text: There is a diurnal pattern since plants also have oxygen-consuming metabolism. The morning has the least available oxygen because there has been no production during the darkness, but there sure has been consumption. The afternoon has the highest diurnal oxygen levels (assuming evenly spread cloud cover), as the setting sun will reach fewer plant surfaces than the midday sun. Come the darkness, only consumption remains, except for buffering in highly productive water bodies (which may build up an oxygen concentration greater than equilibrium with the air, and slowly return to equilibrium over night).