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 toxic2. Properties of gases governed by both chemical and

physical laws3. 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 (atmospheric pressure, elevation: increasing pressure

increases solubility)b. Salinity (increasing salinity reduces solubility)c. Temperature (increasing temperature reduces solubility)

5. Solubility of a gas is independent of the concentrations of other gases in solution

2

Atmospheric vs. Dissolved Gas Concentrations

(% by volume)

Nitrogen 78.08 42 1Oxygen 20.95 35 3Argon 0.934Carbon dioxide 0.033 23 2100Others 0.003

Gas Atmosphere Dissolvedin 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 percentsaturation:

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)• Oxygen drives redox (next slide)• Saturation concentration of dissolved

O2 depends on atmospheric pressure and temperature

• Photosynthesis produces oxygen, respiration consumes it

Potential Energy and Redox

Pot

entia

l ene

rgy

Activation energy

Net energy yield

Ammonium

Nitrate

Oxidizing environment

Going with potential energy

Activation energy

Net energy yield

Ammonium

Nitrate

Reducing environment

Going with potential energy

Pot

entia

l ene

rgy

Activation energy

Net energy cost

Ammonium

Nitrate

Going against potential energy

Activation energy

Net energy cost

Ammonium

Nitrate

Going against potential energy

6

• Which form of N is preferred by primary producers?• How to they convert to the preferred form?

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

1401201008060

4020

1030

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 1.00500 714 1.06

1000 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

10 mg/l O2 at 20OC = 148% (1.20 x 120) saturation at 1500 m (~5000 ft)

7

9

1. Photosynthesis and respiration often result in daily fluctuations in the O2 concentration of surface water

a. May reach 200% saturation in late afternoonb. 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 currents4. Summer stratification may limit amount of dissolved

oxygen in the hypolimnion

Oxygen Losses and Fluctuations

10

Mid-SummerOxygen Profiles

123456789

10

0

0 5 10 15 0 5 10 15

123456789

10

0

O2 mg/l O2 mg/l

Dep

th (m

)

T

T

T

T

O2 O2

O2 O2

1. OrthogradeLow productivity

2. ClinogradeHigh productivity

3. Positive HeterogradeIncreased solubility in the

metalimnion due to temperature

Concentrations of algae in the metalimnion

4. Negative HeterogradeHigh metalimnetic

respiration and/or decomposition

Orthograde

Clinograde

PositiveHeterograde

NegativeHeterograde

Dep

th (m

)

11

*

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 showmetalimnetic 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 2802468

1012141618

02468

101214161820

Temperature OC

Dissolved Oxygen (mg/l)

Dep

th (m

)

TOC[O2]S

[O2][O2]S

[O2]STOC

TOC

[O2]

[O2]

[O2]S

TOC

Crystal Lake,Wisc.

Adelaide Lake,Wisc.

Silver Lake,Wisc.

Akagi Okono, Japan

12

Note areas of DO deficit

Development of a ClinogradeOxygen Curve

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)

IMay

IIJune

IIIJuly

IVAug.

Lake Mendota,Wisc.

Processes responsible for this pattern?

13

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

• Involved in buffering the pH of neutral and alkaline lakes

• The measurement of CO2 in all of its forms is called “Alkalinity”

16

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 water17

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 CarbonInorganic 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 0.03110 0.000 0.757 0.243

Free Bicarbonate Carbonate

21

3 4 5 6 7 8 9 10 11 12pH

0.0

0.2

0.4

0.6

0.8

1.0

Pro

porti

on o

f tot

al in

orga

nic

C CO2 (H2CO3) HCO3-

CO32-

Forms of CO2 in Water in Relation to pH

22

Daily Fluctuations inEpilimnetic 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

CO2O2

1823