What is the largest natural source of dissolved oxygen (DO) in aquatic habitats?

The largest natural source of dissolved oxygen (DO) in aquatic habitats is photosynthesis by aquatic plants. During the process of photosynthesis, these plants, including macroalgae and phytoplankton, absorb carbon dioxide and release oxygen as a byproduct. This process is crucial for aquatic ecosystems as it not only contributes significantly to the overall oxygen concentration in the water but also supports the survival of aquatic animals, which rely on dissolved oxygen for respiration.

While diffusion from the atmosphere can contribute to dissolved oxygen levels, it is typically not as significant as the oxygen produced through photosynthesis, especially in well-lit surface waters where photosynthetic organisms are abundant. Algae also play a vital role in oxygen production, but they are a subset of the broader group of aquatic plants. Volcanic activity, although it can affect oxygen levels in water through various geological processes, does not contribute to dissolved oxygen in the same way that photosynthesis does. Thus, the primary source of dissolved oxygen in aquatic environments comes directly from the photosynthetic activity of aquatic plants.