Describe how fertiliser run-off can cause damage to a lake's ecosystem.

Fertilisers are generally high in nitrogen, to replenish the nitrogen in the soil assimilated by the crop plants. When this is drained into lakes in run-off, it can cause dramatic consequences. Nitrogen availability is usually the limiting factor of surface algae growth. As the nitrogen concentration increases, it becomes an excess, and the algae reproduce uncontrollably. This forms an algal bloom at the surface of the lake. The thick layer of algae absorbs and reflects all light that hits the lake, so photosynthetic organisms below the algae are deprived of light and die. This wide spread death in the lake reduces the biodiversity and alters food chains dramatically, destroying any ecosystem stability. It also creates a large amount of detritus, which feeds the breeding of anaerobic, decomposing microorganisms. These respire anaerobically with toxic waste products, causing the water to be toxic and killing most of the organisms left in the lake. This process is known as eutrophication.