Subtitles section Play video Print subtitles Eutrophication is the process that can happen in a water body, like a river or a lake, when too many nutrients are added to the system. Eutrophication starts with fertilizers, which are rich in nutrients like nitrate and phosphate, are washed into the river or lake system. This leads to an increase in nutrient concentrations in the water. The nutrients are food for algae. The algae grow and reproduce quickly, forming a thick green bloom in the water. This algal bloom will absorb sunlight shining on the water so the sunlight can't reach the bottom. Plants who need this light to photosynthesis will die. The algae will also start to die when they eat up all of the nutrients and run out of food. Next, bacteria start to breakdown the dead plants and algae. That releases more nutrients back into the water continuing the Algal Bloom Cycle. The bacteria, with a continuing supply of food, reproduce into much larger numbers consuming oxygen as they grow and respire. There is not much oxygen in the water to begin with so when the bacteria consume it quickly it might run out completely. Water without oxygen is called Anoxic. If the water turns anoxic all non-bacterial life in the water, including fish and other animals, will die. So, eutrophication happens when nutrients are added to water which causes an algal bloom, cutting off sunlight and feeding bacteria. The bacteria use up oxygen in the water which becomes an anoxic, causing everything living in the water to die. Eutrophication is one reason why we need to be carefull with fertilizers when we are growing crops.