Oxygen is a gas that is very important for living things because we need it to breathe. It is found in the air we inhale and helps our body produce energy. Full definition
This is thought to be due to higher level of oxygen in the atmosphere at that time. (sciencedaily.com)
Also, water is crucial for proper blood circulation in the body, which means that when the body is well hydrated, you have greater levels of oxygen in the bloodstream. (fitnessandpower.com)
This causes excess growth of plant matter and depletion of oxygen levels in the water. (americanhardwood.org)