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Chapter17

DiurnalverticalmigrationDiurnalverticalmigrationisaverycommonphenomenon.AfewexamplesChangesintheabundanceofthecopepodCalanus

finmarchicusinthesurface,between06:00and14:00hduring6weeksfrom21stJuneto11thAugust,1991,offCaliforniancoast§17.1Characteristicsdiurnalvertical

migrationofzooplanktonsamplesofnightplanktonatthesurfacearenotablydifferentfromthosetakenduringtheday,notonlyintheirfaunisticcompositionbutalsoquantitatively.Thenumberofind.colletedinthesurfacelayersbecometentimeshigherbetween16:00and20:00pm.Theonlypossibleexplanationofthesevariationsisverticalmigration,representingarhythmof24h.DiurnalverticaldistributionofthecopepodC.finmarchicusfemalesinLochFyne,ClydeSea,ScotlandDistributionasafunctionofthetimeandmaximumdepthsampledwherethecopepodGaussia

princepsisfound:solidline,regressioncurve:z=zo+a

coswt=487-230coswt.VerticaldistributionofthecopepodC.finmarchicusfromthe14thto16thJuly,1933inGeorgiaBankTop:femalesMedium:CopepodidVBottom:Femaleson17thand18thJuly1933DistanceofDVMinsomeplanktoniccrustaceaGroupsspeciesDistanceofDVM(m)CladoceraDaphnialongispina0.6

CopepodsDiaptomus

shoshomeCalanus

finmarchicusPleuromamma

robustaMetridia

pacifica8.8About100>600400-500MysisBoreomysis

micropsEucopia

unguiculata400400HyporiidVibia

propinqua400EuphausiidNematoscelis

megalopsThysanopoda

acutifrons200600PatternofthemigrationChusing(1951)analysedalltheworkoncrustaceaandconsideredthatitispossibletoarrangetheirpatternsofmigrationinthefollowinggeneralscheme:Risingintheeveningfromthe“daydepth”towardsthesurfaceDeparturefromthesurfaceataboutmidnightorbeforeitRisingtothesurfacejustbeforedawnRapidsinkingatdawntothe“daydepth”Remainingduringthedayata“daydepth”moreorlessvariable§17.2CausesofdiurnalmigrationLightintensityisofprimaryimportance.TheobservationsonDVMathighlatitudeswhereduringpartoftheyearthereisnoalternationbetweendaysandnightsgivesakindof“natural”experiment.Calanusandotherprincipalspeciesofthezooplanktonremaininthesuperficiallayerofwaterwithoutmigrationthisalsoappearsagaininwinterwiththereturnofthechangeofdaytonight.Duringthewinternightcalanusstaysinthedepthsataconstantlevel.Hypothesisofphototaxisratherlargevariationsaroundtheoptimallightintensity.Thisspeciesiscalled“eurylume”species.Thezooplanktonseemtostayatanoptimallightintensityandshowanegativephototaxisabovecertainlevelsandpositiveonebelowthatlevel.HypothesisofOptimallightintensityAnimalsmustrisetosurfaceinacertaintimeavoidingofsinkingtobottomduetogravityRisingisactive,andsinkingispassive.Migrationisakindofdiurnalalternationofperiodsofactivityandinactivity.geotaxisFeedingandavoidingfedZooplanktonrisesupperlayerforPhytoplanktonatdarknight.Zooplanktonsinkstobottomwherethelightisdimforavoidingfedatdaytime.HypothesisoffoodInsummaryoftheDVMreasonsLightiswithoutanyboubtthefundamentalfactor,TemperatureisanimportantaccessoryfactorPressureandgravityareauxilliaryf

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