Phytoplankton blooms beneath the sea ice in the Chukchi sea

16 Pages·2014·2.68 MB·English

by  Kevin R. Arrigo

Deep-SeaResearchII105(2014)1–16
ContentslistsavailableatScienceDirect
Deep-Sea Research II
journal homepage: www.elsevier.com/locate/dsr2
Phytoplankton blooms beneath the sea ice in the Chukchi sea
Kevin R. Arrigoa,n,1, Donald K. Perovichb,c, Robert S. Pickartd, Zachary W. Browna,
Gert L. van Dijkena, Kate E. Lowrya, Matthew M. Millsa, Molly A. Palmera,
William M. Balche, Nicholas R. Batesf, Claudia R. Benitez-Nelsong, Emily Brownleeh,
Karen E. Freyi, Samuel R. Laneyh, Jeremy Mathisj, Atsushi Matsuokak,l, B. Greg Mitchellm,
G.W.K. Mooren, Rick A. Reynoldsm, Heidi M. Sosikh, James H. Swiftm
aDepartmentofEnvironmentalEarthSystemScience,StanfordUniversity,Stanford,CA94305,USA
bEngineerResearchandDevelopmentCenter,ColdRegionsResearchandEngineeringLaboratory,Hanover,NH03755,USA
cThayerSchoolofEngineering,DartmouthCollege,Hanover,NH03755,USA
dDepartmentofPhysicalOceanography,WoodsHoleOceanographicInstitution,WoodsHole,MA02543,USA
eBigelowLaboratoryforOceanSciences,WestBoothbayHarbor,ME04575,USA
fBermudaInstituteofOceanSciences,FerryReachGE01,Bermuda
gMarineScienceProgram,andDepartmentofEarthandOceanSciences,UniversityofSouthCarolina,Columbia,SC29208,USA
hBiologyDepartment,WoodsHoleOceanographicInstitution,WoodsHole,MA02543,USA
iGraduateSchoolofGeography,ClarkUniversity,Worcester,MA01610,USA
jNOAAPacificMarineEnvironmentalLaboratory,Seattle,WA98115,USA
kUniversitéPierreetMarieCurie,Laboratoired’OcéanographiedeVillefranche,Villefranche-sur-Mer06238,France
lTakuvikJointInternationalLaboratory,DépartementdeBiologieandQuébec-Océan,UniversitéLaval,Canada
mScrippsInstitutionofOceanography,UniversityofCaliforniaSanDiego,LaJolla,CA92093,USA
nDepartmentofPhysics,UniversityofToronto,Toronto,Ontario,CanadaM5S1A7
a r t i c l e i n f o a b s t r a c t
Availableonline3April2014 IntheArcticOcean,phytoplanktonbloomsoncontinentalshelvesareoftenlimitedbylightavailability,andare
therefore thought to be restricted to waters free of sea ice. During July 2011 in the Chukchi Sea, a large
Keywords:
Arctic phytoplanktonbloomwasobservedbeneathfullyconsolidatedpackiceandextendedfromtheiceedgeto
Seaice 4100km intothe pack.Thebloomwas composed primarilyof diatoms,with biomass reaching1291mg
Phytoplankton chlorophyllam(cid:1)2andratesofcarbonfixationashighas3.7gCm(cid:1)2d(cid:1)1.Althoughtheseaicewherethe
bloomwasobservedwasnear100%concentrationand0.8–1.2mthick,30–40%ofitssurfacewascoveredby
meltpondsthattransmitted4-foldmorelightthanadjacentareasofbareice,providingsufficientlightfor
phytoplanktontobloom.Phytoplanktongrowthratesassociatedwiththeunder-icebloomaveraged0.9d(cid:1)1
andwereashighas1.6d(cid:1)1.Wearguethatathinningseaicecoverwithmorenumerousmeltpondsoverthe
pastdecadehasenhancedlightpenetrationthroughtheseaiceintotheupperwatercolumn,favoringthe
developmentoftheseblooms.Theseobservations,coupledwithadditionalbiogeochemicalevidence,suggest
that phytoplankton blooms are currently widespread on nutrient-rich Arctic continental shelves and
thatsatellite-basedestimatesofannualprimaryproductioninwaterswhereunder-icebloomsdevelopare
(cid:3)10-foldtoolow.Thesemassivephytoplanktonbloomsrepresentamarkedshiftinourunderstandingof
Arcticmarineecosystems.
&2014ElsevierLtd.Allrightsreserved.
1. Introduction (Maslaniketal.,2011;Nghiemetal.,2007).Thishasproducedan
icecoverthatissubstantiallythinnerandmorepronetomelting
Overthelastseveraldecades,theArcticOceanhasundergone and transport, leading to a markedly extended period of open
unprecedented changes in sea ice, with summer minimum ice water, particularly over the last decade (Arrigo and van Dijken,
extentdeclining 440%since1979(Comisoetal.,2008)andfirst- 2011).
year ice largely replacing the once prevalent multi-year pack ice Associatedwiththelossinseaiceontheseshelveshasbeenan
increaseintheamountoflightpenetratingthesurfaceoceananda
dramatic rise in the productivity of phytoplankton (Arrigo and
nCorrespondingauthor. vanDijken,2011),theorganismsresponsibleforthebulkofArctic
E-mailaddress:[email protected](K.R.Arrigo).
Oceanprimary production and constitute the base of the marine
1DepartmentofEnvironmentalEarthSystemScience,473ViaOrtega,Stanford
University,Stanford,CA94305-4216,USA. food web. This is particularly true for the Pacific sector of the
http://dx.doi.org/10.1016/j.dsr2.2014.03.018
0967-0645/&2014ElsevierLtd.Allrightsreserved.
2 K.R.Arrigoetal./Deep-SeaResearchII105(2014)1–16
Arctic Ocean where annual production increased by 130% in the masspropertiessetwithintheBeringSea(Coachmanetal.,1975;
East Siberian Sea and 48% in the Chukchi Sea between 1997 and Overland and Roach, 1987; Weingartner et al., 2005), they also
2009 (Arrigo and van Dijken, 2011). Because sea ice and snow differsignificantlywithrespecttonutrientconcentrations(Walsh
strongly reflect and attenuate incident solar radiation (Perovich, et al., 1989; Cooper et al., 1997; Codispoti et al., 2005, 2013).
