The needs for light enforce a basic limit on the distribution of all marine organisms that undergo photosynthesis. In order for them to go on living, these organisms must stay in the upper region of the ocean where the sun energy is abundant for photosynthesis will reach them. The deepness of the photic zone is resolve by the aptitude of sunlight to infiltrate the seawater. As a result, this influenced by the variety of conditions such as the atmospheric absorption of light, the angle that between the sea surface and the sun and last, the transparency of water.
The quantity of energy that reach the surface of the sea relies on the conditions of atmosphere such as the dust, clouds and gases that suck up, reflect and spread out a portion of the inward bound solar radiation.
A part of light that makes it through the atmosphere is imitated back into space by the sea surface itself. Beneath the surface of the sea more light energy is lost through absorption of light. Substances that are dissolved, sediments that are suspended and populations of plankton further lessen the amount of light available for photosynthetic activity and cause the deepness of penetration of light to dramatically differ between the coastal and oceanic water.
In some depth, the light intensity is dim that photosynthesis is almost impossible to occur, This depth describes the bottom part of the photic zone and differs from a few meters deep in coastal water in clear tropical seas. The depth somewhere on the upper part of the bottom of photic zone, the rate of photosynthesis is stabled by photorespiration. In area where moderate and low light intensities occur, the photosynthesis of phytoplankton displays a direct relationship to light intensity. In some areas with higher light intensities, photosynthesis stop following the light intensity curve and it may stabilize or even lower down the sea surface because of photoinhibition by strong light.
The phytoplankton from various environments shows some degree of adjustments in photosynthesis to the varying of light intensities. Thus, the dispersion light intensity for any phytoplankton population modifies with changing set of the conditions in environment. The light intensities are also present among the chief groups of phytoplankton. For instance, the dinoflagellates seem to be well adjusted than diatoms to intense light. As it turns out, their relative contribution to the totality of marine primary production is much superior in tropical and subtropical regions.