The removal of biodegradable material, suspended solids and certain cat ions or anions from the water mass by self-purification reactions has two main consequences.

1. It promotes a shift in the ratio of available free chemical and light energy within the system. The steady change of this ratio downstream of a source of organic pollution creates a gradient of ecological conditions which is super imposed on gradients of external factors such as slope, water depth and current velocity, bottom configuration, and temperature.

2. As a result of the above consequence, a biocenological gradient is formed, which is essentially characterized by an increasing ratio of photo-autotrophic to heterotrophic microphytes. This phytosociological shift implies automatically a change of the ratio of photosynthetic O2 production (P) and community respiration (R) with increasing travel distance of the river, a gradual decrease of the absolute amount of oxygen respired per volume of water takes place and will profoundly influence the oxygen balance and the absolute oxygen concentration in the river water.