1998;PerovichandPolashenski,2012),thegrowthofphytoplank- The easternmost Alaska Coastal Water (ACW) is relatively warm
ton at high latitudes is generally thought to begin in the open (1–61C),fresh(So31.8),andnutrient-poor(NO(cid:1)o10mM)duetothe
3
waters of the marginal ice zone (MIZ) once sea ice retreats in inputofriverrunoffandthebiologicaldrawdownofnutrientsinthe
spring, as solar elevation increases and surface waters become eastern Bering Sea. The Central Channel pathway consists of Bering
stratified by the addition of sea ice melt water (Alexander and Shelf Water with moderate nutrients (NO(cid:1)(cid:3)15mM) and salinity
3
Niebauer,1981;Loengetal.,2005;Sakshaug,1997;HillandCota, (31.8–32.5). The westernmost Herald Canyon pathway, containing
2005;Perretteetal.,2011).Infact,allcurrentlarge-scaleestimates mostlyAnadyrWater,hasthehighestsalinity(32.5–33)andnutrient
of primary production in the Arctic Ocean assume that phyto- concentrations(pre-bloomNO(cid:1)425mM)duetotheunder-utilization
3
plankton production in the water column under sea ice is of nutrients in the western Bering Sea (Hansell et al., 1993).
negligible (Subba Rao and Platt, 1984; Sakshaug, 2003; Pabi Upon reaching the shelf break, some of this water turns eastward
et al., 2008; Arrigo and van Dijken, 2011). However, an intense andflowstowardtheBeaufortSeainarelativelyswiftshelfbreakjet
phytoplankton bloom was recently observed in the Chukchi Sea (Weingartneretal.,2005).
growing beneath fully consolidated sea ice ranging in thickness WatermasspropertiesintheChukchiSeaarefurtherinfluenced
from 0.8 to 1.3m (Arrigo et al., 2012). The development of this bytheseasonalcycleofseaice.Inthewinter,brinerejectionduring
bloomhasbeenattributedtoincreasedlighttransmissionthrough seaiceformationmixestheentirewatercolumntoextremelycold
seaice(Freyetal.,2011)aswellashighnutrientconcentrationson temperatures((cid:1)1.81C)(Woodgateetal.,2006).Seaiceformationin
theChukchishelf. polynyas and leads continue to convectively form cold and dense
The Chukchi Sea is an inflow shelf (Carmack and Wassmann, winterwater(WW)ontheChukchishelfthroughoutthewinter.A
2006) that ventilates the upper halocline of the Arctic Ocean large fraction of this becomes nutrient-rich Pacific Winter Water
(Woodgate et al., 2005). Water flows northward through the thatdrainsthroughtheChukchiSeainthespringandeventuallyfills
BeringStraitduetotheseasurfaceheightdifferentialthatresults theArcticOceanhalocline.Asseaiceretreatsinspringandsummer,
fromthesalinitydifferencebetweenthePacificandArcticOceans the water column becomes re-stratified as surface waters freshen
(Coachman et al.,1975), with the volume flux increasing by 50% and warm (Woodgate and Aagaard, 2005). WW remaining on
between 2001 and 2011 (Woodgate et al., 2012). Upon reaching theChukchishelfinthesummerisgraduallyreplacedbyrelatively
the Chukchi Sea, the Pacific-origin water separates into three warmPacificSummerWater(Weingartneretal.,2005).
branches that flow around or between Herald and Hanna Shoals Becauseofitshighnutrientcontent,theChukchiSeaisaregion
(Fig. 1). Although these branches are identified based on water of intense summer biological activity with a rich benthic
150 m
100 m 3000 m
2000 m
80 m 1000 m
60 m
40 m Hanna 40 m
Central Shoal
Channel
40 m
Herald Shoal
20 m
Herald
Canyon 40 m
20 m
40 m
40 m
20 m
80 m
60 m 40 m
Fig.1. MapoftheChukchiSeashowingthemajorbathymetricfeaturesandthepredominantflowpathsfromtheBeringSea.CurrentsmodifiedfromWeingartneretal.
(2005).
K.R.Arrigoetal./Deep-SeaResearchII105(2014)1–16 3
community that supports abundant populations of marine mam- exceeded the surface value by 0.05. Water was collected at
mals and seabirds (Loeng et al., 2005; Dunton et al., 2005; standard depths (5, 10, 25, 50, 75, 100, 150, and 200m) and at
Grebmeier et al., 2006). Depth-integrated primary production the depth of the fluorescence maximum into 30L Niskin bottles.
andchlorophylla(Chla)intheChukchiSeaareamongthehighest Currentspeedanddirectionweremeasuredwithahullmounted
in the world (Springer and McRoy, 1993; Arrigo et al., 2012). OceanSurveyor150KHzADCP(TeledyneRDInstruments,OS150).
Phytoplankton contribute 490% of the total primary production Sea ice thickness. Sea ice thickness was measured directly by
in the Chukchi Sea (Hill and Cota, 2005), although sea ice algal drilling holes through the ice. Surveys of ice thickness were also
productioncanbeimportantearlyintheseason(Gradinger,2009). conducted using a Geonics EM-31 electromagnetic induction
Intensephytoplanktonbloomshavebeenobservedthroughoutthe sensor that determines the ice thickness with an accuracy of
Chukchi shelf, with spatial variation attributed to differences in 70.05mbyexploitingthelargeconductivitydifferencebetween
water masses, nutrient availability, and environmental forcing sea ice and the underlying seawater (Eicken et al., 2001). The
(Cota et al., 1996; Wang et al., 2005; Lee et al., 2007). The instrument transmits a primary electromagnetic field and mea-
southeastern Chukchi Sea and Barrow Canyon have specifically suresthestrengthofasecondaryfieldinducedintheocean.The
been identified as hot spots for primary production (Sukhanova induced field strength is related to the distance from the instru-
et al., 2009), while waters near the coast of Alaska in the ACW menttotheseawaterandthereforetheicethickness.
(Coupel et al., 2012) and off the continental slope generally have
lowerbiomass(Leeetal.,2012).
HerewepresenttheresultsfromtherecentICESCAPE(Impacts 2.2. Phytoplankton
of Climate on EcoSystems and Chemistry of the Arctic Pacific
Environment)programdocumentingingreaterdetailthephysical, Chlorophylla.Underwayseawaterwaspumpedfromadepthof
chemical,andbiologicalcharacteristicsofanintensephytoplank- 8m as part of the Uncontaminated Science Seawater System
tonbloomobservedbelowthefirstyearseaiceoftheChukchiSea (USSS). Some of this seawater was diverted to a WETLabs WET-
(Arrigoetal.,2012).Thepresentpaperprovidesanoverviewofthe STAR flow-through fluorometer. Fluorescence was converted to
observed under-ice phytoplankton bloom. Companion papers in Chl a concentration by calibrating against Chl a concentrations
this issue address more specific aspects of the bloom, including measured in discrete seawater samples (see below) that were
(1)thehydrographicconditionsleadingtothelargeaccumulations collectedatleastfourtimesdailyfromtheUSSS.
of algal biomass beneath the ice (Spall et al., 2014), (2) the ice Samples for fluorometric analysis of Chl a were filtered onto
conditions required for under-ice bloom development (Palmer 25mmWhatmanGF/Ffilters(nominalporesize0.7mm),placedin
etal.,2014),(3)thetypesofphytoplanktonobservedinassociation 5mL of 90% acetone, and extracted in the dark at 31C for 24h.
with the bloom (Laney and Sosik, 2014), (4) the impact of the Chl a was measured fluorometrically (Holm-Hansen et al.,1965)
under-ice bloom on the optical properties of the water column usingaTurner10-AU(TurnerDesigns,Inc.).Thefluorometerwas
(Neukermans et al., 2014), the response of the bacterial commu- calibratedusingapureChlastandard(Sigma).
