Dear Student,
Eutrophication can have serious, long-term effects on the ecosystem. Algal blooms is the most notable feature of eutrophication. When an algal bloom occurs, the stream, lake, river or ocean becomes covered with algae, which is usually bright green in color. In addition to looking pretty ugly, it also blocks light from reaching the water surface. This phenomenon prevents the aquatic plants from photosynthesizing, a process which provides oxygen in the water to animals that need it, like crabs and fishes.
Regards.