Oxygen brings the energy of life. In a biological pool it’s the plants that produce a lot of oxygen. They need daylight and clear water.
As the oxygen content in the water is limited by the water temperature, there is a limited amount of oxygen soluble in water. Solved oxygen in water is not visible. But when the amount of oxygen exceeds 100%, this is the maximum soluble amount, the oxygen bubbles are released like the carbon dioxide in a glass of sparkling water. This situation, often observable in biological pools, shows the photo.