nitytotheunder-icephytoplanktonbloom(Ortega-Retuertaetal., Particulate organic carbon. Particulate organic carbon (POC)
2014),andasatellite-basedanalysisofthefrequencyofunder-ice samples were collected by filtering water samples onto pre-
phytoplankton blooms on the Chukchi shelf in the recent past combusted (4501C for 4h) 25mm Whatman GF/F filters. Filter
(Lowry et al., 2014). Observations made within the sea ice zone blanks were produced by passing (cid:3)50mL of 0.2mm filtered
(SIZ)ontheChukchishelfcontradictthecommonperceptionthat seawater through a GF/F. All filters were then immediately dried
watersbeneaththeconsolidatedicepackharborlittlephytoplank- at 601C and stored dry until analysis. Prior to analysis, samples
ton biomass. Instead, our results suggest that phytoplankton andblankswerefumedwithconcentratedHCl,driedat601Cand
growing beneath a thinner Arctic ice pack composed mostly of packed into tin capsules (Costech Analytical Technologies, Inc.)
firstyearicecancontributegreatlytoannualprimaryproduction, for elemental analysis on a Elementar Vario EL Cube (Elementar
and evenexceedrates measured in openwatersafterthesea ice Analysensysteme GmbH, Hanau, Germany) interfaced to a PDZ
hasretreated. Europa 20–20 isotope ratio mass spectrometer (Sercon Ltd.,
Cheshire,UK).Standardsincludedpeachleavesandglutamicacid.
Fast repetition rate fluorometry. The maximum efficiency of
2. Methods photosystem II (Fv/Fm) of discrete samples was determined by
fast repetition rate fluorometry (FRRf) (Kolber et al., 1998) on a
2.1. Studyregion custommadeinstrument(Z.Kolber).Samplesweredarkadapted
for 30min at in situ seawater temperatures beforemeasurement
During the ICESCAPE program, we sampled the Chukchi Sea with the FRRf. Blanks for individual samples analyzed by FRRf
continental shelf between the Bering Strait and the western werepreparedbygentlefiltrationthrougha0.2mmpolycarbonate
BeaufortSeafrom18Junethrough16July2010andfrom28June syringe filter before measurement using identical protocols. All
through24July2011onboardtheUSCGCHealy.Wesampled140 reported values of Fv/Fm have been corrected for blank effects
stations (including 10 sea ice stations) in 2010 and 172 stations (CullenandDavis,2003).
(including 9 sea ice stations) in 2011. Here we focus on stations Photosynthesis vs. Irradiance. P–E parameters (Pn – maximum
m
46–57(Transect1)and57–71(Transect2)sampledfrom3–8July photosynthetic rate, αn – photosynthetic efficiency, and E –
k
2011 (Fig. 2A) but also refer to stations 38–55 (29–30 June) and photoacclimation parameter) were determined using a modified
71–84 (6–7 July) sampled in 2010 (Fig. 2C). These stations were 14C-bicarbonate incorporation technique (Lewis and Smith,1983;
located along four different transects that extended from open Arrigo et al., 2010). Carbon uptake, normalized by Chl a concen-
waterintothepackiceandallexhibitedevidenceofanunder-ice tration, was calculated from radioisotope incorporation, and the
phytoplanktonbloom(Fig.2). datawerefitbyleastsquaresnonlinearregressiontotheequation
Ateachstation,watercolumnprofilesoftemperature,salinity, of Platt et al. (1980). P–E parameters were used with under-ice
and oxygen were measured using a conductivity, temperature, light profiles to estimate rates of depth-integrated daily gross
anddepthsensor(Sea-BirdElectronics)andoxygensensor(SBE43, primary production. Specific growth rate (m, d(cid:1)1) for a given
Sea-Bird Electronics) attached to a rosette. Mixed layer depth sample was calculated by multiplying the photosynthetic rate
(MLD) was calculated as the depth where potential density (Pn)bytheChla:POCratiofromthatsample.
4 K.R.Arrigoetal./Deep-SeaResearchII105(2014)1–16
170°W 165°W 160°W
74°N July 29 July 22 Beaufort
July 22 Sea
57
59 Chukchi
73°N
61 Sea
73°N 56 63 Transect 2 ALASKA
65
55 67
Transect 1 52 50 69 71 July 1 72°N
48
46
72°N
June 17
71°N
June 3
71°N
74°N
150 m 3000 m
150 m 1000 m
73°N
100 m
73°N 80 m
80 m 60 m
60 m 50 m
72°N
Hanna
Shoal
72°N
Central
Channel
50 m 40 m 71°N
40 m
71°N 40 m
170°W 165°W 160°W
0.01 0.1 0.2 0.5 1 25 10>20
Chlorophyll a (µg L-1)
170°W 160°W 150°W
74°N Aug 10 Aug 3
July 27 73°N
73°N July 20 July 20
July 27
71
7375 July 27 72°N
Chukchi 7779 Aug 3
72°N 81
North transect 84
71°N
July 20
71°N Chukchi South
40 44
38 42 46 50 transect 70°N
53
70°N 55
ALASKA
69°N
June 15
69°N
170°W 160°W 150°W
Fig.2. MapsofthenorthernChukchiSeashowing(A)thedistributionofseaiceon8July2011(MODIS-Aqua)andthelocationofstationssampledduringtheICESCAPE2011
cruise.Blackindicatesopenwater.Linesshowthepositionoftheiceedgeontheindicateddates(AMSR-E).Inthetext,stations46–57arereferredtoasTransect1andstations
57–71areTransect2.(B)Bathymetryofthestudyareaoverlaidbysurfacechlorophyllaconcentrationcalculatedfromfluorescencemeasuredbythecontinuousseawatersystemof
theUSCGCHealyinJuly2011.(C)MapoftheChukchiSeashowingthedistributionofseaiceon8July2010andthelocationofselectstationssampledduringtheICESCAPE2010
cruise(numberedwhitesquares).Anunder-icephytoplanktonbloomofsimilarintensitywasobservedalongtheChukchiNorthtransectduringICESCAPE2010atthesametimeof
year(6–7July2010)andinthesamelocationasTransect1inICESCAPE2011(forreference,otherstationsfromICESCAPE2011areshownasun-numberedgrayboxes).
Unfortunately,theChukchiNorthtransectstartedintheseaiceandprogressedsoutheastwardtowardopenwaterduringICESCAPE2010,withoutenoughtransitintotheicepack
tosamplethefullextentoftheunder-icebloom.
