The amount of oxygen contained within the water of an aquatic environment is called dissolved oxygen. Oxygen dissolves into water due to vegetation within the aquatic environment and the atmosphere. When an aquatic environment is between two and three parts per million dissolved oxygen this puts stress within the organisms living there. If dissolved oxygen gets below two parts per million then there is a potential for organisms to die and this the main cause of fish kills.
Oxygen depletion occurs in aquatic environments more in summer due to warmer temperatures store less and has a higher requirement of dissolved oxygen. The main cause of oxygen depletion is due to excessive algae. This occurs from when there is an excess of nutrients mainly nitrogen and phosphorous that leads to an algal bloom. This causes an extremely low level of dissolved oxygen.
More information regarding dissolved oxygen: