4. Light Penetration
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What’s the spectrum for photosynthetically active radiation (PAR)? Compared to visible spectrum?
400-700nm Mostly overlaps with visible light: 380nm-740nm
What are the wavelengths over which light photons have the ability to affect the outer valence bonds of pigment molecules?
400-700nm PAR spectrum
UV wavelengths= IR (infrared) wavelengths-
UV= 280-400nm IR= 700-3000nm
Incident solar radiation= Differs with:
the solar energy that is received by a specific area over a period of time. Differs with - latitude - time of year (season) - cloud cover
What are 3 main things that affect the PAR entering a waterbody?
• sun angle (time of day). More reflection at dusk and dawn • Ice and snow cover: fresh snow/ white ice reflect a LOT, black ice transmits high % • Waves (small effect)
T/F Once light enters a lake, the degree of light penetration below the water surface is highly variable
true
DOM= aka DOC=
DOM= dissolved organic matter DOC= dissolved organic carbon
Penetration of PAR is regulated by 4 main things:
• absorption by the water molecule (a constant effect) • Absorption by DOM • absorption & scattering by organic particles (algae etc) • scattering by inorganic particles (eg silt in glacial lakes)
Once light enters a lake, a constant ____ of light is lost across subsequent, equal, depth intervals
proportion
What color are deep water habitats in pure waters?
blue
Light attenuation (reduction in light intensity with depth) is most pronounced along the ___ band of the PAR spectrum
reddish
In brown-stained lakes/ bogs, ___ strongly absorbs incident radiation. What PAR is being absorbed? At deeper depths, there is a net shift towards ____ wavelengths
DOM Absorption is most strong in UV and blue regions of the spectrum –> causes a net shift towards longer reddish wavelengths at depth
DOM acts as important “____” in natural waters by attenuating damaging UV light
sunscreen
Give an important light attenuator in a highly eutrophic system
photosynthetic pigments underwater spectra shifts towards green wavelengths at depth, following absorption in the blues and orange-reds by photosynthetic pigments
What are 4 common photosynthetic pigments?
Chlorophyll a: all algal groups Chlorophyll b: green algae Phycocyanin: cyanobacteria Phycoerythrin: cryptophytes and red algae (marine)
Are the mechanisms of light penetration the same in streams/ rivers as they are in lakes?
Yes
T/F The factors that cause light attenuation are often the same in flowing vs still waters
false often different –> mixing in flowing waters can cause particles to have a much greater impact on light penetration
Light penetration is typically controlled by ____ and _____ in the water column
DOM (dissolved organic matter) Particles (including algae)
Light penetration in aquatic systems is important for: 1. 2.
• heat generation • biological function