K.R.Arrigoetal./Deep-SeaResearchII105(2014)1–16 5
Net primary production. Simulated in situ daily net primary using an automated imaging flow cytometric approach. Small
production (NPP) was determined by measuring 14C-bicarbonate volumes of seawater (2.5 or 5mL) were subsampled from bottle
incorporation in water samples collected from different light casts and then analyzed with an Imaging FlowCytobot (Olson and
depths and incubated at corresponding light intensities in an Sosik, 2007). This generated large numbers of images of individual
on-deckincubatorfor24h.Weadded0.74MBq14C-bicarbonateto cells of length scale (cid:3)10 to 4100mm, which were then sorted
150mL of sample in a 250mL Falcon flask and covered the flask manually into appropriate taxonomic classes, typically to the genus
with 0 to 9 layers of neutral density screens to simulate light level.Thebiomasscontributedbycellsineachclassateverystation
intensities of 85, 65, 25,10, 5 and 1% of surface irradiance. After and depth was estimated from the cross-sectional area of cells,
incubation, 30mL of sample was filtered in triplicate under very quantified by automated image processing (Sosik and Olson, 2007)
lowvacuum(o50mmHg).Filterswereacidifiedwith0.1mL6N and calibrated with a fluorescent microsphere standard of known
HCl to drive off inorganic C. After 24h of acidification, 5mL of diameter.Taxonomiccompositionofnano-andmicro-phytoplankton
scintillation cocktail (Ecolume) was added and samples were were also verified onboard ship. Samples of 50–100mL were first
countedafter 43honaPerkinElmerTri-Carbliquidscintillation filteredonto0.4mmpolycarbonatefilters(HewesandHolm-Hansen,
counter.Totalactivitywasdeterminedoneachsamplebycombin- 1983)andthenviewedwithacompoundlightmicroscopeequipped
ing50μLofsamplewith50μLofethanolamine,0.5mLoffiltered withstandardbrightfield,epifluorescence,orpolarizedillumination
seawater, and 5mL of scintillation cocktail. Time zero controls for imaging all cells, chlorophyll/phycoerythrin-containing phyto-
werefiltered(30mLintriplicate)andacidifiedatthestartofthe plankton,orcalcifyingplankton,respectively.
incubationperiod. Specificgrowthrate(m,d(cid:1)1)insurfacewaters
wascalculatedbynormalizingtheNPPratebyPOCconcentration.
Phytoplanktoncommunitycomposition.Thetaxonomiccomposition 2.3. Nutrients,oxygen,anddissolvedinorganiccarbon(DIC)
of the nano- and microphytoplankton assemblage was determined
Nitrate (NO ), Oxygen, and DIC concentration. Discrete water
3
columnsampleswereanalyzedforNO andnitrite(NO )concen-
3 2
trations with a Seal Analytical continuous-flow AutoAnalyzer 3
74°N (AA3) using a modification of the procedure by Armstrong et al.
(1967). Seawater samples for DIC were drawn from the Niskin
73°N samplersintopre-cleaned (cid:3)300mLborosilicatebottles,poisoned
with HgCl to halt biological activity, sealed, and returned to
2
the Bermuda Institute of Ocean Sciences (BIOS) for analysis.
72°N
DIC samples were analyzed using a highly precise ((cid:3)0.025%;
o0.5mmoleskg(cid:1)1) gas extraction/coulometric detection system
71°N (Bates et al., 2005). Analyses of Certified Reference Materials
(provided by A. G. Dickson, Scripps Institution of Oceanography)
70°N ensured that the accuracy of the DIC measurements was 0.05%
((cid:3)0.5mmoleskg(cid:1)1). The oxygen sensor on the CTD rosette
69°N (SBE43)wascalibratedusingstandardWinklertitrations.
NitrateandDICdeficitcalculations.WatercolumnNO andDIC
3
deficitswerecalculatedbyfirstassumingthatthewintertimeNO
170° 165° 160° 155°W 3
and DIC concentrations were equal to the maximum WW con-
centrationsmeasuredalongthestudytransects.WWwasdefined
as water deeper than 10m with a temperature below (cid:1)1.651C.
0 170°W
Thus, WW NO and DIC concentrations over the Chukchi shelf
3
were24.1and2300mmolL(cid:1)1,respectively.Thevalueweusedfor
WW NO was virtually the same as that calculated previously
3
(Hanselletal.,1993)fortheChukchiShelf(23.674.86mmolL(cid:1)1).
50 Assumingthatthewatercolumnwasthoroughlymixedvertically
165°W duringthewinter(Fig.3),NO andDICdeficitswerecalculatedas
3
m) thedifferencebetweenthedepth-integratedwatercolumnNO3or
pth ( DorICDdICurminegastuhreewdidnuterirnagntdhethIeCEdSeCpAthP-Ein2t0e1g1ractreudiswe.aAtellrNcoOl3umanndNDOIC3
e 100 concentrationswerenormalizedtoaconstantsalinityof33.1prior
D
tomakingdeficitcalculationstocorrectformeltwaterinput.
160°W
2.4. Optics
150 Inherentoptical properties.Thebeamattenuationcoefficientat
488nmwasmeasuredinsituwithaC-Startransmissometer(WET
155°W Labs,Inc.)alonga25cmpathlengthwithinseawater.Thespectral
0 5 10 15 20 absorptioncoefficientofcoloreddissolvedorganicmatter(CDOM)
Nitrate concentration (µmol L-1) and particles was determined at 1nm resolution from freshly-
collected Niskin bottle samples. CDOM absorption in the sample
Fig.3. VerticalnitrateprofilesfromtheChukchiSeameasuredduringtheShelf– filtrate (o0.2mm) was measured at 200–735nm using an Ultra-
BasinInteractions(SBI)program.(A)Mapshowinglocationsofstationssampledin Path liquid waveguide system (World Precision Inc.) equipped
Maypriortosignificantphytoplanktongrowth.(B)Nitrateconcentrationsarehigh
with a 2m pathlength. Reference water salinity was adjusted to
insurfacewatersinthewesternportionofthestudyarea,reflectingconvective
thatofthesamplewithpre-combustedNaClandMilliQwater.The
processesontheshallowshelfthatmixnutrientstothesurfaceinwinter.Colors
denotelongitudewheresamplesweretaken. particle absorption coefficient (ap) from 300 to 800nm was
6 K.R.Arrigoetal./Deep-SeaResearchII105(2014)1–16
determinedbycollectingparticlesontoa25mmfilter(Whatman
GF/F) and measuring its optical density relative to a blank
reference filter in a Cary 1E spectrophotometer. Spectral absorp-
tion coefficients were calculated as described in Mitchell (1990).
Following measurement of a , sample filters were extracted in
p
100%methanolandre-measuredtoyielddetritalabsorption(a ).
d
Phytoplankton absorption (a ) was determined by difference as
ph
a ¼a (cid:1)a .
ph p d
Thedrymassconcentrationofsuspendedparticleswasdeter-
minedgravimetricallyonsamplesfilteredontopre-weighedGF/F
filters, rinsed with deionized water to remove sea salt, dried at
601C, and measured with a Mettler-Toledo MT5 microbalance
witharesolutionof0.001mg.Particlenumberconcentrationsand
sizeestimatesweredeterminedwithaCoulterCounterMultisizer
III(Beckman-Coulter)equippedwith30mmand200mmaperture
Bare ice Melt pond
tubes (Reynolds et al., 2010). The estimates of particle number
reported here represent the total concentration of particles over Surface 70% 100% 20% 100%
the size range of 1–100mm. Particle median diameters were scattering
layer
calculatedfromthe volumedistribution overthis size range, and
0.05 m
representthediameterofavolume-equivalentsphere.
