Fertiliser washed into a pond causes a thick green layer of algae to build up over the surface of the water. Below the surface, water plants begin to die. H...
Question 1 Report
Fertiliser washed into a pond causes a thick green layer of algae to build up over the surface of the water. Below the surface, water plants begin to die. How does the thick layer of algae cause the water plants beneath it to die?
Answer Details
The fertiliser washed into the pond supplies nitrate and phosphate, the nutrients that had been limiting algal growth, so the algae multiply rapidly across the surface. The mat they form is thick and opaque, so it blocks sunlight from reaching the water beneath, and the plants below can no longer photosynthesise. Without photosynthesis they make no glucose, run out of energy and die.
This is the first stage of eutrophication. Later, bacteria decompose the dead material and use up dissolved oxygen, which harms fish as well.
The algae shade the plants rather than lighting them, and nothing they do adds oxygen to the deep water. In the exam, link the bloom to light first, then to oxygen.
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