Photic Zone

The photic zone, also called the euphotic or limnetic zone, is the volume of water where the rate of photosynthesis is greater than the rate of respiration by phytoplankton. Phytoplankton are microscopic plants living suspended in the water column that have little or no means of motility. They are primary producers that use solar energy as a food source. The compensation point, where photosynthesis equals respiration, defines the lower limit of the photic zone. Above this point, the phytoplankton population grows rapidly because there is abundant sunlight to support fast rates of photosynthesis. Below the compensation point, the intensity of sunlight is too low and the rate of respiration is faster than the rate of photosynthesis, and therefore the phytoplankton cannot survive. The photic zones of the world's lakes and oceans are critically important because the phytoplankton, the primary producers upon which the rest of the food web depends, are concentrated in these zones.