0.1 m
Under-ice optical measurements. Light transmittance through
theiceandtheupperwatercolumnwasmeasuredatstations55,
56, and 57 (Fig. 2A). At each station, vertical light profiles were
obtained beneath both representative bare ice and melt ponds. Congelation Scattering and
Individual sampling sites werechosen tomaximize the ability to 1.0 m ice layer absorption 0.9 m
uniformly represent each ice surface type (i.e., in the center of a
relativelylargemeltpondorinthecenterofarelativelylargebare
icesurface).AnAnalyticalSpectralDevicesDualDetectorspectro-
radiometer was used to measure transmitted spectral irradiance
overthewavelengthrangeof380–850nm.A modified Compact-
Optical Profiling System (C-OPS, Biospherical Instruments Inc.) Seawater
radiometer with a cosine collector was lowered to a depth of
(cid:3)50m through an auger-drilled (cid:3)25cm hole in the ice. At the 15% 55%
bare ice surface site, the hole was re-filled with ice tailings to
Fig.4. MeltpondsandbareiceintheChukchiSea.(A)Photographtaken13July
mimic the previously undisturbed bare ice surface. At the melt
2011byChristieWood.(B)Schematicshowinglighttransmissionthroughbareice
pond site, the C-OPS was offset horizontally from the hole such andmeltponds.Bareiceconsistsofathingranularsurfacescatteringlayerthat
that the cosine collector on the instrument looked directly up at givesiceitswhiteappearanceandathickcongelationicelayerthatconsistsof
the underside of the ice. The underwater C-OPS and surface columnaricecontainingnumerousbrineinclusions.Seaicebeneathmeltponds
referencesensormeasureddownwellingirradianceat19channels hasnosurfacescatteringlayerandthecongelationiceisgenerallythinner.The
albedoofbareice(70%)andmeltponds(20%)includesspecularreflectionatthe
(320, 340, 380, 395, 412, 443, 465, 490, 510, 532, 555, 560, 625, air–ice interface and scattering of light back out of the ice interior. Light not
665, 670, 683, 710, 780nm, and photosynthetically active radia- backscatteredorabsorbedwithinthecongelationicelayeristransmittedtothe
tion(PAR,400–700nm)).Asurfacereferencewasmountedontop upper water column. Because light transmitted through the ice spreads in all
of a tripod ((cid:3)2.5m above the ice surface) that stood on the ice directions,lightlevelsbelowbareiceandmeltpondsconvergewithin (cid:3)10mof
within (cid:3)1.5mofwheretheC-OPSmeasurementswerecollected. theiceinterface.
stationsalong Transect2.By22July,ourentirestudyregionwas
3. Results ice-free.
Thesnowcoverhadmeltedcompletelybythetimeofourstudy
3.1. Seaice and sea ice thickness increased significantly with distance from
the ice edge. Near the edge at station 55 (Fig. 2A), mean ice
Duringthe2011campaign,theseaiceedgeintheChukchiSea thickness was 0.7770.09m, including melt ponds that were
generallyretreatedfromthesoutheasttothenorthwest(Fig.2A). 15–20% thinner than bare ice (Table 1). As we penetrated
In the vicinity of our study area, sea ice was fully consolidated (cid:3)60km deeper into the pack, ice thickness increased to 1.037
((cid:3)100% concentration) and largely undeformed, with a surface 0.15m (at station 56). By station 57, approximately 120km from
composed of avariable mixof bareice and melt ponds (Fig. 4A). theiceedge,icethicknesshadincreasedto1.2170.10m.Surface
Occasionally, a northerly wind shift would push broken pack ice meltpondfractionatthethreeseaicestationsaveraged(cid:3)0.3.The
southward a short distance, thereby temporarily reducing ice meltpondshadwell-definedboundariesandsurfacesthatwereat
concentrations. However, these events were short-lived and the sealevel,implyingthattheywerebeyondtheinitialfloodingstage
iceedgenearourstudyareawasusuallywell-defined(Fig.2A). (Polashenski,2011).
WhilemuchofthesouthernChukchiSeawasice-freeinmid-
June2011,ourstudyareawasstillcoveredbyseaice(Fig.2A).By 3.2. Phytoplankton
thetimewestartedsamplingTransect1(stations46–71)on3July
2011, ice had retreated northwestward and the southern half of From4–8July2011,wesampledthefull40–150mdeepwater
Transect1wasinopenwater.SamplingalongTransect2(Stations column along Transects 1 and 2, both of which extended from
57–71)endedon8Julyandbythistime,afewmorestationsalong openwaterto 4100kmintotheicepack(Fig.2A).Atthetimeof
Transect 1 were in open water, as were the six southernmost sampling,thesoutheasternendofTransect1(station46)hadbeen
K.R.Arrigoetal./Deep-SeaResearchII105(2014)1–16 7
Table1 Depth-integrated phytoplankton biomass along Transect 2 was
Icethicknessesandmeltponddepthsaatseaicestationsassociatedwithunder-ice even higherthan onTransect1, rangingfrom921 to 1292mg Chl
phytoplanktonbloom. a m(cid:1)2 beneath the ice at stations 59–61 (Fig. 6B). The under-ice
bloom along Transect 2 extended for (cid:3)133km between the ice
Station Allice(m) Bareice(m) Pondedice(m) Meltpond
depth(m) edgeanditsterminationattheshelfbreak.
Mean N Mean N Mean N Mean N 3.3. Lighttransmissionthroughseaice
55 0.7770.09 113 0.8170.07 73 0.6970.06 40 0.0570.03 20
56 1.0370.15 198 1.1070.12 138 0.8870.07 60 0.1170.04 59 Vertical light profiles taken through the sea ice at stations
57 1.2170.10 333 1.2670.06 239 1.0870.05 94 0.1270.04 97 55–57showedthatbareicetransmittedapproximately12.7–17.5%
ofincidentsurfacelighttothewatercolumnbelow(Table2).Due
aMean7standarddeviation. tothelackofsignificantsnowcover,lightattenuationinbareice
wasdominatedbythepresenceofahighlyscatteringsurfacelayer
consistingofdrainedicecrystals.Incontrast,theicebeneathmelt
ice-free for two weeks (Fig. 5A) and was characterized by chlor- pondslackedthesurfacescatteringlayerandwasthinnerthanthe
ophylla(Chla)concentrationsthatwerelow(0.38mgL(cid:1)1)within bare ice (Fig. 4B). Hence, ponded ice transmitted approximately
the 25m upper mixed layer and increased to 2.5mgL(cid:1)1 toward 46.7–58.6% of incident surface light to the underlying water
the sea floor (Fig. 5B). Moving northwest along Transect 1 from column,4-foldmorethanbareice(Table2).
open water toward the ice edge, a prominent subsurface Chl a Interestingly, the euphotic depth (0.1% of visible incident sur-
maximum (SCM) that tracked the shoaling MLD (Fig. 5C) was face light) associated with both bare and ponded ice at a given
increasinglyapparent,withpeakconcentrationsintheSCMrising stationwasidentical(Table2).Thisisbecauselighttransmittedto
from5.4mgL(cid:1)1atstation48to12.9mgL(cid:1)1atstation51.TheSCM the upper ocean through melt ponds scattersintoadjacent areas
and the MLD became progressively shallower with proximity to overlain by bare ice (Fig. 4B). Consequently, with increasing
theice edge.TotalwatercolumnChla alsoincreasedtoward the distancebelowtheundersideoftheice,theunderwaterlightfield
iceedge,from43.6mgm(cid:1)2atstation46to250mgm(cid:1)2atstation became a more uniform mix of light transmitted through both
51(Fig.6A). bareiceandmeltponds(Freyetal.,2011).
