hildcllbmndia rubcaand abrowncrust.Sinceitwasdifficultto determineinthe field ifthetaucrWDSavalidspecies ofRaJjsiaor an encrusting phaseofScytosiphon lomcfltll1iaorPelaJonia/ arcia.itwillhereafterbereferredto as"RaJjsid' (sellsll Lubchenco andCubit1980). Smalltightlypackedcells through outthethallu s and anuhc rhivorcchemicalcompoundsmake both encrustingspeciesresistant to grazing (Bcrtnessetd.19R3).

Scouring bypack ice in thelate winter andearlyspringof 1987 removed almostallof the upright macrophytesfromthetidepools surveyed inthisstudy. Then,

alens offreshwater trappedunderthe packicekilledmost of theinvertebrates inthe intertidalzone excepttheltnortnids whichsurvived by lightlyscalingtheir aperture with the operculum.Theseevents provided me withan opportunity to studythe effectsof grazing byL.liuoreaandL.saxmitison the recolonizationoftidcpoe ls hy macroalgae.

Tidepoolsof intermediatesize in the midd letoupperintertidalwere chose nat 3 sites,tokeepthephysicaldifferencesbetween poolsas small as possible.Low intertidalpoolswere impossible to samplefrequently dueto thesmalltidalamplitude (0.9 m (Anonymous 19&7») andrelativelyexposed locationsofthestudysites. To avoid thetheoret ical and practical problemsof caged manipulations(secHawkins and Hartnoll1983).poolswere selectedwhichvaried inthenumbersoft.utorinatiuorea;

themajorgrazeronrockyshores of theNorthwestAtlantic(Menge 197.5).

Based onthe observationsof grazeralgalinteractions in a functional fonn framework. the following hypothesesweretested:

Theabundance of blade-forming and filamentousalgae will beinver sely related to grazingpressure.

2. The abundanceof encrustingandcalcareousalgae willbedirectly related to grazingpressure.

3. Diversity willbehighest inmoderatelygrazedpools and lowerin heavily and lightly grazed pools.

MATERIALS AND METIIODS

Site Descriptions

Thestud ysites arcall locatedonthecoast oftheAvalonPeninsula, Newfoundland,Canadain the Western NorthAtlantic.They arc inareas of sloping bedrockwhichdropsoffinto10to20 mofwaterjustpast thelow water mark.This typeofhabitatprovid ed a su fficie nt number of suitable tidepocl s . Thethreestudy sites wereatBaulinc (47"44 'N 52°SO'W) and Portugal Cove (47⁰38' N52°52' W) inCo nception Bay and BayBulls (47⁰18 ' N 52"47'W)on the castcoast(Figure1.).

Pools wereselected on thebasis ofaccess ibility,relative positionin thelittoral lone.size,andfloralandfaunalcommunity(Table1),Accessibility was ofobvious importancesince the pools wouldbesampledmonthlyfromspring until fall. The pools were located inthemidtoupper intertidalzone; extremeupper andlow er intertidal pools wereno t studied. Poolsof interm ediatesizewere selectedsince these we re the most common andhad the best developed alga lco mmunities. Smallpools

«

0.75minlength ) were susceptible tolarge salinitychangesdue to evaporation and rainfall.and tolarge temperaturefluctuations.Large pools (>5min length ) weretoo rareto beincluded.Alltidepools at thethreestudysites withinthissizeandlittoral rangewereincluded in the study.

8aulinc

Baulincisthemostexposed site, with.a northwestaspect.Theshore is open, backed byhigh. steep cliffs withamore gently slopin garea extend ing 10MeanLow

Figure l,Locationofstudysitesand individual pools.All maps are orientedwith trueNorth atthe top of thepage.

Table1.Physicalcharacte ristics of individualtidepools.' Tidepool

Light Moderate H,avy

BB3 BB4 PCl BB2 PC2 BA2 BA4 RA3 HBlBAl Max.lengthtm) 2.0 1.5 3.1 I.' 1.3 0.8 1.2 1.1 1.7 '.9 Max.width(m) 0.7 0.9 1.6 0.8 0.7 05 0.8 0.6 1.2 3.3 Max.depth(em) 18 27 20 IS 17 11 36 2. 23 2.

Height(m) above M.L.W. 1.0 0.9 1.3 1.3 1.4 0.9 0.9 1.6 1.3 1.2 Max.temperature(cC) IS 15 20 2. 25 25 22 20 19 27 , Poolsin ordero(in creasinggrazingintensity. ^{- - _ . -}- - _ . ~ --..

_- -

Water(MLW)andthen dropping rapidly10-20 mdepth.Fourpoolsvarying in exposure to wave actionwere sampledat thisSlIC;Pool I(SAl }andPool2(BA2) are more exposed. Pool3(SA 3)andPool 4(BA4) arc less exposed,

BaulinePool I is the largest pool. A 2 m high wall on itssoutheastside shadesabouthalf of the poolformost of the morning,Poot3issmaller,located ina Imdepressionin thebedrock.Itsrecessed positionprovides moreprotect ionihanit wouldotherwisehave.Pool 4is locatedin a smallcovein thecoastline,and is protectedby a fewlarge boulders,BaulinePool5islocated in a very exposed positionon the slopingbedrockand is exposedto insolatio nformos t ofthe day.All ofthesepools wereregularlyinundated by swellsduring samplingcttow tide.

Day BuJis

The Bay Bullssite hasmorevaried topography tha ntheBau llncsue.Overall.

itis a slightlylessexposed site,havinga south aspect.but theran ge ofexposure amongIhe four poolsis much greaterthan at Baulinc. The least exposed pool in Bay Bulls. Pool 1(8B I) is situated in a cove withinthe bay and ispro tected byIIsmall spitof bedrock directlyseawardofthe pool.The other3 poolsarclocated on a more exposed shoremadeup of alternating surgechan nelsandspits ofbedrock outsideof the cove,There are also many large boulders onthis gent lyslopingshore. Pool2 (BB 2)isthe least exposed ofthe oute rpoolsbeing located lit thehead of one of the surgechannelsand protectedby alarge boulder.Pool 4 (884)isthe next most exposed. locat ednearthe mouthof a protectedsurgechannel,Themostexposed pool,

Pool3 (8 B3), islocated attheendof oneofthespitsbetwee n the surgechannels.It waltregularlyinundated by swellsat lowtide andthereforeoftendifficult to sample.

PoolsIand2weretheonlypoo lsthat werenot oftensuddenly and unpredictably inundated bywaves atlowtide.

PortugalCove

The west facing PortugalCove shorelineistheleast exposed of allthe sites.

Thetwo poolswere rarelyinundatedby swellsduringsampling visits atlowtide, Steepcliffsdrop down to anirregu larbedrockshorelineprovidingvariable degrees of protect iontotidepoolsinthe area.Both PoolI(PC I)and Pool 2(PC2) arelocated in protectedareasofthe shoreline.Thecliffs behindthese poolsalso shadethemfrom directinsolation formost of themorning.Pool I periodicallyexperie ncedsome terrest rial runoffresultinginathin low salinitylayeronthepoolsurface.

