Microzooplankton community structure and grazing impact on major phytoplankton in the Chukchi sea and the western Canada basin, Arctic ocean

Abstract

Weinvestigatedthemicrozooplanktoncommunityanditsgrazingimpactonmajorphytoplankton groups intheChukchiSeaandinthewesternCanadaBasinduringtheperiodJuly？August2010.The study areawasdividedintothreeregionsbasedontopography,hydrographicpropertiesandtrophic conditions: (1)aproductiveregionovertheChukchiSeashelf(CSS)withhighphytoplanktonbiomass dominated bydiatoms,(2)anoligotrophicregionovertheNorthwindAbyssalPlain(NwAP)withlow phytoplanktonbiomassdominatedbypicophytoplankton,and(3)theNorthwindRidge(NwR),over which watersweredominatedbypicophytoplanktonanddiatoms.Thespatialdistributionofmicro- zooplankton biomassanditscompositionwererelatedtodifferencesinphytoplanktonbiomassand assemblage compositioninthethreewatermasses.Heterotrophicdinoflagellates(HDF)andciliates weresignificant componentsofmicrozooplanktonpopulations.AthecateHDFwasthemostimportant component intheCSS,wherediatomsweredominant.Nakedciliatesweredominantmicrozooplankton in theNwR.Microzooplanktongrazingratevariedbytheassemblagecompositionofbothphytoplankton and microzooplankton.Microzooplanktonwascapableofconsuminganaverageof71.7717.2%ofdaily phytoplanktonproduction.Growthratesofsmallerphytoplankton(i.e.,picophytoplanktonandauto- trophic nanoflagellates)andgrazingratesonthemwerehigherthanratesfordiatoms.Microzooplank- ton grazedmoreonpicophytoplankton(PPgrazed=89.3720.5%) andautotrophicnanoflagellates (PP grazed=82.3722.5%) thanondiatoms(PPgrazed=62.5720.5%). Thedynamicsofpredatorand preypopulationswerealmostbalancedinwatersinwhichsmallerphytoplanktonsweredominant. Picophytoplanktonproductionwasconsumedbymicrozooplanktonallowingtransfertolargercon- sumers. Onaverage,microzooplanktongrazed62.5%ofthediatomproductioninthewaterswestudied, indicating thattheclassicalfoodchain(withcarbon flux fromdiatomstocopepods)islikelyoperational and ofsignificance inthisregion.Overall,microzooplanktongrazingwasanimportantprocess controlling phytoplanktonbiomassandcompositionintheChukchiSeaandthewesternCanadaBasin during earlysummer.