Oxygen levels refer to the concentration of oxygen in the atmosphere or in water. In the air, oxygen makes up about 21% of the total volume. This is essential for the survival of most living organisms, as they rely on oxygen for respiration. In aquatic environments, oxygen levels can vary significantly. Fish and other aquatic life depend on dissolved oxygen in water, which can be affected by factors like temperature, salinity, and pollution. Maintaining healthy oxygen levels is crucial for the well-being of ecosystems both on land and in water.