The most striking thing as you descend into the sea is the fact that it progressively becomes darker. The "white" daylight which permits us to see on dry land is, in fact, composed of a mixture of colours, as can be seen in a rainbow. In water, however, red light is absorbed in the top 5 metres (15 feet) of the sea surface ; so no red light can be seen at greater depths. At a depth of 15 metres (45 feet), green light has disappeared as well and only blue light is left. By a depth of 50 metres (150 feet) below the surface, conditions are really quite dim and there is complete darkness at depths greater than 150 metres (450 feet).
Plants disappear in the right photograph as compare to the left one when
the available light is insufficient. So light is essential for the life-processes of
all green plants. At a depth of 20 metres (60 feet) only 5 percent of the sunlight
arriving at the surface is still available for plants.
Coastal Biology / Underwater Prairies