Ocean Water: Optics
Most of the organisms in the ocean depend on sunlight. Plants and
bacteria, such as kelp, seagrass and photosynthetic plankton, use light to make energy through a process called photosynthesis.
These are then eaten by larger animals, which are in turn eaten
by larger animals and so on. Remember the food chain? Sunlight is
the basis (beginning) for this food chain. Sunlight also warms the
ocean's surface. This is important because it makes the water warm
enough for animals to live in it, and it is a driving force for
As sunlight enters the ocean, it starts to be absorbed. The ocean can
be broken down into three vertical zones based on how much light
it receives. The first zone, or euphotic zone,
extends from the water's surface to about 50 meters depth, depending
on the time of year, the time of day, the clarity (clearness) of
the water and the presence of clouds. This is the part of the water
column where there is still enough light for plants to photosynthesize.
All plankton, kelp forests and seagrass beds are found in the euphotic
by E. Widder/ HOBI
The next zone is the dysphotic zone,
which extends from about 50 meters, or wherever the euphotic
zone ends, to about 1,000 meters. In this zone, there is
enough light for organisms to see, but it is too weak for photosynthesis
to happen. If we were to venture into the dysphotic zone, we could
watch the visible light disappear as we traveled deeper. Once we
reached the aphotic zone, there would
be no light. This zone extends from about 1,000 meters depth to
the ocean bottom. Animals in this zone are rare, but they do exist.
Think about the thriving hydrothermal vent communities, which live
and prosper without sunlight.
Some animals in the aphotic zone create their own light. This is called bioluminescence.