Large dams or reservoirs occur in humid, populated areas of the world. Their biological productivity and diversity is generally limited due to their depth and frequent and large changes in water level. Shallow zones have the highest diversity, with simple food webs of algae, waterbugs, birds, frogs and aquatic plants, often supporting introduced fish species. Plankton live at the surface, but life is scarce in the depths. Algal blooms may be common if there are high nutrient inputs from rivers.