What is ocean deoxygenation and what are its potential consequences?

• A. Increases dissolved oxygen in seawater leading to more marine life.
• B. Is caused by salinity alone.
• C. Decreasing dissolved oxygen in seawater due to warming and stratification; harms marine life and disrupts ecosystems.
• D. Only occurs in freshwater systems.

Answer: (C)

Ocean deoxygenation is the decline of dissolved oxygen in seawater, driven by warming and the resulting stratification of the ocean. Warmer water holds less oxygen, so as surface temperatures rise, the amount of oxygen that can stay dissolved drops. At the same time, increased warming strengthens stratification, creating more pronounced layers that mix less between the oxygen-rich surface and the deeper waters that rely on this exchange. That reduced mixing means deeper waters don’t get replenished as effectively, while respiration and microbial breakdown of organic matter consume oxygen, often in coastal zones where nutrients from land increase organic material.

The consequence is a loss of habitat for many aerobic (oxygen-using) marine organisms, leading to hypoxic or even anoxic conditions in parts of the ocean. This can shift where species live, disrupt food webs, reduce biodiversity, and impact fisheries and ecosystem services. It also affects important biogeochemical processes, including carbon and nutrient cycling, and can help expand oxygen-minimum zones over time.

The other ideas don’t fit because deoxygenation is not about more dissolved oxygen, not caused by salinity alone, and it isn’t limited to freshwater systems.