Algae blooms and the rapid proliferation of photosynthetic plankton are the consequence of eutrophication. Eutrophication is a response of an ecosystem to the excess of nutrients which comes from fertiliser run-off. Algae and photosynthetic plankton use the presence of increased nutrients and enhance their own growth. It is known that they produce an oxygen in the photosynthesis, so it is not a problem while they are still alive. But when these organisms die, their decomposition begins. For the process of decomposition, the oxygen is needed and ultimately it will be depleted from a body of water. This will lead to the death of aquatic animals that depend on oxygen.