Eutrophication occurs when water is enriched by added nutrients getting into water bodies like rivers, dams or estuaries. This can be caused by run off from farmlands or chemical spills from mineral processing plants. Nitrate and phosphorus enrichment in particular causes water plants to bloom.
This fast growth is paired with fast decay of plant matter in the water which depletes the oxygen content of the water particularly at night when large amounts of oxygen is used by the plants during respiration. During the day, carbon dioxide is extracted from the water when photosynthesis occurs in the plants.
At times of oxygen depletion water animals like fish can die from lack of oxygen ie. Suffocation. Nutrient enrichment can affect the pH of the water and increase turbidity.
Lake Erie algal blooms, August 2011