How do photosynthesis and respiration change the amount of carbon dioxide in seawater?

Photosynthesis and respiration are fundamental biological processes that significantly influence the concentration of dissolved carbon dioxide (CO₂) in seawater.

• Photosynthesis: Marine organisms such as phytoplankton and algae perform photosynthesis in the sunlit upper layers of the ocean. During this process, they consume dissolved CO₂ to create organic compounds and tissues for growth, effectively removing carbon from the water and sequestering it.123
• Respiration: Conversely, marine organisms—including zooplankton and other consumers that feed on photosynthetic producers—release CO₂ back into the seawater through the process of respiration.24
• Decomposition: When marine organisms die, their organic matter sinks. The subsequent decomposition of this material by microorganisms releases CO₂ back into the water column, often at greater depths.124

These processes are part of a complex cycle that, alongside atmospheric exchange and the formation of calcareous shells, regulates the total inorganic carbon content and the pH of the ocean.24