SamplingTechniques

A flexiblesheet of clearplastic,0.25 x 0.25 m, similar totheoneusedby Menge (19'/6), wasusedas aquad ratto samplethe algalandgrazercommunities in the ttdcpocts.The quadra twasweighted withleadsinkerstopreventitfromfloating andtominimizeits movement during sampling.100 whitedotswere placed on the quadratin a stratifiedrandom manner 10 preventpotentialexcessiveclumping which couldoccurwitha completelyrand om scheme.To determinetheplacement of the dots, a pieceof graphpapercutto 0.25 x0.25 m was divided into4 even squares.25

\0

pairs of coordinateswere selected from atable ofran domnumbers torepresent the coordi natesofthe dotsineachof thefour squares.After all100 points were selected, thequ ad rat wasplaced overthe graphpaperand a whitedot was marked onthe quadratateach point.

The quadr at wasrandomlyplacedinthe poolandthe speciesof algaeunder eachdot was recorded.Identifica tionand classification ofthemacroph ytes follows SOI.Jlh and Hooper (1980). Algaewerethe nplacedinone of thc morphologica l groups: blade-fo nning, filamentous,calcareo us. andencrusting(Table2). Ind ividuals of eachspeciesof mobilegrazer orsessileinvertebratewithinthequad ratwerethen countedand recorded.This proccsswasrepeatedfor atotal of5 quadratsineachpool oneachsampli ng dale, Pools weresampledat towtide asclo se tomonth ly as possibleconsidering timeof tides andweather.A swellof 1,5to 2.0min this area withameantid alrange of0.9 m(A nonym ous, 1987) was not uncommo n, andoften impeded sampling. Water and airtemperatu re were recordedusing amercury thermom eter each timethe pools were sam pled.Sali nity was measured in the fieldon some sampledatesusing aY.S.I,salinitymeter.Tida lheight was measured relative to a pointof know nheight on the shoreusingan incli nometeronatripod .

The current taxono mic statusofLittorina.~«<aliIi.~isin question . Onthe shores ofnorth westernEurope. fourspeci esarecurrently recognized inihcL. saxal tis speciescomplex(Hilland Grnhame1990). Gilkinsonand Methven (1991)have described the biologyofa subtidalpopulatio nofL.sw:otilisformsausiiisinTrinity

11

Table 2.Species of macrophyticalgaeineachmorphologicalgroup.

-Blade FormingAlgae

Mtainentou.sAlgae-

-CalcareousAlgae EncrustingAlgae

Acrosiphoniaareta C/athromorphumeircumscriptum Ceramiumrubrum Corallinaofficinalis Chaesomorpha Unum

CladophiJrasericea Fragellaria Pilayella littoralis Polysiphonia vroeotoea Sphaeelariaplumasa Tuce dwelling diatom Alarioesculenta

Aseophyllumnodosum Chondruscrispus Chordariajlagelliformis Devalariaramenlaeeum Dicty03iphon/oe nicu1aceus Bnuromoepha imestinalis Fucusdisuchusdisliehus Fucus spiralis Fucus vesicuiosus N Leathesiadifformis

MonoslTomagrevillei MonostromaunduJatum Palmariapalmata Petalonia/ascia Pela/oniasosterfolia Porhyra umbilicaUs Saeehorizadermatodea

Scylosiphon /omenlaria _

Greencrust Hildenbrandiarubra Ralfsiajimgifonnis

"Ralfs;a"

Bay, Newfoundland,Throughoutthe rest of thisthesis.Linorinasatati/jswillbeused tode scribe therough periwinkle found in the intertidalzone, withthe complete unders tanding that the taxonomicstatusmaybechanged in thefuture.

Similarly, the taxonomic status ofNotoocmoeatestudindisis alsoinquestion.

This designationhas been chosenforthe presentstudy tobeconsistentwithother works from thecast coast of NorthAmerica (ex. Petraitis19 89).

StatisticalAnalysis

A grazingindex was devisedlISa measure of grazing intensity. Yurnada and Mansour (1987) have shownthatL, littorea andL.saxailisofthesamesize havethe same abilityto reducealgalstandingcrop. In the present study, the ratioofL.

.soxouttswetweight toL,linoreawetweight was 0.083 (171L. littoreaand172L soxauissampledat BBIandSA Irepresenting the fullsize rangefor each species).

Therefore, the grazing indexwas calculatedas, Glp= nL1+(0,083"I..)

where Glpisthe grazing index forpoolP, "u isthe mean number ofL.liuorea;and '\.isthemean numberofLsaxatilisinpool P.

The poolswere dividedintothree grazing intensity levels basedonthe mean value of the grazing indexoverallquadratssamp ledin eachpool:light,moderate, andheavy graz ing. Tabl e 3 clearlyshowsthe differencesingra7£rabundancebetween intensitylevels. Inthelightly grazedpools,few if anyL.tinorea wereprese nt and numbe rs ofL.saxaiiiswere low. In the moderately grazed pools, numbers ofL

13

Table3.Meangrazingindexandnumber of grazers perm^2:\:;1SOgroupedbygrazingintensity.

Pool Grazing Number of Numberof Index Littorina Littorina Jitrorea soxatilis

(perml) (pe~

high!

BBJ 0,020 0.0:1:0.0 3.8:l::10.6 BB4 0.266 0.0:1:: 0,0 51.2:1::41.4 PCI 0,415 0.0:f::0.0 f.0.O:f::93.2 BB2 0.914 2.1.:1:5.5 158.9 :1::104.6 Mggerate

PC2 1.668 O.O± O.O 321.6:1::231.3 BA2 2.01. 5 0.6:1::3.2 410,2:1::279.5 BA4 2.259 20.5:1:25,9 190,7:1::157.4 BAJ 4,246 39.S::1::31.7 342.9::1::279.7 fum

BBI \5,259 360,4::1::466.5 159,2 ::1:: 167.1 BAI 15.307 330.1:l::25\.8 208.0:1::210.2

14

(iUortawerelowin(WOof

ue

^poolsbutslightlyhigherintheOlhertwo poets.

However,numbersofL sat«iliswere high. Intheheavily grazedpools,numbersof Lhnore awerehigh and nu'llbcrofLsaxai iliswererredcrate(i.e.lcwer jhan inlhe moderatelygrazedpools but higherthan in thelightlygrazedJlOO!5).

Todeterm inethesignificallCeofdifferencesin!heabundance ofalgae:inthe4 morphologicalgroupsandofbarerodbetweengrazing intensitylevels,one way analysisofvaria ncewasused onindividualmorphologicalgroups,Ifa signilicant overallgrazingeffect wasfoundat the0,05probabilitylevel,aScheffe'slestwas used to determinewhich levelsof grazingweresignificantlydifferentfromeachother, Percentcoverdatawerearcsinetransformedbeforestatisticalanalyseswercperformed (Sakal andRohlf1969).Allfiguresshowuntransfo rmcddata.

TheShan non-wie nerindexofdivers ity.H',wascalculatedusinga macroin MINITAB.Theformulaforibcindex is.

H'=-E p,lo& P,

wherePiisthepropor1ionoftheco mmunitybelonging10thei'"species (Shannonand Wuver.I949).

