2.5 Light in the Ocean
The Fading Light
Light is essential for photosynthesis and vision, but it's rapidly absorbed and scattered in seawater. Only the upper ~200m receives enough light for photosynthesis (euphotic zone). Below 1000m, the ocean is in perpetual darkness.
Light Attenuation
Beer-Lambert Law
$$I(z) = I_0 e^{-Kz}$$
I = intensity, Iā = surface intensity, K = attenuation coefficient, z = depth
Absorption
Light energy converted to heat. Water absorbs red wavelengths most strongly.
Scattering
Light redirected by particles and water molecules. Makes water appear blue.
Light Zones
Sufficient light for photosynthesis. All primary production occurs here.
Twilight zone. Some light for vision, none for photosynthesis.
No sunlight. Only bioluminescence. 90% of ocean volume.
Ocean Color
Why is the ocean blue?
Water absorbs red wavelengths, scatters blue. Clear ocean water appears deep blue.
Why is some water green?
Phytoplankton contain chlorophyll, which absorbs blue and red, reflects green.
Satellite Ocean Color
Satellites measure ocean color to estimate chlorophyll concentration, phytoplankton abundance, and primary productivity globally.
Bioluminescence
Many marine organisms produce their own light through chemical reactions. Bioluminescence is found in bacteria, dinoflagellates, jellyfish, fish, and squid.
Defense
Startle predators, burglar alarm effect
Predation
Lure prey (anglerfish), illuminate targets
Communication
Species recognition, mating signals