The photic zone is the surface layer of a body of water. It has enough light for organisms to photosynthesize. In the ocean, around 90% of the life can be found in this zone. The depth of this zone can vary tremendously, depending on a number of different factors. Depth of the photic zone is measured with a device called a Secchi disc; the disk is dropped in the water and observers make note of the point at which the pattern on the disc cannot be clearly distinguished.
Also called the euphotic zone, the photic zone harbors organisms which need light for energy, along with organisms which rely on these organisms for sustenance. This includes plants, bacteria, algae, and many animals. Some inhabitants of this area have developed creative adaptations like pigments which help them utilize sunlight more effectively, so that they can survive in lower light levels.
In the ocean, beyond the photic zone lies a phantasmagoria of curious ocean creatures which inhabit the aphotic zone. These creatures have adapted to live in total darkness, and can withstand the extreme cold and pressure found in the depths of the ocean. One area of surprising activity in the deep ocean is around hydrothermal vents, which support thriving communities of organisms which use chemosynthesis for energy. These communities were discovered by accident by researchers who were astounded to find organisms flourishing in such extreme conditions.
One thing which can impact the depth of the photic zone is turbidity. Turbidity is determined by disturbances in the water such as silt and mud which may darken the water, reducing the photic zone's depth significantly. Some bodies of water are naturally turbid, while others tend to be more clear. The number of organisms in the water can also make a difference. Huge colonies of microorganisms like algae can literally cloud the are, reducing available light.
Researchers are naturally interested in the photic zone since it can provide important information about ocean health. Protection of this zone is important for fisheries and other industries which use the ocean, and for the survival of the environment as a whole. Those microorganisms which inhabit this area, for example, account for a sizeable percentage of the world's carbon dioxide consumption, respiring oxygen as a waste product and keeping the composition of gases in the Earth's atmosphere stable. Phytoplankton are believed to be responsible for as much as 90% of the world's oxygen production.