Allstatist icalanalyseswerepcrfonncdu.\ingMINITABandSPSS'onthe DigitalVAX mainframecomputer at Memorial University ofNewfoundland.

IS

RESULTS

TheGrnzerCo mmunity

The graze rcommu nitiesin the tidcpoolsweredominatedbyLiUOri' IQlittorea andUftm1l1tl .fcuot ili Jbutanumberofothergrazerswerealsopresentatvariouslimes duringth estudy.Gammaridamphipods occ u rredinsmall numbersinallpools except BayBullsPoolIandPortugalCovePool 2. Theamphipodsonlyreachedlarg e numbersin BayBullspo o ls3 and 4 in OCtoberamongChordariajfagellifonnis. Early in the ye a r,Notoa moeafe.fWl li llmi,fwaspresentin low numbersinBay BullsPoo lst and 2 andBaulinePools1and 4 andSfITmgyloc elltro tllsdmebodue nsiswas presentin [ow num bersinBaulinePools1and4.The se speci eswere both killedbythe freshwater runoff and did nol retu rntothepools duri ng thecourseofthestudy.

Lcu'lIIWvinctawas sporadicallypresentin lownumbers«5 mol)througho ut thestud yperiodin Bauli ncu- 'Bay Bullspools but was abse nt fromthePortuga l Covepools. Ho we ver,afall recruitmentresu ltedinlargenumbersoftin yL.vincmin Baulinc Pool 2 (9818m·²)and Pool3(5453 m·^l).Nosaruplin gwasdone after the settlementofL.villela,sotheir effectsonthe algal communitycouldnotbe determinedandtheyarcthe refore excludedfro mthegrazerindex.Littodnaobtusaa was obse rved on atleastIoccasionin allpoolsexceptPortugalCovePoolIbutwas onlyabu ndantinBay BullsPool2 (49.6m·^l)and Bau linePool4(26.2m·^l) .Bo th of these poolshad a healthy growth of fueoid al gae,thepreferred habitat ofLootusaa (Srcncck and Watling1982.Hawkinsand Hartnoll1983).

LittonnaIiUomJandLinonnasaailiswerethemostahundantgrazerswith mean valuesoverall quadra tsof97and 193individua lsp..'1'm~respectively.L littoreawasonlyabundantin BayBullsPool1i1J1dBaulinePoo l Ibutin

nesc

two pools,ithadanave rage abundance of360m·: and 330m.Jrespectively.Thenun~'!'5 ofLlinoreadeclinedsteadilythrou g hthestudyperiod(Fig.2).Thereason forthb declineisunclearsincepredatorsofLlinoreawerera relysee n inthetidcpools. L.

littareadidnotbecomeactiveuntilthe water temperatureinthepool!>climbedabove SoCattheend of April.Before thistime.theywereobservedtobeclumpcdtogcuer incracksandcrevicesin aninactive stale.

Littatina saxauawasobserv edin allpoolsandwasabundant (>150m·~ )in allbut 3pools.Itwasbyfarthemostabundantgrazerinthemedcrmcly grazccl pools.butwas lessabundantintheheavilygrazed poolswhereLfi1trm'uwa\

abundant (Table3).Duringthespri ngandea rlysummer, L.<;(mt ifismigratedfrom its overwintering cracks andcrevices intheupper int c nidal.tothe pools.Theninthe fa ll,therough winklesmigratedoulof!hepoolsagain(Fig.2).

Panemsof AlgalAbundance

Blade-formingAlgae

Themeanabundance ofblade- forming algaewas significanlly lower in the moderate lyandheavily grazedpoolsthanin thelightlygraze dpools(Table4,Fig.). Therewas.however, nosignificant differenceinabundance ofblade-for mingalg ae between moderate lyandhe avilygra ze dpools(p<O.OS.SchcffeTc~l).

11

600 Bay Bull s1

60 0

400

N

8

"

²¹¹⁰

·

'"

"

·

N

J A S

a

N

~

^{M A M J}

'0

^{60 0} _{Bauli ne1}

"

·

"8

⁶⁰⁰

Z0 40 0

200

M A M J J A S

a

N Mon th(1 967 )

Figure2.MeanabundanceofIitlorinidsoneach samplingdate at BayBullsPool1 andBaulinePool1,

18

Table4. Analysisof varianceomcoverofmopholollicalgroupsandbareTOCkbetweenlevels of grazingintensity.\

~- -••",.,= = =-="-,-""",,,,_ ---',. '

Bl.deFo rmloEAlg.e _,_...Q!.~ng-'-~l}!.i.IL

Sumof M~ F Prob. Light Moderate Heavy SOUICt D,F. "'~. ^{Squares Ratio}

BelWeen Groups 2 91 97 8 4.6489 47.496<0.0001 47.57 1M2 17.75 WilhinGroups 264 25.840 1 0.097 9

Tolal 266 35.1379

FUlmen toulA lgae G_rll7.i.nslnl!lllsity

Sumof M'M F Plob. liGht Moderate Ilcavy Source D.F. Squalts SqllUeS Ratio

Be!Ween GIOUp! 2 2.7338 1.3669 1B.099<0.0001 31.Slt 26.115 7.736 WithinGroups 264 19.9378 0.07:5.5

'''''

^{266 22.6716}

[ocrustlngAlgae 9r.~inll!n.lensit)'

Sumof Mean F Prob. Ught Moderate Hc~v)'

Source D.F. Squaxc s SqUIIl"(:S Ralio

Be~en GIOUp! 2 0.4594 01297 15.545 <0,0001 7.195 13.73 17.03 Within Groups 26' 3,9456 0.0148

r.., 269 4,4051

C.lcareoUI Algae ____..9~~lIS!n~ity_

Sumof M~ F Prob. LiShl Moderate Heavy Source D.F. Squares Squares Ratio

Between Groups 2 105805 0.7903 33.503<0.0001 12.55 24.765 30.2 WithinGroups 26' 6.2980 0.0236

Tolal 26. 7.8785

BaftRock ___9~.~1~1I1.n~en~~ty

Sumof M'M ^F Prab. Ughl Modern!e Heavy

Source D,F. Squares Squarer Ratio

BetweenGroups 2 2.6926 1.3463 33.628<0.0001 10.25 27.46 31,HI WithinGl'ou ps 26' 10.6984 0.0400

TOlal 26. 13,3821

IY.ltesformQtJlhologlcalVOUplilatthdcruing!IIk l'di1)'IcvclurCllIIlnUUfOll"cdmeans.-U~de-;ij~~d~c~anl";';

~gnific.anllydirrerentat1hc O.O.5pl'llNbillty1~~el 'l$tnI SehcfTe·llelt.

