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Lec 5: Gases (DO & CO2) and pH
• Factors affecting Oxygen Concentrations• Inorganic & Organic Carbon and the Carbonate Cycle
Wednesday:Cole, J.J. et al. 1994. Carbon dioxide supersaturation in the surface waters of lakes. Science 265:1568-1570.
Dissolved Gases1. Gases constitute one class of chemical impurities of
water: some essential for life, some inert, others toxic
2
2. Properties of gases governed by both chemical and physical laws
3. Gases tend toward equilibrium between the concentration in the atmosphere and that dissolved in water
4. Equilibrium (saturation) amount of each gas dissolved in water dependent on:a. Pressure
b. Salinity
c. Temperature
5. Solubility of a gas is independent of the concentrations of other gases in solution
Atmospheric vs. Dissolved Gas Concentrations
(% by volume)
Nitrogen 78.08 42 1
Oxygen 20.95 35 3
Argon 0.934
Carbon dioxide 0.033 23 2100
Others 0.003
Gas AtmosphereDissolvedin water
RelativeSolubility
Nitrogen and Phosphorus are important plant nutrients3
Oxygen• 90% of water (by weight) but not biologically
available or important in this form• Probably the most important single indicator of
aquatic conditions for biota• Concentration in water generally expressed as PPM
(Parts per million) = mg/l, or as percent saturation:
100%*Amount PresentSolubility
• Determination
– DO Probe and meter
– Chemically (Winkler method and modifications)
4
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Oxygen - Forms and Transformations
• 21% of atmosphere is O2
• Aerobic/anaerobic - oxic/anoxic (hypoxic)• Saturation concentration of dissolved O2 depends on atmospheric pressure
and temperature• Photosynthesis produces oxygen, respiration consumes it• Oxygen drives redox
Potential Energy and Redox
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• Which form of N is preferred by primary producers?• How to they convert to the preferred form?
Using potential energy
Creating potential energy
Factors affecting Oxygen Conc. 1. Diffusion from atmosphere (Often less important than
photosynthesis). Diffusion rate depends on:
a. Wave action (rate increases with increasing wave action)
b. Atmospheric pressure (rate increases with increasing atmospheric pressure)
c. Oxygen saturation of water (rate decreases with increasing saturation)
d. Salinity (rate decreases with increasing salinity)
e. Moisture content of air (rate decreases with increasing humidity)
2. Photosynthesis (Often more important than atmospheric diffusion). May contribute more than 50% of the oxygen in water. Photosynthesis may contribute 5mg O2/cm2/day
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Nomogram for Determining Saturation of Oxygen at Different Temperatures
0 10 20 305 15 25
140120
10080
6040
2010
30
50
0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
0 2 3 4 5 6 7 8 9 10 11 121
Temperature (degrees C)
% Saturation
Oxygen (mg./liter)
Oxygen (cc./liter)
0 760 1.00 500 714 1.061000 671 1.131500 631 1.202000 594 1.282500 560 1.36
Elev.(m)
Pressure(mm Hg) Factor
10 mg/l O2 at 20OC =
123% saturation at sea level
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10 mg/l O2 at 20OC =
148% (1.20 x 120) saturation at 1500 m (~5000 ft)
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1. Photosynthesis and respiration often result in daily
fluctuations in the O2 concentration of surface water
a. May reach 200% saturation in late afternoon
b. May fall to 50% saturation by dawn
Oxygen Losses and Fluctuations
10
2. Oxygen losses due to:
a. Respiration
b. Decomposition
4. Summer stratification may limit amount of dissolved oxygen in the hypolimnion
3. Oxygen distributed in the water column mostly by currents
Mid-SummerOxygen Profiles
4. Negative HeterogradeHigh metalimnetic
respiration and/or decomposition
0 5 10 15
O2 mg/l
T
O2
NegativeHeterograde
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1
2
3
4
5
6
7
8
910
0
T
O2
Orthograde
Dep
th (
m)
1. OrthogradeLow productivity
T
O2
Clinograde
2. ClinogradeHigh productivity
0 5 10 15
1
2
3
4
5
6
7
8
910
0
O2 mg/l
Dep
th (
m)
T
O2
PositiveHeterograde
*3. Positive Heterograde
Increased solubility in the metalimnion due to temperatureConcentrations of algae in the metalimnion
O2 Profiles for Shallow Dimictic Lakes
• Crystal Lake: unproductive, transparent, with deep photosynthesis
• Other Lakes - range from moderately productive to highly productive
• All lakes except Adelaide show metalimnetic oxygen maxima
0 1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0 2 4 6 8 10 12 14 16 18 20 22 24 26 2 4 6 8 10 12 14 16 18 20 22 24 26 280
2
4
6
8
10
12
14
16
18
0
2
4
6
8
10
12
14
16
18
20
Temperature OC
Dissolved Oxygen (mg/l)
Dep
th (
m)
TOC[O2]S
[O2]
[O2]S[O2]S
TOCTOC
[O2]
[O2]
[O2]S
TOC
Crystal Lake, Wisc.
