The amount of oxygen in aquatic systems in dependent on several variables, these include the atmospheric pressure, the altitude, temperature, mineral concentration in the water, etc.
Looking at processes that consume oxygen in aquatic systems, oxygen is generally consumed during respiration and decomposition. When living organisms in the aquatic system breathe they consume oxygen and give off carbon dioxide. Plants, in addition to consuming oxygen during respiration also release oxygen by the process of photosynthesis. Decomposition of organic matter is also responsible for a large decrease in the level of oxygen in aquatic systems.
In addition to these naturally occurring processes, the release of several chemicals that are not naturally present in the aquatic systems can also drain the dissolved oxygen.