What is eutrophication and how could it negatively impact plant life in rivers?

Eutrophication is the process whereby excess fertiliser is added to fields by farmers. This fertiliser contaisn nitrates which are very sioluble and so easily runs off into rivers and lakes. Plants use nitrates to sythesise proteins and therefore excess nitrates cause plants to grow very quickly. Algal blooms often are a result, algae covers the water with a layer of algae and prevents any light from reaching the plants living on the river/lake bed. This means that the plants cannot photosythesise, due to lack of light so they die.