Nearstation52,weenteredtheSIZ,whichhadawell-defined Euphoticdepthdidvarybymorethanafactorofthreebetween
ice edge coinciding with the westward limit of the warm water stations (Table 2) as a result of changes in light attenuation by
(Fig. 5D) being advected northward in the upper portion of the particulate and dissolved materials within thewatercolumn. For
shelfbreakjet(Fig.5E)andaMLDofapproximately10m(Fig.5C). example, beam attenuation at 488nm increased 6-fold from
Althoughweexpectedthatreducedlightavailabilitybeneaththe 0.26m(cid:1)1 in surface waters outside the ice pack (station 46) to
0.7770.09m thick first-year ice at the edge of the SIZ would 1.54m(cid:1)1withintheunder-icephytoplanktonbloom(station56),
result in lower phytoplankton abundance, water column Chl a due to large increases in both particle scattering (5-fold) and
continued to increase as we traveled further into the ice pack particle absorption (3.5-fold). Light absorption within the bloom
(Fig. 2B). Between stations 52 and 56, depth-integrated phyto- wasalmostentirelyattributable to particles (91%), mainlyphyto-
planktonbiomassbeneaththeseaicerosefrom212mgChlam(cid:1)2 plankton (78%), and to a lesser extent by CDOM. The patterns in
(8.8gC,m(cid:1)2)toaremarkable1016mgChlam(cid:1)2(28.7gCm(cid:1)2) particle absorption and scattering are consistent with observed
at station 56 (Fig. 6A), despite sea ice cover of 100% and an differences in particle number (0.9–1.8(cid:4)1011m(cid:1)3), particle dry
increaseinseaicethicknessto1.0370.15m(Table1).Mostofthis mass(0.4–2.7mgL(cid:1)1),andparticlemediandiameter(2.2–3.6mm)
phytoplankton biomass was within the upper 30m of the water betweenstations(moredetailscanbefoundinNeukermansetal.,
column, with the highest Chl a concentrations near the ice/ 2014). Due to the strong light attenuation within the water
seawater interface (Fig. 5B). The depth over which this bloom column, the euphotic depth shoaled considerably within the
extended(30–70m)andthehighdepthintegratedphytoplankton under-ice phytoplankton bloom, decreasing from 46m at station
biomassdistinguishesthisunder-icebloomfromthemuchsmaller 46(inopenwater)toonly (cid:3)10matstation56.
bloomsthathavebeenobservedpreviouslyinshallowmeltwater
lensesbeneaththeicethatonlyextend (cid:3)1mbelowtheicewater
interface (Gradinger, 1996; Spilling, 2007). The ship’s underway 3.4. Dissolvednutrientsandgases
fluorometershowedthatthismassivediatombloomcontinuedfor
another32kmnorthofstation56beforeterminatingjustnorthof Along Transect 1, the nitracline (Fig. 5C) largely tracked the
the shelf break near station 57 (Fig. 2B). In total, the under-ice depthofthemixedlayer(Fig.5C)andwasdeepestinopenwater
phytoplankton bloom along Transect 1 extended for (cid:3)105km southeast of the sea ice edge, extending to (cid:3)30m at station 46.
from the ice edge to the shelf break. Diatoms were the most Surface water NO concentrations at this stationwere belowour
3
abundantphytoplanktongroupbeneaththeice,includingspecies detection limit (0.01mmol L(cid:1)1), increasing to 1.65mmolL(cid:1)1 at a
inthegeneraChaetoceros,Thalassiosira,andFragilariopsis,andata depthof27mandreachingamaximumof10.5mmolL(cid:1)1nearthe
few stations, Odontella (more details can be found in Laney and seafloor(44m).Thenitraclineshoaledasweapproachedtheice,
Sosik.,2014). reaching (cid:3)10mattheiceedge(station52),withNO concentra-
3
Transect 2, which also terminated north of the shelf break at tions of 0.3mmolL(cid:1)1 at the surface and 4.5mmolL(cid:1)1 at 11m.
station57,wassampledatahigherspatialresolutionwithintheSIZ Near-bottom NO was also higher at this station (18mmolL(cid:1)1)
3
(Fig.2A)andexhibitedaspatialpatterninphytoplanktonbiomass than they were at station 46. The nitracline was shallowest
that was almost identical to Transect 1. Starting in open water beneath the sea ice, reaching its minimal depth ((cid:3)7m) near
southeast of the SIZ (station 71, Fig. 5H), the SCM was located station 55 (Fig. 5C). The nitracline along Transect 2 exhibited a
progressively closer to the surface with proximity to the sea ice similarspatialpatterntoTransect1,beingdeepestinopenwater
edge (station 66, Fig. 5I) and depth-integrated Chl a increased southeast of the sea ice edge. Unlike Transect 1, however, the
dramaticallyfrom54mgm(cid:1)2to576mgm(cid:1)2(Fig.6B).Beneaththe nitricline depth (25–30m) exceeded the MLD ((cid:3)10m) in open
ice,phytoplanktonbiomassincreasedwithdistanceinfromtheice water,indicatingthattheMLDhadshoaledafterthenitriclinehad
edge despite increasing ice thicknesses (to 1.2170.10m, Table 1). formed (Fig. 5J). Maximum nitracline depths ((cid:3)40m) were also
8 K.R.Arrigoetal./Deep-SeaResearchII105(2014)1–16
100 100
3 July 3 July
50 Sea ice Sea ice 50
4 July 4 July
0 concentration (%) concentration (%) 0
57 56 55 52 50 48 46 57 56 55 52 50 48 46
0 30
20
25
15
50 20
m)
h ( 15 10
pt
e
D 100 10
5
Transect 1 5 Transect 1
Chlorophyll a (µg L-1) Nitrate (µmol L-1)
150 0 0
0
4 20
3
50 10
2
m)
h ( 1 0
ept 100 0
D
-10
Transect 1 -1 Transect 1
Potential temperature (°C) Cross-track velocity (cm s-1)
150 -2 -20
0 700 2200
2150
600
m) 50 2100
h ( 500 2050
pt
e
D 100 2000
400
Transect 1 Transect 1 1950
Oxygen (µmol L-1) DIC (µmol L-1)
150 300 1900
250 200 150 100 50 0 250 200 150 100 50 0
Distance along transect (km) Distance along transect (km)
100 100
50 Sea ice 7 July Sea ice 7 July 50
8 July 8 July
0 concentration (%) concentration (%) 0
57 59 61 63 65 67 69 71 57 59 61 63 65 67 69 71
0 30
20
25
m) 50 20 15
h ( 15
pt 10
e
D 100 10
5
Transect 2 5 Transect 2
Chlorophyll a (µg L-1) Nitrate (µmol L-1)
150 0 0
200 150 100 50 0 200 150 100 50 0
Distance along transect (km) Distance along transect (km)
Fig.5. HydrographicsectionsfromICESCAPE2011showingunder-icephytoplanktonbloomcharacteristics.(A–G)arefromTransect1andshow(A)seaiceconcentration,
(B)chlorophylla,(C)nitrate,(D)potentialtemperature(θ),(E)cross-trackvelocity(positivevaluesdenotenortheastwardflow),(F)dissolvedoxygen,and(G)dissolved
inorganiccarbon(DIC).(H–J)arefromTransect2andshowconcentrationsof(H)seaice,(I),chlorophylla,and(J)nitrate.Stationnumbersareaboveeachpanelandblack
dotsrepresentsamplingdepths.Solidcontourlinesin(B),(D),and(E)arepotentialdensity(sθ).Theblacklinein(C)and(J)denotesthedepthofthemixedlayer.
somewhatgreateralong Transect 2than onTransect1,with NO withproximitytotheiceedgeandwasshallowestundertheice,
3
being completely consumed (i.e. below the detection limit) to a althoughnitraclinedepthsweremorespatiallyvariablethanalong
depthof26matstations69and70.Thenitraclineagainshoaled Transect1.
