The mobilisation of sediment and benthic infauna by scallop dredges01013-main - 图文

MarineEnvironmentalResearch90(2013)104e112

ContentslistsavailableatSciVerseScienceDirectMarineEnvironmentalResearchjournalhomepage:www.elsevier.com/locate/marenvrevThemobilisationofsedimentandbenthicinfaunabyscallopdredgesF.G.O’Neilla,*,M.Robertsona,K.Summerbella,M.Breena,L.A.RobinsonbabMarineScotlandScience,FishingTechnology,AberdeenAB119DB,Scotland,UKUniversityofLiverpool,LiverpoolL693BX,UKarticleinfoArticlehistory:Received2May2013Receivedinrevisedform20June2013Accepted22June2013Keywords:MobilisationofsedimentBenthicinfaunaScallopdredgesSedimentresuspensionBenthicimpactTowed?shinggearsabstractWepresenttheresultsofexperimentstoassesstheimmediateimpactofscallopdredgingontheseabedsedimentandontheinhabitinginfauna.Thepassageofthescallopdredgeisshowntohomogenisetheseabed,?atteningsandripples.Theturbulentwakeentrainsuptotheequivalentofa1mmlayerofsedimentperunitofsweptwidth,althoughananalysisofthe?nerparticlesmaterialimpliesthatthesuspendedsiltmaterialmustoriginatefromdepthsofatleast10mm.Thespeciesmostabundantinthesedimentplumeeitherswimactivelyinthewatercolumnorarefoundin,oron,theupperlayersofthesubstrate,whereasthosemostabundantincoresamplestakenfromthesediment,butnotpresentinthenetsamples,arealmostalltube-buildingordeepburrowing.Theverticalstrati?cationofsedimentconcentrationandofanimalnumbersinthewatercolumnsuggeststhatevenifsomeofthesespeciesrespondactivelytothepresenceofthedredge,onceentrained,theyaretransportedmoreorlesspassivelyinthesamewayasthelargersedimentparticles.Therewasnodifferencebetweenthecoresamplestakenbeforeoraftertowingsuggestingthatani-malsmobilisedbythedredgeresettleinthetowpath.Ouranalysisdoesnotprovideanyinformationregardingthefateoftheseanimals.CrownCopyrightó2013PublishedbyElsevierLtd.Allrightsreserved.1.IntroductionTheenvironmentalandecologicalimpactsoftoweddemersal?shinggearshavebeenthesubjectofmanystudiesandanumberofreviewshavebeenpublishedcollatingandassessingtheimpactthesegearscanhaveonbenthichabitatsandcommunities(Kaiseretal.,2006,2002;L?kkeborg,2005;Collieetal.,2000;AusterandLangton,1999;JenningsandKaiser,1998;Hall,1999).Whilemoststudieshavefocussedonbiologicalimpacts,physicaloneshavealsobeenconsideredandthedisturbancetotheseabedduetotrawlinganddredginghasbeenmeasuredusingarangeofopticalandacoustictechniques(LucchettiandSala,2012;Ivanovi??cetal.,2011;Lambertetal.,2012;BoulcottandHowell,2011).Themobilisationofsedimentisanimportantfactorinthisregardasitisrelatedtothereleaseofnutrients,benthicinfaunalmortalityandtheresus-pensionofphytoplanktoncystsandcopepodeggs(Brownetal.,2013).IntheFirthofLorne,Scotland,particularconcernshavebeenraisedaboutthesedimentmobilisedduringscallopdredgingforPectenmaximus,whichitisthoughtmayresettleandsmotherthefeedingandrespiratoryorgansofsomeofthebenthicspecieson*Correspondingauthor.E-mailaddress:b.oneill@marlab.ac.uk(F.G.O’Neill).nearbycobbleandrockyreefs(Daleetal.,2011).ThescallopdredgesusedinScotlandhavenineteethonaspringloadedbarbehindwhichamatoflinkedsteelringsis?xed.Aheavynettingcoverisattachedtotheringmattoformabaginwhichthescallopsareretained(Galbraithetal.,2004).Typicallytwotowingbeams,witharoundeightdredgesshackledtoeach,aretowedfromeithersideofthe?shingvessel,although,theexactnumberwilldependonthesizeofthevessel(Figs.1and2).Thespringloadedteethandthesteelringmatwillloosenandpenetratethesediment,whichmaymakeiteasiertomobiliseandmakedeepersedimentmoreavailable.Anumberofauthorshaveexaminedthesedimentputintothewatercolumnbehindgearssuchasscallopdredges?shedinsoutheasternAustralia,aclamharvestinggearintheVeniceLagoonandcommercialandsurveytrawlsinarangeof?sheries(BlackandParry,1999;Pranovietal.,2004;Dounas,2006,2007;Dellapennaetal.,2006;DurrieudeMadronetal.,2005).Althoughthesestudiesareveryinformative,theyareconcernedwithspeci?cgearsonspeci?csedimenttypesusingdifferentmethodologiesanditisdif?culttogeneralisetheirresults.ToaddressthisO’NeillandSummerbell(2011)lookatthesedimentremobilisedinthewakeofsomeoftheelementsofademersaltrawlthatareincontactwiththeseabedonsedimentsclassi?edasbeingsand,muddysandandsandymud.Theydemonstratethatthemassofsedimententrainedbehindtowed?shinggearelementsisdirectlyrelatedtothe0141-1136/$eseefrontmatterCrownCopyrightó2013PublishedbyElsevierLtd.Allrightsreserved.http://dx.doi.org/10.1016/j.marenvres.2013.06.003F.G.O’Neilletal./MarineEnvironmentalResearch90(2013)104e112105

