Your neighbor has fertilized her lawn. several weeks later, she is alarmed to see that the surface of her ornamental pond, which sits at the bottom of the sloping lawn, is covered with a green layer of algae

The reason why there is a green layer of algae was because of the reason that there may be too much fertilizer applied in the lawn. When having too much fertilizer, and it had been rained down—there could have been a possibility of run-off to be flown down on the lawn of which has been sloped and made its way on the pond. By this, it increases the nutrients in the pond, making the algae to grow on it.