The amount of oxygen dissolved in water at a certain time, expressed in ppm mg/L. If your objective is to ascertain the quality of your water, analyzing the dissolved oxygen is one of the most effective parameters in doing so. DO is indicative and extremely important to the health of aquatic organisms such as fish, sharks, and other living matter. DO arises due to the prevalence of high speed winds that produces a lot of oxygen which dissolves in surface water. Oxygen can also become incorporated with the water as a byproduct of aquatic plant photosynthesis.