Scientists believe phytoplankton in the Earth's oceans produce between 50 and 85 percent of the planet's oxygen. The exact figure is difficult to calculate because scientists do not know how much phytoplankton exists on Earth. Phytoplankton, including algae, make oxygen by photosynthesizing sunlight.
Researchers do know how much oxygen individual phytoplankton cells give off. The reason why it is hard to calculate how many phytoplankton exist is due to their microscopic size. Most of these individual plants cannot be seen with the naked eye, yet huge algae blooms appear in the spring when more sunlight is available and nutrients rise from the ocean's depths.
Algae blooms vary in size. Sometimes, the plants just float on the surface of the ocean. Other times, phytoplankton reach more than 100 yards down into the ocean. Most phytoplankton are single-celled organisms that live in groups that make them seem like large plants.
Trees also provide a lot of the Earth's oxygen, but land only accounts for 29 percent of Earth's surface. Plus, not all land is suitable for trees. The same is true for algae and phytoplankton - not all of the Earth's oceans are plentiful with green plants. Trees, like algae, have periods of great growth and periods of decline with the change of seasons, lower temperatures and less light.