The phytoplankton community drives the dissolved oxygen, carbon dioxide and pH cycles and directly affects the concentration of nitrogenous water products such as ammonia and nitrite. Phytoplankton produce oxygen during photosynthesis. The oxygen is then released into the water. Dissolved oxygen concentrations reach their highest concentrations in the late afternoon. The carbon dioxide cycle is the opposite of that of oxygen.
Phytoplankton take up carbon dioxide during the light hours for photosynthesis.
The total amount of photosynthesis in the pond overshadows the total pond respiration during the day. Therefore, there is little or no carbon dioxide in the pond in the afternoon and evening. Respiration of all pond organisms, including phytoplankton, results in the release of carbon dioxide. This continues during the dark hours while photosynthesis is not occurring. The carbon dioxide cycle directly affects the pH cycle.