Isotope methodology to estimate aquatic
production
Boaz Luz
Institute of Earth Sciences, Hebrew University, Jerusalem
ISEEQS May, 2005
Collaboration:
E. Barkan
Y. Yacobi
Y. Sagi
Aquatic production: Some terminology
P – gross production (C-fixation or O2-formation)
R – “respiration” (C-oxidation or O2-consumption)
P-R = N – net production
P
CO2 + H2O
CH2O + O2
R
Our goal is to estimate community net and gross production
In practice we run:
- Bottle incubations with: 14C, 15N and 18O
- Field measurements of [O2], [Ar] and oxygen isotope ratios
Which bottle incubation method is better?
- The 14C method is relatively easy, but
- Suffers from uncertainty due to recycling of the 14C spike
- The 15N method measures net nitrogen fixation
- The 18O measures gross production and also net as a byproduct
Graphic presentation of the 14C method
14C
1
?
?
consumption
on
i
t
p
m
consu
2
n
o
i
t
p
m
u
s
? con
3 ption
m
u
s
? con
Case
1
2
4
n
o
i
ct
u
d
o
r
p
N
14C
production estimates
P
Between P and N
3
~N
4
<N
R
P
[Cell Carbon]
Graphic presentation of the 18O method
18O/16O
(δ18O)
consumption
on
i
t
uc
d
pro
N
R
P
[O2]
250
Peridinium bloom
y = 7.53x
-1
P( O), mmol d m
-3
200
150
100
18
y = 1.92x
50
0
0
10
20
14
30
-1
P( C), mmol d m
40
-3
Collaborators: Barkan, Sagi, Yacobi (L&O 2002)
50
NCP (µ mol h -1 L-1)
NCP <
Peridinium bloom
NCP >> P(14C)
P(14C)
5
4
6
1:1
4
3
2
1
0
2
0
-2
0
1
2
3
4
5
P( C) (µ
µ mol h L )
14
-1
-1
0
1
2
3
P(14C) (µ
µ mol h-1 L-1)
How these (bottle) rates compare with true NCP?
4
Atmospheric O2
N = K([O2]diss – [O2]eq)
P
N=P–R>0
R
[O2]diss > [O2]eq
Plankton
Net production (N = P – R)
[O2]diss – measured concentration of dissolved oxygen in
[O2]eq – equilibrium oxygen concentration
K – gas transfer velocity
N = K([O2]diss – [O2]eq )
NCP – L. Kinneret
Date
field
bottle
11-Jul-96
0
1.25
26 Sept 96
0
-0.19
17-Apr-97
0.01
-0.02
06-May-97
0.03
0.36
04-Apr-98
0.03
0.93
03-May-98
0.07
1.31
14-Jun-98
0.07
2.84
04-Aug-98
0
1.74
We measure biological oxygen
saturation directly from O2/Ar
(as a byproduct of the isotopic
measurements) and calculate
NCP – net community production
In L. Kinneret, bottle estimates of NCP
are un-realistically exaggerated because
much of the respiration in the lake is not
represented in the bottle measurements
4
Bottle measurements
do not indicate true NCP
N C P (b o ttle )
3
2
1
0
y = 18.29x + 0.55
R2 = 0.30
-1
0
0.01
0.02
0.03
0.04
NCP (field)
0.05
0.06
0.07
0.08
Bottle measurements do not indicate true NCP
What about gross community production?
The answer is possible by using a natural spike to
measure gross production
The spike is found in the 17O/16O and 18O/16O in air O2
250
0.52
slope
>>
ln(δ
δ17O +1)
Non-mass-dependent transfer of
isotopes from O2 to CO2 in the
stratosphere
0
0.52
~
e
op
l
s
n
tio
a
r
i
p
Res
Bio steady
state O2
17∆
max = 250
Atmospheric O2
17∆ = 0
Seawater
ln(δ
δ18O +1)
0
Atmospheric O2 is different than O2 affected only by
photosynthesis and respiration
The deviation – 17∆ – can be used to estimate gross production
just as δ18O is used in 18O spike experiments
∆ v s A ir o x y g e n
200
17∆
max
150
17
100
50
0
0
50
100
150
Day
200
Atlantic Ocean station BATS
(Bermuda Atlantic Time-series Study)
Atlantic Ocean, Bermuda
15
20
25
30 -200
0
-100
0
0
0
50
δO2/Ar
o
Temp. ( C)
100
17
∆
50
50
100
100
100
150
150
200
200
250
250
250
300
300
300
equilibrium
50
150
200
equilibrium
0
150
200
17
0
∆ Mar-Sep 2000
40
80
120
160
0
50
Sep
Jul
100
Mar
May
150
200
No O2 production
below this depth
250
300
Large scale “incubation experiment”
(17∆eq)F
Atmospheric O2
(17∆diss)F
(17∆max)P
Plankton
(17∆diss)R
At steady state 17∆ flux balance is:
(17∆eq)F + (17∆max)P = (17∆diss)R + (17∆diss)(F + N)
P(17∆max – 17∆diss) = F(17∆diss -17∆eq);
F = KCo
P = (17∆diss – 17∆eq)/(17∆max – 17∆diss)KCo
Gross Production in L. Kinneret
2.5
C
O2 Gross Production (from H218O incubations)
2.0
1.5
mol m-2 d-1
1.0
0.5
0.0
2.5
B
2.0
O2 Gross Production (from 17∆)
1.5
1.0
0.5
0.0
per meg
160
A
140
17∆
of dissolved O2
120
100
80
J
F
M
A
M
J
J
A
S
O
N
D
J
Conclusions
1. It is not clear what 14C incubations measure.
(Gross production? Net production? In between?)
2. [O2] incubations measure net O2 production in the bottle but not
true community production
3. H218O incubations measure gross O2 production in the bottle and
also in the field as confirmed by the 17∆ method
4. The 17∆ method gives integrated estimates of gross community
production and O2/Ar measurements give net community
production and there is no need for bottle incubations