Over-enrichment of nutrients in a water body chiefly due to pollution by sewage, run-off from the land, and industrial wastes (e.g. inorganic nitrates and phosphates). These compounds act as nutrients, stimulating algal growth which may result in algal blooms. Their subsequent decomposition reduces the oxygen content in the water, so killing animals with a high oxygen requirement. Much of the nitrate and phosphate settles to the bottom, to promote further growth at a later stage.