There are several geographic factors (as in the location of the water) that can reduce the amount of dissolved oxygen in water. For example, the faster a body of water is flowing, the more dissolved oxygen it has because it gains oxygen bubbles as it churns over rocks; thus, slower moving bodies of water have less dissolved oxygen. Additionally, water at higher altitudes has less dissolved oxygen because atmospheric pressure makes it more difficult for oxygen to dissolve.
A variety of organisms can affect the amount of dissolved oxygen in water. Plants are a major factor in both creating and absorbing dissolved oxygen because they release dissolved oxygen during photosynthesis, and absorb it during respiration. Thus, if a body of water has a lot of plants, it will have a noticeable reduction in dissolved oxygen at night because plants go through photosynthesis during the day. Additionally, the more fish there are in a body of water, the less dissolved oxygen there is because they are breathing in the oxygen. Also, organic waste from fish will cause the amount of dissolved oxygen to drop because the bacteria in the waste will absorb the oxygen.
Temperature is also a factor in reducing the amount of dissolved oxygen in water. The higher the temperature, the less dissolved oxygen. This is because the temperature causes the water's volume to reduce, and thus oxygen is released from the water. Other climate factors can affect the amount of dissolved oxygen. For example, if there is not a lot of rain, there will be less dissolved oxygen because water levels will decrease, and thus the aggregate amount of dissolved oxygen will be less.
Human beings have also invented several conditions that can reduce the amount of dissolved oxygen in water, with the most common being vacuum sealed environments, which do not occur naturally on Earth. For example, a joint experiment by the University of South Wales and the Norwegian University of Science and Technology found that a vacuum-sealed environment, with the chemical polymethylpentene added, would reduce dissolved oxygen in water. Also, Lucent Technologies has created a "Membrane Contractor" system that reduces dissolved oxygen by forcing the water through a vacuum tower.