Eutrophication: in fresh water environment
Eutrophication is the process of excess nutrients building up in a lake or pond of stagnant water. During this process, the excess nutrients fuels the rapid growth of large amounts of algae, which in turn covers the surface and prevents sunlight from reaching plants under this thick, mat-like layer. This stops plants from carrying out photosynthesis, thus reducing the amount of oxygen in the water and causing other organisms and fish to die off. If no more nutrients are introduced to the pond or lake, the ecosystem will return to normal.