Dissolved Oxygen | Introduction
Introduction
What turns a body of water into a "dead zone" where nothing can live? One condition that can wipe out most living things in a stream, river, or lake is a low level of dissolved oxygenOxygen molecules that have dissolved in water.. The term dissolved oxygen refers to molecules of oxygen that have been dissolved in water. Some of these molecules enter the water from the surrounding air, especially if the water tumbles over falls and rapids. Other dissolved oxygen in the water is a "by-product" of photosynthesisChemical process by which plants containing chlorophyll use sunlight to manufacture their own food by converting carbon dioxide and water into carbohydrates, releasing oxygen as a by-product.. During photosynthesis, green plants, including those that live in the water, use the energy in sunlight to combine carbon dioxide and water to produce carbohydrates and oxygen. The oxygen is expelled by the plant and enters the water.
The level of dissolved oxygen in water can reach as high as 8 or 9 parts per million. The United States Environmental Protection Agency (EPA) considers water to be healthy if it contains at least 5 parts per million of dissolved oxygen. When the level falls below 4 parts per million, the water quality is considered to be poor. At 2 parts per million, fish become stressed and grow more slowly, and some die.