daTextBox=Light is important for most life in the ocean.  Photosynthetic organisms at the base of a marine food web require light, just like photosynthesizers at the base of food webs on land.  Photosynthesizers are most concentrated near the surface of the ocean that is lit by the Sun.  They cannot survive at depth, where the ocean is totally dark.
Each component of color in sunlight passes through seawater differently.  Red light doesn't go very far, while blue light goes farther.  Below 200 meters, there isn't enough light for photosynthesis to take place.
Where does the light energy go? After all, the energy contained in light doesn't just disappear.
Light disappears at depth when materials in seawater - dissolved chemicals, solids, and organisms - either reflect it out the ocean or absorb it.  At places where materials that reflect and absorb sunlight are concentrated, it becomes darker at shallower depths.
The sunlit portion of the water column is called the "photic zone".  The thickness of the photic zone varies from place-to-place.  Below the photic zone, the marine environment is dark.