Fig.1.ThescallopdredgeusedinScotlandto?shforPectenmaximus.Atypicalvesselwill?shabouteightdredgesfromeachside.

hydrodynamicdragoftheelementandthetypeofsedimentoverwhichitistowed.Thehydrodynamicdragisrelatedtothemo-mentumtransferredfromthegearcomponenttothe?uidandmanifestsitselfasturbulenceandacceleratedregionsinthewake,whichdependingonthebedshearstressandsedimentsizeanddensitymayresultinsedimentbeingmobilised.Whilethesepro-cessesareprimarilyhydrodynamictheremayalsobeothergear-Fig.2.Thedimensions(inmm)ofthescallopdredgesusedinthesetrials.

speci?cmechanismsinvolvedthatwillhaveabearingontheamountandcompositionofsedimentputintothewatercolumn.Thesituationwiththebenthicinfaunaismorecomplex.Someanimalsmayreacttotheoncominggearandenterthewatercol-umnwhereasthosethatremaininthesedimentwillbesubjecttothesamephysicalforcesasthesedimenttheyinhabit.Aproportion,dependingontheirsizeanddensity,thebedshearstress,andadditionalfactorssuchaswhethertheyareattachedtoanything,willbemobilisedpassivelyinasimilarwayasthesediment.Thefateofalloftheseanimalswilldependonwhethertheyareatanincreasedriskofpredation,wheretheyresettleiftheyhavebeenputintothewatercolumnand/orthedamageandstresstheysustain.AllbutoneofthestudiesoftheimpactsofscallopdredgingoninfaunalcommunitiesreviewedbyL?kkeborg(2005)demon-stratedadecreaseintheabundanceofsomespeciesandchangestocommunitystructure.ThestudyofEleftheriouandRobertson(1992),whilenotshowinganysigni?cantchangesinabundanceorbiomassoftheinfaunalcommunity,reportedthatlargenumbersoftheepifaunalandlargeinfaunalorganismsweredestroyed.Amorerecentstudy(Hinzetal.,2012)ofthreedifferentqueenscallop(Aequipectenopercularis)gears?ndnoevidenceofadecreaseofinfaunalabundancebutdid?ndadecreaseofbiomassforoneofthegears.Here,weinvestigatetheimmediateimpactofscallopdredgingontheseabedsedimentandontheinhabitinginfauna.Wefocusongainingabetterunderstandingofthephysicalprocessesinvolvedandinparticularonquantifyingthesedimentandinfaunaentrainedinthewakeofascallopdredge,measuringthelargescaledimensionsoftheplume,characterisingthephysicalalterationtotheseabedandassessingtheshorttermimpacttoinfaunalspecies.2.MaterialsandmethodsExperimentaltrialstookplaceonboardtheRVClupeaonsandysedimentgrounds(withbrokenshellandpebbles)northeastofColonsay,Scotlandatdepthsofapproximately22mduringSeptember2007(Fig.3).Scallopswerepresentonthegrounds,butnotincommercialquantities.Threedifferenttypesofexperimentswerecarriedoutandarelabelled,(i)thetowedunderwaterdivers’vehicle(TUV)experiments,(ii)thebenthicsledgeexperimentsand(iii)thebefore-aftercontrol-impact(BACI)coresampling(withthelaserstripemetrology).ADaygrabwasalsousedtoobtainsamplesoftheundisturbedsedimenttocomparetheparticlesizecompo-sitionoftheplumewiththatoftheseabed.2.1.TUVexperimentsTheTUVisaworkingplatformfordiversusingSCUBAthatal-lowsthemtobetowedalongsideandworkincloseproximitytotowed?shinggears(Fig.4).OneofthediverspilotstheTUVandcandirectithorizontallyandverticallybyadjustingthehydrofoilsandmoveitforwardsandbackwardsbycommunicatingwiththetowingvesseltopayinoroutthetowingcable.IntheseexperimentsaLISST100Xwasattachedtoa‘wing’ontheportsideoftheTUV.TheLISST100Xisaninsituparticlesizerandmeasurestheparticlesizedistributionandconcentrationofthesedimentsuspendedinthewakeofascallopdredge.Itusesthelaserdiffractionprincipletoestimateparticlesizeandtheresultingconcentrationofparticles(measuredinml/l)ispresentedin32logarithmicallyincreasingsizerangesbetween2.5and500mm(microns)(Pottsmith,2007).Fourtowstookplacewherescallopdredgesweretowedfromabeam.Thepilot‘?ew’theLISST100Xintothesedimentplumeatdistancesofapproximately2,5,10,20,30and50mfromthescallopdredgetomeasuretheparticlesizeandconcentrationofthe106F.G.O’Neilletal./MarineEnvironmentalResearch90(2013)104e112

Fig.3.ThesiteinthewestofScotlandwherethetrialstookplace.

