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Oxygen and Water
Oxygen is an odorless, colorless gas that was discovered in 1774. When one molecule of oxygen is combined with two molecules of hydrogen (another natural element), it forms the compound, water. So at base, oxygen is important in water to establish water's very existence. A ripple effect is that all living organisms must consume water in varying amounts for their existence as well.
In addition to this foundational link to water, oxygen plays a different role when it is dissolved in water as a separate, independent substance.
Dissolved Oxygen
Contrary to popular belief, aquatic animals do not breathe the oxygen that creates the water they live in. This is because when oxygen is used to form water, the oxygen is bound to the hydrogen in such as way that it makes the oxygen unavailable for any further use -- including for respiration. Instead, fish and all other aquatic animals depend on the oxygen dissolved into the water from the air above and from aquatic plant photosynthesis. Water also becomes aerated as it tumbles in rapids and falls.
Where dissolved oxygen is produced by photosynthesis, atmospheric absorption and certain water movement, it is depleted by respiration and the decomposition of aquatic plants and animals. Natural, interrelated factors that cause dissolved oxygen levels to vary include daylight hours, water depth and seasonal changes.
Daylight Hours and Water Depth
Photosynthesis only occurs in the presence of light. So, as photosynthesis in taking place during daylight hours, dissolved oxygen levels are being increased. Conversely, during night hours, dissolved oxygen levels taper off.
These levels also vary between the upper portions and subsequent depths of water. Wherever sunlight penetrates in a body of water, photosynthesis can occur. Thus the upper levels of water have higher concentrations of dissolved oxygen, whereas the other depths steadily experience lower concentrations.
Seasonal Changes
Warmer temperatures of spring and summer increase the rates of decomposition --- which depletes oxygen. However, simultaneously, warm temperatures also increase photosynthesis --- which increases oxygen levels. On the other spectrum, ice cover, water level and flow volumes change during cooler seasons. These conditions also cause variations in dissolved oxygen concentrations throughout bodies of water.