K.R.Arrigoetal./Deep-SeaResearchII105(2014)1–16 9
Table2
1200 TUrnadnesr-eiccet 1 30000 -2m) Oblpotoicma.lcharacteristicsaatseaicestationsassociatedwithunder-icephytoplankton
bloom C
-2)mg m 1800000 2205000000 on (mg Station Icethickness(m) LBiagrhetitcreansmPitotnadneced(i%ce) EBuarpehoicteicdepPthon(mde)dice
b
a ( ar 55 0.7770.09 17.570.7 53.871.3 17.570.06 17.970.40
yll 600 15000 c c 56 1.0370.15 12.770.5 46.771.3 10.070.03 11.070.03
h ni 57 1.2170.10 13.670.7 58.671.2 32.470.43 33.271.01
p a
o 400 10000 g
or or Euphoticdepthincludesthicknessofoverlyingseaice.
Chl 200 5000 ate Threelightcastsweredoneateachstation.
ul aMean7standarddeviation
c
0 0 arti
58 56 54 52 50 48 46 P Fv/Fm
0.0 0.1 0.2 0.3 0.4 0.5 0.6
Transect 2 ) 0
1500 35000 -2m Station 55
Under-ice C 10
a-2hyll (mg m) 1692000000 bloom 12235050000000000000 nic carbon (mg Depth (m) 234000 SSttaattiioonn 5662
p a 50
o g
or 10000 or
hl 300 e 60
C 5000 at
ul 70
c
0 0 arti Fig. 7. Depth profiles of the efficiency of electron flow through photosystem II
57 59 61 63 65 67 69 71 P
(Fv/Fm)withintheunder-icephytoplanktonbloom.Allvaluesexceed0.4,suggesting
Station number thatphytoplanktonwerephysiologicallycompetentthroughoutthewatercolumn.
the nitracline, the depth of maximum O concentration shoaled
1 2
with proximity tothe ice edge.Beneaththe ice pack, O concen-
mean = 46.4 2
trations were highest in the upper (cid:3)25m of the water column,
mode = 20-40
reachingvaluesashighas583.1mmolL(cid:1)1atthesurfaceofstation
n = 12,048
on 0.1 61. This station also exhibited enhanced O2 concentrations down
cti toadepthofapproximately75m.
un Dissolved inorganic carbon (DIC) concentrations in the deep
y f samples along Transect 1 varied from approximately 2250 to
sit 2303mmolL(cid:1)1. The DIC depletion pattern (Fig. 5G) strongly
n 0.01 mean of gray max of gray
y de area in B area in B sreosuetmhebalsetdothfatthoefNseOa3,icweitehdlgoew(D2I0C6v4a–l2u0e9s8inmmopoelnLs(cid:1)u1rfaatcestwataitoenrss
bilit 46–48)andsubstantialdepletiondowntoatleast30m.Thedepth
a
b ofDICdepletiondiminishedtowardtheiceedge,eventuallyshoaling
o 0.001
Pr to (cid:3)10m at ice edge station 52. There was less DIC depletion in
waters beneath the sea ice (except station 57), with surface DIC
valuesreducedto2105–2190mmolL(cid:1)1atstations52–55.
0.0001
3.5. Phytoplanktonphotophysiologyandgrowthrate
0 200 400 600 800 1000 1200 1400
Depth-integrated chlorophyll a (mg m-2)
Microscopic analysis indicated that under-ice phytoplankton
Fig.6. Depth-integratedchlorophylla(diamonds)andparticulateorganiccarbon at all stations were healthy, with few observations of empty
(squares) along (A) Transect 1 and (B) Transect 2. Gray areas denote samples or senescing diatom frustules. This conclusion is supported by
collectedwithintheunder-icephytoplanktonbloom.Probabilitydensityfunction observationsofgenerallyhighefficiencyofelectronflowthrough
(C) for depth integrated water column Chl a from the global SeaBASS pigment
photosystem II (Fv/Fm), which exceeded 0.4 at all depths shal-
database(http://seabass.gsfc.nasa.gov/).Notelogscaleofy-axis.Themeandepth-
integratedbiomassintheunder-icebloomalongTransect2ranksinthetop0.1%of lower than 70m and exceeded 0.5 in 12 out of the 15 samples
globalvalues.Depth-integratedChlaatstation60ishigherthananyvalueinthe whereFv/Fmintheunder-icebloomwasmeasured(Fig.7).
database. Photosyntheticparametersdeterminedforphytoplanktonsampled
atthesurfaceandat (cid:3)25m(theapproximate depthof theSCMin
Dissolved oxygen (O ) concentrations were very high along openwater)atagivenstationwerenotsignificantlydifferent(t-test,
2
bothTransect1(Fig.5F)andTransect2(notshown).Atstation47, p40.05)acrossourstudyregion(Table3).However,photosynthetic
locatedsoutheastoftheiceedge,O concentrationswerehighest parameters of under-ice phytoplankton differed significantly from
2
at a depth of 25m (466.1mmolL(cid:1)1), although there was still those of phytoplankton in nearby open water. The maximum Chl
significant supersaturation in shallower waters. Like Chl a and a-specificphotosyntheticrates(Pn)withintheunder-icephytoplankton
m
10 K.R.Arrigoetal./Deep-SeaResearchII105(2014)1–16
bloom(1.4070.33mgCmg(cid:1)1Chlahr(cid:1)1)weredoublethoseoftheir Table3
openwatercounterparts(0.7170.14mgCmg(cid:1)1Chlahr(cid:1)1),despite PhotosyntheticParametersofPhytoplanktonatthesurfaceandtheSCMfromthe
growing beneath heavysea ice cover (t-test, po0.05). Similarly, the under-icebloom(bloom)andadjacentopenwater(non-bloom).
plahrgoetotshyantthoefttihceeirffiocpieenncwy(aαten)rocfouunntdeerrp-aicretsp(ht-ytteospt,lapnokt0o.n05w),aasvtewraicgeinags Pnm αn Ek m
0.02470.007 and 0.01270.004mg C mg(cid:1)1 Chl ahr(cid:1)1 (mmol Surface
photonsm(cid:1)2s(cid:1)1)(cid:1)1,respectively(Table3).Themeanphotoacclima- Bloom 1.35(0.30) 0.021(0.006) 67.6(22.4) 0.85(0.47)
tionparameter(E)fortheunder-icebloom(61.2718.0mmolphotons Non-bloom 0.68(0.11) 0.011(0.003) 67.9(25.1) 0.05(0.02)
m(cid:1)2s(cid:1)1) was nokt significantly different (t-test, p40.05) from that SCM
Bloom 1.45(0.38) 0.027(0.007) 54.9(10.0) 0.92(0.30)
of phytoplankton growing in open waters (66.0726.6mmol Non-bloom 0.74(0.17) 0.013(0.005) 64.0(31.9) 0.24(0.23)
photonsm(cid:1)2s(cid:1)1). The high Pn, high αn, and low E indicate that All
m k
the under-ice phytoplankton were well adapted to grow at the Bloom 1.40(0.33) 0.024(0.007) 61.2(18.0) 0.89(0.39)
Non-bloom 0.71(0.14) 0.012(0.004) 66.0(26.6) 0.15(0.18)
reducedlightlevelsencounteredbelowtheArcticicepack.
