Water of a lake discoloured by blue green algae ^ICyanophyceae. ^i This phenomenon is also known as water bloom, an abnormal growth of algae. It may have natural causes, such as an unusual amount of sunshine, or it may be due to a surplus of phosphorus or nitrogen derived from human activities such as agriculture. Some species of ^ICyanophyceae^i liberate a harmful toxin for zooplankton, fishes, aquatic birds and human beings. Water bloom may also lead to eutrophication, the progressive consumption of oxygen in the water, resulting in the death of all animal and vegetable species. Photographed near Norwich, England.