Eutrophication

• A large lake is surrounded by fields. These fields are separated from each other by hedges. One

  hundred years ago the lake was a habitat for many plants, invertebrates and fish. Today the lake has

  no fish and few plants or invertebrates.

  Explain how increased use of inorganic fertilisers on the fields may have led to these changes. (5)

  1. leaching of nutrients
  2. leads to increase growth of algae
  3. competition for light
  4. death of algae
  5. increases food supply
  6. respiration of microorganisms uses up oxygen
  7. fish die due to lack of oxygen
• Nitrate from fertiliser applied to crops may enter ponds and lakes. Explain how nitrate may

  cause the death of fish in fresh water. (5)

  1. Growth of algae blocks light
  2. No photosynthesis so submerged plants die
  3. Saprobiotic microorganisms
  4. Aerobically respire
  5. Less oxygen for fish to respire