Water is purified in large part by the routine actions of living organisms.
Energy from sunlight drives the process of photosynthesis in aquatic plants, which produces oxygen to break down some of the organic material such as plant and animal waste. This decomposition produces the carbon dioxide, nutrients and other substances needed by plants and animals living in the water. The purification cycle continues when these plants and animals die and the bacteria decompose them, providing new generations of organisms with nourishment. Unfortunately, there are many toxic substances which are affected only slowly, or not at all, by this and other processes. These are called persistent and are of great environmental concern.