The largest physiological difference between the under-ice aMean7standarddeviation.
phytoplanktonandtheiropenwatercounterpartsistheirmaximum Pnm–mgCmg(cid:1)1Chlahr(cid:1)1,αn–mgCmg(cid:1)1Chlahr(cid:1)1(mmolphotonsm(cid:1)2s(cid:1)1)(cid:1)1,
biomass-specific growth rate, calculated from both short-term Ek–mmolphotonsm(cid:1)2s(cid:1)1,m–d(cid:1)1.
estimates of P–E parameters and from 24-h NPP measurements. Bloomstations:52,54,55,56,58,60,62,and64.Non-bloomstations:46,48,57,and70.
Light-saturated growth rates (near the ice/water interface) within
theunder-icebloomalongTransect1wereextraordinarilyhigh,rang- is further supported by the coincidence of extraordinarily high
ingfrom0.86d(cid:1)1neartheiceedgeto1.59d(cid:1)1atthehigh-biomass algalbiomassandlargenutrientdeficitsintheupper25–30mof
station56.Light-saturatedgrowthratesalongTransect2weresome- the water column beneath the ice, which can only be the result
whatlowerbutstillveryhigh,varyingfrom0.21–1.20d(cid:1)1.Fiveof of nutrient uptake by phytoplankton growing within the water
the eight stations associated with the under-ice bloom exhibited column. Sea ice algae simply cannot deplete nutrients from that
phytoplankton light-saturated growth rates 41.0d(cid:1)1, which are farbelowtheice/waterinterface.Itispossiblethatalgaereleased
muchhigherthanexpectedforpolarwatersnearthefreezingpoint from the sea ice may have helped to seed the under-ice phyto-
of (cid:1)1.81C(0.53d(cid:1)1,Eppley,1972). planktonbloom,althoughwemeasuredverylowicealgalbiomass
The mean light-saturated growth rate within the under-ice duringourstudy.
bloom was 0.89d(cid:1)1 (Table 3), approximately 6-fold higher than The most intense phytoplankton blooms previously reported
the mean growth rate of phytoplankton collected in open water inpolarwatersweregenerallyassociatedwithopenwatersalong
(0.15d(cid:1)1).ThislargedifferenceisdueinparttotheelevatedPOC theMIZ(AlexanderandNiebauer,1981;Sakshaug,2004;Perrette
concentration associated with a higher detrital content in the et al., 2011), where meltwater-induced surface stratification cre-
older open water phytoplankton blooms (m¼NPP/POC or Pn(cid:5)Chl ateslightconditionsthatarefavorableforphytoplanktongrowth.
a/POC).However,itisalsoareflectionofthefactthatphytoplankton Occasionally, these MIZ blooms can become ice covered if either
growinginopenwatershadalreadyconsumedmuchoftheNO in seaiceadvancesbackintowaterswhereaMIZbloomhasalready
3
near surface waters by the time of our cruise and were concen- developed(Mundyetal.,2009)orlocalcirculationadvectstheMIZ
tratedatthesub-surfacenitraclinewherebothnutrientsandlight bloom beneath the ice pack. As a result, it is important to
levels are relatively low. In contrast, phytoplankton growing distinguish ice-covered MIZ blooms from phytoplankton blooms
beneaththeicehadnotyetconsumedalloftheavailablenutrients thatdevelopentirelybeneaththeseaice.Onthebasisofageand
fromwithin the euphotic zone and were able to maintain much location of the bloom relative to both the ice edge and bottom
highergrowthrates. topography,andnutrientandbiogenicgasconcentrationsbeneath
the ice and in the adjacent open water, we determined that the
under-ice phytoplankton bloom we observed during ICESCAPE
4. Discussion 2011wasneitheraresidualMIZbloomnoranopenwaterbloom
thathadbeentransportedbeneaththeice.
4.1. Phytoplanktonundertheice First,satelliteobservationsshowthattheseaiceintheChukchi
Sea consistently retreated in a northwestward direction prior to
It is noteworthy that the mean biomass associated with the oursamplingthebloom,rulingoutthepossibilitythaticehadever
under-ice phytoplankton bloom we observed during ICESCAPE reversed direction and drifted over a previously developed MIZ
2011 was comparable to depth-integrated values from the most bloom.Second,phytoplanktonbiomassassociatedwiththeunder-
productive pelagic ecosystems in the global ocean. In fact, the icebloomalongbothTransect1and2reacheditsmaximumvalue
maximum depth-integrated biomass we measured at station 60 approximately 120km from the ice edge at a location where
was higher than any of the 12,048 open ocean values currently nutrientswereenhancedduetoshelfbreakupwellingfromdepth
contained in the SeaBASS pigment database managed by NASA (see below). This bathymetric constraint strongly supports the
(Fig.6C).Althoughthesehighbiomasslevelswereobserveddeep conclusion that the bloom developed beneath the ice, with its
within the ice pack under relatively thick (0.8–1.2m) ice, it is northwesternboundary(theboundarylocateddeepestwithinthe
importanttodeterminewhetherthisbloom(1)originatedasasea icepack)controlledbythepositionoftheshelfbreak.
ice microalgal bloom that was subsequently released into the Finally, if the bloom had advected beneath the ice from open
water column, (2) advected beneath the ice from the site of an water, it would have been in a much later stage of development
earlierMIZbloom,or(3)developedexclusivelybeneaththeice. thanweobserved,giventhehighgrowthrateofthephytoplank-
The majority of the stations within the under-ice the bloom tonbeneaththeiceandthelocalcirculationpatterns.Theshortest
were overwhelmingly (480% bycell cross-sectional area) domi- route from ice-free waters to our study region beneath the ice
natedbypelagic diatomsof the genera Chaetoceros,Thalassiosira, would require advection of the bloom to the northwest in the
and Fragilariopsis, indicating that this was not a remnant sea ice direction of sea ice motion. This is unlikely since no known
algalbloomthathadsloughedoffintothewatercolumn,although currentsinthevicinityofourstudyregionflowfromthesoutheast
the diatom Odontella aurita was abundant at station 56 and is to the northwest. Instead, one current flows northward through
occasionallyfoundinseaice(McMinnetal.,2008).Thisconclusion Central Channel then northeastward around Hanna Shoal while