What is the importance of dissolved oxygen in water? How does oxygen reach water?

The concentration of dissolved oxygen in water is a useful tool for the support of aquatic life. The fish growth is inhibited if the dissolved concentration of oxygen in water is below 6 ppm.
Oxygen reaches water through two sources:
The first source is that oxygen dissolves at the surface of the water from the atmosphere. Still, water takes up oxygen slowly whereas turbulent water takes it up more rapidly since bubbles are often submerged.
The second source is from photosynthesis, where there are many aquatic green plants present, the water often becomes super saturated with oxygen during daylight. At night, photosynthesis stops but the plants continue to respire and actually reduce the amount of dissolved oxygen.