Promo%ngphytoplanktonbloomsintheopenocean:

Resultsfromnutrientaddi0onexperiments

Dr.ClaireMahaffeyDr.KarinBjorkmanandProf.DavidKarl

Introduc7on• Fixa7onofCO2byphytoplankton(PRIMARYPRODUCTION)dependentupontwofactors:lightandnutrients

SEASURFACECOLOURASAPROXYFORCHLOROPHYLLACONCENTRATIONS

60%ofsurfaceocean:carbonfixa7onlimitedby

nutrientsupply

YET:subtropicalgyresaccountforupto50%of

totalorganiccarbonexportofglobalocean

(Sub)tropicaloceanenvironment

Nitrate

0.2‐2µm

2‐10µm

>10µm

0.2‐2µm

2‐10µm

>10µm

CarbonfixaGon

Phytoplanktonbiomass(Chlorophylla)

• Permanentlystra7fied

• Nutrientimpoverishedsurfaceocean

• Picoplankton(<2μm)responsiblefor>90%ofchlorophyllaand>70%ofcarbonfixa7on

Communitysizestructureandexport

FromKarl,1999

>10μmcellsize <2μmcellsize

HIGHEXPORTPRODUCTION

LOWEXPORTPRODUCTION

Bloomsin“oceandeserts”

• Summer7meincreaseinwatercolumnstability• MesoscaleeddiesandRossbywaves(Churchetal.,2008,Wilsonetal.,2003)• Winter7mesupplyofphosphorus(Doreetal.,2008)

WHATDRIVESTHESEBLOOMS?

• Episodicbloomsofdiatomsandnitrogenfixingcyanobacteria(Churchetal.,2008,Doreetal.,2008,

Whydoweneedtounderstandbloomsinoceandeserts?

•  Oceandesertsrepresent60%oftotaloceanarea(EppleyandPeterson,1979)•  Responsiblefor~50%ofglobaloceancarbonexport(Emersonetal.,1997)

•  Periodicbloomsdriveupto50%ofannualexport(Karletal.,1997,Doreetal.,2008)

HOWEVER:• Fewdirectobserva7onsofbloomsduetotheirstochas7cnature• Uncertaintysurrounding:

‐ Speciescomposi7onandsuccession‐ Carbonfixa7onandfate‐ Speedofdevelopment‐ Reproducibility

Oceanfer7lisa7on• Addi7onofnutrientcocktailordeep‐seawatertosurfaceoceantos7mulatecarbonfixa7onandsequestra7on

Nutrientaddi7onexperiments

• Dissolvednutrients• Biomassparameters(chlorophylla)• Communitysizestructure,0.2,2and10μm• Speciescomposi7on(pigmentsandflowcytometry)• Ratemeasurements(carbonfixaGon)

Stn. ALOHA (22°45’ N, 158°W)

5%vol:vol

QuesGon:Inanutrientlimitedsystem,canwepredictablys7mulateaphytoplanktonbloombyaddingnutrient‐richdeepseawater:

‐increaseinbiomass(chlorophylla)

‐increaserateofcarbonfixa7on

‐changecommunitysizestructurefromsmall(0.2‐2μm)tolarge(>10μm)cells

‐increasethepoten7alforexportoforganiccarbon?

Bloomresponse

• 70‐100%nutrientassimila7on

• 20±4foldincreaseinchlorophyll• 23±7foldincreaseincarbonfixa7on

0

20

40

60

80

100

120

0 20 40 60 80 100 120 140 160

Percen

t(%)a

ssim

ilaGon

Time(hrs)

nitrate

phosphate

Series4

Series5

SizestructureCarbonfixaGon

0.2–2μm

2–10μm>10μm

Chlorophylla

INITIAL INITIALFINAL FINAL

• 3cruises

Bloomresponse

• 70‐100%nutrientassimila7on

• 20±4foldincreaseinchlorophyll• 23±7foldincreaseincarbonfixa7on0

20

40

60

80

100

120

140

160

HOT161 HOT183 HOT191

Prim

aryprod

ucGon

(ugCL‐1 d

‐1)

Cruise

SizestructureCarbonfixaGon

0.2–2μm

2–10μm>10μm

Chlorophylla

INITIAL INITIALFINAL FINAL

• 3cruises

Bloomresponse

• 70‐100%nutrientassimila7on

• 20±4foldincreaseinchlorophyll• 23±7foldincreaseincarbonfixa7on

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

HOT161 HOT183 HOT191

Chloroph

yll(ugL‐1)

