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1. How do temperature, thermal stratification

and biological activity influence oxygen profiles

in lakes?

2. What are the major chemical reactions involved in

the inorganic carbon complex?

3. How is DIC related to pH?

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6 CO2 + 6 H2O C6H12O6 + 6 O2

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Why care about oxygen in lakes?

Required for respiration

Toxic to anaerobic organisms

Influences chemical processes (Redox)

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Why care about dissolved inorganic carbon (DIC)?

1. It influences pH

2. It determines the amount of inorganic

carbon available for photosynthesis

3. Inorganic carbon complexes are major

anions in fresh water

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Oxygen flux:

Diffusion

photosynthesis

O2

O2 respiration

Respiration includes bacterial respiration associated with

decomposition

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Diffusion across the air-water boundary

depends on the concentration gradientand turbulence

Turbulence increases the surface

area of the water

With more surface area, more

potential for diffusion

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How much oxygen the water holds is determined by:

Henrys Law:

Cs = KP

Cs = the saturation concentration of a gas

K = the solubility constant

P = partial pressure

Where:

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Saturationthe amount of a gas that can be

held by water in equilibrium with the

atmosphere at a given temperature, pressureand salinity

Subsaturationthe condition where the water holdsless than the saturation concentration of a particular

gas

Supersaturationthe condition where the waterholds more than the saturation concentration of a

particular gas

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Solubility of oxygen in pure water

0

2

4

6

8

10

12

14

16

0 10 20 30 40 50

Temperature (C)

Oxygen(m

g/L)

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Supersaturation occurs during

periods of intense photosynthesis

Can have flux of oxygen out of water

(by diffusion)if water is supersaturated

Can result in strong changes in

oxygen concentration in a 24 hour

period

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The distribution of oxygen in lakes is influenced by:

Productivity

Morphometry

Temperature

Mixing pattern

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Eutrophic high productivity and green

water

Lakes are classified based on productivity:

Oligotrophic low productivity and

clear water

In oligotrophic lakes, oxygen concentrations arestrongly influenced by temperature

In eutrophic lakes, oxygen concentrations aremodified by photosynthesis and decomposition

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Wetzel 2001

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Oligotrophic

lakes often

exhibit an

orthogradeoxygen curve

during

midsummer

Hypolimniontemperatures

are colder, can

hold more

oxygen

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Eutrophic

lakes often

exhibit a

clinogradeoxygen

curve by

midsummer

Decompositionin the

hypolimnion

depletes the

oxygen

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Meromictic lakes have extremeclinograde curves

Never any oxygen in the monimolimnion

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0

5

10

15

20

25

30

35

0 2 4 6 8 10 12 14

Oxygen (m g/L)

Depth(m)

Metalimnetic oxygen

maximum

Accumulation of algae

at the thermocline

0

5

10

15

20

25

30

35

0 2 4 6 8 10 12

Oxygen (mg/L)

Depth(m)

Metalimnetic oxygen

minima

Accumulation ofdecomposing

organisms at

thermocline

Heterograde oxygen curve peak at an intermediate depth

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Morphometry

What are the volumes of the epilimnion and hypolimnion?

This decomposition removes oxygen

Dead organisms fall into the hypolimnion where they decompose

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Lakes become anoxic from the bottom up:

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Most fish cannot survive at oxygen

concentrations < 2 mg/L

Winterkillsnow and ice block out sunlight. No

photosynthesis but continued respiration

Summmerkillin late summer, macrophytes dieand begin to decompose. Most common in

shallow basins.

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Flux of carbon dioxide (CO2)

CO2

CO2

Diffusion

respiration

Carbon dioxide not only dissolves in water, it reacts with it.

photosynthesis

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There is normally much more carbon dioxide in water

than expected by Henrys Law.

--most lakes are supersaturated with CO2

--like oxygen, CO2 is less soluble in warm water

Inorganic carbon occurs in several forms in

addition to CO2

Many plants and phytoplankton can only use

CO2 for photosynthesis

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CaCO3 Calcium carbonate

CO2 Carbon dioxide

H2CO3 Carbonic acidCO3

2- Carbonate

HCO3- Bicarbonate

H+ Hydrogen ion

OH- Hydroxyl ion

Ca(HCO3)2 Calcium bicarbonate

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CO2 (air) CO2 (dissolved) + H2O

When carbon dioxide enters the water:

Some of the dissolved CO2 hydrates (reacts with

water) to from carbonic acid:

CO2 + H2O H2CO3

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Carbonic acid will dissociate:

Forming bicarbonate and a free hydrogen ion

H2CO

3HCO

3

- + H+

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Bicarbonate further dissociates:

HCO3- CO3

2- + H+

to carbonate and a free hydrogen ion

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Free H+ ions will also react with free OH- ions (when

available) to form water

These reaction equations illustrate why

the carbon cycle is tied to pH

Free H+ ions produce acidity

R i

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pH ranges from 7 = alkaline (lots of OH- ions)

Review:

Measure acidity as concentration of H+

pH = - log [H+]

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These reactions get a bit more complicated:

Also have reactions that result in the formation of the

hydroxyl ion

HCO3- + H2O H2CO3 + OH

-

Bicarbonate Carbonic acid

CO32- + H2O HCO3- + OH-

Carbonate Bicarbonate

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Changes in pH can change the dominant form of

carbon in water

If change the dominant form ofcarbon, can change the pH

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Calcium bicarbonate (Ca(HCO3)2) is very soluble

Also get DIC from the watershed

Dissolution of limestone (calcium carbonate):

CaCO3

Calcium carbonate is not, so forms a precipitate

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At high pH (> ~8.5), calcium carbonate can re-

precipitate due to high photosynthesis in the

epilimnionUsually happens in late summer (warm

temperatures, recall relationship between

solubility and temperature)

Ca(HCO3)2 CaCO3 + H2O + CO2

(Solid)

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Can coat macrophytes, make them look and feel

crusty

This precipitate can accumulate at lake edges,

called marl

Can remain in suspension and cause the lakes

to look milky, called whiting

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http://earthobservatory.nasa.gov/Newsroom/NewImages/Images/seawifs_lake_mich_2001_tn.jpg

Th B ff S t

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Ability to resist changes in pH with respect to the

addition of acid is called

AlkalinityorAcid neutralizing capacity-ANC

The Buffer System

Lakes that have a lot of carbonate can resist

changes in pH with the addition of acids

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Lakes in limestone regions have high bufferingcapacity and are therefore not as impacted by Acid

Rain. Lakes on granite are highly impacted.

ANC is measured by titrating with acid

until the pH changes to 4.5

The more acid needed to reach 4.5, the more

buffered a lake is against changes in pH

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Terms to Know

pH

DIC

Henrys Law

saturation

supersaturation

subsaturation

solubility

productivity/production

oligotrophiceutrophic

othograde

clinogradeheterograde

anoxic/anoxia

winterkillsummerkill

marl

alkalinity/ANC