3 mg/l are harmful to most of the water life.
In the atmosphere, about one out of 5 molecules is oxygen; in water, about one out of every 10 million molecules is oxygen. Mixing of the water and air, such as in churning waters, increases the amount of dissolved oxygen. Photosynthesis by aquatic plants also makes dissolved oxygen increase. Dissolved oxygen is taken in by aquatic life such as fish, plankton, and bacteria that decompose materials. Usually dissolved oxygen levels are lower in warmer water. For example, at 25 degrees Celsius dissolved oxygen solubility is 8.3 mg/L, whereas at 4 degrees the solubility is 13.1 mg/L.
Dissolved oxygen levels can vary not only with different water temperatures, but also with the rate of flow, water depth, altitude and seasonal and daily cycles. Fast moving water contains more dissolved oxygen than slow moving water. Shallow water has a greater ratio of surface water to total volume, so it makes sense that it would have more dissolved oxygen compared to deeper water. Water at high altitudes holds less oxygen because of the thinner air. Water has the most dissolved oxygen in late afternoon. Throughout the night oxygen continues to be consumed by aquatic animals and decaying matter, while photosynthesis stops. When the sun rises plants begin to photosynthesize again and the dissolved oxygen levels increase throughout the day reaching a high point before nightfall and then falling to a minimum before dawn.
Too much dissolved oxygen is never a problem in naturally flowing streams but it can be a problem in streams that have been altered by humans. Turbulent waters from hydroelectric dams create supersaturated conditions that can be dangerous to fish. Fish can develop a condition called “Gas Bubble” disease by swimming through turbines to get upstream. When fish swim through these waters, the oxygen in their blood level rises. Then when they leave these areas, the oxygen in the blood stream reverts to gas form, creating bubbles inside the blood vessels. This can cause serious harm to fish.
