4. Light Penetration

Question 1

What’s the spectrum for photosynthetically active radiation (PAR)? Compared to visible spectrum?

Answer 1

400-700nm
Mostly overlaps with visible light: 380nm-740nm

Question 2

What are the wavelengths over which light photons have the ability to affect the outer valence bonds of pigment molecules?

Answer 2

400-700nm

PAR spectrum

Question 3

UV wavelengths=

IR (infrared) wavelengths-

Answer 3

UV= 280-400nm
IR= 700-3000nm

Question 4

Incident solar radiation=

Differs with:

Answer 4

the solar energy that is received by a specific area over a period of time.

Differs with
- latitude
- time of year (season)
- cloud cover

Question 5

What are 3 main things that affect the PAR entering a waterbody?

Answer 5

1. sun angle (time of day). More reflection at dusk and dawn
2. Ice and snow cover: fresh snow/ white ice reflect a LOT, black ice transmits high %
3. Waves (small effect)

Question 6

T/F

Once light enters a lake, the degree of light penetration below the water surface is highly variable

Answer 6

true

Question 7

DOM=
aka DOC=

Answer 7

DOM= dissolved organic matter
DOC= dissolved organic carbon

Question 8

Penetration of PAR is regulated by 4 main things:

Answer 8

1. absorption by the water molecule (a constant effect)
2. Absorption by DOM
3. absorption & scattering by organic particles (algae etc)
4. scattering by inorganic particles (eg silt in glacial lakes)

Question 9

Once light enters a lake, a constant ____ of light is lost across subsequent, equal, depth intervals

Answer 9

proportion

Question 10

What color are deep water habitats in pure waters?

Answer 10

blue

Question 11

Light attenuation (reduction in light intensity with depth) is most pronounced along the ___ band of the PAR spectrum

Answer 11

reddish

Question 12

In brown-stained lakes/ bogs, ___ strongly absorbs incident radiation.
What PAR is being absorbed?
At deeper depths, there is a net shift towards ____ wavelengths

Answer 12

DOM

Absorption is most strong in UV and blue regions of the spectrum
–> causes a net shift towards longer reddish wavelengths at depth

Question 13

DOM acts as important “____” in natural waters by attenuating damaging UV light

Answer 13

sunscreen

Question 14

Give an important light attenuator in a highly eutrophic system

Answer 14

photosynthetic pigments

underwater spectra shifts towards green wavelengths at depth, following absorption in the blues and orange-reds by photosynthetic pigments

Question 15

What are 4 common photosynthetic pigments?

Answer 15

Chlorophyll a: all algal groups
Chlorophyll b: green algae
Phycocyanin: cyanobacteria
Phycoerythrin: cryptophytes and red algae (marine)

Question 16

Are the mechanisms of light penetration the same in streams/ rivers as they are in lakes?

Answer 16

Yes

Question 17

T/F
The factors that cause light attenuation are often the same in flowing vs still waters

Answer 17

false
often different
–> mixing in flowing waters can cause particles to have a much greater impact on light penetration

Question 18

Light penetration is typically controlled by ____ and _____ in the water column

Answer 18

DOM (dissolved organic matter)
Particles (including algae)

Question 19

Light penetration in aquatic systems is important for:
1.
2.

Answer 19

1. heat generation
2. biological function