Resource Ratios and Primary Productivity in theOcean

George I. Hagstrom, Simon Levin, Adam Martiny

Princeton UniversityDepartment of Ecology and Evolutionary Biology

Stoichiometry Couples Nutrient Cycles

I Photosyntheis in surface ocean pumps carbon to the deep.

I Phytoplankton require nitrogen, phosphorus, iron, andsometimes other nutrients (sillicon)

I Depletion of these nutrients in surface ocean slows biologicalpump, couples Carbon cycle to nutrient cycles.

I Strength of coupling is elemental stoichiometry ofphytoplankton.

C OO

C OO

C OO

Nutrient Timescales

Each major nutrient has different chemistry in the ocean:

I Inorganic Phosphorus: Residence time of 105 years.

I Inorganic Nitrogen: N-Fixation and denitrification.

N2 + 8H+ + 8e− + 16ATP→ 2NH3 + 16ADP + H2

I Iron residence time 100 years.

Redfield-Tyrrell Paradigm

I Biologists: N is ULN

I Geochemists: P is ULN

dBp

dt= Bp

(γp − m

),

dBd

dt= Bd (γd − m)

dNS

dt=

(ND − NS )

τS+

fN

DS

+ (rS − DN )m(Bp + Bd ) − γpBp

dPS

dt=

(PD − PS )

τS+

fP

DS

+(rSm − γp)Bp

(N:P)org+

(rSm − γd )Bd

(N:P)org

dND

dt= τ

−1D (NS − ND ) + mrD (Bp + Bd )

DS

DD

dPD

dt= τ

−1D (PS − PD ) + mrD

Bp + Bd

(N:P)org

DS

DD

− kPPD

I P is ultimate limiting nutrient

(TPP) = m(Bd + Bp

)=

fP (N:P)pkPDD (1−rS )

I Homeostasis

(N:P)deep ∼ (N:P)p

(1 − DN

1−rS

). (N:P)p

Challenges to Tyrrell/Redfield: Iron Limitation andStoichiometry

I Widespread iron limitation, HNLC regions and diazotrophs.

I High (N:P)org in subtropical gyres, low (N:P)org in subpolargyres.

Simple Biogeochemical Model

I Three nutrients: N, P, Fe

I Three phytoplankton types: diazotrophs, prokaryotes,eukaryotes

I Three ocean regions: High latitude, low-latitude, deep ocean.

Ultimate Limiting Nutrient Controlled by Supply andDemand

I Resource Supply:

JP,L =1

τL(PD − PL) +

fP,L

dL, JFe,L =

1

τL(FeD − FeL) +

fFe,L

dL, JN,L =

1

τL(ND − NL) +

fN,L

dL

JP,U =1

τU(PD − PU ) +

fP,U

dL, JFe,U =

1

τL(FeD − FeU ) +

fFe,U

dU, JN,U =

1

τU(ND − NU ) +

fN,U

dU

Normalize by resource demand:

φP,L = (N:P)pJP,L. φFe,L = (N:Fe)pJFe,L, φN,L = JN,L

φP,U = (N:P)uJP,U , φFe,U = (N:Fe)uJFe,U , φN,U = JN,U

I Limiting nutrients set by lowest supply to demand ratio:

WL =P,Fe (φP,L, φFe,L), WU =N,P,Fe (φP,U , φFe,U , φN,U)

TPP = α1ALφWL

(1− rS)+ α2fN,L +

AUφWU

(1− rS)

I α1 = 1, α2 = 0 when (N:P)p = (N:P)d or(N:Fe)p = (N:Fe)d .

Iron Supply Shifts Nutrient Limitation Scenarios

Deep Ocean N Regulated by Limiting Nutrient Supply toLL

I Fe Limited: ND

JFe,LτL= (N:Fe)p − DN

1−rS

((N:Fe)p +

JFe,UJFe,L

(N:Fe)u)

I P Limited: (N:P)deep = (N:P)p + DN

1−rS

(−(N:P)p − kU

kL(N:P)u

)

Reconciliation: Iron limitation, high kUkL

, lateral transport of P depleted

waters (Weber and Deutsch).

Response to Nutrient Flux Changes

I How would ocean respond to increases in nutrient fluxes?

I Redfield picture: Rapid transition to P limitation, no changein TPP.

I John Martin and others: Iron/nitrate fertilization may beimportant.

I Perform experiments: biogeography and stoichiometry givenew mechanisms.

Future Directions: Evolution of PhytoplanktonStoichiometry

I Many Directions to GoI Stoichiometry more plastic than indicated here.

I Frugality?I Growth Rate Hypothesis?I Temperature, Phylogeny, Luxury Storage?

I Incorporate eco-evolutionary feedbacks.

I Could the ocean evolve to colimitation?

Thanks!