Eutrophication is a natural aging process for most lakes and ponds. It involves the accumulation of nutrients in the water and the bottom sediments. As the nutrient availability increases, more plant growth is supported in the lake; this can include both benthic, or bottom-rooted, plants, and also those that float. When plant growth increases, shed or dead plant material falls to the bottom and as it decays it contributes to even more plant growth. Over time, the lake may accumulate so much sediment that it fills in and becomes a swamp or a meadow. Generally this takes many centuries or even millenia to occur.
Natural eutrophication is a very slow process, and is somewhat temperature dependent. Cultural eutrophication happens when the amount of nutrients in the water and/or the water temperature are changed due to human activity, and the eutrophication process begins to run at high speed.
Runoff from agriculture and from lawns, sewage disposal, erosion, and chemicals and waste heat from industry all are important contributors to cultural eutrophication.