The primary reason for the productivity in the Photo zones is due to the influence of major factors like the Nutrients, light, and grazing of animals. The rate at which the plants are eaten by herbivorous.
Upwelling of cool, water from the bottom and interaction of warm waters from the above that derives nutrient-rich water to the surface. These upwelling can be consciously be caused by the winds and the depth of light penetrates down the layer of the water.
As the intensity of water decrease with depth and with the concentration of nutrient the photic zone becomes shallower.
