resource ratios and primary productivity in the ocean - george i. hagstrom, simon levin, adam...

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!