When the water in a stream enters a pond, microorganisms in the pond begin to metabolize (break down) organic matter, consuming oxygen in the process. This is another form of oxygen cycle - oxygen enters water in rapids and leaves water in pools.
Oxygen uptake rate (O.U.R.) is the rate at
which oxygen is consumed by living organisms in the water. Since
organisms are constantly using up oxygen in the water and oxygen
is constantly reentering the water from the air, the amount
of oxygen in water remains relatively constant. In a healthy
ecosystem, the rates of oxygen transfer (being used up) and
oxygen uptake are balanced in the water.