1. Lec 5: Gases (DO & CO2) and pH
Factors affecting Oxygen Concentrations
Inorganic & Organic Carbon and the Carbonate Cycle
Wednesday:
Cole, J.J. et al Carbon dioxide supersaturation in the surface waters of lakes.
Science 265: 1

2. Dissolved Gases
1. Gases constitute one class of chemical impurities of water: some essential for life, some inert, others toxic
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:
Pressure
Salinity
c. Temperature
5. Solubility of a gas is independent of the concentrations of other gases in solution 2

3. Atmospheric vs. Dissolved Gas Concentrations (% by volume)
in water
Relative
Solubility
Gas
Atmosphere
Nitrogen
Oxygen
Argon
Carbon dioxide
Others
Nitrogen and Phosphorus are important plant nutrients 3

4. 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 Present
Solubility
Determination
DO Probe and meter
Chemically (Winkler method and modifications) 4

5. 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 5

6. Potential Energy and Redox
Which form of N is preferred by primary producers?
How to they convert to the preferred form?
Using
potential
energy
Creating
potential
energy 6

7. 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 8

8. Nomogram for Determining Saturation of Oxygen at Different Temperatures
Elev.
(m)
Pressure
(mm Hg)
Factor 10 20 30 5 15 25
140
120
100 80 60 40 50 2 3 4 6 7 8 9 11 12 13 14 16 17 1
Temperature (degrees C)
% Saturation
Oxygen (mg./liter)
Oxygen (cc./liter)
10 mg/l O2 at 20OC = 123% saturation at sea level
10 mg/l O2 at 20OC = 148% (1.20 x 120) saturation at m (~5000 ft) 7

9. 9

10. Oxygen Losses and Fluctuations
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
2. Oxygen losses due to:
a. Respiration
b. Decomposition
3. Oxygen distributed in the water column mostly by currents
4. Summer stratification may limit amount of dissolved oxygen in the hypolimnion 10

11. Mid-Summer Oxygen Profiles1 2 3 4 5 6 7 8 9 10 T O2
Orthograde
Depth (m)
1. Orthograde
Low productivity T O2
Clinograde
2. Clinograde
High productivity 5 10 15 1 2 3 4 6 7 8 9
O2 mg/l
Depth (m) T O2
Positive
Heterograde *
3. Positive Heterograde
Increased solubility in the metalimnion due to temperature
Concentrations of algae in the metalimnion
4. Negative Heterograde
High metalimnetic respiration and/or decomposition 5 10 15
O2 mg/l T O2
Negative
Heterograde 11

12. O2 Profiles for Shallow Dimictic Lakes
Temperature OC
Crystal Lake: unproductive, transparent, with deep photosynthesis
Other Lakes - range from moderately productive to highly productive
All lakes except Adelaide show metalimnetic oxygen maxima 2 4 6 8 10 12 14 16 18 20
Akagi Okono, Japan
Crystal Lake, Wisc.
TOC
[O2]S
[O2]S
TOC
[O2]
Depth (m)
Silver Lake, Wisc.
[O2]
Note areas of
DO deficit
TOC
Adelaide Lake, Wisc.
[O2]
[O2]S
[O2]S
TOC
Dissolved Oxygen (mg/l) 12

13. Development of a Clinograde Oxygen Curve
Processes responsible for this pattern? 2 4 6 8 10 12 14 16 18 20 22
Depth
(m)
Dissolved Oxygen (mg/l)
Lake Mendota, Wisc. IV
Aug.
III
July II
June I
May 13

14. Productive and Consumptive Aspects of Lake Morphology
High volume to surface area ratio lakes
Low volume to surface area ratio lakes
Productive Aspect
Consumptive Aspect
What other factors might affect this balance? 14

15. Carbon Forms of Carbon Transformations of Carbon
A General Introduction to Nutrient Cycling and the Carbon Cycle 15

16. Carbon Dioxide
Generally, the most important source of carbon for photosynthesis
Involved in buffering the pH of neutral
and alkaline lakes
The measurement of CO2 in all of its forms is called “Alkalinity” 16

17. Lake Nyos Disaster
1700 people and many livestock died near Lake Nyos in Cameroon in 1986
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 17

18. The Carbon Dioxide Cycle
Carbon dioxide in Solution O 2 O 2
(photosynthesis)
Plants
respiratory CO 2
Plants
(respiration) O 2
respiratory CO 2
Animals O 2
respiratory CO 2
dissolved
Bacteria
organic
material CO 2
non-biological oxidation
Organic Carbon
Inorganic Carbon 18
(mainly CO ) 2

19. Forms of Carbon Inorganic Carbon-bicarbonate equilibrium
Carbon dioxide: CO2
Carbonic acid: H2CO3
Bicarbonate: HCO3-
Carbonate: CO32-
Organic Carbon
CO2 + H2O H2CO3 HCO3- + H+ CO H+
-In which direction will PP drive these reactions? 19

20. Carbon Dioxide Cycle in Lakes
CO2
H2O+CO2<—>H2CO3<—>HCO3– + H+<
<—>2HCO3<—>CO3= +
Ca++
Phytoplankton (Euphotic Zone)
H2O
CaCO3
Sediments 20

21. Proportions of the forms of CO2 in Relation to pH
Free
Bicarbonate
Carbonate
pH CO2 HCO3– CO3=
x 10-9
x 10-7
x 10-5
x 10-4
x 10-3 21

22. Forms of CO2 in Water in Relation to pH22

23. Daily Fluctuations in Epilimnetic O2 and CO260
360
Sunset
Sunrise 50
350 40
340
CO2
(µm)
CO2 O2 30 O2
330
(µm) 20
320 10
310
300
1800
2400
600
1200
1800
Time 23 18

24. Consider these relationships when
we are processing the data from the
Hensley Reservoir field trip