Eutrophication can be a natural process and occurs naturally through the gradual accumulation of sediment and nutrients.  Naturally, eutrophication is usually caused by the natural accumulation of nutrients from dissolved phosphate minerals and dead plant matter in water.
Natural eutrophication has been well-characterized in lakes. Paleolimnologists now recognise that climate change, geology, and other external influences are also critical in regulating the natural productivity of lakes. A few artificial lakes also demonstrate the reverse process (meiotrophication), becoming less nutrient rich with time as nutrient poor inputs slowly elute the nutrient richer water mass of the lake. This process may be seen in artificial lakes and reservoirs which tend to be highly eutrophic on first filling but may become more oligotrophic with time. The main difference between natural and anthropogenic eutrophication is that the natural process is very slow, occurring on geological time scales.