The hypolimnetic oxygen depletion rate describes changing dissolved oxygen concentrations in the hypolimnion (lowest stratum) of lakes and reservoirs. Dissolved oxygen concentrations in the hypolimnion are especially significant because of their effect on fish. Dissolved Oxygen: The amount of oxygen that is dissolved in water. This term also refers to a measure of the amount of oxygen available for biochemical activity in a waterbody, and is an indicator of the quality of that water