The increased nutrients promote rapid growth of algae when they are deposited in rivers, lakes and streams. The excessive growth of algae is often called an algal bloom.
Algal blooms endanger the ecosystem in 2main ways. 1, The densely populated algae cover the water surface and block sunlight. Unable to carry out photosynthesis, the aquatic plants die. When these plants die, aquatic animals lose their food sources and habitats. As a result, the food chain and ecosystem change. Algae also reduce the level of dissolved oxygen in the water. 2, When the algae and other aquatic organisms die, decomposing organisms especially saprophytic bacteria use up oxygen to decompose the dead organic materials. This reduce the oxygen content in the water.
A depletion in oxygen results in an increase in biochemical oxygen demand (BOD). Eutrophication increase the BOD value in the water. BOD refers to the amount of oxygen utilised by microogranisms to oxidise all organic matter in one litre of water. The higher the BOD value, the more polluted the water as the dissolved oxygen level decrease.