What does eutrophication mean? A simple definition of it

Eutrophication is the process in which a water-body acquires a large concentration of nutrients, specifically phosphates and nitrates. As is known, both these nutrient compounds promote the growth of excess algae in water-body. Due to this excess growth of algae, which usually grow on the surface of water, they reduce the amount of sunlight reaching aquatic plants, which photosynthesize to produce the oxygen required by marine animals in order to survive. Hence, the excess of growth of algae has a negative impact on life of many aquatic organisms.

A (very) brief definition of Eutrophication is the over-enrichment of a river or lake with nutrients, typically as a result of surface run off from nearby fields that have been fertilised artificially.

This leads to excessive algae growth, which, through a lot of steps, basically kills everything in the lake.