Eutrophication and water quality of lakes: Control by water residence time and transport to sediments

Steady-state concentration in lake water is determined by the rate of input, residence time of water, and rate of removal into lake floor sediments. For a given input (loading) rate, a steady-state concentration is a ‘warning sign’ or a limit to which the lake will tend. For 22 lakes in North America and Western Europe, the upper limits of steady-state concentrations of phosphorus are in the ‘eutrophic’ range from 0·03 to 0·3 mg P/l., at the existing input rates of 10 ⁻²to 10⁻¹mg 1.⁻¹year⁻¹. A natural baseline input of phosphorus to lakes, owing to the chemical weathering of continents, is lower, 10⁻⁴to 10⁻³mg 1.⁻¹year⁻¹. The rates of removal of phosphorus to lake floor sediments are of the order of 25–50 per cent of input load. For sodium and chloride, mean rate of removal into sediment is less than 0·5 per cent of input load; for Sr⁹⁰, 1–4 per cent; and for Cs¹³⁷, 25–65 per cent or higher. The higher rates of removal into sediments result in lower concentration in lake water.