19

'0

PercentCo\er30

~O

:0

Heavy

:\lnrrh nluci clIl C;rUUJI

Figure3.xtcun abundance01'algaeineach morphotoaic a!~rntlrant! harerock,It eachgrMm~intensitylevelovertheenti re\wd~ Fl'~'o(l

20

In the lightly grazedpools.the abundance of blade-formingalgae washigh in March.then dropped precipitouslyto May.climbed to October andthen dropped again to November(Fig.4).The spring peakwascaused byF,/C1/.~.tistichus,Ii.~ticllll.~in Bay BullsPool2(the only lightly grazed pool sampledinMarch)wheretheprotected.

inland locationof the pool prevented removalof the perennialbyicc scouringup to this time.However,a few days afterthe poolwassampledin March,itlargechunk of ice(-2m indiameter) was observed inthepool. Movementof this iccchunkat high tideresulted in a decreaseinabundanceofFucliSdistichusdiuichusfrom84%cover inMarchto37%coverinApril.Theincreasein abundance ofblade-fonn ing 1Ilgoeto October(Fig. 4)was causedbyaslightincrease inFucusdiuichustli.'iricflll.'iinBay BullsPool2 duetogrowth ofmatureplantsand regenerationfrom holurests,und a dramaticrise inClloroaria flagellifonnisto 85%cover in Bay BullsPools3 and 4.

The decreaseinabundanceof blade-formingalgaefrom OctobertoNovemberinthe lightlygrazedpoolswas asampling artifact.Only Portugal CovePoolIundBay Bulls Pool 2weresampledinDecember.These poolsboth hadlow abundancesof Chordariapageflifonnisthroughout thestudy.

In the moderatelygrazedpools.the b1ade-fonn ing algaeclimbedgradually to a peak in July andthen decli nedslightly(Fig. 4).Monostmm a grevilleideclinedafter L.tinoreabecameactiveandL.saxailisbecame abundant.f:distichust1i.'ilichuJ increasedslightly throughthe studyperiod. Thepeakabundanceof blade-forming algae.inJuly,wasdue toScytosiphonlomentariainBaulinePool2 (41%cover).

21

Month (1987)

April June Au,

Figure 4.Meanabundance of bladefonningalgae ateachgrazingintensitylevelper month.

22

C. flagellifonniswas not observed in anyof the moderatelygrazed poolsin spiteof ample baresubstrate on whichto scuteand a healthy parent populationinthe adjacent middle intertidal zone.

In theheavilygrazedpools,blade-formingalgae decreased from Marchto November (Fig.4). Monostromagrevilldwasresponsible forthe highabundanceof blade-formi ngalgae in thespring.ThP,high abundance ofMmrmtmmuRI'evifleiwas unexpectedsinceit is a preferred food ofL.lirtOl'eo(Lubchenco 1978).However,M.

grevilfeigrew earlyin theyear reachingpeakabundanceinMarchwhileL. linorea wasstilldormant andL.saxailisnumberswerelow,M.grevitleideclinedrapidly oncethe littorinidsbegan activelygrazingand was not seen afterMay ineitherpool.

C,flagellif' m nisdid not appear in either ofthe heavilygrazedpoolsdespitethe presenceof ample baresubstrate onwhichtosculc and the abundanceofparentplants in the adjacent middleintertidalzone,

FtlamcntousAlglJC

Theabundanceof filamentousalgae wassignificantly lower in theheavily grazed poolsthaninthe moderatelyandlightly grazed pools (p<O.05.ScheffeTest ).

There wasnosignificantdif ferencebetweenthemean abundanceof filamentousalgae in thelightl y andmode rately grazed pools(Table 4, Fig.3).

Inthe lightly andmoderatelygrazed pools,the abundanceof filamentousalgae increasedto a peakin summerand thendeclined inlate summerand increasedagain

23

in Nove mber(Fig.5).The secondpeak was dueentirelytoCladophoro sereciain the lightly grazed PortugalCove PoolI (45%cover) and the moderately grazedPortuga l Cove Pool2 (44%cover).The peak in May for the lightlygrazed poolsinFig.5is duetothe highabunda->-(68%cover)ofAcrosipllonia orelainPortugalCovePool I.the only lightly grazedpoolsampledin May.

A.wrtawas abundantduringthe summer (peak abundance>30%cover)in allof the lightlyand moderately grazedpools with the exception ofBayBullsPool2 (maximum20%cover) andBaulinePool2(maximum6%cover). Inallof these pools, it was present when the poolWB~firstsampledin the spring and persisted at least untilAugust (OctoberinBay BullsPool 3),

Inthe heavily grazed pools,the filamenlousalgaepeaked earlierandatlower abundancethaninthe lightlyand moderately grazed poolsand then declined(Fig.5), A.curtadid not become abundan tduringthe summer.In Bay BullsPoolI.it increasedtoapeak of26%cover inMay,dropped afterL.lit/areabecame active and number]; ofLsaxaitisincreased but persistedinthepool untilAugust(9%cover).In BaulincPoolI,A.cuctoclimbed to a maximum abundance of6%cover in June and

wasnot,",-mdin thepool after thatdate,

Encm~ti ngAlgae

There was significantly less encrusting algae in the lil'iJtlygrazedpools than in the moderatelyand heavily grazed pools (p<O.05, Scheffe Test). There wasno

24

··

~ I

^{Gm'. ,Intens"y}

0 • L1lh t

S 80 .. Mod unte

~ ^{• Beau}

~ _~

⁸⁰

{Y\/

~

· 1

~

⁴⁰

·

⁰

^{- ..}

~ _{20 .~} .... . .-»

!>

0.. --

~~/

^{...•••___}

. _.~

:9

§

'"

§ ^{0F,b April Jun e Au, 0"}

::<

Month (1987)

Figure5.Mean abundanceoffilamentous algaeat eachgrazingintensity level per month.

25

significantdifferencebetween themeanabundanceof encrustingalgaeinthe moderatelyandheavilygrazedpools(Table4, Fig. 3).

In thelightlygrazedpools.theabundance of encrusti ngalgae waslowest during thesummer monthswhe n the ephemeralblade-formingandfilamentousalgae weremost abundant(Fig.6).Inthe moderately grazed pools.the encrustingalgae.as a group.increased in abundanceduringthe spring and then maintained asteadylevel throug hout the studyperiod (Fig6). "Ralfsia~increasedthroughou tthe studyin the moderately grazed pools.Bauli nePool 2 showed a peak in abundanceof 42%cover in June. by farthe highestlevelobservedin the study.Inthe most heavily grazed pool.theencrustingalgaedecreasedfrom March toApril. increaseduntil September andthendeclinedto November(Fig. 6).A green crustinBay BullsPool I was responsiblefor the high abundanceof encrustingalgaein the spring. In the most heavily grazed pool.BaulinePoolI.H.rubragradually increased in abundancefrom 10%coverin Aprilto22%cover inNovember demonstrating thatit cansurvivefrom springuntil latefall and thatit is notnegativelyaffectedbyintenselittorinid grazing.

In Bay Bulls Pool I.H.tubrawas absentuntil June.In all of theotherpoolswhereit occurred.H.rubrawas observedonthe firstsampling date. This allowed vegetative spreading toincrease abundanceratherthan new planktonic settlement asoccurred in Bay BullsPoolI.Therefore.itisnot surprisingthat themaximum abunda nce inBay Bulls Pool 1was only6% coverin October.

26

, ..

l

e r U ID. ln l e n. lt,

• Uabl

.. Mod erate

• H..vy

April

·

~ 100 ,- - - - - - - - r.:==;==;=,=",==;l

w

] 80

u_

~

; '0 8

^eo

·

^U~•

· .

