Why is the runoff from fertilized agricultural fields, even if free of pesticides, often harmful to the ecosystems of temperate lakes?

Fertiliser contains nutrients for plant growth, typically nitrogen and phosphorus. Much of the fertiliser applied to farmland washes off into rivers and lakes during rainfall events. A high concentration of nutrients in a river or lake is called eutrophication. Algae, like plants, need nutrients to grow, and their growth in lakes is usually limited by a lack of nutrients. This means, any additional input of nutrients into a lake can result in an algal bloom. An algal bloom typically changes the trophic and ecological state of a lake, and can be harmful to other organisms who are adapted to less nutrient-rich conditions. Once the algae deplete the nutrients, a mass algal die-off will occur. The algal biomass will decompose, resulting in anoxic conditions, which places additional stress on aquatic organisms.