The highest net primary productivity in terrestrial environments occurs in swamps and marshes and tropical rainforests; the lowest occur in deserts. Net primary production is the rate at which all the autotrophs in an ecosystem produce net useful chemical energy. As noted, it is equal to the difference between the rate at which the plants in an ecosystem produce useful chemical energy (GPP) and the rate at which they use some of that energy during respiration. The sunlit zone of the ocean is called the photic zone (or euphotic zone). This is a relatively thin layer (10-100 m) near the ocean's surface where there is sufficient light for photosynthesis to occur. In the deep ocean, productivity is limited by availability of light.