] t

'0

~e:.

!.>

~ '"

^C

· '"

Month(19 87)

Figure6. Meanabundanceof encrustingalgaeateachgrazing intensity levelper month.

27

The two heavily grazed pools haddifferent abundancepatterns for"Ralf sid'.It first occurred in Bay BullsPool I in Juneat13% cover andincreasedto 26%cover in October.Thetimingof the lateoccurrence of"Ra1fsia~just asScytosiphonlomentaria was decreasingin abundancesupportsthe hypothesisthatthesetwospecies arc alternatestagesin thelife cycle of thesame plant (Lubchenco and Cubit1980).

"RaJfsja~first occurred in BaulinePool Iin Maywith 2% cover. thenincreased to3%

coverin June andthen disappeared.

CalCaNOW;alj;l1lC

Theabundanceofcalcareousalgae wassignificantly differentat all 3 levelsof grazingand increasedwith increasedgrazingpressure (p<0.05. Scheffc Test) (Table 4, Fig. 3).In Baulinc Pool I,Corollinaofficina/isincreased from2%coverin Aprilto 14%coverin November.In contrast,C.ofJicilUl!is wentfrom 13%coverin April down to alow of 0.1%cover in Julywhen the ephemeralA.tvl:'taandC.

f/ageflijonni.f were abundant in Bay Bulls Pool 3.Poolswithintermediate levels of grazing showed intermediatepatternsofabundancefor C.officina/is.

CIa/111mI/O/pili/illcircumsctiptumwasmore abundantthanC.officinalis and had differentmean abundances betweenthe threegrazing intensitylevels(fable4).In BayBullsPool 3 and 4, it wasabundant ;nitiallyand then declinedasthe ephemerels became more abundant. In Portugal Cove Pooltand BayBulls Pool2,the abundance

2H

ofC.circumsctiptumwaslow throughout the study.Ithad ahigherabundancein the moderatelygrazedpools and remainedrelativelysteadythroughout the studyperiod.

C.circumsctiptumwasmostabundantin the heavilygrazed pools(Fig.7).In Bay BullsPoolI.its abundancestayedrelatively constantat23%coverthroughout the study. In BaulinePool I. its abundance increased from12%cover inApril to a maximu mof 32%cover in July and then declined to17% coverinNovember.

BareRock

In lightlygrazedpools,there was significantly lessbare rockthanin moderately andheavilygrazed pools(p<O.05, SehcffeTest) (Table4,Fig.3).

In theabsenceof grazing(only 6L.sawtilisand noL.tiuoreaobserved during theentirestudyperiod) inBay Bullspool3,the large amountof hare rockobservedin April(31% cover) was reducedto 0.4%cover inJuly onceAcrosipltolliaeur:taand ChorrJariuflagellifonllissettled.In theotherlightly grazedpools.the amountofbare rock remainedlow throughout the studywith the exceptionof Bay BullsPool 2 where barerock was 20%coverthroughoutthe study.

The moderatelygrazed pools hadlarger amountsofbare rock.These levels remained relativelyconstantthroughthestudy period with adip inmid summer inall pools.This dipreflectedthepeaksinabundanceoftheephemeralblade-formingami filamentousalgae.

29

[

Gra zi n gInllluit y

• Light ... Moderate

• He· "7 u

.

:t

April Jun e Au, oer 0"

Mon lh (1g8? )

Figure7.Meanabundanceof calcareousalgaeateach grazingintensity levelper month.

30

Aslighlly highermean amountof bare substrate was observedin theheavily grazed poolsdue10 thelargeamountofbare rock present in BaulinepoolI.The amount of barerock rangedfrom 55%coverin Mayto ::19%coverin July in thispool.

In BayBullsPoolI.the meanamountofbare rock waslower (21%cove r)ducmainly tohigherlevelsofA,aretaandScytasiptumlamentaria:Even so. thcamount of bare rock peaked at33%coverinJuly.

Effects ofLittorinid Grazers onSpecies Illverslty

The Shannon-Wienerdiversityindexintidcpoolswas greatly influenccdbythe grazing activityof L,liltoreaand L.smatiUs.In moderately and heavily graze d pools.

thediversit y remained fairlyconsistent overtheentire studyperiod(1.59±0.25 sdand 1.36± 0.28sd respectively).In contrast,pools withlowgrazer populationshadII low erdiversity withlargeseasonal fluctuation s(1.22±0.49sd).The poolswith the greatest fluctuat ions were PortugalCovePoolI, BayBullsPool4and BayBullsPool 3(Figure 8,).In allofthese pools.oncspecies orassemblage obtained amaximum abundance of 85% coverorgreater(A.areta/C.Unumin PortugalCove Pool IandC.

flagellifo m ;islD.foeniculacells in Bay Bulls Pool3and 4),In atlcases, diversity decreaseduntilthe dominantassemblagereached its maximum abundance.

In Bay BullsPool2.FucusdistichusdistichusdominatedCormost of the sa mple period.However. it onlyreacheda peak abundanceof 53%coverexcept for the period in Marchbefore the poolwasscouredbyicc,Thislevelofdomina ncc by

31

IolA)'(.l'.l'AS ON IolAMJJASON MHtJJAS ON MA/of,j' J A SON

B.O

c

^{B" Bu'Io'}

1.1S..

v.,...

^~---.

1.0

o .e

0.o

t.(UUJA S O N

J

^~

"

^1.5

C ^V ' ;::' ^·

.~ 1.0

a

^O.IS

l:l 0.0

~ MUU JASO N

il

~

Month (ID87)

Baullne 2 Bauline4 Ba u lin e3

L

^{MAMJJASO N}

L L

JUMJ JA S ON MAM JJA S ON Bautfne1

l.;

MA M J JASON Month (1987)

Figure8.ShannonDiversityIndex(H')oneach samplingvisitforeachtidepool.

32

onespecies wasnOIhigh.enough 10excludemany other species.Also.the ice scouringleft patchesof baresubstrate whichwereutilizedby cphcmcrals.

In themoderatelygrazed pools.no singlespecieseverdominated,therefore diversitywas higherthroughout the seasonthan inlightly grazed pools.Inthemost heavily grazed pools.diversitywaslower than inmoderately grazed pools.Species suchasCllordaria flagel/ifo"niswereprevented fromsettlingand otherspecies such,IS

Acrosiplwniaarctawereremoved earlierby the grali nglinorinds.

As observed byLubchenco(1978),the greatestdiversityoccurredat intermediatelevelsof grazingintensity.1n the presentstudy,thelowestdiversity was seeninthelightly grazed poolsattimeswhentheyweredominated by 2 species (Portugal CovePoo l1.BayBullsPool3 and Bay Bulls Pool 4).Diversitywas highestin poolswheredominance byasinglespecies waspreventedby grazingbut grazingwasnotso severe as tototallyeliminatemany species.Most ofthe ulgul speciespresent wereephemeralswhich havebeenshownby Luhchc nco(I97R) tohe preferred foods ofL. littorea.

