What is dissolved oxygen?
Dissolved oxygen is the amount of oxygen gas dissolved into liquid water. Scientists sometimes abbreviate dissolved oxygen as D.O. Oxygen isn’t the only gas that can dissolve into water, but it is the one most important for life. Just like land organisms depend upon oxygen for respiration, so do aquatic organisms. Aquatic critters have adaptations which help them to get the oxygen they need to breathe. Some have little snorkels (like mosquito larvae), to help them breathe near the surface. Some have gills, and some have breathable membranes on their bodies which take in oxygen. Not all aquatic organisms come to the surface to get the oxygen they need to survive (like newts). Aquatic insects like dragonfly larvae and tadpoles need to use the dissolved oxygen in the water instead.
How does the oxygen get dissolved in the water?
Well, there are several ways. Even on the surface of a seemingly still pond, air molecules are bouncing around and bombarding the surface of the water. This aerial assault can diffuse oxygen gas into the water, though it is not the fastest way. Mechanical aeration, from natural sources such as water being churned up from rapids, spilling over a waterfall or cascading over rocks will infuse oxygen into the water simply by carrying bubbles of oxygen into the water where they are dissolved.
Oxygen can also be dissolved into the water as a byproduct of photosynthesis. Aquatic plants use sunlight and water to produce sugar as a carbon-rich food source. Green plants (plants with chlorophyll) produce oxygen gas as a waste product in the process. Aquatic organisms then use this dissolved oxygen “waste” why are we going this direction in the discussion? If we just call it a “byproduct” in the first place, this seems to clear up the whole concept. in respiration. What bubbles to the surface or comes out of solution is fair oxygen-game for land lubbers like us. Take a deep breath… (Thank a plant). Now exhale and return the favor. What a nice exchange!