Oceans Produce Half of the World’s Oxygen

Caitlin Dempsey


The world’s oceans play a crucial role in sustaining life on our planet. Not only do they provide a habitat for countless species, but they also contribute significantly to the amount of oxygen in the atmosphere.

According to researchers, the oceans are responsible for producing over half of the oxygen produced in the world.

Why are the oceans producing oxygen?

Photosynthesis is the primary process responsible for oxygen production in the oceans. This process occurs when tiny plants called phytoplankton convert carbon dioxide and sunlight into energy.

A view under the microscope showing phytoplankton.
A view under the microscope showing phytoplankton. Photo: Seth Benson, USGS, public domain.

What are phytoplankton?

Phytoplankton are tiny, photosynthetic organisms that are found in the Earth’s oceans, lakes, and rivers.

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Phytoplankton are single-celled or colonial organisms that are typically less than 0.02 inches (0.5 millimeters) in size. They are capable of photosynthesis, which is the process of converting light energy into chemical energy that can be used by the cell.

There are many different types of phytoplankton, each with their own unique characteristics and ecological roles. Some common types of phytoplankton include diatoms, dinoflagellates, and cyanobacteria.

How many plankton are there in the oceans?

According to researchers with MIT, there are a “billion billion billion phytoplankton in the world’s ocean.”

How phytoplankton contribute oxygen

As they photosynthesize, phytoplankton release oxygen into the water. This oxygen then diffuses into the atmosphere, contributing to the world’s oxygen supply.

Other sources of ocean oxygen

A girl with a white sweatshirt and black leggings dances in the shallow water of the beach by the ocean. Her back is to the camera and the sun is shining with a clear blue sky.
The world’s oceans are a major source of oxygen thanks to the presence of phytoplankton and other photosynthesizing organisms. Photo: Caitlin Dempsey.

While phytoplankton is the main source of the ocean’s oxygen production, other marine organisms such as seaweed and algae also contribute to oxygen production through photosynthesis.

What affects the ability of oceans to produce oxygen?

The amount of oxygen produced by the oceans varies depending on several factors.

Ocean water temperature

One of the most significant factors is the temperature of the water. Colder water holds more dissolved oxygen, which means that areas with colder water tend to have higher oxygen concentrations.

Map of ocean heat content in the upper ocean (from the sea surface to a depth of 700 meters, or 2,300 feet) for 2017 relative to the 1993–2017 baseline. Source: NASA.
Map of ocean heat content in the upper ocean (from the sea surface to a depth of 700 meters, or 2,300 feet) for 2017 relative to the 1993–2017 baseline. Source: NASA.

Conversely, warmer water holds less dissolved oxygen, which can lead to areas of low oxygen known as hypoxic zones.

As the world’s oceans warm due to climate change, this will adversely affect the ability of the planet’s largest source of water to hold oxygen.

Nutrient availability in the oceans

Another factor that can affect oxygen production in the oceans is nutrient availability.

Phytoplankton require certain nutrients, such as nitrogen and phosphorus, to grow and photosynthesize. If these nutrients are in short supply, phytoplankton growth and oxygen production can be limited.

Satellite image of an algae bloom off the coast of New York and New Jersey, August 3, 2015.
Every day, approximately 1.7 billion gallons of treated sewage is emptied into the ocean via the inlet connecting Sandy Hook, New Jersey and Rockaway, New York, contributing to algae blooms. This algae bloom occurred on August 3, 2015. Image: Landsat 8, NASA, public domain.

On the other end of the spectrum, excessive nutrient input, often caused by agricultural runoff or wastewater discharge, can lead to algal blooms and oxygen depletion in certain areas (which can lead to dead zones).

Cyanobacteria, also known as blue-green algae, are a type of phytoplankton. Cyanobacteria are able to fix atmospheric nitrogen, which means they can convert nitrogen gas into a form that is usable by other organisms.

Cyanobacteria can form large blooms, which can have harmful effects on aquatic ecosystems.

Increase in human activities

Human activities such as fossil fuel burning and deforestation also contribute to decreased oxygen levels in the atmosphere.

A view over the South Pacific Ocean from the International Space Station. Photo: NASA, public domain
A view over the South Pacific Ocean from the International Space Station. Photo: NASA, public domain

As carbon dioxide levels increase, the oceans absorb more of this gas, which can lead to ocean acidification. This process can reduce the ability of phytoplankton to photosynthesize, which in turn reduces the amount of oxygen produced by the oceans.

The oceans help to regulate the Earth’s climate

In addition to producing oxygen, the oceans also play a critical role in regulating the Earth’s climate.

The oceans act as a massive heat sink, absorbing and storing large amounts of heat from the sun. This heat is then transported by ocean currents, which can have a significant impact on regional and global weather patterns.

The world’s oceans are threatened

A satellite image showing runoff into the ocean after a storm off the coast of California. The oceans have light green swirls from the runoff.
Activities like climate-driven wildfires, deforestation, and flooding can increase soil runoff into the oceans. Post-storm runoff of soil off the coast of California on January 17, 2023. Image: Aqua satellite, NASA, public domain.

Despite their importance, the oceans are facing numerous threats from human activities.

Climate change, overfishing, pollution from such sources as plastic waste, runoff, and sewage, and habitat destruction are just a few of the factors that are impacting the health of our oceans. These threats not only impact marine ecosystems but can also have far-reaching consequences for the world’s planetary health.


Breitburg, D., Levin, L. A., Oschlies, A., Grégoire, M., Chavez, F. P., Conley, D. J., … & Zhang, J. (2018). Declining oxygen in the global ocean and coastal waters. Science359(6371), eaam7240. DOI: 10.1126/science.aam7240

Dutkiewicz, S., & Krol, A. (2022, March 14). Phytoplankton. MIT Climate Portal. https://climate.mit.edu/explainers/phytoplankton

Evidence points to widespread loss of ocean oxygen by 2030s. (n.d.). NSF – National Science Foundation. https://www.nsf.gov/news/news_summ.jsp?cntn_id=138396

How much oxygen comes from the ocean? (23, February 1). NOAA’s National Ocean Service. https://oceanservice.noaa.gov/facts/ocean-oxygen.html


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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.