The amount of oxygen that can be dissolved in water depends on several factors, including: water temperature, the amount of dissolved salts present in the water and atmospheric pressure. Cold water can hold more dissolved oxygen than warm water. In winter and early spring, when the water temperature is low, the dissolved oxygen concentration is high. In summer and fall, when the water temperature is high, the dissolved-oxygen concentration is often lower. The biological importance of the presence of dissolved Oxygen and Carbon dioxide in river water is that the aquatic life can use the oxygen from the water for their respiration and exhale the carbon dioxide which goes into the water. Oxygen dissolves in surface water due to the aerating action of winds. Oxygen is also introduced into the water as a byproduct of aquatic plant photosynthesis. When dissolved oxygen becomes too low, fish and other aquatic organisms cannot survive.

The biological importance of the presence of dissolved Oxygen and Carbon dioxide in river water is that the aquatic life can use the oxygen from the water for their respiration and exhale the carbon dioxide which goes into the water. The amount of oxygen that can be dissolved in water depends on several factors, including water temperature, the amount of dissolved salts present in the water and atmospheric pressure. Total dissolved gas concentrations in water should not exceed 110 percent. Concentrations above this level can be harmful to aquatic life. Fish in waters containing excessive dissolved gases may suffer from "gas bubble disease"; however, this is a very rare occurrence. It is essential for the survival of fish and other aquatic organisms. Oxygen dissolves in surface water due to the aerating action of winds. Oxygen is also introduced into the water as a byproduct of aquatic plant photosynthesis. When dissolved oxygen becomes too low, fish and other aquatic organisms cannot survive. Dissolved oxygen concentrations are constantly affected by diffusion and aeration, photosynthesis, respiration and decomposition. While water equilibrates toward 100% air saturation, dissolved oxygen levels will also fluctuate with temperature, salinity and pressure changes. Dissolved oxygen and carbon dioxide are vital for marine life. Marine plants use dissolved carbon dioxide, sunlight and water to make carbohydrates through the process of photosynthesis. This process releases oxygen into the water. Dissolved oxygen concentration will increase as pressure increases. This is true of both atmospheric and hydrostatic pressures. Water at lower altitudes can hold more DO than water at higher altitudes.