Oxygen

**__ Oxygen __** What is Oxygen? 
 * Oxygen enters the water from air.
 * Oxygen is produced during photosynthesis, and consumed during respiration and decomposition.
 * Usually expressed as mg/L.
 * Found in microscopic bubbles of oxygen that are mixed in the water.
 * The amount of dissolved oxygen in a body of water depends on the time of day, the speed of the current, and temperature. For example. as the temperature increases the oxygen that will dissolve in water at atmospheric pressure decreases.

How does oxygen affect aquatic life?


 * Organisms such as fish, insects, spiders, and worms need oxygen to breath.
 * Ways in which organisms absorb oxygen are: Fish as water passes over their gills, beetles and other insects get oxygen through air tubes, or directly from the water. Worms absorb oxygen through their skins.
 * Each organism depends on a certain amount of dissolved oxygen, depending on its species, the temperature of the water, pollutants present, and the state that the organism is in.
 * Organisms have adapted to survive with a lower amount of oxygen.
 * Some organisms can increase the current that flows over their respiratory systems. This helps the organisms, because when the current increases, more oxygen is available.
 * The generally accepted minimum amount of dissolved oxygen that will support a large population of various fishes is from 4 to 5 mg/l.



How human activities have impacted oxygen.
 * Human activities have decreased the amount of dissolved oxygen in a river, by fertilizer running off of a farm field, or lawns. Since fertilizer is used to to grow plants, it will do the same for aquatic plants, and when there is a time period of cloudy days, the plants will use much of the oxygen, while failing to photosynthesize, because there is no sun.
 * Once, this happens there is an increased amount of aquatic plants that will eventually die, and their large amounts of bacteria will use large amounts of dissolved oxygen.
 * Low levels of this oxygen, may also be because of bacteria, such as untreated sewage, that may enter the water, and will use up large amounts of dissolved oxygen.