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Readings: Snucins & Gunn 2000

Lec 2: Light and Heat

I. Light and Transparency

II. Stratification:Vertical Temp. Gradients

III. Circulation

Solar RadiationAll-important influence on in-lake conditions

• Solar Spectrum: Differing wavelengths and intensities

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• Variation in the solar spectrum• PAR: Photosynthetically Active Radiation• Infrared: Main heat sourcePAR

Selective Absorption of the Solar Spectrum by 1 meter of Pure Water

100

50

0300 400 500 600 700

UV IRV B G Y O R

%Absorbed

Wave Length, nanometers

50% of remaining light is absorbed for each additional meter, yet:•30% blue light remains after 70m•6% yellow light remains after 70%•0% orange light remains after 17 m•0% of red light remains after 4 m 3

Selective Light Transparency in Different Lakes

Tahoe, CA-NV

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2

3

4

5

Depth(m)

0.1 0.5 1.0 5 10 50 100Percent Incident Light

• Transparency of water depends on:– Wave length (water is

differential in its absorption of certain wave lengths)

– Suspended materials– Dissolved materials

• Different lakes tend to have different light absorption characteristics Long, MN

Crystal, WI

Montezuma Well, AZ

Itasca, MN

Little Triste, AZ

Secchi Disk

Saguaro, AZ

Seneca, NY

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Determination of Transparency

• Transparency– Secchi Disk (20cm diameter) - measures depth

of 95% light absorption – range <5 cm - >40 m (Crater Lake, Oregon, has the greatest transparency of any North American Lake)

– Light meter typically measures in photons or calories (lakes have light profiles just as they have oxygen & temperature profiles)

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S

Quantification of Transparency Extinction Coefficient

• Based on Bouguer’s (a.k.a. Lambert’s) & Beer’s laws, where:Io = intensity of entering lightId = intensity of light at depth Z

e = base of natural logarithms (approx. 2.7)

k = extinction coefficient

• The “Extinction Coefficient” (k) is the proportion of the original light absorbed at a depth

• The proportion of light transmitted through a depth is called the “Transmission Coefficient” (k is more commonly used)

kZod eII

kZ

o

d eI

I Z

IlnIlnk Zo

“Secchi Depth”

k = 1.7 / Zsd

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Turbidity• Turbidity is a measure of water’s cloudiness• Caused by suspended materials in water• Often varies seasonally, affected by:

– Water movements– Stream discharge– Plankton populations

• Settling time for suspended materials vary:– sand: 10 cm/second (still water)– colloids: <0.5 cm/year (still water) 7

Light is Attenuated More Rapidly in Eutrophic Lakes

0 20 40 60 80 100

Light (% of incoming)

7

6

5

4

3

2

1

0

Dep

th (

m)

Olig

otro

phic

Mes

otro

phic

Eutrophic

A

1 10 100Light (% of incoming)

Olig

otro

phic

Mes

otro

phic

EutrophicB

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Photic Zone

Compensation

Depth

• Affected by water clarity• Important for system metabolism• Important habitat determinant

-Heat-Dissolved oxygen (DO)

Possible to calculate from Secchi Depth?

ddpz

k

6.4

k

100lnZ

99

Thermal Characteristics of Lakes• Light and heat represent a continuum with wave lengths

e.g. >700 nm (infrared) = heat• Water selectively absorbs in the infrared

– at 820 nm 91% absorbed within the 1st meter– 99% absorbed within the 2nd meter

• Based on the absorption of light, you would expect the following temperature profile of a body of water at uniform temperature exposed to the sun:

Temperature

Depth

1010

• Lakes generally do not show heat distributions that directly reflect the relative absorption of light with depth

• Many lakes (esp. deep)stratify during part of the year

Thermal Characteristics of Lakes

• This results in a characteristic thermal profile:

Epilimnion (upper water)Metalimnion (middle water)Hypolimnion (lower water)

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Stratification• Layering based on differences in density

(temperature or salinity)

• Stratification alters biogeochemistry and ecology

• Lake with all same temperature called isothermal

• Thermal stratification into three layers

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A Thermally Stratified Lake

0 2 4 6 8 10 12 14 16 18

Temperature (0C)

25

20

15

10

5

0

Dep

th (

m)

Epilimnion

Metalimnion (thermocline)

Hypolimnion

Defined by at least1OC / m

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Principles governing thermal stratification1. Heat enters and leaves the lake (mostly) from the surface2. Temperature affects water density3. Warmer water has a much greater difference in density per degree change than cold water

Thermal Characteristics of Lakes

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• Amictic - no mixing. Applies only to lakes that permanently are ice covered. Arctic climates only

• Cold Monomictic - Temperature never exceeds the temperature of maximum density (4OC). Ice covered from late fall through late spring, mixes all summer. In very cold climates.

• Dimictic - Spring and Fall mixing periods. Lake surface freezes in winter, lake is thermally stratified in summer

• Warm Monomictic - Lake never freezes. Mixes over winter. Stratified from early spring through late fall

• Oligomictic - Circulates irregularly. Mostly in the tropics• Polymictic - Continually circulates at low temperatures

Lakes at high elevations near the equator

Classification of Lake Mixing RegimesFunction of latitude, elevation, morphometry

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Lake Thermal Profile - Time and Depth

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Diagrammatic Representation of Dimictic Mixing Regime

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Wind

Hypolimnion

MetalimnionEpilimnion

-Wind mixes surface heat down -Density differences cause resistance to vertical mixing-The work need to mix depends on the different desities of the strata-However, much more work is needed to mix 25o to 15o vs. 15o to 5o

Why? When is mixing most likely to occur? Effect of wind fetch?

Circulation Patterns in a Stratified Lake 18

Annual Temperature Cycle of a Dimictic Lake Represented as Temperature-Depth Profiles

0O 4O 0O 4O 0O 4O 0O 4O

Depth

Temperature

SummerStratification

FallOverturn

WinterStratification

SpringOverturn

Ice

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0O4O 0O4O 0O4O 0O4O

Depth

Temperature

SummerStratification

Fall WinterMixing

Spring

20O 20O 20O 20O

Annual Temperature Cycle of a Warm Monomictic Lake

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Importance of Heat and it’s Distribution

• High heat retention – due to specific heat of water

• Most biological processes have Q10 values of 2-3

• Influence on DO concentrations (Important habitat variable)

• Determines who, when, & where re: community composition and ecosystem processes

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Horizontal Lake Zones and Biota

psammon

macrophytes

benthos

Shallow & deepwater emergents

Floatingplants

Submerged plants

Sublittoral zone

Profundal zone 22