Dissolved Oxygen and cycles

Oxygen dissolves into water from two sources: the atmosphere and from plants in the water. The primary source of oxygen for a pond is from microscopic algae (phytoplankton) or submerged plants. The CO2 and O2 levels alternate and the O2 levels are high when the sun is out and low at night. Photosynthesis is the reason why these levels are affected by the time of day. When the sun is out, plants are photosynthesizing and producing oxygen.