Algae bloom

A phenomenon whereby excessive nutrients within a river, stream, or lake causes an explosion of plant life that results in the depletion of the oxygen in the water needed by fish and other aquatic life. Algae bloom is usually the result of urban runoff (of lawn fertilizers, etc.). The potential tragedy is that of a “fish kill,” where the stream life dies in one mass execution.

Source: Handbook of water and wastewater treatment plant operations