mobilisedsediment.AstheTUVmovedbetweenthesestations,theparticlesizedistributionoutsidetheplumewasmeasured.Inthreeofthetowsasinglescallopdredgewasattachedtothebeam.However,owingtopoorweatheritwasonlypossibletocarryoutonetowwiththreedredgesinplace.Thediversalsotookvideorecordingsandestimatedthelargescaledimensionsoftheplumeateachstation.2.2.BenthicsledgeexperimentsEighttowswereperformedwhereasinglescallopdredgeandabenthicsledge(Fig.5)weretowedfromthesamebeam.Therewerealsotwocontroltowswithoutascallopdredge.ThebenthicsledgeisthesameasthatusedbyO’NeillandSummerbell(2011)andcanbeusedasaplatformforanumberofsamplingdevices.Intheexperimentsherethesledgewas?ttedwiththeLISST100Xand,forthreeofthetows,withthreeplanktonnets.ItwastowedfromthesamebeamasasinglescallopdredgebytwothreemetrelengthsofchainandpositionedintheplumeofsuspendedsedimentbehindthedredgesothattheLISST100Xandtheplanktonnetswere1mbehindtherearofthedredge.Oftheeighttowswithadredge,theLISST100Xwas?ttedsothatit’ssamplingheadwasinthelowestposition(35e40cmabovetheseabed)forfour,theintermediateposition(45e50cmabovetheseabed)forthreeandthehighestposition(55e60cmabovetheseabed)forone.TheLISSTwasintheintermediatepositionforthetwocontroltows.Theplanktonnetshadameshsizeof250mmandtheirmouthopening,whichwas12.9cmhighby29cmwide,couldbeopenedandclosedremotelyusingacousticreleases.Theywerepositionedoneontopoftheotherandtheopeningswere26e39,49e62and72e85cmfromtheseabed(Fig.4)andsampledtheplumeforperiodsofbetween10and12min.Theresultingsamplesweresievedonboardovera0.5mmmeshandtheresiduepreservedin4%bufferedformalininseawater.2.3.BACIcoresamplingandlaserstripemetrologyThreetowstookplacewhereaBACIcoresamplingexperimentwascarriedouttoassesstheshorttermimpactofscallopdredgingontheinfaunalspecies.Priortoeachtowa50mbaselinetransectwasestablishedperpendicularlyacrosstheplannedtowpathandmarkedwithtwobuoys.SixsamplecoresofsedimentwithinhabitinginfaunawerecollectedbySCUBAdivers.The?rstcorewastaken8mfromtheinitialmarkerbuoyandtherestat3mintervalsalongthetransect.Theseactedasbaselinesamples(pre-tow)fortheBACIcomparisonandtheirpositionsweremarkedwithsmall?ags.Thescallopdredgewasthentowedacrossthetransectandasclosetotheinitialmarkerbuoyastheskipperjudgedpossible.Immediatelyafterthescallopdredgecrossedthetransect,diversdescendedtheinitialmarkerbuoy,swamalongthetransectandFig.4.Thetowedunderwaterdiversvehicle(TUV).

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2.4.AnalysisofsedimentdataAsdescribedabovetheLISST100Xmeasurestheconcentrationofparticlesinthewatercolumn(c)inml/l.Thecorrespondingmassofsedimentmobilisedpermetretowed,m(kgmà1),canbeesti-matedasfollowsm?2:65phpwcr=106whereitisassumedthatthesedimenthasarelativedensityof2.65,phandpwareestimatesoftheheightandwidthoftheplumeobtainedfromvideocamerasattachedtothesledgeandr(kgmà3)isthedensityofwater.Ifwedividethisquantitybythesweptwidth,sw,(whichhereweassumetobe0.82m,thedistancebetweentheoutermostteeth)wecancalculatethemobilisedsedimentinventoryperunitsweptarea.Thethicknessofthesedimentlayer,ts,equivalenttothismassofsedimentcanbeestimatedwithts?phpwcswe1àfT106wherefisthesedimentporosity(whichwetaketobe0.4)andwhereweassumethatonlysedimentinthesweptwidthisremobilised.2.5.AnalysisofinfaunaldataOnreturntothelaboratorytheanimalsfromthepreservedcoreandnetsamplesweresortedandidenti?edtothelowestpossibletaxonomiclevelandbiomassandabundanceperspeciesrecorded.ThePRIMER6statisticalpackage(ClarkeandGorley,2006)wasusedtocalculatearangeofunivariatestatisticalindices(ofwhichwepresenttotalspecies,totalindividualsandspeciesrichness).Totestwhethertherewasasigni?cantdifferencebetweensamplesofthedifferenttreatmentgroups(thecontrol,insidetowpathandoutsidetowpathofthecoresamples;andtheheightsofthenetsamples),aone-wayANOSIM(analysisofsimilarities)wasappliedtoaBrayCurtissimilaritymatrixgeneratedfromthesquareroottransformedabundancedatasets.Forthecoredatathiswascarriedoutatthehaullevel.3.Results3.1.TUVexperimentsTheaverageconcentrationofsedimentincreasedfrom154ml/latapproximately2mbehindthedredgetoamaximumof176ml/lbetween5and10mbehindandthendeceasedtoavalueof90ml/lat30m(Fig.6).Thedashedlinerepresentsthetowwiththreedredges.Theresults,althoughslightlylower,aresimilar.Fig.5.ThebenthicsledgewiththeLISST100X?ttedinthecentre.Inthisimagewatersamplerbottlesandplanktonnetsarealso?tted.