Cruise

SizestructureCarbonfixaGon

0.2–2μm

2–10μm>10μm

Chlorophylla

INITIAL INITIALFINAL FINAL

• 3cruises

Non‐bloomresponse• 7cruises

• <50%ofnutrientsassimilated

• 4±1foldincreaseinchlorophyll

• 5±1foldincreaseincarbonfixa7on

SizestructureCarbonfixaGon

0.2–2μm

2–10μm>10μm

Chlorophylla

INITIAL INITIAL

0

20

40

60

80

100

120

0 50 100 150 200

Percen

t(%)a

ssim

ilaGon

Time(hrs)

nitrate

phosphate

Series4

Series5

Series7

Series8

FINAL FINAL

Non‐bloomresponse• 7cruises

• <50%ofnutrientsassimilated

• 4±1foldincreaseinchlorophyll

• 5±1foldincreaseincarbonfixa7on

SizestructureCarbonfixaGon

0.2–2μm

2–10μm>10μm

Chlorophylla

INITIAL INITIAL

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

HOT163 HOT164 HOT165 HOT167 HOT174 HOT175 HOT186

Chloroph

ylla(u

gL‐1)

Cruise

FINAL FINAL

Non‐bloomresponse• 7cruises

• <50%ofnutrientsassimilated

• 4±1foldincreaseinchlorophyll

• 5±1foldincreaseincarbonfixa7on

SizestructureCarbonfixaGon

0.2–2μm

2–10μm>10μm

Chlorophylla

INITIAL INITIAL

0

20

40

60

80

100

120

140

160

Prim

aryprod

ucGon

(ugCL‐1 d

‐1)

Cruise

FINAL FINAL

Experimentsummary

Parameter Bloomresponse Non‐bloomresponse

Nutrients 70‐100%assimila7on <50%assimilated

Chlorophyll 20±4foldincrease 4±1foldincrease

Carbonfixa7on 23±7foldincrease 5±1foldincrease

Sizecomposi7on Dominatedby>10um Dominatedby0.2–2um

Months MaytoJuly SeptembertoJanuary

Bloomresponse,Summer(MaytoJuly)–rapidandcompleteassimila7onofnutrientsandbloomforma7on

Non‐bloomresponse,Winter(SeptembertoJanuary)–slowandincompleteassimila7onofnutrientsandnobloomformed

Poten7alcarbonfixed•  Nitrateadded =2μM•  Maximumpoten7alcarbonfixed =174±19μgCL‐1

Accumulatedcarbonfixedduringsummerandwinterincuba7ons

*Expressedaspercent(%)ofmaximumpoten7alcarbonfixed(174μgCL‐1)

Comparingsummerandwintercarbonpoten7al:

3xcarbonfixedforwholecommunity 10xmorecarbonfixedbylargecells(>10μm)

Season Wholecommunity* >10umcells*

Summer 115±18 59±15

Winter 31±4 5±1

Whatarethemechanismsdrivingthisvaria7onincommunityresponse?

• Ini7alnutrientcondi7ons• Communitystructureandcomposi7on• Sizecomposi7on

NOSIGNIFICANTDIFFERENCEBETWEENWINTERAND

SUMMERCRUISES

0

20

40

60

80

100

120

140

0 2 4 6 8 10 12

Mized

LayerDep

th(m

)

Months

• Watercolumnstabilityandstructure

SeasonalalteraGonofwatercolumnstabilitymaycauseFINE‐SCALEchangesin

nutrientfieldsandcommunitystructure

Poten7almechanisms

‐0.100

‐0.080

‐0.060

‐0.040

‐0.020

0.000

0.020

0.040

0.060

0.080

‐10

‐8

‐6

‐4

‐2

0

2

4

6

8

0 1 2 3 4 5 6 7 8 9 10 11 12 13

• Watercolumnsweepingbyexportevents

• Summer‐7mebloomsdrivelargeexportevent

• “Sweep”thewatercolumnofseedpopula7onscapableofrapidassimila7onofnutrients

• LightfluxEm‐2d‐1‐Summer: 47.3±2.3‐Winter: 31.2±3.1

SIGNIFICANTDIFFERENCEINLIGHTFLUX

Par7clefluxanomalymeasuredat150m

Conclusions

•  Addi7onofnutrient‐richdeepseawatertosurfacephytoplanktoncommunitygeneratedbloomsduringsummeronly

•  Mechanismsunderlyingvaria7oninphytoplanktoncommunityresponsebetweensummerandwinterremainsuncertain

•  IMPLICATIONS:–  finescaleupwelling,e.g.mesoscaleeddies

–  Ar7ficialfer7lisa7onoftheopenocean

Thankyou

Carbonpumps•  Solubilitypump:‐seawatertemperature

‐thermohalinecircula7on

•  Biologicalpump