Eutrophication explained - WELS (Waterpedia Environmental Learning Series)

Oh, in a body of water like a river or lake, when too many nutrients are added to the system,

eutrophication

begins when nutrient-rich fertilizers like nitrates and phosphates are washed into the river or lake system, leading to a increase in nutrient concentrations in the water the nutrients are food for the algae and the algae grow and reproduce rapidly forming a thick green bloom in the water this algae bloom will absorb sunlight shining into the water so that the light of the sun cannot reach the bottom the plants that need this light to carry out photosynthesis will die the algae will also begin to die when they consume all the nutrients and run out of food, then the bacteria will begin to decompose the dead plants and algae and that will release more nutrients back into the water, continuing the algae bloom cycle, the bacteria with a continuous food supply reproduce in much larger quantities by consuming oxygen as they grow and respire.

There isn't much oxygen in water to begin with, so when bacteria consume it quickly, it may be completely depleted. Water without oxygen is called anoxic if the water becomes completely anoxic. Bacterial life in the water, including fish and other animals, will die, so

eutrophication

occurs when nutrients are added to the water, causing an algal bloom that cuts off sunlight and feeds the bacteria. Bacteria consume oxygen in the water, which becomes anoxic and causes everything that lives in the water to become anoxic. Eutrophication is one of the reasons why we must be careful with fertilizers.