identi?edthedredgepathandnotedtheproximityoftheclosestmarker?ags.Threecoresampleswerethentakenfromthedredgepathandafurtherthreeoutside,adjacenttodredgepath.Asabove,thesesixsamplesalongwiththethreeclosestpre-towonesweresievedonboardovera0.5mmmeshandtheresiduepreservedin4%bufferedformalininseawater.Inaddition,diversmeasuredtheseabedtopographybeforeandafteradredgetowusinganunderwaterlaser-linemetrologysys-tem.Thissystememploysatriangulationtechniquewherebyacameraandalaserlinemoduleare?ttedtoaframethatdiverscanpositionabovethatpartoftheseabedtheywishtopro?le.Itentailscomparingtheimageofthelaserlinewiththecorrespondingpo-sitionontheimageofarectangulargridthathaspreviouslybeenplacedintheplaneofthelaserstripe.Thesystemisaccuratetowithin0.5mmandisdescribedindetailinO’Neilletal.(2009).Fig.6.Themeanvolumeperlitreofwaterofsedimentparticlesinthesizerangebetween2.5and500micronsplottedagainstdistancefromthedredge.Thethinlinesaretheindividualhaulswhereonedredgewastowed;thedashedlineisthehaulwiththreedredges;andthethicklineistheaverageofthethinlines.Thegreyextensionatthelefthandsideofthethicklineistothemeasurementmadeduringthebenthicsledgeexperiment.

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mean volume concentration (ul/l)101520253005101520253005mean volume concentration (ul/l)mean volume concentration (ul/l)mean volume concentration (ul/l)2.723.23.784.465.276.217.338.6510.212.114.216.819.823.427.632.538.445.353.563.174.587.9101520253005101520253002.723.23.784.465.275.276.217.338.6510.212.114.216.819.823.427.632.538.445.353.563.174.587.91041221441702012372803313904.463.783.22.7252.723.23.784.465.276.217.338.6510.212.114.216.819.823.427.66.217.338.6510.212.114.216.819.823.432.538.445.353.563.174.587.910412214417020123728033139046027.6haul 9haul 12haul 14haul 2032.538.4particle size (micron)particle size (micron)particle size (micron)45.3F.G.O’Neilletal./MarineEnvironmentalResearch90(2013)104e112

particle size (micron)5 m2 m30 m20 m10 m5 m20 m10 m46053.563.174.587.9104104122144170201237280331122Fig.7.Particlesizedistributionofthesedimentintheplumebyhaulanddistancefromthedredge.

3904601441702012372803315 m2 m20 m10 m39050 m4602 m5 m2 m30 m20 m10 m

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Thebackgroundsedimentconcentrationstakenbetweensta-tionsoutsidethesedimentplumerangedbetween1.8and5.0ml/landarecomparablewithcontrolmeasurementsfromthebenthicsledgeexperiments.Whiletheconcentrationsoflargerparticles(>100mm)reachedamaximumbetween2and10mbehindthedredge(Fig.7),themaximumconcentrationsofthe?nerparticles(<100mm)occurredfurtherback,between10and30m,wheretheconcentrationofthelargerparticlesbegantodecrease.Theheightoftheplumeincreasedfromabout0.4mat2mbehindthedredgetoabout1.5mat30mand2mat50mandthewidthincreasedfrom0.82matthedredgetoabout2m,30mbehindit(Fig.8).Thesevaluesmustbetreatedwithcautionastheplumeisturbulentandvariableandtheestimateshavebeenmadebydifferentdivers.Nevertheless,theyareindicativeandcanbeusedtoprovideanestimateofthesuspendedsedimentinventory.Ifweassumethesedimenthasaspeci?cdensityof2.65,thesus-pendedsedimentinventorypermetrelengthofplumehasamaximumof0.85kgwhichoccursabout20mbehindthedredge(ifwetakethesweptwidthtobe0.82m,thedistancebetweentheoutermostteeth,themaximumsedimentinventoryperunitsweptareais1.03kgmà2).Similarly,ifweassumeasedimentporosityof0.4,andthatonlysedimentinthesweptwidthisremobilised,thenthemaximumsedimentinventoryisequivalenttoasedimentlayerof1mmthickness.3.2.BenthicsledgeexperimentsTheaverageparticleconcentrationmeasuredduringthecon-trolswas4.7ml/l.Withthescallopdredgeinplacethisrosebyanorderofmagnitudeto41.2ml/latthelowestpositionandaveraged14.5ml/lintheintermediateposition.Atthehighestpositiontheparticleconcentrationwas5.9ml/l,onlyslightlyhigherthanthebackgroundvalue.TheseresultsaresupportedbyvideoevidencewhichshowthatatthelowestpositionthesamplingheadoftheLISSTwasfullyimmersedinthesedimentplumewhereasthein-termediatepositionwasontheborderandthehighestpositionwasoutside.TheANOSIManalysisoftheplanktonnetdataindicatedthereweresigni?cantdifferencesinspeciescompositionsbetweenthesamplestakenatthethreedifferentnetheights(p<0.005),withthePairwiseRtestresultsinexcessof0.5andreachingto1.00,re?ectingthedecreaseobservedintotalspecies,abundanceandspeciesrichnesswithnetheight(Tables1and5b).Thetop20speciesfoundinthenetsweredominatedbyhighlymobilecrus-taceansfrom?vefamilies(amphipodse10,caprellids,mysids,decapodsandcumaceanse7plus2polychaeteandasinglegastropodmolluscspecies)(Table2).Mostofthespeciesswimactivelyinthewatercolumnorburrowintheupperlayersofthesubstrateandarecapableofstrongswimmingbehaviour.Themostabundantanimalsinthesesamplesaretwospeciesofcaprellids,whichareknowntoattachtotheuppersurfacesofthelocalsub-strate(VolbehrandRachor,1997)andhencearelikely,asstrongTable1Abundancedata,totalnumberofindividualsandspeciesrichnessfromthethreeplanktonnetsforeachofthreereplicatehaulstowed.HaulN1PlanktonnetpositionTopMiddleBottomTopMiddleBottomTopMiddleBottomTotalspecies316082344960375962Numberofindividuals397123838584911055261456211032092Speciesrichness5.018.299.815.336.907.505.698.287.98N2N3swimmers(Keith,1971),tohavemovedintothewatercolumnwhendisturbedorhavebeenpassivelyentrainedalongwiththesediment.OthersmallCrustaceainthenets(theAmphipodaandCumacea)liveinoronthesedimentsurfaceandarealsocapableofspendingextendedperiodsoftimeinthewatercolumn(LawrieandRaffaelli,1998)whiletheshrimpfamily,theHippolytidae,ishighlymobileasarethemysids(O’Brien,1988)whichmayevenberegardedasfullyhyperbenthic(possiblypelagic).Thepercentageofanimalssampledateachnetpositionisveryconsistentbetweenspeciesandaveraged7,24and69%forthetop,middleandbottomnets,respectively.Thesevaluesarealsoverysimilartotheincreaseofsedimentsampledatsimilarpositions,with3,21and77%oftheincreasebeingmeasuredatthetop,middleandbottomsamplingpositions,respectively.Thereareanumberofspeciesthatappearinrelativelyhighnumbersinthenetsbutinlownumbersinthecores.Inmostcasesthesearehighlymobileanimalswhichmayhavebeendisturbedbythepresenceofdiverswhencoring.Twogroups,however,thepolychaetefamily,theTerebellinae,andthegastropodmollusc,theRissoidae,areabsentfromthecoresamples.TheTerebellinamaypassthrough2larvalstagesandpresumablywereinoneoftheseplanktonicstagesofdevelopmentatthetimeoftheexperimentaltrialshoweverthegastropodsencounteredseemedtobefullydevelopedwhenexaminedunderabinocularmicroscope.Weareunabletoexplaintheirpresenceinthenetsbuttotalabsenceinthecoresamples.3.3.BACIcoresamplingandlaserstripemetrologyTherewasnosigni?cantdifferenceininfaunalcompositionbetweenthecontrol,insidetowpathandoutsidetowpathtreat-mentgroupsateitherthehaulorthecombined-haullevel(p?0.7,0.39,0.4andp?0.92)whichre?ectsthesimilarityintotalspecies,abundanceandspeciesrichnessofthesesamples(Table3).TheseconclusionsarealsosupportedbytheANOSIManalysiswithGlobalRresultslessthan0.25indicatingthattherewasnoclearseparationbetweentreatments(Table5a).Thetop20speciesinthecoresamplesweredominatedbypolychaetes(10)andamphipods(8),therewasonecaprellidFig.8.Theaverageplumeheightandwidthasestimatedbythedivers.

