Oxygen is produced whenever green plants grow. Plants use carbon dioxide and water to produce simple sugars and oxygen, using sunlight as the energy source. Chlorophyll, the green pigment in plants, absorbs sunlight and serves as the oxygen production site. Respiration Photosynthesis occurs only during daylight hours and only to the depths where sunlight penetrates. The amount of photosynthesis depends on the quantity of plants, nutrient availability, and water temperature.
Higher temperatures speed up the process.
Plants and animals also constantly use oxygen to break down sugar and obtain energy by a process called respiration , basically the reverse of the photosynthetic reaction as shown in Equation 2 . The combination of these two reactions largely determines the amount of oxygen and carbon dioxide present in lakes at different times of day and at different depths. At lake depths below the reach of sunlight, the only reaction that occurs is oxygen-consuming respiration. The deep hypolimnic waters of productive lakes often experience oxygen depletion. Lakes with high biological activity undergo greater fluctuations than lakes with few plants and animals.