Eutrophication (DP IB Biology)

Eutrophication

• When lakes, rivers, estuaries and coastal waters receive artificially large inputs of nutrients, such as nitrates and phosphates, this results in excess growth of plants and phytoplankton, e.g. algae

  • For example, when the mineral ions from excess fertilisers leach from farmland into waterways

• This growth of algae, known as an algal bloom, can block out sunlight and stop it penetrating below the water surface, so aquatic plants below the surface of the water start to die as they can no longer photosynthesise

  • The algae also start to die when competition for nutrients becomes too intense

• As aquatic plants and algae die in increasing numbers, decomposing bacteria feed on the dead organic matter and also increase in number

• As they respire aerobically, these bacteria use up the dissolved oxygen in the water

  • The respiring bacteria create an increased biochemical oxygen demand, or BOD

• As a result, the availability of dissolved oxygen in the water rapidly decreases, so aquatic organisms such as fish and insects may be unable to survive

  • Dead zones in both oceans and freshwater can occur when there is not enough oxygen to support aquatic life

Eutrophication process diagram

Eutrophication can result from the leaching of agricultural fertilisers into lakes and rivers