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Table2Thetwentymostabundantspeciesintheplanktonnetsamples.Theirnumber,proportionsateachnetlevelandtheirnumbersampledinthecores.TotalTop/allMiddle/allBottom/allCoresPhtisicamarinaa48760.080.250.6723Pariambustypicusa12260.040.230.7228Terebellinaespjuv9720.360.320.320Bathyporeiaspa7850.060.230.70205Hippolytidaesppjuv5420.080.250.671Amphilochusneapolitanus5140.050.280.671Pseudocumalongicornis4460.070.200.733Synchelidiummaculatum3830.040.210.742Perioculodeslongimanusa3460.060.250.6917Laniceconchilega3280.990.000.0022Mysidaceasp3250.050.380.560Gammaropsispalmata2920.060.190.752Megaluropusagilis2340.060.200.743Bodotriasp2330.040.210.754Rissoidaespjuv1840.010.150.840Aoridaespindet(female)a1660.050.240.7142Crangonbispinosusneglecta1350.040.160.790Stenothoemarina1190.040.340.621Paramphilochoidesodontonyx1000.040.250.711Abludomelitaobtusata910.010.240.756Averageproportions0.070.240.69aSigni?esspeciesthatwerealsoamongthetwentymostabundantinthecoresamples.TheaverageproportionsdonotincludetheLaniceconhilegadataofwhichallbuttwowerefoundinonenetduringonehaul.speciespresentinsubstantialnumbersandonespeciesofbivalvemollusc(Table4).Mostofthesespecies(withtheexceptionofBathyporeiasppandPhtisicamarina)aretubicolousorburrowintothelocalsedimentsandaresedentaryand/orpoorswimmers.P.marinaareknowntoattachtotheuppersurfacesofthelocalsubstrateandalthoughtheBathyporeia(themostabundantgenusinthecores)areconsideredtobegoodswimmers,theyarethoughttobeselectivedepositfeeders,typicallyfeedinglyingupsidedowninasmallcavityinthesediment.Ofthesespecies,thosewhichhavefewindividualsinthenetsamplesarealmostalltubebuildingordeepburrowing.Table3Abundancedata,totalnumberofindividualsandspeciesrichnessfromthecoresamplestakenbeforeandafterthethreehaulsoftheBACIexperiment.HaulTreatmentTotalspeciesNumberofindividualsSpeciesrichnessC1Control21684.7424495.9131587.39Outside19474.6821475.1916344.25Inside18584.1923485.6825685.69C2Control23425.8931916.6524625.57Outside25595.89361127.4229736.53Inside22645.0524466.0132717.27C3Control24456.0424555.7428576.68Outside21276.0718265.2224326.64Inside15294.1627496.6815214.60Table4Thetwentymostabundantspeciesinthecoresamples.Theirnumber,proportionsineachtreatmentgroupandtheirnumbersampledinthenets.TotalControlOutsideInsideNetBathyporeiasppa2050.310.400.30785Spio?licornis1890.450.300.252Corophiumcrassicorne970.340.240.423Aricideaminuta640.280.410.310Poecilochaetusserpens550.350.270.380Aoridaespindet(female)a420.430.360.21166Cochlodesmapraetenue380.340.340.320Clymenurajohnstoni340.320.350.320Magelona?liformis330.240.480.270Photislongicaudata310.100.100.8148Siphonoeceteskroyeranus300.470.270.2736Pariambustypicusa280.390.290.321226Exogonehebes240.580.250.170Phtisicamarinaa230.220.390.394876Ampharetelindstroemi220.360.410.230Laniceconchilega220.270.410.32328Nephtysspjuv210.290.330.3815Ampeliscabrevicornis210.330.290.383Exogonenaidina200.350.400.2533Perioculodeslongimanusa170.180.290.53346Averageproportions0.330.330.34aSigni?esspeciesthatarealsoamongthetwentymostabundantinthenetsamples.Therewere?vespecieswhichareamongthetwentymostnumerousinboththecoreandnetsamples,twocaprellidsandthreeamphipods.Theseareconsideredtobeeither(i)goodswimmersthatalsoareassociatedwiththesubstrate(Bathyporeiaspp),(ii)burrowers/inhabitantsofthesuper?ciallayersofthesediment(Phtisicamarina,PariambustypicusandPerioculodeslongimanus)or(iii)interfacefeeders(Aoridaespp).Ascanbeseenfromthecameraimages,thepassageofthescallopdredgetendstohomogenisetheseabed(Fig.9).Thelaserlineanalysisdemonstratesthattheunimpactedsandripples,whichhaveacresttotroughamplitudeofbetween1.5and2cm,are?attenedandtheresultingdifferencebetweenthehighestandlowestpointsinthetowpathislessthan1cm(Fig.10).Further-moreitispossibletoidentifythetracksleftbythedredgeteethbothintheimagesthemselvesandinthesubsequentanalysiswhichhavelocalminimumpointscoincidingwiththeteethpositions.4.DiscussionTheseresultsdemonstratehowthepassageofascallopdredgemobilisesthesedimentandtheinhabitinginfaunaintoitstrailingwake.Aturbulentregionwithhighshearbedstressisproducedbehindthedredgethatentrainssedimentandfaunainaplume.Thesedimentconcentrationandplumedimensionsincreasequicklyandthesedimentinventorygoesfromabout0.04kgpermetreofplumeimmediately(0e1m)behindthescallopdredgeto0.18kgmà1between1and3mfromthedredgetoamaximumof0.85kgmà1about20mbehindthedredge.Thismaximumvaluecorrespondstoasedimentlayerofabout1mmdepthofthesweptwidth(of0.82m)beingputintosuspension.AtthispointbehindTable5aOnewayanalysisofsimilarities.(ANOSIM)ofcoresampleabundancedata.HaulnumberGlobalRC1à0.062C20.029C30.07F.G.O’Neilletal./MarineEnvironmentalResearch90(2013)104e112

