Eutrophication

Eutrophication is the artificial enrichment of an aquatic system by the addition of nutrients[?]. The effect is often unintentional and harmful, indirectly causing the death of aquatic plants and animals.

It is primarily caused by the leaching of phosphate or nitrate containing fertilisers into lakes or rivers. Some algae and blue-green bacteria thrive on the excess ions and a population explosion known as algal bloom occurs. This growth is unsustainable, however. Perhaps because another nutrient becomes limiting, massive death among the bacteria happens, and they float to the surface.

Oxygen is required by all respiring animals in the water and it is replenished by photosynthesis of green plants. The oxygen level is already low because of the population explosion and further oxygen is taken up by microorganisms which feed off the dead algae. The layer of dead algae also forms a surface layer through which little light can pass, meaning the rate of photosynthesis decreases and the oxygen level becomes depleted. Due to this, fish suffocate. The new anaerobic conditions can promote growth of bacteria such as Clostridium botulinum which produces toxins deadly to birds and mammals.