Adelaide Lake, Wisc.
Silver Lake, Wisc.
Akagi Okono, Japan
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Note areas of DO deficit
Development of a Clinograde Oxygen Curve
IMay
IIJune
IIIJuly
IVAug.
0 1 2 3 4 5 6 7 8 9 10 11 12
0
2
4
6
8
10
12
14
16
18
20
22
Depth(m)
Dissolved Oxygen (mg/l)
Lake Mendota, Wisc.
Processes responsible for this pattern?
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Productive and ConsumptiveAspects of Lake Morphology
Productive Aspect Consumptive Aspect
High volume to surface area ratio lakes
Low volume to surface area ratio lakes
What other factors might affect this balance?14
Carbon
• Forms of Carbon
• Transformations of Carbon
• A General Introduction to Nutrient Cycling and the Carbon Cycle
15
Carbon Dioxide
• Generally, the most important source of carbon for photosynthesis
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• Involved in buffering the pH of neutral and alkaline lakes
• The measurement of CO2 in all of its forms is called “Alkalinity”
Lake Nyos Disaster
• 1700 people and many livestock died near Lake Nyos in Cameroon in 1986
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• A survivor reported a 25m high water surge and odor of rotten eggs• Caused by catastrophic release of supersaturated CO2 from the hypolimnion
• CO2 probably came from volcanic activity
• Landslide or cool weather released the gas• Building up again, using pipes to release pressurized water
The Carbon Dioxide Cycle
(photosynthesis)
Plants
(respiration)
Plants
Animals
dissolvedorganicmaterial
Bacteria
O2
O2
O2
O2
Carbon dioxide in Solution
respiratory CO 2
respiratory CO 2
respiratory CO 2
non-biological oxidationCO2
Organic Carbon
Inorganic Carbon(mainly CO 2 ) 18
Forms of Carbon• Inorganic Carbon-bicarbonate equilibrium
– Carbon dioxide: CO2
– Carbonic acid: H2CO3
– Bicarbonate: HCO3-
– Carbonate: CO32-
• Organic Carbon
CO2 + H2O H2CO3 HCO3- + H+ CO3
2- + 2H+
-In which direction will PP drive these reactions?
19
Carbon Dioxide Cycle in Lakes
Phytoplankton (Euphotic Zone)
H2O+CO2<—>H2CO3<—>HCO3– + H+<<—>2HCO3<—>CO3
=
CO2
H2O
+Ca++
CaCO3
Sediments
20
Proportions of the formsof CO2 in Relation to pH
pH CO2 HCO3– CO3
=
4 0.996 0.004 1.26 x 10-9
5 0.962 0.038 1.20 x 10-7
6 0.725 0.275 0.91 x 10-5
7 0.208 0.792 2.60 x 10-4
8 0.025 0.972 3.20 x 10-3
9 0.003 0.966 0.031
10 0.000 0.757 0.243
Free Bicarbonate Carbonate
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3 4 5 6 7 8 9 10 11 12
pH
0.0
0.2
0.4
0.6
0.8
1.0
Pro
port
ion
of t
otal
inor
gani
c C CO2 (H2CO3) HCO3
-
CO32-
Forms of CO2 in Water in Relation to pH
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Daily Fluctuations in Epilimnetic O2 and CO2
60
50
40
30
20
10
0
360
350
340
330
320
310
3001800 2400 600 1200 1800
CO2
(µm)
Sunset Sunrise
O2
(µm)
Time
CO2
O2
1823
Consider these relationships when we are processing the data from the Hensley Reservoir field trip