In this zone, there is enough light for photosynthesis to take place. In fact, most of the life in the ocean is found in this zone and there is heaps of food, although it is the smallest ocean zone in terms of volume of water.
Most of the oxygen is produced by phytoplankton. Phytoplankton are the first link in the food chain in the ocean. Because of this food source, many animals also live in this zone. Photosynthesis is a process in which sunlight and carbon dioxide gas are converted into food (chemical energy contained in carbohydrates) and oxygen. We know about this with plants on land but it is just as an important process under water. They call it the “biological pump” here. The taking of light and carbon dioxide and converting it to oxygen.