Discription Slide 9-16
On land, oxygen is primarily produced by plants, e.g. trees and grass while in the water it many comes from algae but to some extent also from seagrass.
Second picture: Seagrass.
In a food pyramid, the lowest level consists of the smallest organism: algae (also called phytoplankton). It is these algae that produced most oxygen in the ocean.
Pictures: Algae. They appear in different forms and sizes. In order to see them one often needs a microscope since they are barely visible with the naked eye.
A second producer of oxygen are the corals. More exactly, the small plants in them.
The third source in the water is the already mention seagrass.
This food pyramid is not just important because of the oxygen but it also shows how animals control each other. At the top are the so called super predators, e.g. sharks, orcas and others. These animals feed on smaller predators which themselves feed on smaller animals and so on. At the very bottom are the phytoplankton, consumed by zooplankton and other small organism.
But independently who eats whom, the sharks are the ultimate controller due to their abundance and worldwide distribution. Picture : Caribbean reef shark.