Table5bOnewayanalysisofsimilarities(ANOSIM)ofnetsampleabundancedata.NetcomparisonTopvsMiddleTopvsBottomMiddlevsBottomPairwiseR0.6671.0000.741111

thedredge,theturbulencehasdecayedtotheextentwherethelargerparticles(>100mm)begintodropoutandtheirconcentra-tiondeceasesbuttherecontinuestobeentrainmentofthe?nerparticles,whichreachamaximumconcentrationbetween10and30mbehindthedredge.AcomparisonoftheparticlesizeanalysisofthesedimentsamplesobtainedwiththeDaygrabandtheparticlesizedistri-butionsoftheremobilisedsedimentsdemonstratethatthepro-portionof?nerparticlesputintosuspensionbehindthescallopdredgeismuchgreaterthantheirproportionintheseabed(Figs.7and11).Silt(<63mm)makesuponly4.1%oftheDaygrabsamples,however,itcomprisesfrom20%oftheparticulatematterintheplumesamplesimmediatelybehindthedredgetoabout37%atabout20mand48%at30m.Thesedimentinventoryintheplumeatthesepointsbehindthedredgeimpliesthatthesus-pendedsiltmaterialoriginatesfromalayerwhosedepthisatleast10mm.Presumably,thissiltismadeavailablebythedredgeteethwhichrakethrough,loosenandbreakuptheupperlayersofthesediment.Theseresultsareconsistentwiththoseofthebenthicinfaunalsamplingwherethespeciesmostabundantintheplanktonnetseitheractivelyswiminthewatercolumnorburrowintheupperlayersofthesubstrate,andwherethosemostabundantinthecoresamples,butnotinthenetsamples,arealmostalltube-buildingordeepburrowing.ThesimilaritybetweentheproportionofsedimentandtheproportionofanimalssampledatthedifferentpositionsaboveFig.9.Ontheleftistheun-impactedseabed,whereasontherightistheseabedafterthepassageofascallopdredge.Themarksleftbythedredgeteethcanclearlybeseen.

