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. They cannot rot without oxygen. The oxygen debt in the bottom layer will grow. Typically, a stratified pond undergoes fall turnover in September or October in Oklahoma. Weather activities can reduce the surface water temperature.