PhysicalCharacteristicsof 1idepools

One mightreasonably assume that some ofthebioticdifferencesdescribed abovemightbedue10differences inphysical characteristicsofthe tidcpoolssuch as distanceaboveMLW orpoolsize.However.although physical characteristicsvaried

33

betweenindividualtidepooJs (TableI),there werenosignificant differences between levels ofgrazing intensity(ANOVAp>O.05).

34

DISCUSSION

InNew England.grazingpressure,definedasthe probabilitythatanindividualalgi!

willberemovedbyherbivoryduring a giventimeperiod(Lubchcnco andCubit1980), exerted byLittorinaliuoreais maximalduringthesummer and minimalinthewinter.

The winter minimummaybe caused byadecreasein numbers due toprcdeuen.cold temperaturemortalityoremigration(Menge1972, Dcthtcr1982.GardnerandThomas 1987), orfactorsdecreasingforaging efficiency.Inthepresentstudy,L.Iirt(lfr!il became activein Aprilwhen the daytimetemperature in thepoolsreachedSoC.

During thewinter,thesnailscongregatedin an inactive stale in crevicesand depressionsinthe pools.The littorinids continued 10actively grazethroughtile summer andfalluntilNovemberwhentemperaturesdeclinedto5°C lind foraging activityceased.Menge(1975)described a decreasein grazingactivity due tostorm generatedwaveactioninthe winter.Williams(1964) notcdamarked reductioninthe growthrateofL.liuoeeaduringthe winterin wetes.This "lasdueto areduction in the numberof feedingexcursionscaused bylethargybroughton by coldsea te mperatures(_ROC).Newell (1958) describeda completecessationofactivityat temperaturesbelows-c,

Sincethe numberof commonperiwinklesdecreasedthroughout the yearwith nopeak in numbersin summerandsinceVCI}'fcw smallL.liuoreawere seen.a failurein recruitmentfrom theplankton in thespring andsummer seemslikely(Hayes 1929).Recruitmentfrom the planktonispatchy andirregularfor many species

(Dcthier 1984).Sincespatsettle in thesubtidalorlowinlertidal zone,andlater migrateupintothemiddle inte rtidalzone(Lambert and Farley19 68, Gardner and 'rhomas1987),spring ice scou ring may decreaserecruitmenlby killing the snails dinxtlyorthroughstarvation by removing the macrophytes.Thisdoesnot howevcr cxplainthe gradualdecreasein numbersduringthestudy. Unexplained changes in population sizeand growthratebetweenclimatically simila ryearshavebeenobserved byothers (Williams 1964)and maybetheresult ofnormal yearly fluctuation sof populationsoccupyingtheharshand variableintertidalzone.

In thespring,Liuorinasasailismigratedfrom cracks andcrevices inemergent SUbstrata. into thepools tofeed and reproduce. Themaximum summer pop ..tenon size varied betweenpoolsandbetweensitesdepending.primarily,onthepool's proximity toalarge populationofL.S4tOlifison emergent substrataintheupper intertidalzone.

Recruitment ofsmallL.saxarilisinearlyandmid summeradded10thepeak populationsize.Thedecline intheL.saxailispopulationaftermids ummer, waslikely caused by mortalityand/or emigration.

Althoughfew sea starswere observedatthestudysites, otherpredators includingsea gulls,cunners(Tal/og o/ohnaadsfWT:fUS),andsticklebacks(Gastervst eus uc"/~ltlls)wcn::observedonoccasion insomeof thepools.Blue mussels,sea urchin s andcrabs. not gastropods,made upmost of the debris foundat gullanvilsinthe vicinityof the study sites suggesting that Iittorinids werenot apreferredprey item.

Gastropods have been foundin thegut ofT.odspemu,but they onlymade up asmall

36

proportion of thetotalgutcontents(Chao1973. Ollaetd.1975).Sticklebac ksmny have eatensome ofthesmallestlittorinidsinthe2 pools (PCI andDBI)in which they occurredbutis unlikelytobeIIsourceof mortality inmostpool s. In themosr heavily grazed pools,starvation may have resultedinthe dea thofso me roug h periwinklesas describedforlimpetsin Oregon (Cubit1984). This,migrationoutof the poolsinto the cracksand crevicesof the surroundingshorewhere Lsaxantis overwi ntersseems themost probablecause of itsfall declinein the pools.

The differences in the dist ributionof

Inc

twoIittorinids betwee nsitesami pools (Lsaxatilisin allpoolsandL.littoreainonly two pools)is dueinpart 10 differences in larvaldevelopment. Planktonicdevelo pment inL.Iittoreatakes aboutfourweeks (F'reller and Graham 1980),and dispersal by currentscanresult intoofewla rvae settling in a givenareatomainta in a breedingpopulation.On the etherhand.the larvae ofL..eaxa ilisarcbroodedinternall yand hatch as post-metamorphicjuven iles (F'retterandGraham 1980).If oneormore gravidL.saxatilisare carried to a new area via rafting onfloating macro a lgae, forexample.the likelihoodofestablis hing a breedingpopulation is muchgreater, Joh annesse n(1988) showedtha tfhcpre senceof L.saxaiilisand the absence ofL.Iinoreaina numberof isolatedloc a tions fromSouth AfricatoGreenland wasduetothediffe re nt modes oflarval development.Similar mechanismslikely explainthe ubiquitous distributionofLtnosinasaxauiisand the patchy distributionofL.liuoreaonthe cast coast of Newfoundland.

37

The popula tionsizeof linort mdgraze rsexertsa majorstructuringinfluenceon thealga lcommunitiesofthesetldcpools. In heavilygrazedpools,encrustingand calcareousspecies dominate.LubchcncoandCubit(1980)suggestthai theprimary adaptivevalueofcrusoscalgae istheir ability to persist throughtime whileupright algaearebeingremovedbygrazers. The calcareousspecies,Clathromorphwll cirrumscriplr/ l1landCorollinaofficinalisdominated in theheavilygrazed pools inthe current study. Littler and Littler(19 80)havedemonstrated thatlowcalorificvalueand toughness duetohigh levelsof CaCO)decreasethe susceptibilityofcalcareous macrophy testograzers.The removalof the upright algae by grazingoffsetsthe lower growth rate oftheencrusting andcalcareousforms(LittlerandArnold19 82)thereby decrcestn g competition forprimary space.

