A large number of fish are suddenly found floating dead on a lake. There is no evidence of toxic dumping but you find an abundance of phytoplankton. Suggest a reason for the fish kill.
Large quantities of phosphates and nitrates increase the growth of phytoplanktons. These phytoplankton use so much oxygen that it is not sufficiently available for other organisms to use in respiration. Moreover, a large population of bacteria decomposes organic matter such as leaves, grass, etc., in water. During this process, they consume the oxygen dissolved in water. Microorganisms may become so abundant that they form a mat covering on the water surface and preventing sunlight from penetrating the surface and thereby preventing photosynthesis. In all these processes concentration of dissolved oxygen in water decreases. When the concentration of dissolved oxygen of water is below 6 ppm, the growth of fish gets inhibited and they cannot survive.