Pollution is usually harmful to one or more species of animal or plant.
However, releasing nutrients into rivers may be beneficial to some organisms while it is harmful to others. Sewage contains lots of organic substances which are a source of food for bacteria and fungi. The result is that the bacteria and fungi thrive. Since they use up all the oxygen in the water, most of the animals will die. I hope that you will be able to remember most of the facts on this page, but you must memorise the definition of pollution.