Fig.10.Pro?leofun-impactedandpostimpactedseabedmeasuredusingthelaserstripepro?ler.Theverticallinerepresentthepossiblepositionofthescallopdredgeteethandcoincidewithlocalminimaofthepostimpactpro?le.

Fig.11.AverageparticlesizedistributionoftheDaygrabsedimentsamples.

112F.G.O’Neilletal./MarineEnvironmentalResearch90(2013)104e112

theseabedandtheconsistencybetweenspeciesisnoteworthy.Itsuggeststhat,evenifsomeofthesespeciesrespondactivelytothepresenceofthedredge,onceentrainedinthewake,theturbulentforcesaresuchthattheyaretransportedmoreorlesspassivelyinthesamewayasthelargersedimentparticles.Astheturbulencedissipatesanddiffuses,thesespecies,dependingontheirswimmingability,sizeanddensity,arelikelytoswimactivelyagainorfalltotheseabed.Thefactthattherewasnodetectabledifferencebetweenthecoresamplestakenbeforeoraftertowingsuggeststhatanimalsmobilisedbythedredgeresettleinthetowpath.Wemustbecareful,however,inter-pretingtheseresultsasouranalysisdoesnotprovideanyinfor-mationregardingthefateoftheseanimals.Theindividualssampledmayhavebeendamagedorstressed(andsubsequentlymorelikelytohavebeenpredated)ortheymayhavebeendead.Indeed,wewouldalsoneedtobecautiousifwehadobservedareductioninabundanceasanytideorcurrentcouldtransporttheplumeawayfromthetowpathbeforesomeoftheentrainedanimalscouldresettle.Itisdif?culttomakedirectcomparisonswiththeresultsofotherstudiesof?shinggearinducedsedimentsuspension.Thedifferentcomponentsofa?shinggearincontactwiththeseabed,thecharacteristicsofthewakehydrodynamicsandthesedimenttypewillalldeterminethenatureoftheplume.Nevertheless,thevaluesofsuspendedsedimentconcentrationandinventoryaregenerallycomparablewiththoseofothertrials(citedinsection1).Theoneexceptionisthe‘Peninsula’scallopdredgeexaminedbyBlackandParry(1999)whosemeasure-mentsareanorderofmagnitudegreater.Thereareprobablytwoexplanationsforthis.Firstly,themudcontentoftheseabedwasgreaterandvariedfrom7.2to30.1%overthethreesitestheyexamined.Secondly,andperhapsmoreimportantly,thesedredgeshavea‘cutterbar’thatisangledat45??andwhichex-tendstotheleveloftheskids.Thecutterbartrimsoffthehighregionsofanunevenseabedmakingmoresedimentavailableformobilisationintotheplume.Thereare,however,similaritieswithourtrials.Inparticular,theyalsofoundthatmaximumconcentrationsoccurredatabout20mbehindtheirdredgesuggestingthatsedimentcontinuestobeentrainedatleastuptothisdistancebehindtheirgear.Althoughonlyonetowtookplacewiththreedredgesattachedtothebeam,theplumeconcentrationmeasurementsweresimilartothosewithonedredge.Thus,toestimatethemassofsedimentremobilisedbytowinganumberofthesedredgesitisprobablyenough(atleasttoagood?rstorderapproximation)tosumthecontributionoftheindividualdredges.Thismeansthatatypicalscallopdredger?shingeightdredgesoffeachsidewouldputabout13.6kgofsedimentintothewatercolumnpermetreofseabedtowed.Itisnoteasytointerprettheseresultsdirectlyinrelationtoscallop?sherymanagement.Themassofsedimentmobilised,theextenttowhichitremainsresuspendedandtheimpactofre-settlementonthebenthosissitespeci?canddependsontheparticlesizedistributionofthesediment,thelocalhydrographiccharacteristicsandthemake-upofthebenthiccommunity.Furthermore,asdiscussedabove,theanalysiscarriedoutdoesnotprovideanyinformationregardingthefateofthebenthicinfauna.Hence,inordertoaddressbroaderenvironmentalcon-cernsandissuessuchasecosystemfunctioningitwouldbenecessarytoinvestigatetheassociatedmortalityoftheanimalsimpacted.ReferencesAuster,P.J.,Langton,R.W.,1999.Theeffectsof?shingon?shhabitat.In:AmericanFisheriesSocietySymposium22.Black,K.P.,Parry,G.D.,1999.Entrainment,dispersalandsettlementofscallopdredgesedimentplumes:?eldmeasurementsandnumericalmodelling.Ca-nadianJournalofFisheriesandAquaticSciences56,2271e2281.Boulcott,P.,Howell,T.R.W.,2011.Theimpactofscallopdredgingonrocky-reefsubstrata.FisheriesResearch110(3),415e420.Brown,L.,Bresnan,E.,Summerbell,K.,O’Neill,F.G.,2013.Thein?uenceofdemersaltrawl?shinggearsontheresuspensionofdino?agellatecysts.MarinePollutionBulletin66,17e24.Clarke,K.