Nitrates and phosphates help plants to grow. However, if these nutrients accumulate in water, algae use them as a source of food and multiply rapidly. The lake soon becomes choked by a green slimy mass of green algae. This process is called 'eutrophication'.Algae use up oxygen. They also produce oxygen during photosynthesis.After a while algae start to die. Large number of bacteria feed on plant material and use up so much of oxygen that there is no oxygen left of other aquatic animals. This leads to death of these animals.Bacteria also thrive and eventually die. Thus, the water resource suffering from eutrophication eventually becomes stagnant and lifeless.