Lecture 12

Primary Production – Nutrient Stoichiometry

TopicsStoichiometryBiolimiting Elements

Surface NItrate

Q. Why high NO3 ocean areas?

Surface Phosphate

Surface Silicate

Chemical Composition of Biological Particulate Material

Hard Parts - ShellsName Mineral Size (mm)

PlantsCoccoliths CaCO3 Calcite 5Diatoms SiO2 Opal 10-15Silicoflagellates SiO2 Opal 30

AnimalsForaminifera CaCO3 Calcite ~100

and AragoniteRadiolaria SiO2 Opal ~100Pteropods CaCO3 Aragonite ~1000Acantharia SrSO4 Celestite ~100

Soft Parts - protoplasm

from Redfield, Ketchum and Richards (1963) The Sea Vol. 2Also for particles caught by sediment traps.

The Redfield or "RKR" Equation (A Model)

The mean elemental ratio of marine organic particles is given as:

P : N : C = 1 : 16 : 106

The average ocean photosynthesis (forward) and aerobic ( O2 ) respiration (reverse) is written as:

106 CO2 + 16 HNO3 + H3PO4 + 122 H2O + trace elements (e.g. Fe)

light (h n) ( C106H263O110N16P ) + 138 O2

or (CH2O)106(NH3)16(H3PO4)

Algal Protoplasm

The actual chemical species assimilated during this reaction are:HCO3

- NO3- PO4

3-

NO2-

NH4+

1. This is an organic oxidation-reduction reaction - during photosynthesis C and N are reduced and O is oxidized. During respiration the reverse occurs. There are no changes in the oxidation state of P.

We assume C has an oxidation state of 0 which is the value of C in formaldehyde (CH2O), that N has an oxidation state of -III and that H and P do not change oxidation states.

2. Photosynthesis is endothermic. This means it requires energy from an outside source. In this case the energy source is the sun. Essentially plants convert the photo energy from the sun into high energy C - C bonds. This conversion happens in the plants photosystems.

Respiration is exothermic. This means it could occur spontaneously and release energy. In actuality it is always mediated by bacteria which use the reactions to obtain their energy for life.

3. Stoichiometry breakdown of oxygen productionCO2 + H2O (CH2O) + O2 C : O2 1 : 1H+ + NO3

- + H2O (NH3) + 2O2 N : O2 1 : 2

4. Total oxygen production: 106 C + 16 N x 2 = 138 O2

Oxidation State of C?

5. If ammonia is available it is preferentially taken up by phytoplankton.

If NH3 is used as the N source then less O2 is produced during photosynthesis106 CO2 + 16 NH3 + H3PO4 + 122 H2O + trace elements

light (hn)

(CH2O)106(NH3)16(H3PO4) + 106 O2

The relationship between O2 and NO3/NH4 is 2:1 (as shown in point #3)

16 HNO3 + 16 H2O = 16 NH3 + 32 O2

Dissolved seawater data from Atlantic GEOSECS Program (Broecker and Peng, 1982)

small deficit in NO3

Remarkable congruence between ratios in the ocean and plankton composition.

Nutrient stoichiometry from US JGOFS EqPacSuspended Matter and sediment trap particlesLines show Redfield slopes

Murray et al, 1992, unpublished EqPac

Why the Redfield Ratios?? From a chemistry point-of-view.

Each class of organic compounds has its own unique stoichiometry

carbohydrates are C rich but N and P poor (e.g. (CH2O)n)proteins are C and N rich but P poor (e.g. amino acids)nucleotides are rich in both N and P (e.g. 4 bases)lipids are C rich

Questions:

Why 16:1? Why not 6:1 or 60:1? See Arrigo 2005 (to be read later)How does an organism end up with a certain composition?What happens if one constituent is not available in adequate amounts?

