At other times, cloudy skies block sunlight. Sometimes algae below the surface can't get enough oxygen to live. If enough die simultaneously, the rotting of their bodies uses all the oxygen. Stratification occurs when water at the pond bottom can't circulate to the surface. The surface layer of water becomes warmer than the bottom layer.
Cool water is heavier than warm water.
The weight difference resists circulation. Therefore, oxygen rich surface water and oxygen poor bottom water do not mix. All the plants and animals that die in the pond, sink to the bottom. Bacteria use all the oxygen in the bottom layer of water to rot the bodies. More bodies will continue to settle to the bottom.