Upright ephemeralalgaedominatedin less intenselygrazedpools. The filamentousor sheetlike thallus.Jack of metabolic burdenofproducingchemicalor structura ldefenses,and limitedattac hment points alloweda high ratioof photosynt heticareaperunit biomass(LubchcncoandCubit1980 ).Therefore,these species couldgrowfasterandcompeteforspacemoreefficientlythan theencrusting andcalcareous algae.

e/lOmoriaflagellifomu's is anephemeralspecieswhichis common in environmentallyharsh, physicallydisturbedareas.It is a mejorcomponentofthe successio nalcommunityfollowing ice scouringon thenorth western Atlantic coast (Hooper1981,Lobban and Henle1984,Munda1992).TheabilityofC.flagellifonnis

38

10take upnitrogenwhen nutrient concentrations inthewaterarcvery lowduringthe summerallows ittoprosperandgrow (Probyn andCha pman1983)when the grow th of manyotherspeciesofmacrophytes and phytoplankton is severelydepressed(PM I 197 I. Chap man andCraigie 1977).Despiteamplelower emergent intertidal population s atallthree sites,C.jlagellijOnlli.fonly bec ameabundant in th e twoleast intenselygraze dpoo ls.In both cases, itwasfirst observed in June when grazing pressure in the otherpoolswas reach ingitspeak.It islikelythat inpoolswithlarger grazer popul ations. somegermlingsofC.jlagellijonnis(lowerin herbivore defense chemicalsthan adu lts(Geiselman1980» wereeaten bythe littori nids. Geiselman (1980),Watson and Norton(1985),andLubchenco (1983)haveshownhowlittori nid grazershavedislodged juvenile macrophyteswhilethey were searchingforother types offood.

Withone exception(BayBulls Pool2). the perennialFUClI.f di.flic h ll .f(Ii.'ticillu wasrare inthe tide pools surveyed.Scouringby pack iceand sto rmgener a ted waves removedmost of the adult plants duri ngthe winter,Inallcaseswherethe abunda nce ofF.distichusJisticllUSincreased duringthe study,yo ung plantsregrewfrom the remaining holdfasts which survived the winter. Nonewl~1settledjuveniles were observed. Thisis mostlikelyexplain edbythcpropensi tyofL.tiuoreato dislodge juvenile fucoids while crawling acrossthesubstratum(Geiselman1980, Watsonand Norton 1985).How ever, in BayBullsPool2.protected bythe topographyof the surroundingshore, the establishedcanopyofF.distichusdistichus dominated.

39

Although adultF.distichus dis/jehus thallidid not sufferany obvious negative effects ofgrazing, nonewspo rclings settledinthis poolduringthestudyperiodsu ggestin g physic aldisturbancebylittorinids.

F.distichssdistichuswas comple telyabsent from the heavily grazed pools.

AllhaughadultFueus sp. havealowgrazing preferencerating,L.lit/area willcat lar ge quantities of germlings inthe absence ofany othermacrophytes(BarkerandChapman 1990). In themost heavilygrazedpool (BaulincPool1),while upright macrophyte s were almost completelyabsentanygenn lingswhichsettled wereeaten. In BayBulls PoolI,dislod gement probablyplayed a greater role inpre ventingF.distichus disnchus from attaining a footho ldsince duringmostof the study, morehighly preferredalgal speci es werepresent. These ephemeralsprovidedenoughhighqualityfoodthatthe littorinidsdid not havetograzethe less attractive and nutritionallypoorerF.distichus dist jchl/s.

Thegreateraverage abundanceofMonos/m iliagreviff eiinheavily grazed pools

Wli Sanartifactof the sampling not an effectofgrazing. Most ofthe modera tely and lightly grazedpoolswere notsampled until afterM. greviJIeihad reachedits peak abundance.The heavily grazedpoolsweresamp ledearlie r whenabundances of the blade-forming Chlorophytewere higher.Monostroma grevlllelis similarin morphologyto thehighly preferred Ulva toauca (Lubchenco1978, watsonand Norton 1985) and thus shouldbeeasyto graze, providing a high returnof nutrientsper unit time. Itoccu rred earlyin the ye ar,persis teduntilmid sum mer in thelightly grazed

40

pools.butdisappe ared bylatespring in the moderatel y andheavily grazedpools. The decline of},t.grevilleiwas due to se nescenceand reproductionin the lightl y grazed pools.Since thedecline in the growthrare ofM.grevilleicoincided withthe increase ingrazingactivityof L littoreaand the migrationof L.fatalili s into thepools.there was arapiddecrease inM.81?villeiin the moderately and heavilygrazedpools.

However.the impact ofthe muchsmaller L. .saxa ilis onthe abundance of M. grevillti wasless dramatic thanthatofLliuorea.

Scytosiphonlomentariowasm~stabundant during the timeofpeakgrazing intensity inBaulinePool2and Bay BullsPoolI.Since S,fomentariais aspecies preferred byLtittona(Lubchenco1978).one wouldexpectto sec a low abundance of upright fonns duringthe summerwhengrazers are abundantand activeand a higher abundanceduringthe winter(Lubch enco andCubit 1980). Forinstance,Lobbanand Hanic(1984)foun d thatScytosiphon/omentariareco lonizedanicc scoured rocky shore inPrince EdwardIsl and in the fall aftera succ essionof mostly ephemeral species .Itwasnot seen duringthesummerwhen grazingwas mostintense. Howeve r.

inthepre sent studyand others(Shannon etd.1988.Villard-Bo hnsackandHarlin 1992.Hooper19 81, Bolton1983)S.lomentariahasbeenobserved duringspring and/orsummerwhengrazing pressureis hig h. lara and Moreno (1984) observed Scytosiphoninherbivore addition and contro lplotsbut not in grazer exclusionplots.

Thusgraz ingalo necannotexplain the distribu tionofS./omentariainNewfoundland

41

tidepools.The rolethatphysicalfactors andcompetitiveinteractionsplay requ ires fun her study.

Liuodna liuoeadecreased the abundance offilamentous algal:but the mechanism ofalgal removal was different.For most of theblade-fonn ingspecies, low abundancewas maintainedthroughpreventionof successfulrecruitment. In contrast, most ofthefilamentousspecies were presentas adultplants beforethe grazing intensity increasedand thusthe littorinids had to cat the adultplants rather than juveniles.Thetacnioglossanradula of the littorinidsis well designed foreating adult filamentousplantsas wellas juveniles(Steneckand Walling1982) andmany filamentou s macrophytesare preferredfoods of littorinids. Theabundance of Acrasiphonia orctawas drasticallyredueed whenL.lit/areabecame active in the heavily grazedpe els. No other filamentous algaebecame abundantoncethe grazers began acti vely feeding in thepools.Incontrast,Acrosipholliaaraa andChaeroJnOlp1la linum reachedmaximumabundances inmid summer inthelightlyand moderately grazed pool s.AlthoughL.saxauttshas been reported to eatfilamentousalgae(Secchi ettil.19 81 ) it hada moresubtle effect than didL.littorea. Insteadof removing the filamento usalgae altogether,L. saxaJ:iis decreased the maximumabunda nceand reduced the persistence lime.The inabilityofL. saxali/is 10 significantlydecreasethe abundanceof adultfilamentousalgaecombinedwith its negative affectonthe establishment ofnew blade-fanningalgaefurthersupportsitsclassification by Hawkins andHartnoll(1983).as a mlcrogrezer.

42

Hddenbnndia mbraand~Ralfsia"were the mostcommon and abundant encrus ting algaepresent during thecourse of thestudy. Both species nrc relntively grazerresistant due to a thallusconstructed of smalltightlypacked cells.This structure offersconsiderable resistanceto scratchingand thuscausesIthighdegree of wear to theradular teeth of lirtnrinidsgrazing on them (HennessetaI.19R3). The increased wear on the radularteeth makes them less effectiveforgrazingthe nutritionallyricher uprightepheneralsand thereforethe cost of grazingthecrusts is high. Bcrtne sstlaI.(1983) foundthat extracts ofHildellbmJlllia rubmandRalf.ria verrucoseembedded in agar were eaten at lower ratesth an agar controlswithlower nutritivevaluesuggestingthepresence of chemical defensesinthe macrophytes as wellasstruc tural defenses.

