Eutrophication is what a water-based ecosystem does when too many nutrients are added to it. Eutrophication is mainly caused by two nutrients, phosphorus and nitrogen. These are brought to aquatic ecosystems as runoff from fertilized agricultural areas, erosion from river banks, river beds, clearing of land (deforestation), or sewage that ends up in aquatic environments. This causes algal blooms, the accumulation of algaer. When the algae die, they deplete the dissolved oxygen in the water.