Which of the following is not a primary source of dissolved oxygen in outdoor systems?

• A. photosynthesis
• B. wind/wave action
• C. aeration
• D. fish respiration

Answer: (D)

Dissolved oxygen in outdoor systems comes from processes that add oxygen to the water or move it into the water from the air. Photosynthesis by aquatic plants and algae releases oxygen as a byproduct of converting light energy into chemical energy. Wind and wave action mix atmospheric air into the water, increasing the rate of gas exchange at the surface. Aeration devices actively introduce air or increase the dissolved oxygen level by mechanical means. In contrast, fish respiration uses oxygen to fuel metabolism; it does not add oxygen to the water. So the process that is not a source of dissolved oxygen is fish respiration, which can actually reduce DO, especially when photosynthesis is low.