The amount of oxygen in water is influenced by flow and turbulence as well as temperature and pressure. For example, cold water will hold more oxygen than warm water. The percent saturation determines how readily the oxygen will cross gill or other respiratory surfaces and therefore how available it is to aquatic organisms. Organisms vary tremendously in there requirements for oxygen. Some stream organisms that live in still water or wetlands may be adapted to much lower levels.
All aquatic organisms have a range of pH within which they can exist.
Changes in pH, for example, by acid precipitation can lead to the changes in the community structure of an aquatic system. Temperature - Aquatic organisms may be found over a wide range of temperatures from hot springs where some bacteria exist in water that is close to the boiling point to water that is always ice covered. Different organisms are adapted to particular ranges of temperature. Increasing temperature by addition of hot water to natural systems may cause native species to be eliminated. Warm water contains less oxygen and the increased temperature will increase the metabolic activities of many of the organisms in this water.