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Light in Water

1. Why study the amount of light penetration inlakes?

2. What 5 things determine how much lightreaches a lake?

3. What factors influence light attenuation?

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Light provides energy for photosynthesisphytoplanktonmacrophytes

Sunlight heats the water

Zooplankton use light for orientation

Vision of predators

Why study light in lakes?

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Properties of light(or more precisely, solar radiation):

In photochemical reactions (photosynthesis and vision)it is necessary to consider light as

discrete packets of energy (quanta)

In the atmosphere and in waterits behavior is best explained by assuming a wave

A quantum of light is called a photon

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n = frequency (Hz)

The energy (E) content of a photon varies withwavelength:

E = hn

h= Plancks constant (6.625 x 10-34 J sec)

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Frequency and wavelength are related by:

n = c/l

l = wavelength

c = speed of light

2.9972 x 1010 (cm/s) in air2.2492 x 1010 (cm/s) in water

n = frequency

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Spectrum of solar radiation:Wetzel 2001

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Ultraviolet (UV) 100400 nm

UV-C < 280 nmUV-B 280-320 nmUV-A 320-400 nm

damaging effects on organisms

Short wavelength = high frequency =high energy photons

Small fraction (~ 3 %) of the daily energy distribution

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Photosynthetically active radiationPAR

Visible 400-700 nm

VBGYORshort to long

~ 46 % of the daily energy distribution

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Infrared radiation 700-3000 nm

transfers heat to the surface waters

long wavelength = low frequency =low energy photons

~ 51 % of the daily energy distribution

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(1) Latitude

How much light reachesthe surfaceof the lake depends on 5

things:

(2) Season

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(3) Time of day

How much light reaches the surfaceof the lake depends on 5 things:

(4) Altitude

(5) Meteorological conditions

Earth

AtmosphereThe atmosphere

scatters and absorbslight

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Some is reflectedThe rest enters the lake

Of the light that the reaches thesurface of the lake:

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Surface characteristics of the water

What determines howmuch light is reflected?

Angle of incidence

Surface reflection (%)

Can increase reflection 30-40%

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Absorbed as heat

What happens to the light that enters the lake?

Scattered by suspended particles

Transferred to other energy sources (photosynthesis)

Deeper in the lake, there is less radiant energythis is called light attenuation

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In pure water (no scattering, no photosynthesis)for every given wavelength

a constant fraction of light is absorbed (transferred to heat)

with each increase in depth

In this example, the attenuation coefficient is 0.1 (i.e. 10%)

% of surface light

0

5

10

15

20

25

30

35

0 25 50 75 100

depth(m)

% of surface light

0

5

10

15

20

25

30

35

1 10 100

depth(m)

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Where:

I0

= light intensity at the lake surfaceIz = light intensity at depth zkd = vertical attenuation coefficientz = depth (m)

This decrease of amount of light with depthcan be estimated by:

Iz= I0e-k

dz

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I0 and Id are measured with a light metersolve for kd

kd =

ln I0 ln Iz

z

This is exponential decayand kd is the rate of decay

The higher the kdthe faster light is attenuated

with depth

% of surface light

0

5

10

15

20

25

30

35

0 25 50 75 100

depth(m)

10%

20%

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Water is acolor filter

Red isabsorbed most

quickly

Bluepenetrates the

furthest

h l h f b

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Dissolved organic compounds in the water have high

absorption of UV and blue

This relationship is imperfect in nature because:

Sunlight is not monochromatic

Lake water has suspended particles that scatterand absorb the lightThey are often concentrated into discrete layers

C i f

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Comparison ofcomposite kd among

lakes

Kalff 2002

Which of these lakes is

the clearest? How doyou know?

Why is it importantto know how far

light penetrates in

a particular lake?

S hi di

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Secchi disc:

Named for an Italian priest/scientist (1800s)

20 cm disk, either all white or black and white

Secchi depth correspondsroughly to 10% of surface light

Depth at which the disk is no longervisible from the shady side of the boat

(without sunglasses)

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Varies with lakes and with season

Crater Lake 40 m

Lake Superior 20 mTypical small lake 1-8 mProductive pond a few cm

ljea.org/ljsecchi.html

http://ljea.org/ljsecchi.htmlhttp://ljea.org/ljsecchi.html

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Terms and Concepts to Know

PhotonUltraviolet radiationInfrared radiation

Angle of incidenceLight attenuationExponential decayKdSecchi disc/Secchi depth