Oxygen availability refers to the presence and concentration of oxygen in a given environment, which is essential for the survival of most living organisms. In the atmosphere, oxygen makes up about 21% of the air we breathe. It is produced primarily by plants through the process of photosynthesis, where they convert sunlight, carbon dioxide, and water into oxygen and glucose. In aquatic environments, oxygen availability can vary significantly. Oxygen is dissolved in water and is crucial for the survival of fish and other aquatic life. Factors such as water temperature, salinity, and the presence of organic matter can influence the levels of dissolved oxygen, impacting the health of ecosystems like lakes, rivers, and oceans.