R.,Gorley,R.N.,2006.PRIMERv6.PRIMER-ELimited.Collie,J.S.,Escanero,G.A.,Valentine,P.C.,2000.Photographicevaluationoftheimpactsofbottom?shingonbenthicepifauna.ICESJournalofMarineScience57,987e1001.Dale,A.C.,Boulcott,P.,Sherwin,T.J.,2011.Sedimentationpatternscausedbyscallopdredginginaphysicallydynamicenvironment.MarinePollutionBulletin62(11),2433e2441.Dellapenna,T.M.,Allison,M.A.,Gill,G.A.,Lehman,R.D.,Warnken,K.W.,2006.Theimpactofshrimptrawlingandassociatedsedimentresuspensioninmuddomi-natedshallowestuaries.Estuarine,CoastalandShelfScience69(3e4),519e530.Dounas,C.G.,2006.Anewapparatusforthedirectmeasurementoftheeffectsofottertrawlingonbenthicnutrientreleases.JournalofExperimentalMarineBiologyandEcology339(2),251e259.Dounas,C.,Davies,I.,Triantafyllou,G.,Koulouri,P.,Petihakis,G.,Arvanitidis,C.,Sourlatzis,G.,Eleftheriou,A.,2007.Large-scaleimpactsofbottomtrawlingonshelfprimaryproductivity.ContinentalShelfResearch27,2198e2210.DurrieudeMadron,X.,Ferré,B.,LeCorre,G.,Grenz,C.,Conan,P.,Pujo-Pay,M.,Buscail,R.,Bodiot,O.,2005.Trawling-inducedresuspensionanddispersalofmuddysedimentsanddissolvedelementsintheGulfofLion(NWMediterra-nean).ContinentalShelfResearch25,2387e2409.Eleftheriou,A.,Robertson,M.,1992.Theeffectsofexperimentalscallopdredgingonthefaunaandphysicalenvironmentofashallowsandycommunity.NetherlandsJournalofSeaResearch30,289e299.Galbraith,R.D.,Rice,A.,Strange,E.S.,2004.Anintroductiontocommercial?shinggearandmethodsusedinScotland.ScottishFisheriesInformationPamphlet.No25.Hall,S.J.,1999.TheEffectsofFishingonMarineEcosystemsandCommunities.BlackwellScience,Oxford,UK,p.274.Hinz,H.,Murray,L.G.,Malcolm,F.R.,Kaiser,M.J.,2012.Theenvironmentalimpactsofthreedifferentqueenscallop(Aequipectenopercularis)?shinggears.MarineEnvironmentalResearch73(1),85e95.Ivanovi??c,A.,Neilson,R.D.,O’Neill,F.G.,2011.Modellingthephysicalimpactoftrawlcomponentsontheseabed.OceanEngineering38,925e933.Jennings,S.,Kaiser,M.J.,1998.Theeffectsof?shingonmarineecosystems.Ad-vancesinMarineBiology34,201e352.Kaiser,M.J.,Collie,J.S.,Hall,S.J.,Jennings,S.,Poiner,I.R.,2002.Modi?cationofmarinehabitatsbytrawlingactivities:prognosisandsolutions.FishandFish-eries3,114e136.Kaiser,M.J.,Clarke,K.R.,Hinz,H.,Austen,M.C.V.,Somer?eld,P.J.,Karakassis,I.,2006.Globalanalysisofresponseandrecoveryofbenthicbiotato?shing.MEPS311,1e14.Keith,D.E.,1971.SubstrateselectionincaprellidamphipodsofsouthernCaliforniawithemphasisonCaprellacalifornicaandC.equilibra.Paci?cScience25,387e394.Lambert,G.I.,Jennings,S.,Hinz,H.,Murray,L.G.,Lael,P.,Kaiser,M.J.,Hiddink,J.G.,2012.Acomparisonoftwotechniquesfortherapidassessmentofmarinehabitatcomplexity.MethodsinEcologyandEvolution4,226e235.Lawrie,S.M.,Raffaelli,D.G.,1998.InsituswimmingbehaviouroftheamphipodCorophiumvolutator(Pallas).JournalofExperimentalMarineBiologyandEcology224(2),237e251.Lucchetti,A.,Sala,A.,2012.ImpactandperformanceofMediterranean?shinggearbyside-scansonartechnology.CanadianJournalofFisheriesandAquaticSci-ences69,1806e1816.L?kkeborg,S.,2005.ImpactsofTrawlingandScallopDredgingonBenthicHabitatsandCommunities.FAOTechnicalPaperNo.T472.FoodandAgricultureOrga-nizationoftheUnitedNations,Rome.O’Brien,D.P.,1988.Directobservationsofclustering(schoolingandswarming)behaviourinmysids(Crustacea:Mysidacea).MarineEcologyProgressSeries42,235e246.O’Neill,F.G.,Summerbell,K.,2011.Theremobilisationofsedimentbydemersalottertrawls.MarinePollutionBulletin62(5),1088e1097.O’Neill,F.G.,Summerbell,K.,Breen,M.,2009.Anunderwaterlaserstripeseabedpro?lertomeasurethephysicalimpactoftowedgearcomponentsontheseabed.FisheriesResearch99,234e238.Pottsmith,C.,2007.LISST-100XUsersManualeVersion4.65.SequoiaScienti?cInc..Pranovi,F.,DaPonte,F.,Raicevich,S.,Giovanardi,O.,2004.AmultidisciplinarystudyoftheimmediateeffectsofmechanicalclamharvestingintheVeniceLagoon.ICESJournalofMarineScience61,43e52.Volbehr,U.,Rachor,E.,1997.TheassociationbetweenthecaprellidPariambustyp-icus.Hydrobiologica355(1),71e76.

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