Density differences results in two layers being formed within a pond. Oxygen Dissolved oxygen refers to oxygen gas that is dissolved in water. Fish need oxygen just like land animals do. Fish absorb oxygen directly into their bloodstream using their gills. Fish cannot live without oxygen.
Oxygen depletion is the cause of many fish kills.
Low oxygen stress is the cause of disease outbreaks. The amount of oxygen in water decreases as temperature and altitude increases. As temperatures increase, fish metabolism will increase so they consume more oxygen. So both the increase in temperature and increase in fish metabolism may cause oxygen depletion in the summer. Oxygen is produced during the day when sunlight shines on the plants in the water through photosynthesis.