Step 1: Fertilisers contain nitrates and phosphates, which are plant nutrients. Step 2: When they enter a pond, algae multiply rapidly and form a thick green layer (algal bloom). This process is called eutrophication. Step 3: The layer blocks sunlight, so underwater plants die. Bacteria decomposing the dead matter use up the dissolved oxygen. Step 4: With less oxygen, fish and other aquatic animals die. Answer: Algal bloom, fall in dissolved oxygen and death of fish.