Download Lecture 12

Lecture 12
Primary Production – Nutrient Stoichiometry
Topics
Stoichiometry
Biolimiting Elements
Surface NItrate
Q. Why high NO3 ocean areas?
Surface Phosphate
Surface Silicate
Chemical Composition of Biological Particulate Material
Hard Parts - Shells
Name
Mineral
Size (mm)
Coccoliths
Diatoms
Silicoflagellates
CaCO3 Calcite
SiO2 Opal
SiO2 Opal
5
10-15
30
Foraminifera
CaCO3 Calcite
and Aragonite
SiO2 Opal
CaCO3 Aragonite
SrSO4 Celestite
~100
Plants
Animals
Radiolaria
Pteropods
Acantharia
~100
~1000
~100
Soft Parts - protoplasm
from Redfield, Ketchum and Richards (1963) The Sea Vol. 2
Also 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:
HCO3NO3PO43NO2NH4+
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 production
CO2 + H2O  (CH2O) + O2
H+ + NO3- + H2O  (NH3) + 2O2
C : O2  1 : 1
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 photosynthesis
106 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 EqPac
Suspended Matter and sediment trap particles
Lines 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 Plankton
65% protein
19% lipid
16% carbohydrate
Average formula for plankton biomass
C106H177O37N17PS0.4
Oxidation consumes 154 moles of O2
106 CO2 + 17 HNO3 + H3PO4 + 122 H2O + trace elements
light (h n)
( C106H177O37N17PS0.4 )

+
154 O2
Hedges et al (2002) Marine Chemistry
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)
B = VmCd + VrCr - VmCs
B
(1-f)B
fB
see Fig. 2 of 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
Mass balance for surface box
Vs 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)
fraction of input
to surface removed
as B
because fB = VrCr
fraction of element removed to
sediment per visit to the surface
fxg
In this model Vr = 10 cm y-1
Vmix = 200 cm y-1
so
Vmix / Vr = 20
Here are some values:
g
f
N
0.95
0.01
P
0.95
0.01
C
0.20
0.02
Si
1.0
0.01
Ba
0.75
0.12
Ca
0.01
0.12
See Broecker (1971) Table 3
fxg
0.01
0.01
0.004
0.01
0.09
0.001
From 14C mass balance (next slide)
Q. Explain these values and
why they vary the way they do.
How large is the transport term:
If the residence time of the deep ocean is 1850 yrs (from 14C)
and t = Vold / Vmix fraction of total depth
volume
that is deep ocean
then:
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 it
evaporates and returns as river flow.
Example – 14C Deep Ocean Residence Time
vmix in cm yr-1; vC in cm yr-1 x mol cm-3
substitute for B
Rearrange and
Solve 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
for h = 3200m
A = ocean area
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?
The nutrient concentration of
the deep ocean will adjust so that
the fraction of B preserved in the
sediments equals river input!
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
Example: Perturbation analysis – Mass Balance Control
Double Upwelling Rate
Double rate of ocean mixing
VrCr = fB at the beginning and at the end!
The deep concentration (Cd) is cut in half
sequence of events
Paleo record
Example:
Perturbation Analysis
1. Double River Input
2. CaCO3 burial increases
and Carbonate
Compensation Depth (CCD)
deepens
R = RKR
P = Protein
L = Lipid
C = Carbohydrate
E = Equatorial Pacific
A = Arabian Sea
1-3 = Southern Ocean
1a = Anderson et al
From Hedges et al (2002)