In eutrophication, fish die because

  1. algae poison them
  2. water becomes too cold
  3. nitrates kill them directly
  4. dissolved oxygen falls as decomposers respire
Answer

D) dissolved oxygen falls as decomposers respire

Decomposers use up oxygen. O Level Biology · Topic 19 Relationships of Organisms with One Another and with the Environment · 19.4 Effects of humans on ecosystems

Biology MCQ · More O Level Biology: Organisms and the Environment MCQs · Practise