Question

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

Answer 1

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.