Lakes, resevoirs, rivers and streams contain organic matter like decaying leaves and sewage. This organic matter can be chemically broken down into nutrients by aquatic organisms such as bacteria, fungi, and phytoplankton. These decay organisms need oxygen in order to live, in order to perform respiration. It is this need that creates the demand in BOD.
The rate of decomposition cycles with the seasons, as the amount of dissolved oxygen varies with the temperature of the water. Fertile lakes, those with large amounts of organic matter, have a high BOD due to high rates of decay, and can develop an oxygen deficit. This can greatly affect the distribution of larger animal such as lake trout.