Using Stable Isotopes of Oxygen and Dissolved Inorganic Carbon to ...

Using Stable Isotopes of Oxygen
and Dissolved Inorganic Carbon to
Trace Respiration and
Photosynthesis Processes Under Ice
Cover at Georgetown Lake,
Montana
Bill Henne 1 , Chris Gammons 1 , Simon Poulson 2
1 Montana Tech, Butte, MT
2 University of Nevada‐Reno, NV

• Vertical mixing in summer
• Stratification in winter
• LONG winter!
• Ice cover from early November to mid‐May

Comer’s Pt.
46⁰10′N
113⁰20′W
0 1 mile
0 1 2 km
3 water depth, m
GT‐2
6
3
Georgetown
Thrust
Flint Creek
Granite Co.
Deer Lodge Co.
Dam
GT‐1
many
Yc
Ps
Stuart Mill Spring
and Creek

So Why Isotopes?
• Hydrogen and oxygen isotopes of water are
widely used as tracers of hydrogeological
processes such as precipitation, groundwater
recharge, groundwater‐surface water
interaction, basin water hydrology, etc.
Old idea
• Stable isotopes of dissolved oxygen (DO) and
dissolved inorganic carbon (DIC) can provide
new insights into lake metabolism because
photosynthetic and respiratory processes
have opposing effects on the ratios of heavier
to lighter isotopes
New idea

Dissolved Oxygen, mg/L
Dissolved oxygen: historical trends
All data from March
12
10
8
6
4
2
0
0 1 2 3 4 5
Depth, meters
1997-2000
2001
Montana Fish, Wildlife & Parks, unpublished

Date
Field
Parameters
PAR Alkalinity Phosphate Ammonia Sulfide δ 18
O‐DO δ 18
O‐H2O δD‐H2O δ 13
C‐DIC δ 34 S‐H2S
6 Nov 10 X X X X X X X X
16 Dec 10 X X X X X
24 Jan 11 X X X X X X X X X
15 Feb 11 X X X X X X X
21 Feb 11 X
16 Mar 11 X X X X X X X X X X X
2 Apr 11 X
15 Apr 11 X X X X X X X
30 Apr 11 X
13 May 11 X X X X X X
27 May 11 X X X X
5 Jun 11 X X X X X X X

Simon Poulson
Chris Gammons

Results: Field
Parameters and
Chemistry

Depth (m)
Depth (m)
Temperature o C
GT‐1
GT‐2
Ice
Ice

Depth (m)
Depth (m)
Specific Conductance (μS/cm)
GT‐1
GT‐2
Ice
Ice

Depth (m)
Depth (m)
Dissolved Oxygen (mg/L)
GT‐1
GT‐2
Ice
Ice

Depth (m)
0
2.5
5
7.5
10
Photosynthetically Active Radiation (PAR)
PAR (μE m
0 10 20 30
‐2 s ‐1 )
24‐Jan‐11
15‐Apr‐11
0
2
4
6
8
0 8 PAR (μEm 16 24
‐2 s ‐1 )
GT‐1 GT‐2
16‐Mar‐11
15‐Apr‐11

Other changes with depth
• Decreases in pH, Eh
• Large increases in HCO 3 ‐ , Ca 2+ ,SiO2(aq)
• Dissolution of calcite, diatoms in sediment
• Moderate increases in NH 4 + , SRP, H2S
• Euxinic layer, bottom 1‐2 m
• BSR and/or fermentation (putrefaction)
• Small increases in Mn 2+ , Fe 2+
• Reductive dissolution of oxy‐hydroxides

Photosynthesis → O2 Aerobic respiration
diffusion
Ice
H 2S, NH 4 + , PO4 3‐ Ca 2+ , HCO 3 ‐
Anaerobic Respiration & Decay
Comer’s site (GT‐2)
‐ Conceptual Model
Aerobic
respiration
diffusion
Calcite dissolution
shallower water (~ 6m), flat bottom
Oxic
Photosynthetic H 2S
oxidation
Anoxic

Filters from March sampling @ Comer’s
Shallow water: green‐brown
solids on filter (phytoplankton)
Deep water (5.5 m): purple solids on
filter (photosynthetic H 2S‐oxidizing
bacteria!)
This proves that there is enough light
reaching the bottom of the lake
through the ice for photosynthesis.

Results:
Isotopes

δD‐H 2O (‰)
‐100
‐110
‐120
‐130
‐140
‐150
Water Isotopes
y = 4.8653x ‐ 49.17
R² = 0.979
‐19 ‐18 ‐17 ‐16 ‐15 ‐14
δ 18 O‐H 2O (‰)
Lake, November
(~ 15% evaporated)
Tributary and Springs
GT‐1 & GT‐2 (November)
GT‐1 & GT‐2 (January)
GT‐1 (March)
GT‐2 (March)

Isotopes of DO and DIC
• Photosynthesis consumes light DIC and
produces light DO
• Respiration consumes light DO and
produces light DIC

Depth (m)
Depth (m)
δ 18 O ‐ Dissolved Oxygen (‰)
GT‐1
GT‐2
Ice
Ice

δ 13 C‐DIC (‰)
‐3.5
‐4.5
‐5.5
‐6.5
‐7.5
δ 18 O of air‐saturated water ‐ 24.2‰
y = ‐0.1714x ‐ 1.2091
R² = 0.7839
15 20 25 30 35
δ 18 O‐DO (‰)
Nov GT‐1
Jan GT‐1
Mar GT‐1
Nov GT‐2
Jan GT‐2
Mar GT‐2

δ 18 O‐DO (‰)
35
30
25
20
15
10
R
y = ‐1.7194x + 33.153
R² = 0.93
y = ‐0.4363x + 23.856
R² = 0.40
P
0 2 4 6 8 10 12
DO (mg/L)
GT‐1
GT‐2
‐ Steeper slope for
Dam site is
consistent with
consumption of
DO by respiration
in absence of
photosynthesis
(R >> P)
‐ Shallower slope for
Comers shows both
respiration and
photosynthesis are
important (R ≥ P)

Using isotopes to estimate
photosynthesis:respiration ratios
X g = α g*(R atm*α s ‐ (DO/DO sat)*R DO)/(1‐DO/DO sat)
P/R = (R DO*α r ‐ X g)/(R W ‐ X g)
αr is O‐isotopic fractionation from respiration. This can vary. Use range of 0.977 to 0.98 (same as Wang).
αg is ratio of 18 O‐ 16 O to 16 O‐ 16 O gas transfer velocities, = 0.9972 at 20 o C
αs is the ratio of 18 O‐ 16 O to 16 O‐ 16 O solubility in water, = 1.0007 at 28 o C
RDO is 18 O/ 16 O ratio of DO (Measured by Simon)
Ratm is 18 O/ 16 O ratio of air
Rwater is 18 O/ 16 O ratio of water in G‐Town Lake
Quay et al.(1995)

Summary:
• Big DO drops (worse at GT‐1 than GT‐2)
• Increases in DIC, Ca 2+ , Si, ammonia, H 2S, P
towards lake bottom = diffusion from sediment
pore water
• DO and DIC isotopes show expected trends
• DO isotopes suggest higher P:R ratio at GT‐2
– P:R ratios support this
• Photosynthesis happens through the ice at
Georgetown Lake!

Thanks to:
• Professor Chris Gammons
• Professors Glenn Shaw & Steve Parker
• Professor Simon Poulson (UNR)
• MT Tech Geological Engineering
Department

Questions???