The answer is algal blooms.
Algae blooms are the consequence of large amounts of farm fertilizers in the rivers. Algae 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 could lead to the death of aquatic animals that depend on oxygen.
