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 normally 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. The major consequence of eutrophication are algal blooms. 