Stoichiometry based on organic composition

Average Plankton65% protein19% lipid16% carbohydrate

Average formula for plankton biomassC106H177O37N17PS0.4

Oxidation consumes 154 moles of O2

Hedges et al (2002) Marine Chemistry

106 CO2 + 17 HNO3 + H3PO4 + 122 H2O + trace elements

light (h n) ( C106H177O37N17PS0.4 ) + 154 O2

Controls on Atmospheric CO2

Remarkable consistency for glacial/interglacial concentrations of CO2.A main Control on atm CO2 is the B flux! We need to understand B…PI CO2 = 280 ppm PI CO2 w/o B = 970 ppm!

How do we get from the marine food web to a global assessment of CO2 flux???

With great difficulty!

Broecker two-box model (Broecker, 1971)

v is in m y-1 then flux is mol m-2y-1

Flux = Vmix Csurf = m yr-1 mol m-3 = mol m-2 y-1

see Fig. 2 of Broecker (1971)

B

fB

(1-f)B

B = VmCd + VrCr - VmCs

Mass balance for surface boxVs dCs/dCt = VrCr + VmCd – VmixCs – B

At steady state:B = VrCr + VmixCd – VmixCs and fB= VrivCriv

Broecker (1971) defines some parameters for the 2-box model

g = B / input = (VmixCD + VrCr – VmixCs) / VmixCd + VrCr

f = VrCr / B = VrCr / (VmixCd + VrCr - VmixCs)

f x g

In this model Vr = 10 cm y-1

Vmix = 200 cm y-1

so Vmix / Vr = 20 From 14C mass balance (next slide)

Here are some values:g f f x g

N 0.95 0.01 0.01P 0.95 0.01 0.01C 0.20 0.02 0.004Si 1.0 0.01 0.01Ba 0.75 0.12 0.09Ca 0.01 0.12 0.001

Q. Explain these values andwhy they vary the way they do.

because fB = VrCr

fraction of element removed to sediment per visit to the surface

fraction of inputto surface removed as B

See Broecker (1971) Table 3

How large is the transport term:If the residence time of the deep ocean is 1850 yrs (from 14C)and t = Vold / Vmixthen:

Vmix = (3700m/3800m)(1.37 x 1018 m3) / 1850 y = 0.72 x 1015 m3 y-1

If River Inflow = 3.7 x 1013 m3 y-1

Then River Inflow / Deep Box Exchange = 3.7 x 1013/72 x 1013

= 1 / 19.5

This means water circulates on average about 19.5 times through the ocean (surface to deep exchange) before itevaporates and returns as river flow.

volumefraction of total depth that is deep ocean

Example – 14C Deep Ocean Residence Time

substitute for Bvmix in cm yr-1; vC in cm yr-1 x mol cm-3

Rearrange andSolve for Vmix

Use pre-nuclear 14C data when surface 14C > deep 14C(14C/C)deep = 0.81 (14C/C)surf

Vmix = (200 cm y-1) A A = ocean areafor h = 3200m

thus age of deep ocean box (t)t = 3700m / 2 my-1 = 1850 years

Why is this important for chemical oceanography?What controls ocean C, N, P?g ≈ 1.0

Mass Balance for whole ocean:C/ t = VRCR – f B

CS = 0; CD = CD

VU = VD = VMIX

Negative Feedback Control:if VMIX ↑VUCD ↑B ↑f B ↑ (assumes f will be constant!)assume VRCR then CD ↓ (because total ocean balance

VUCD ↓ has changed; sink > source)B ↓

CS

CD

if VMIX = m y-1 and C = mol m-3

flux = mol m-2 y-1

The nutrient concentration of the deep ocean will adjust so thatthe fraction of B preserved in thesediments equals river input!

Example: Perturbation analysis – Mass Balance ControlDouble Upwelling Rate

sequence of events Paleo recordDouble rate of ocean mixing

VrCr = fB at the beginning and at the end!The deep concentration (Cd) is cut in half

Example: Perturbation Analysis

1. Double River Input

2. CaCO3 burial increasesand Carbonate Compensation Depth (CCD)deepens

R = RKRP = ProteinL = LipidC = Carbohydrate

E = Equatorial PacificA = Arabian Sea1-3 = Southern Ocean1a = Anderson et al

From Hedges et al (2002)