Which processes control dissolved oxygen in aquatic systems?

• A. Photosynthesis
• B. Respiration
• C. Decomposition
• D. All of the above

Answer: (D)

Dissolved oxygen in aquatic systems is set by a balance between oxygen-producing and oxygen-consuming processes. Photosynthesis by algae and aquatic plants adds oxygen to the water, especially during daylight, raising DO levels. On the other hand, respiration by fish, zooplankton, and microbes consumes oxygen as they metabolize organic material, lowering DO. The breakdown of organic matter by microbes (decomposition) is another major oxygen sink, linking nutrient and organic loading to oxygen depletion. Because all three processes influence DO, the overall control is by their combined effects. This also explains diurnal patterns: DO tends to rise in the day with photosynthesis and fall at night when photosynthesis stops but respiration and decomposition continue. Environmental factors like temperature and mixing modulate the extent, but the fundamental controls are these processes.