Biological Consequences of Water Pollution (Edexcel IGCSE Biology)

• Minerals from agricultural fertilisers can be leached into water bodies, such as lakes and streams, resulting in a process known as eutrophication 
• The following events occur during eutrophication:
  • fertilisers are high in nitrogen, an essential mineral for plant growth, so this can result in the overgrowth of aquatic plants and algae at the water surface
  • aquatic plants below the surface die due to reduced light levels, and are broken down by decomposers, e.g. bacteria and fungi
  • decomposers increase in number, and the increased respiration of these organisms uses up oxygen in the water, reducing dissolved oxygen levels
  • the water no longer contains enough oxygen to support other organisms, so many aquatic organisms die

Fertiliser run-off can cause eutrophication in lakes and rivers

• Water pollution can occur when sewage is washed into waterways, e.g. when sewers overflow or agricultural waste is washed off fields due to heavy rain
• This can have harmful effects on aquatic ecosystems
  • Sewage can cause an increase in growth of aerobic bacteria, which feed on biological waste
  • These bacteria reduce the availability of dissolved oxygen in water
  • Aquatic organisms that are sensitive to oxygen levels die, leaving only organisms that can survive at low concentrations of oxygen
  • The aquatic ecosystem decreases in biodiversity
• Sewage in waterways can also result in an increase in the number of pathogenic bacteria present