The abundance of the slow growing perennialH. mbmwas extremelylowin thelightly grazed pools. Ephcmeralsovergrew the crust and primaryspace was at a premium.Itwas moreabund ant inthemoderately grazed poolsdue to the increasein primaryspace caused by Lsaxailisremoving the ephemeral algae or preventing them from settling.In the most heavily grazedpool.H. mbmincreased in abundance by latera l spreadingduring the studyperiodwhen allof thecompeting upright macrophyteswerere moved by grazing.InBay BullsPoo lI,however,H.rubrawas not presentbefore thegrazers became activeandthushad to recruitfrom the plankton intheface ofintcnsegrazingpressure.Itrecruitsvery slowly(Bcrtncssetd.1983 ) andlikerr an y other macrophytes,the juvenilesarc probablymore attractive to grazers

4J

tha nlheadult plan ts(BariterandChap man1990.Lubchenco 1983. Watson andNon ce 1985).OnceH.IUb mappeared in this poolinmid su mner,an dreacheda sizeat which it was nolo n ger u susceptible to grazing,itgraduallyincreasedin abundance.

Althoughthere:wa.,nosignific ant differencebetween mean abundancesof

"RaJ/sia"between pools.there was aconsiste nt grazer-dependen tseasonal panern of ab undance. Inthelighllygrazedpools.abunda nce was lowestduring the summer whenthe ephemerals werepresent.In themode rately grazed poolsand the heavily grazed Bay Bulls Pool I,removalofthe faste rgrowing ephemer a l!allowed~Ralfsia~

to increaseinabun dance throughthe studyperi od. Only in one of themoderately grazed poolsdid itpeakduringthepea k ingrazing inten sity asLubchenco andCu bit (1980)obse rved.Inthe most intenselygrazedpool.ho wever."RaJ/lia"was eliminated inJune.Extremelevelsof grazingmay preve n tencrust ingalgaefrommonopoliz ing bare rock intidepools.Bert nesset0/.(1983) fo undsimilar dcnsitics ofL liuoreato limitabundanceofalgal crusts inprotect ed rocky beachesin sou thernNew England.

CalcareousredalgaesuchasConi/ina officina/isandClathromorphwn circul'tlscrip twnca rrystl\lCturalandchemicaldefenseswhM:bprotectthem from Iiuorin id grazers.Theonlyweak poin tonC. offidnolisistheun calcified articula te d join tbut sincethe radutaofmostgrazersis toowidetofit betweenthecalcareous seg ments, the thallu sis almostimpene trable to grazers (Watson andNorto n1985). C.

officil/aJiswas foundtohavethelowestrankin g of allalg ae testedby Watsonand Norton(19 85)forbothedibilityand attractiven e ss.It wasnot ea te nbyL.liuarea

44

evenafter50daysofstarvation(WatsonandNorton1985). Cofflrinn/isgrewlillie duringthepresentstudybutremovalof epiphytessuchasM.gfrvillnbythelinorindli mayhaveaidedthelimited growth. No recruitme nt was observedbuI thestructural strengthofC.offirinaJ iJmade itleususceptibletoiccdamagethanmost oftheoche r perennialmacrophytes.Tidepoolson theiccscourednorthWC$!coast

or

Icel and have Com/lina off icintiisasoneof theirchie f components (Munda 1992).

Cfalhro/1/Ofphu/1/cirrumscriptllmhas amuhilaycredepuhalluswhichprotects themerisernatlccellsfromgrazinggastropods(S tencck 1982).Howeverthe lhnllusill pronetoepiphytism in the absenceofgraz ingbythelimpet,NtJfoucmuea1e'.WII//illllli.t (Stcneck 1982).Inthepresentstudy,limpets we reallkilledbythefreshwa terrunoff inthe spring. MostoftheC.cimmlscriptumin the poolsdidnOI grow,waspalein colourandappeared heavilyepiphytised.However, in theIll(l\Ihea vilygrazedpools.

CcirrumsCrip tllfflincreased throughthestudyperiodandeventuallytook onahealthy pinkappearance.Thehigh density of linerinidgrazers inthesepoolsmayhavc mimickedthebereflc ial effects of grazingbyN.testudindls01\C.dmunscriplllm.

Algalspecies divenityvariedbetweenpoolsdependingonthe graringinlcnliity.

With low grazingpressure(low numbers oflluorinaMXatilisandnoLtittolr a), diversity decreasedto a minimumlitmidto late summerwhenoneoftheephemeral species became dominant. In contrast,inpoolswithintermediategrazing pressure (large numbersofLsma/illsbutlownumbersofL.lilforea),diversity remainedhigh and co nstant throughthe yearsincethegrazers removedenoughof the dominant

45

ephemeralalgaeto decreasecompetition for space and reduce thefrequencyof competitiveexclusions.Diversitywasalso lowin poolswithgrazingpressureshigh enoughto remove most of theupright alga e(large numbersofL.littoeea and moderate numbersofL.saxatifis). Lubchcnco(1978) firstdescribed thistype ofrelationsh ip betweengrazers anddiversityinNewEng landIidepoolswhere L.litlrlreawas the only sisnificanl herbi vore. Inthepresentstudy,Lmonnasaxonttswasalsofoundto play a majorrole indeterminingalgal diversity.

Summary

The struct ureof algalco mmuni ties in tidepoolsonNewfoundland'seastcoast is determined by a numberofinteractingbiotic andabioticfactors.Frequent ice scouringpreventssomeperennia lspeciesfromestablishing by removingparen t popula tionsin localised areas. Repealeddisturbance can preventsomepopula tions fromreachingmaturityandreproducing.Ontheotherhand,scouringcanbenefit algal populat ionsbyreducingthenumberof grazersascan freshwater inflows duringspring.

Grazing by theherbivorous gastropods.Littorina linoreaand L.

seattts

varies seasonallyandspatially.Themo re Intense lygrazedpoolswere dominatedby encrustingandcalcareousspecies adapted to persist throughtimewhileuprigh talgae wereremovedbygrazers.Uprightperenn ialsare prevented fromdomi natingbya combin ationofgrazersremoving adultsan dsettling propagul es and by physica l removalthroughwaveactionandicescouringorlarger plantswhich have fou nda

46

temporalorspatialescapefromgrazing.Inthe lessinte nsely grazedpools. opportunis ticspecie s with few structural orche mical defenses and high fecundity.and high grow thratesoutcompetetheslo wergrow i ngperen nials for primary space.

Diversitywas highestin pools wi\hintermediategrazing pressure.and was maintainedata stab leleve l through theyear sincethe dominantephemeral species arc thepreferredfoods of the grazers.Diversity was lowerin lightlygrazed poolswhere one algal speciesdominated andinheavilygrazed poolswherealluprightmacroph ytes were removed bygrazing.

47

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