Once absorbed, oxygen is either incorporated throughout the water body via internal currents or is lost from the system. Flowing water is more likely to have high dissolved oxygen levels than is stagnant water because of the water movement at the air-water interface. Because stagnant water undergoes less internal mixing, the upper layer of oxygen-rich water tends to stay at the surface, resulting in lower dissolved oxygen levels throughout the water column. Oxygen losses readily occur when water temperatures rise, when plants and animals respire, and when microbes aerobically decompose organic matter. Dissolved oxygen may play a large role in the survival of biota in temperate lakes and reservoirs during the summer months, due to a phenomenon called stratification.
Seasonal stratification occurs as a result of water's temperature-dependent density.
As water temperatures increase, the density decreases. At the beginning of the summer, the hypolimnion will contain more dissolved oxygen because colder water holds more oxygen than warmer water. However, as time progresses, an increased number of dead organisms from the epilimnion sink to the hypolimnion and are broken down by microorganisms. Continued microbial decomposition eventually results in an oxygen-deficient hypolimnion. If the lake is in a eutrophic state, this process may be accelerated and the dissolved oxygen in the lake could be depleted before the summer's end.