How the Ocean Produces Oxygen

Scientists estimate that the ocean produces about half of the oxygen generated on Earth. Most of this oxygen comes from plankton, including drifting plants, algae, and some bacteria. These organisms use sunlight to make food, a process called photosynthesis. One tiny bacterium, Prochlorococcus, produces as much as 20% of the oxygen in the biosphere, more than all tropical rainforests together.

It is difficult to measure exactly how much oxygen the ocean produces because production levels keep changing. Satellite images help researchers follow plankton and estimate how much photosynthesis happens at sea, but they do not show everything. Plankton levels change with the seasons and respond to nutrients, water temperature, and other conditions. Oxygen levels at some locations also change during the day and with the tides.

Although the ocean produces at least half of Earth's oxygen, marine life uses roughly the same amount. Animals need oxygen to breathe, and both plants and animals use it to get energy from food. As dead plants and animals decay in the sea, this process also uses oxygen. People and other land animals get most of the oxygen they breathe from the atmosphere, where oxygen has built up over hundreds of millions of years.

A problem can occur when an algal bloom ends and the algae decay, using oxygen faster than it can be replaced. This can create areas with very low oxygen, called hypoxic zones or dead zones, where there is not enough oxygen for most marine life. NOAA researchers study and forecast algal blooms and low-oxygen areas to help reduce harm to marine ecosystems and people.

Why can an algal bloom lead to an area where most marine life cannot survive?