What is the primary source of nutrient loading in lakes?
Runoff is a major pathway carrying nutrients from a lake’s surrounding watershed into the water. Rainfall or snowmelt moving over and through soil mobilizes phosphate, nitrate, ammonium, eroded sediment, manure, sewage residues, and fertilizer from farms, lawns, roads, and disturbed land. Streams and drainage systems then concentrate these materials and deliver them to the basin. Groundwater can also provide important nutrients in particular lakes, and direct atmospheric deposition or rainfall contributes nitrogen, but watershed runoff is usually the broad primary source considered in nutrient-loading questions. Aquatic animals mostly recycle nutrients already present rather than supply the main external load. Once nutrient input exceeds uptake and long-term burial, phytoplankton and macrophyte production rises. The resulting organic matter is decomposed by microbes, which may consume deep-water oxygen and promote harmful blooms, a sequence called cultural eutrophication when accelerated by humans. The magnitude of runoff loading depends on land use, slope, soil, rainfall intensity, vegetation cover, and riparian buffers. Thus, hydrological transport from the catchment directly connects terrestrial nutrient sources to lake productivity.
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 2-3