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. 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

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

Potential Energy and Redox

6

• 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

8

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

7

10 mg/l O2 at 20OC =

148% (1.20 x 120) saturation at 1500 m (~5000 ft)

9

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

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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

11

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

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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?

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Carbon Dioxide Cycle in Lakes

Phytoplankton (Euphotic Zone)

H2O+CO2<—>H2CO3<—>HCO3– + H+<<—>2HCO3<—>CO3

=

CO2

H2O

+Ca++

CaCO3

Sediments

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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