Thus O2 is more uniform in the air we breathe. In a watershed, the oxygen available is influenced by the temperature variability through out the stream, the pH changes, the riffles which mix the water with air, and the surface area exposed to air. Also the partial pressure of oxygen in the air above the water affects the DO in water. Therefore watersheds at higher elevations with less oxygen in the air will have less DO in the water. 2 Biological processes such as respiration and photosynthesis can significantly affect O2 concentrations.
Photosynthesis usually increases the DO in water.
For example, bacteria consume organic pollutants and consume excessive amounts of oxygen in the process, thus lowering DO levels in the water. Productivity is seen as a measure of how much photosynthesis occurs by the plants living in the water at any given time. The primary productivity of an ecosystem is defined as the rate at which sunlight is stored by plants in the form of organic compounds. Only those organisms possessing the pigment chlorophyll can use solar energy to make new organic compounds. We will use the light and dark bottle method to measure the rate of oxygen production.