What happens during eutrophication?

The addition of nutrients such as nitrate means that nutrients no longer limit plant growth.Algae begins to bloom which blocks out sunlight. As a result, plants cannot photosynthesise (or photosynthesis is reduced) and they begin to die. As a result, the oxygen levels in the water fall.The dead plants are decomposed by microorganisms whose aerobic respiration further depletes oxygen.Eventually, the oxygen levels drop so significantly that fish cannot respire and die as a result.The level of anaerobic organisms increases, further decomposition occurs and the water becomes putrid.