Eutrophication is a natural process that occurs to all lakes over time as the weathering of rocks and soils from the surrounding catchment area leads to an accumulation of nutrients in the water and associated sediments.
Young lakes (and man made reservoirs) usually have low levels of nutrients and correspondingly low levels of biological activity. Such lakes are referred to a being oligotropic from the Greek work oligos meaning little or few. Literally oligotrophic means little-nourished.
Old lakes usually have high levels of nutrients and correspondingly high levels of biological activity. Such lakes are referred to as being eutrophic from the Greek word eu meaning well. Literally eutrophic means well-nourished.
The natural time scale for the aging of a lake from being oligotrophic to eutrophic is of the order of thousands of years. However, a high rate of input of nutrients (from human activities) can increase the rate of aging significantly resulting in eutrophic conditions developing after only a few decades. This artificial eutrophication has already happened in many parts of the world including the Norfolk Broads and parts of Holland, Denmark and Norway.
To renew all the water in a lake may take up to a hundred years compared to a few days for the renewal of the water in a river. Consequently, lakes are particularly susceptible to pollution such as artificial eutrophication.