Eutrophication is when there are excessive amounts of nutrients in a lake. The effects of this are that algae will bloom (grow quickly). Having a lot of algae will mean that there is not enough oxygen for other organisms, they will also struggle to find enough light as algae covers the surface. More organisms will die then usual- more algae to die/ less oxygen and light so fish die- so decomposers will thrive; these decomposers will also use a lot of oxygen from the water. In the end there will not be enough oxygen for fish.
Nutrient get leached into rivers from soil as rain water runs off land into rivers and lakes taking nutrient with it. If fertiliser has been put in the soil then the soil will be rich in certain nutrient, especially nitrogen: so rain water runs off fertilised soil it will bring high amounts of nutrient into surrounding rivers or lakes causing eutrophication.