Major contribution of diatom resting spores to vertical flux in the sub-polar North Atlantic

The masssinkingofphytoplanktoncellsfollowingbloomsisanimportantsourceofcarbontotheocean's interior,withsomespeciescontributingmoretothe flux ofparticulateorganiccarbon(POC)thanothers. During the2008NorthAtlanticBloomExperimentintheIcelandBasin,weexaminedplankton community compositionfromsurfacewatersandfromsedimenttrapsatdepthsdownto750m. Samples collectedwithneutrallybuoyantLagrangiansedimenttrapscapturedamajor flux event. Diatoms comprised Z99% ofcell flux intothesedimenttraps,withvegetativecellsandrestingsporesof the genus Chaetoceros contributing 5095% ofcell flux. Restingsporesofonespecies,identified as Chaetoceros aff. diadema, weredominant,comprising3592% ofcell flux. The flux ofrestingspores rangedfrom2to63mgCm2 day1 and wassignificantly correlatedwithPOC flux (p0.003).Overthe course of10days,the flux ofrestingsporesincreasedby26fold,suggestingthatthecellssankenmasse, possibly inaggregates.Incontrast,vegetativecellsof C. aff. diadema sampled fromsurfacewatersduring the periodprecedingthe flux eventgenerallycomprised o1% ofthediatomcommunityandnever exceeded5.2%.Restingsporesof C. aff. diadema wererarelyobservedinsurfacewatersbuttheir concentrations increasedwithdepth(to200m)belowthemixedlayer.Thisincreaseinrestingspore abundance, coupledwithincreaseddissolvedsilicicacidconcentrationsatdepth,suggestthatthe morphological changesassociatedwithsporeformationmayhaveoccurredinthemesopelagiczone, while cellsweresinking.Thevaluesofvariable fluorescence (Fv/Fm) measuredonsedimenttrapmaterial dominated byrestingsporeswereamongthehighestvaluesmeasuredinthestudyareaatanydepth. This, incombinationwiththerapidgerminationofrestingsporesinship-boardincubations,suggests that vegetativecellswerenotphysiologicallystressedduringsporeformation.Thedegradation-resistant, heavilysilicified restingsporevalvesexplainthehighrelativecontributionof C. aff. diadema resting spores tototalplanktoncarbonatdepth.Thesedataemphasizetheephemeralnatureoforganiccarbon flux eventsintheopenoceanandhighlighthownon-dominantspeciesandtransientlifestagescan contribute moretocarbon flux thantheirmoreabundantcounterparts.