When nutrients build up in a lake, eutrophication occurs, and this generally occurs in the top layer of a lake. The nutrients come both naturally and artificially and usually contain phosphates. The artificial nutrients can come from sewage and fertilizers, from agricultural runoff. Phosphorus from the phosphates causes algae to grow rapidly and spread throughout the top layer of the lake. Algal blooms have negative effects on both the aesthetics and the ecology of the lake. Aesthetically, the lake is not pleasing because it is covered with algae. Ecologically, eutrophication causes organisms in the lake to die because the algae deplete the dissolved oxygen in the lake.